physical hydraulic model investigation of critical submergence for

Thesis presented in partial fulfilment of the requirements for the

degree of Master of Engineering (Civil) at Stellenbosch University

PHYSICAL HYDRAULIC

MODEL INVESTIGATION OF

CRITICAL SUBMERGENCE

FOR RAISED PUMP INTAKES

by

SH KLEYNHANS

FEBRUARY 2012

DEPARTMENT OF CIVIL ENGINEERING

STUDY LEADER:

Prof. GR BASSON

ii

PHYSICAL HYDRAULIC MODEL INVESTIGATION OF CRITICAL SUMBERGENCE FOR RAISED PUMP INTAKES

SINOPSIS

Oor die afgelope vier dekades is verskeie ontwerpriglyne vir die berekening van minimum

watervlakke, om werwelvorming by pompinlate te voorkom, gepubliseer. Hierdie ontwerpriglyne

vereis dat die klokmond van die pompinlaat nie hoër as 0.5 keer die deursnee van die klokmond

(D) bokant die kanaalvloer geleë moet wees nie.

Sandvang kanale vorm ‘n integrale deel van groot riveronttrekkingswerke, met pompinlate wat

aan die einde van hierdie kanale geleë is. Die kanale word aan die stroomaf kant van die

pompinlaat voorsien met sluise sodat die kanale gespoel kan word. Hierdie sluise is tipies

1.5 m hoog. Dit is derhalwe nodig om die hoogte onder die klokmond dieselfde te maak as die

hoogte van die sluis sodat die klokmond die spoelwerking nie beïnvloed nie. Die vraag is egter

– wat is die impak op die minimum vereiste watervlakke indien die klokmond op ‘n hoër vlak

installeer word?

‘n Fisiese hidrouliese model met ‘n 1:10 skaal is gebruik om die minimum watervlakke te bepaal

waar tipes 2, 5 en 6 werwels aangetref word vir prototipe inlaatsnelhede van 0.9 m/s tot 2.4 m/s

en klokmond hoogtes van 0.5D, 1.0D en 1.5D bokant die kanaalvloer. Vier klokmond

konfigurasies is getoets. Die minimum vereiste watervlakke was die laagste vir die tradisionele

plat klokmond met ‘n lang radius buigstuk en was dus die voorkeur klokmond.

Die eksperimenttoetsresultate vir die voorkeur klokmond is met die gepubliseerde

ontwerpriglyne vergelyk om te bepaal watter van die ontwerpsriglyne van toepassing sal wees

vir verhoogde klokmond installasies.

Uit die eksperimenttoetsresultate is dit duidelik dat die vereiste watervlakke skielik verhoog

sodra die klokmond installasie ‘n seker hoogte bokant die kanaal vloer oorskry. Daar is bevind

dat hierdie verskynsel by al vier klokmond konfigurasies voorkom sodra die verhouding tussen

die prototipe klokmond inlaatsnelheid teenoor die snelheid in die kanaal hoër as 6.0 is.

Daar word aanbeveel dat die minimum vereiste watervlak vir pompinlate met dieselfde

geometrie as die fisiese model, met Knauss (1987) se vergelyking bereken word, naamlik

S = D(0.5 + 2.0Fr), waar die snelheidsverhouding tussen die klokmond en kanaal 6.0 nie

oorskry nie, en dat die vergelyking gepubliseer deur die Hydraulic Institute (1998),

S = D(1 + 2.3Fr), gebruik word waar die snelheidsverhouding 6.0, so bereken met Knauss

(1987) ser vergelyking, wel oorskry. Die prototipe klokmond inlaatsnelheid moet ook beperk

word tot 1.5 m/s.

Stellenbosch University http://scholar.sun.ac.za

iii


SYNOPSIS

Various design guidelines have been published over the past four decades to calculate the

minimum submergence required at pump intakes to prevent vortex formation. These design

guidelines also require the suction bell to be located not higher than 0.5 times the suction bell

diameter (D) above the floor.

Sand trap canals are an integral part of large river abstraction works, with the pump intakes

located at the end of the sand trap canals. The canals need to be flushed by opening a gate,

typically 1.5 m high, that is located downstream of the pump intake. This requires the suction

bell be raised to not interfere with the flushing operation, which leads to the question – what

impact does the raising of the suction bell have on the minimum required submergence?

A physical hydraulic model constructed at 1:10 scale was used to determine the submergence

required to prevent types 2, 5 and 6 vortices for prototype suction bell inlet velocities ranging

from 0.9 m/s to 2.4 m/s, and for suction bells located at 0.5D, 1.0D and 1.5D above the floor.

The tests were undertaken for four suction bell configurations with a conventional flat bottom

suction bell, fitted with a long radius bend, being the preferred suction bell configuration in terms

of the lowest required submergence levels.

The experimental test results of the preferred suction bell configuration were compared against

the published design guidelines to determine which published formula best represents the

experimental test results for raised pump intakes.

It became evident from the experimental test results that the required submergence increased

markedly when the suction bell was raised higher than a certain level above the floor. It was

concluded that this “discontinuity” in the required submergence occurred for all the suction bell

configuration types when the ratio between the prototype bell inlet velocity and the approach

canal velocity was approximately 6.0 or higher.

It is recommended that, for pump intakes with a similar geometry to that tested with the physical

hydraulic model, critical submergence is calculated using the equation published by Knauss

(1987), i.e. S = D(0.5 + 2.0Fr), if the prototype bell inlet velocity/approach canal velocity ratio is

less than 6.0, and that the equation published by the Hydraulic Institute (1998), i.e.

S = D(1 + 2.3Fr), can be used where the ratio, as determined with Knauss’ (1987) equation,

exceeds 6.0. It is also recommended that prototype bell inlet velocities be limited to 1.5 m/s.


iv


ACKNOWLEDGEMENTS

The author would like to acknowledge the contributions of the following individuals and funding

institution:

Prof GR Basson, for the support and guidance he provided as study leader;

The laboratory personnel of Stellenbosch University, especially Christiaan Visser, for

arranging the construction of the physical model and assisting with modifications during the

test phase;

TCTA, for funding the construction of the physical hydraulic model;

Charlie Espost, for providing the pumpset and variable speed drive;

My colleague, Schalk van der Merwe, for encouraging me to enrol for a Master’s degree and

for his continuous support at work while I completed my studies on a part-time basis; and

My wife, Maré – this thesis would not have been possible without your support and

encouragement over the past two years.


v


CONTENTS

DECLARATION ........................................................................................................................... i

SINOPSIS ................................................................................................................................... ii

SYNOPSIS ................................................................................................................................ iii

ACKNOWLEDGEMENTS .......................................................................................................... iv

CONTENTS ................................................................................................................................ v

LIST OF FIGURES .................................................................................................................. viii

LIST OF TABLES ..................................................................................................................... xiii

LIST OF ABBREVIATIONS ....................................................................................................... xv

1. INTRODUCTION ................................................................................................................ 1

1.1 Background to the research project ........................................................................................ 1

1.2 Motivation for the study ............................................................................................................. 3

1.3 Objective of study ....................................................................................................................... 7

1.4 Layout of the thesis .................................................................................................................... 7

2. METHODOLOGY ................................................................................................................ 9

2.1 Physical model study ................................................................................................................. 9

2.2 Modelling scenarios ................................................................................................................... 9

2.3 Criteria to decide on minimum submergence in proto-type ............................................... 12

3. LITERATURE REVIEW ......................................................................................................13

3.1 Fundamentals and theory of vortex formation ..................................................................... 13

3.1.1 Introduction ........................................................................................................................ 13

3.1.2 Vortex classification and strengths ................................................................................ 15

3.1.3 Acceptance criteria for pump intakes ............................................................................ 16

3.1.4 Dimensionless parameters ............................................................................................. 17

3.2 Impacts of scale effects in physical models ......................................................................... 20

3.2.1 Physical similarity ............................................................................................................. 20

3.2.2 Similarity laws ................................................................................................................... 22

3.2.3 Modelling criteria .............................................................................................................. 26

3.2.4 Scale of proposed physical model ................................................................................. 28


vi


3.3 Design guidelines to calculated critical submergence ........................................................ 30

3.3.1 The hydraulic design of pump sumps and intakes (Prosser, 1977) ......................... 30

3.3.2 Centrifugal pump handbook (Sulzer Brothers Limited, 1987) ................................... 30

3.3.3 Pump intake design (Hydraulic Institute, 1998) ........................................................... 30

3.3.4 Pump station design (Jones et al., 2008) ..................................................................... 30

3.3.5 Pump handbook (Karassik et al., 2001) ........................................................................ 31

3.3.6 Swirling flow problems at intakes (Knauss, 1987) ....................................................... 31

3.3.7 Gorman-Rupp design guidelines (Strydom, 2010b) ................................................... 31

3.3.8 KSB design guidelines (Gouws, 2010) ......................................................................... 32

3.3.9 Collection and pumping of wastewater (Metcalf and Eddy, 1981, provided by

Strydom, 2010a) ............................................................................................................................... 32

3.3.10 Design recommendations for pumping stations with dry installed submersible

pumps (Flygt, 2002) ......................................................................................................................... 33

3.3.11 Werth and Frizzell (2009) ................................................................................................ 33

4. EXPERIMENTAL SET-UP AND TEST PROCEDURES .....................................................36

4.1 Experimental set-up ................................................................................................................. 36

4.2 Instrumentation ......................................................................................................................... 41

4.3 Test procedure .......................................................................................................................... 42

4.3.1 Submergence for different types of vortices ................................................................. 42

4.3.2 Dye injection tests ............................................................................................................ 45

4.3.3 ADV measurements ......................................................................................................... 48

5. EXPERIMENTAL TEST RESULTS ....................................................................................52

5.1 Experimental test results ......................................................................................................... 52

5.2 Repeatability of tests ............................................................................................................... 64

5.3 Visual observations .................................................................................................................. 71

5.4 Analysis of experimental test results ..................................................................................... 75

5.4.1 Comparison of submergence for different suction bell heights ..................................... 75

5.4.2 Explaining the increase in submergence when raising the pump intake ..................... 83

5.5 Comparison of test results of four bell intake configurations ............................................. 94

5.6 Acoustic Doppler velocimeter measurements ................................................................... 100


vii


5.6.1 ADV measurements at edge of suction bell ................................................................... 100

5.6.2 ADV measurements along approach canal .................................................................... 107

6. COMPARISON OF TEST RESULTS AGAINST DESIGN GUIDELINES .......................... 116

7. CONCLUSIONS AND RECOMMENDATIONS................................................................. 127

7.1 Conclusions ............................................................................................................................. 127

7.2 Recommendations ................................................................................................................. 128


viii


LIST OF FIGURES

Figure 1.1: Dimension variables for pump intake (Hydraulic Institute, 1998) ............................. 2

Figure 1.2: Proposed layout of Lower Thukela abstraction works .............................................. 3

Figure 1.3: Physical model of the proposed Lower Thukela abstraction works .......................... 4

Figure 1.4: Proposed raised suction bell installation in sand trap canal ..................................... 5

Figure 2.1: Flat bell installation for flows ≤ 1 m3/s (Sulzer Brothers Limited, 1987) ...................10

Figure 2.2: Slanted bell installation for flows > 1 m3/s (Sulzer Brothers Limited, 1987) .............10

Figure 3.1: Rankine combined vortex (Prosser, 1977) .............................................................14

Figure 3.2: Free surface and sub-surface vortex strength classification (Hydraulic Institute,

1998) ........................................................................................................................................16

Figure 3.3: Critical submergence versus bell inlet velocity .......................................................34

Figure 3.4: Approach velocities versus bell inlet velocity ..........................................................34

Figure 4.1: Plan view of physical model ...................................................................................36

Figure 4.2: Sectional view of physical model ............................................................................36

Figure 4.3: View of pump model ...............................................................................................38

Figure 4.4: View of stilling basin and flow straightener pipes ....................................................38

Figure 4.5: Dimensions of suction bell configurations ...............................................................39

Figure 4.6: Rear view of suction bell configurations (from left to right: Type 1B, Type 1A, Type

2B, Type 2A) .............................................................................................................................40

Figure 4.7: Side view of suction bell configurations (from left to right: Type 2A, Type 2B, Type

1A, Type 1B) .............................................................................................................................40

Figure 4.8: ADV support frame .................................................................................................42

Figure 4.9: Vectrino instrument ................................................................................................42

Figure 4.10: Flow diagram of daily test procedure ....................................................................44

Figure 4.11: Path of dye injected upstream of suction bell .......................................................47

Figure 4.12: Evidence of a coherent dye core behind the suction bell ......................................48

Figure 4.13: Positions of ADV measurements at inlet bell ........................................................49

Figure 4.14: XYZ coordinates of ADV instrument (Nortek, 2004) .............................................50

Figure 4.15: Position of ADV measurements in sand trap canal ...............................................51

Figure 5.1: Type 1A bellmouth, 0.5D above floor – prototype type submergence .....................53


ix




Figure 5.4: Type 1B bellmouth, 0.5D above floor – prototype type submergence .....................56






Figure 5.10: Type 2B bellmouth, 0.5D above floor – prototype type submergence ...................62



Figure 5.13: Type 1B bellmouth, 0.5D above floor – prototype type submergence for Type 2

vortices .....................................................................................................................................65


vortices .....................................................................................................................................66


vortices .....................................................................................................................................67


vortices .....................................................................................................................................68


vortices .....................................................................................................................................69


vortices .....................................................................................................................................70

Figure 5.19: Type 2 vortex (surface dimple) .............................................................................72

Figure 5.20: Type 5 vortex (pulling air bubbles to intake) .........................................................72

Figure 5.21: Type 6 vortex (full air core to intake) ....................................................................73

Figure 5.22: Break-away caused when water level is at joint on segmented bend ...................74

Figure 5.23: Type 1A suction bell – submergence required for Type 2 vortices at bell heights of

0.5D, 1.0D and 1.5D .................................................................................................................75


0.5D, 1.0D and 1.5D .................................................................................................................76


0.5D, 1.0D and 1.5D .................................................................................................................76


x


Figure 5.26: Type 1B suction bell – submergence required for Type 2 vortices at bell heights of

0.5D, 1.0D and 1.5D .................................................................................................................77


0.5D, 1.0D and 1.5D .................................................................................................................78


0.5D, 1.0D and 1.5D .................................................................................................................78


0.5D, 1.0D and 1.5D .................................................................................................................79


0.5D, 1.0D and 1.5D .................................................................................................................80


0.5D, 1.0D and 1.5D .................................................................................................................80


0.5D, 1.0D and 1.5D .................................................................................................................81


0.5D, 1.0D and 1.5D .................................................................................................................82


0.5D, 1.0D and 1.5D .................................................................................................................82

Figure 5.35: Type 1A suction bell – submergence against bell/approach velocity ratios for Type

2 vortices ..................................................................................................................................86


5 vortices ..................................................................................................................................86


6 vortices ..................................................................................................................................87

Figure 5.38: Type 1B suction bell – submergence against bell/approach velocity ratios for Type

2 vortices ..................................................................................................................................88


5 vortices ..................................................................................................................................88


6 vortices ..................................................................................................................................89


2 vortices ..................................................................................................................................90


5 vortices ..................................................................................................................................90


6 vortices ..................................................................................................................................91


xi



2 vortices ..................................................................................................................................92


5 vortices ..................................................................................................................................92


6 vortices ..................................................................................................................................93

Figure 5.47: Comparison of submergence between Type 1A and 1B suction bell configurations

for Type 2 vortices ....................................................................................................................94











Figure 5.53: Comparison of submergence between Type 1B and 2B suction bell configurations






Figure 5.56: X, Y and Z velocities for 0.9 m/s bell inlet velocity at Position 2 .......................... 102

Figure 5.57: Plan view of resultant velocities at suction bell for prototype bell inlet velocities of

0.9 m/s to 2.4 m/s ................................................................................................................... 106

Figure 5.58: Position 1C: X, Y and Z velocities for 0.5D and 1.2 m/s inlet bell velocity ........... 111

Figure 5.59: Position 1C: X, Y and Z velocities for 0.5D and 1.2 m/s inlet bell velocity for sample

numbers 195 to 295 ................................................................................................................ 112

Figure 5.60: Average resultant velocities along centrelines 1-4-7, 2-5-8, and 3-6-9 ............... 113

Figure 5.61: Average resultant velocities along Planes A, B and C ........................................ 114

Figure 5.62: Path of particle approaching the inlet bell ........................................................... 115

Figure 6.1: Comparison of submergence between design guidelines and measured Type 2, 5

and 6 vortices for a bell height of 0.5D .................................................................................... 116


xii






Figure 6.4: Comparison of submergence between design guidelines and measured Type 2

vortices for bell heights of 0.5D, 1.0D and 1.5D ...................................................................... 119


xiii


LIST OF TABLES

Table 1.1: Recommended dimensions for pump intakes ........................................................... 2

Table 1.2: Recommended dimensions versus proposed dimensions for Vlieëpoort and Lower

Thukela sand trap canals ........................................................................................................... 6

Table 2.1: Modelling scenarios .................................................................................................11

Table 2.2: Repeat tests performed to verify reliability of results ................................................12

Table 2.3: ADV recording scenarios .........................................................................................12

Table 3.1: Prototype information and scale effects ...................................................................29

Table 3.2: Critical submergence recommended by Gorman-Rupp ...........................................32

Table 3.3: Critical submergence recommended by KSB ..........................................................32

Table 3.4: Critical submergence recommended by Metcalf and Eddy (1981) ...........................33

Table 4.1: Configuration set-up of ADV instrument ..................................................................41

Table 5.1: Test results – inlet bell Type 1A, 0.5D above floor ...................................................52



Table 5.4: Test results – inlet bell Type 1B, 0.5D above floor ...................................................55






Table 5.10: Test results – inlet bell Type 2B, 0.5D above floor .................................................61



Table 5.13: Repeatability test results – inlet bell Type 1B, 0.5D above floor, Type 2 vortices ...65



xiv






Table 5.19: Reynolds Numbers for approach velocities ............................................................84

Table 5.20: Water levels for ADV tests at suction bell ............................................................ 100

Table 5.21: Summary of ADV measurements (model) at suction bell for prototype 0.9 m/s bell

inlet velocity ............................................................................................................................ 102


inlet velocity ............................................................................................................................ 103


inlet velocity ............................................................................................................................ 104


inlet velocity ............................................................................................................................ 104


inlet velocity ............................................................................................................................ 105


inlet velocity ............................................................................................................................ 105

Table 5.27: Water levels for ADV tests along approach canal ................................................ 107

Table 5.28: Summary of ADV measurements (model) along approach canal ......................... 108

Table 6.1: Bell inlet velocity/approach canal velocity ratios for Type 1B suction bell located 0.6

m (prototype) above canal floor ............................................................................................... 121






xv


LIST OF ABBREVIATIONS

a Acceleration

A Cross-sectional area of inlet bell

ADV Acoustic Doppler velocimeter

AWWA American Water Works Association

B Distance from back wall to pump inlet centreline

C Distance from floor to underside of the inlet bell

Cd Discharge coefficient

CFD Computational fluid dynamics

d Suction pipe diameter

D Inlet bell diameter

E Euler Number

F Force

Fg Force due to gravity

Fp Force due to pressure

Fr Froude Number at suction bell

Fst Force due to surface tension

Fv Force due to viscosity

g Gravitational acceleration

HDPE High density polyethylene

Hz Hertz

kW Kilowatt


xvi


L Length (linear dimension)

Lr Geometric scale

ℓ/s Litres per second

m Metre

MCC Mokolo Crocodile Consultants

MHz Megahertz

mm Millimetre

m/s Metres per second

m3/s Cubic metres per second

p Prototype

P Pressure

PVC-U Un-plasticised polyvinyl chloride

Q Flow rate

R Radius

Re Reynolds Number

rpm Revolutions per minute

S Submergence

Sc Critical submergence

T Time

TCTA Trans Caledon Tunnel Authority

V Velocity

VSD Variable speed drive


xvii


Vt Tangential velocity

Vx Velocity in x-direction

Vy Velocity in y-direction

Vz Velocity in z-direction

W Width of the inlet channel

We Weber Number

Scale factor

% Percentage

°C Degrees Celsius

σ Surface tension of liquid

ρ Mass density of liquid

ⱱ Dynamic viscosity of liquid

Γ Circulation

ω Angular velocity


Page | 1


1. INTRODUCTION

1.1 Background to the research project

The design of pump intakes is often based on published design guidelines and empirical

formulae that were derived from physical model or scaled prototype studies. In a few instances,

designs are also based on existing pump intakes that are operating satisfactorily, provided that

similar site conditions exist.

These design guidelines and empirical formulas have specifically been developed to deal with

hydraulic phenomena that could cause problems at pump intakes, for example (Hydraulic

Institute, 1998):

Submerged vortices;

Free-surface vortices;

Uneven velocity distribution in approach channels;

Excessive pre-swirl of flow entering the pump;

Non-uniform spatial distribution of velocity at the impeller eye; and

Entrained air of gas bubbles.

The presence, duration and magnitude of the above phenomena at pump intakes could impact

negatively the operation of the pumping system and result in a reduction in pump performance

(i.e. both flow rate and pressure), excessive vibrations, increased noise and higher power

consumption.

Figure 1.1 shows a typical single pump intake with the dimension variables to design the pump

intake. Table 1.1 provides a description of each of the dimension variables and the

recommended dimensions based on the guidelines published in the American National

Standard for Pump Intake Design (Hydraulic Institute, 1998).


Page | 2


Figure 1.1: Dimension variables for pump intake (Hydraulic Institute, 1998)

Table 1.1: Recommended dimensions for pump intakes

Dimension variable Description of dimension Recommended

dimension

B Distance from the back wall to the pump

inlet centreline

B = 0.75D (1)

C Distance between the inlet bell and the floor C = 0.3D to 0.5D

D Inlet bell diameter See Note 2

Fr Froude number at suction bell -

H Minimum liquid depth H = S + C

S Minimum submergence at pump inlet bell S = 1D + 2.3Fr

W Entrance width of pump inlet bay W = 2D

X Length of pump inlet bay X = 5D minimum

(1) This recommended dimension is not achievable for suction bells fitted with bends inside the wet well.

(2) The inlet bell diameter can be determined from the following recommended velocity ranges:

a. Inlet bell velocities could range from 0.6 m/s to 2.7 m/s for flows less than 315 ℓ/s per pump;

b. Inlet bell velocities could range from 0.9 m/s to 2.4 m/s for flows exceeding 315 ℓ/s but less than

1 260 ℓ/s per pump; and

c. Inlet bell velocities could range from 1.2 m/s to 2.1 m/s for flows exceeding 1 260 ℓ/s per pump.


Page | 3


The “minimum submergence at the pump inlet bell” is also referred to in the literature as “critical

submergence”, which is defined as “submergence at which, after the flow reached steady

conditions of depth and discharge, air from free-surface is ingested either continuously or

intermittently through the agency of the vortex” (Denney, 1956; cited in Rajendran & Patel,

2000).

It should be noted that, in Table 1.1, all pump intake dimensions are expressed as a function of

the inlet bell diameter, D. This ensures geometric similarity of the hydraulic boundaries and

dynamic similarity of the flow patterns.

The question that arises is – what will the impact be on the minimum submergence when any of

the recommended pump intake dimensions listed in Table 1.1 are changed?

1.2 Motivation for the study

Figure 1.2 shows the main components associated with the abstraction works proposed for the

Lower Thukela Bulk Water Supply Scheme, i.e. a weir, a boulder trap, a gravel trap, sand trap

canals and a fishway (Basson, 2011). A trash rack is provided at the inlet to the sand traps,

with the raw water pumps located at the end of the sand trap. Figure 1.3 shows the physical

model constructed for the abstraction works of the Lower Thukela Bulk Water Supply Scheme.

Figure 1.2: Proposed layout of Lower Thukela abstraction works

Gravel trap

Submerged slot

Boulder trap

Crump weir:

low notch

Fishway

Sand trap


Page | 4


Figure 1.3: Physical model of the proposed Lower Thukela abstraction works

A similar abstraction works arrangement, with six sand traps, is proposed for the river

abstraction works located at Vlieëpoort on the Crocodile River (Basson, 2010).

The sand trap canals are designed with velocities of less than 0.2 m/s at the minimum operating

level to trap sediment larger than 0.3 mm in diameter. The sand trap canals also have bed

slopes of 1:80 to allow flushing of sand and gravel under gravity by opening the downstream

gate (see Figure 1.3). Furthermore, in order to effectively flush the sand trap, the raised

downstream gate opening should be at least 1.5 m (Basson, 2010; Basson, 2011).

Dry well pump installations are proposed for both the Lower Thukela and Vlieëpoort abstraction

works. This would require the installation of a suction bell inside the sand trap canal (see

Figure 1.3). The suction bell, however, needs to be raised to not interfere with the flushing

operation and to ensure an effective gate opening of 1.5 m, as shown in Figure 1.4.

Upstream gate

Downstream gate

Suction bell

Sand trap canal


Page | 5


Figure 1.4: Proposed raised suction bell installation in sand trap canal

Table 1.2 summarises the dimensions proposed for the Lower Thukela and Vlieëpoort sand trap

canals in comparison with the dimensions recommended in the American National Standard for

Pump Intake Design (Hydraulic Institute, 1998).

It is evident from Table 1.2 that, for the Vlieëpoort and Lower Thukela sand trap canals,

dimension variables “B” and “C” are not in accordance with the dimensions recommended in the

American National Standard for Pump Intake Design (Hydraulic Institute, 1998). This leads to

the question – what impact does the raising of the suction bell have on the minimum

submergence required to prevent air entrainment? This question is the motivation for this study.

Raising the suction bell to ensure the effective flushing of the sand traps would, however, be

a deviation from the published pump design guidelines.


Page | 6


Table 1.2: Recommended dimensions versus proposed dimensions for Vlieëpoort and Lower Thukela sand trap canals

Dimension

variable

Dimension

description

Vlieëpoort sand trap canal

(2 m3/s per pump)

Lower Thukela sand trap

canal (0.467 m3/s per

pump)

Recommended

dimension (1)

(mm)

Proposed

dimension

(2) (mm)

Recommended

dimension (1)

(mm)

Proposed

dimension

(mm)

D Inlet bell

diameter

1 500 1 200 700 700

W Entrance width

of pump inlet

bay

3 000 2 400 1400 2 000

B Distance from

the back wall to

the pump inlet

centreline

1 125 2 600 525 1 500

C Distance

between the

inlet bell and the

floor

750 1 500 350 1 500 (3)

S Minimum pump

inlet bell

submergence

2 200 2 180 1 450 1 200

(1) Recommended dimensions refer to the dimensions recommended by the Hydraulic Institute (1998).

(2) The sand trap canals were initially designed for a flow of 2.5 m3/s per pump, an inlet bell diameter of 1.4 m and a

pump inlet bay entrance width of 3.0 m. The dimensions were reduced as the design flow reduced to 2 m3/s per

pump.

(3) The initial consideration was to install the suction bell at a height of 350 mm (as reported by Basson, 2011) and

to use a guiderail system to raise the suction bell during times of flushing. This system has the risk that air could

be entrained on the suction pipework should the guiderail system malfunction. It was later decided to rather

install the suction bell at a fixed height of 1 500 mm above the floor.


Page | 7


1.3 Objective of study

The objective of the study was to determine, by means of a physical hydraulic model, the

minimum submergence levels required to prevent air entrainment for suction bell inlets located

at different heights above the canal floor. The measured submergence levels were compared

against the design guidelines that are available to calculate minimum submergence, after which

recommendations were formulated for the design criteria to be applied for raised pump intake

installations, which are similar in geometry to the physical model.

The following variables were investigated in the physical model study:

Different bell inlet heights above the floor level;

Different bell inlet velocities; and

Different bell configurations.

The physical model study did not consider aspects such as the widening or narrowing of the

sand trap canal, or velocity distributions at the impeller eye. The testing and modelling of

changes in geometry, and the determination of velocity distributions at different locations along

the pump intake, can be performed more effectively (both from a time and cost perspective) with

three-dimensional computational fluid dynamics (CFD). Such a CFD model would first have to

be calibrated against the results obtained in the physical model study. Therefore, a further

objective of the physical model study was to obtain sufficient information on velocity distributions

along the sand trap canal to assist with defining the boundary conditions, and the calibration, of

the CFD model. The calibration and application of the CFD model forms part of another study

(Hundley, 2012) and is not covered in this thesis.

1.4 Layout of the thesis

The report is structured as follows:

The methodology to achieve the study objective is described in Section 2;

Section 3 covers the literature review, which focuses on the theory and causes of vortex

formation, the possible scaling effects associated with physical hydraulic models, and the

available design standards to calculate the required submergence;

The experimental set-up, instrumentation and test procedures are described in Section 4;

The experimental test results are presented and discussed in Section 5;


Page | 8


Section 6 compares the available design standards with the results obtained from the

physical model testing; and

Section 7 contains the conclusions of the study and recommendations for further work.


Page | 9


2. METHODOLOGY

2.1 Physical model study

The Hydraulic Institute (1998) recommends that physical hydraulic model studies be performed

when the sump and piping geometry deviate from published norms, as is the case with the

suction bell configurations at the Vlieëpoort and Lower Thukela abstraction works. This

recommendation is supported by Prosser (1977), who stated that the main factor influencing the

need for model testing is whether the proposed scheme varies radically from existing

satisfactory designs.

In light of the deviations from published pump intake design norms, the Trans Caledon Tunnel

Authority (TCTA) instructed Mokolo Crocodile Consultants (MCC) to undertake a physical

hydraulic model study, as well as a CFD model study, of the proposed Vlieëpoort sand trap

canals with the raised suction bells, to determine the minimum submergence required to prevent

air entrainment. MCC appointed Stellenbosch University to construct the physical model, but

the TCTA decided to postpone the laboratory testing, and subsequent design of the Vlieëpoort

abstraction scheme, due to uncertainties related to the water demands of end users, which

could vary from 1 m3/s to 2.5 m3/s per pump. The TCTA did, however, agree that the physical

model could be used by Stellenbosch University for other research projects, such as this one,

which focussed primarily on the Vlieëpoort scheme.

2.2 Modelling scenarios

The minimum submergence levels required in the sand trap canals will be the maximum water

level required to:

Prevent air entrainment due to vortex formation; or

Prevent priming problems, i.e. the minimum water level should be above the pump volute.

It should, however, be noted that, in practice, the height of the weir must not only be designed to

ensure that the minimum submergence level in the sand trap canals is achieved, but also needs

to be designed to ensure a sufficient hydraulic head across the abstraction works to flush the

boulder trap, gravel trap and sand trap canals. The latter requirement was not dealt with in this

study.


Page | 10


The following variables were considered in the design of the overall study methodology and

determined the scenarios to be tested:

The height of the suction bell above the floor level to be altered to test various heights;

The flow rate to be varied from 1.0 m3/s to 2.5 m3/s per pump, or alternatively the bell inlet

velocities to be varied from 0.9 m/s to 2.4 m/s;

The preferred suction bell configuration to be determined, as Sulzer Brothers Limited (1987)

recommend the use of a slanted bell for flows exceeding 1.0 m3/s (see Figures 2.1 and 2.2

for details of different bell configurations); and

The radius of the suction pipe bend to be determined, as it determines the height of the

pump volute. Sulzer Brothers Limited (1987) recommends that the suction pipe radius

needs to be equal to or greater than 1.5 times the diameter of the suction bell.

Figure 2.1: Flat bell installation for flows ≤ 1 m3/s (Sulzer Brothers Limited, 1987)

Figure 2.2: Slanted bell installation for flows > 1 m3/s (Sulzer Brothers Limited, 1987)


Page | 11


Table 2.1 summarises the modelling scenarios analysed, based on the variables listed above.

Table 2.1: Modelling scenarios

Suction

bell type

Radius of suction

bend (1)

Bell inlet velocities

(m/s)

Height above

canal floor

Bell type

reference (2)

Flat bell 1 x diameter of

suction pipe

0.9; 1.2; 1.5; 1.8; 2.1;

2.4

0.5D; 1.0D;

1.5D

1A

Flat bell 2 x diameter of

suction pipe

0.9; 1.2; 1.5; 1.8; 2.1;

2.4

0.5D; 1.0D;

1.5D

1B

Slanted

bell

1 x diameter of

suction pipe

0.9; 1.2; 1.5; 1.8; 2.1;

2.4

0.5D; 1.0D;

1.5D

2A

Slanted

bell

2 x diameter of

suction pipe

0.9; 1.2; 1.5; 1.8; 2.1;

2.4

0.5D; 1.0D;

1.5D

2B

(1) The radii of the bends were based on the suction pipe diameter and not the bell diameter as proposed by Sulzer

Brothers Limited (1987), as the bends are manufactured in accordance with AWWA C208 where the radius is

given as a function of the suction pipe diameter, e.g. 1d, 1.5d, 2.0d, etc., where “d” is the diameter of the suction

pipe.

(2) The bell type reference shown in the last column is used in the report to refer to the different bell configurations.

The water level at which air was entrained was established in the physical model for each of the

72 scenarios listed in Table 2.1.

The formation of vortices is unsteady and unstable, i.e. vortices form intermittently at different

locations near the pump intake and vary in strength over time. Therefore, the water levels at

which the different types of vortices form, required subjective interpretation through observation

of the predominant type of vortex present at a given water level. As these subjective

interpretations might impact on the reliability of the test results, and in order to verify the

reliability and repeatability of the tests, the tests for the two long radius (i.e. two times the

diameter of the suction pipe) inlet bells, situated 0.5D above the floor, were repeated three (3)

times for all six (6) bell inlet velocities. The decision to repeat the tests only for suction bells

located 0.5D above the floor was based on the fact that almost all conventional pump intakes

are located at this level. Table 2.2 summarises the additional tests that were performed to

validate the reliability of the test results.


Page | 12


Table 2.2: Repeat tests performed to verify reliability of results

Suction

bell type

Radius of suction bend Bell inlet velocities (m/s) Height above

canal floor

Flat bell 2 x diameter of suction pipe 0.9; 1.2; 1.5; 1.8; 2.1; 2.4 0.5D

Slanted bell 2 x diameter of suction pipe 0.9; 1.2; 1.5; 1.8; 2.1; 2.4 0.5D

As the objective of the study was to also take sufficient velocity readings that could be used for

calibrating the CFD model, an acoustic Doppler velocimeter (ADV) was used to take three-

dimensional velocity readings at different positions and heights along the sand trap canal. The

ADV readings were only taken for the preferred inlet bell configuration at the velocities and

heights indicated in Table 2.3. The decision on the bell inlet velocities, for which ADV readings

were taken was based on the fact that the majority of pump intakes are designed for bell inlet

velocities ranging from 1.0 m/s up to 1.5 m/s. ADV readings were also taken for a bell inlet

velocity of 1.8 m/s for calibrating the CFD model for higher bell inlet velocities.

Table 2.3: ADV recording scenarios

Suction bell type Radius of suction

bend

Bell inlet velocities

(m/s)

Height above canal

floor

Preferred option Preferred option 1.2 0.5D, 1.0D, 1.5D

Preferred option Preferred option 1.8 0.5D

2.3 Criteria to decide on minimum submergence in proto-type

The minimum submergence levels required to prevent air entrainment due to vortex formation,

were determined in the physical model study for each of the four suction bell configurations

(refer to Table 2.1). A comparison was made between the results of the four suction bell

configurations to select the configuration with the lowest required submergence levels for all

velocities and heights above the canal floor.

The results of the preferred suction bell configuration were then compared against published

design guidelines that are generally used to calculate minimum submergence levels. The

published formula that best represents the experimental test results for raised pump intakes was

identified, after which recommendations were formulated on the design criteria to be applied for

the prototype design of the Vlieëpoort and Lower Thukela sand trap canals and pump intake

configurations.


Page | 13


3. LITERATURE REVIEW

The purpose of the literature review is to (a) review the causes of vortices, (b) investigate the

likely scale impacts on the formation of vortices in a physical hydraulic model, and (c) obtain

design guidelines for the calculation of critical submergence to avoid the entrainment of air.

This section of the report will therefore address:

The fundamentals and theory of vortex formation (Section 3.1);

The impact of scale effects in physical models (Section 3.2); and

The design guidelines available to calculate critical submergence (Section 3.3).

3.1 Fundamentals and theory of vortex formation

3.1.1 Introduction

Vortex formation at pump intake structures has been studied for more than 50 years. The

guidelines for the “Hydraulic Design of Pump Sumps and Intakes” were developed by the British

Hydromechanics Research Association (Prosser, 1977) and referenced research work

performed in the 1950s, e.g.:

“Studies of submergence requirements of high specific-speed pumps” (Iversen, 1953);

“Hydraulic problems encountered in intake structures of vertical wet-pit pumps and methods

leading to their solution” (Fraser and Harrison, 1953); and

“The prevention of vortices and intakes” (Denny and Young, 1957).

The three fundamental causes of vortices were defined by Durgin and Hecker (1978; cited in

Knauss, 1987) as (a) non-uniform approach flow to the pump intake due to geometric

orientation, (b) the existence of high velocity gradients, and (c) obstructions near the pump

intake.

It was further recommended by Prosser (1977) that flow approaching a pump intake that

changes from free surface to a closed conduit should be:

Uniform flow – the velocity, in magnitude and direction, of fluid particles should be the same

at all points across the section considered;

Steady flow – the velocity, in magnitude and direction, should not change with time; and

Single phase – there should be no entrained air.


Page | 14


The equations used to describe the motion of vortex flow are complex, as velocities could vary

spatially (i.e. in terms of the depth and width of the approach canal) and local geometric

features could influence flow fields, which all impact on the calculation of the resulting

circulation. A simplified analytical approach was proposed by Prosser (1977) to calculate

circulation. Figure 3.1 shows a schematic representation of vortex flow that consists of a

relatively small central portion of fluid that is rotating as a highly viscous solid body (also

referred to as a “forced” vortex), combined with a non-viscous “free” vortex region extending

radially outward from this central portion.

Figure 3.1: Rankine combined vortex (Knauss, 1987)

In the central area of the vortex, the fluid is assumed to rotate so that the tangential velocity, vt,

varies linearly with the radius, r, so that

vt = ω r (3.1)


Page | 15


where

vt = tangential velocity (m/s)

ω = angular velocity in radians per unit time

r = radius (m).

In the free vortex area, the tangential velocity varies inversely with the radius and directly with

the circulation. For a circular curve of radius r, the circulation, Γ (m2/s), is expressed as,

Γ = 2 π r vt (3.2)

Figure 3.1 also shows the variation in tangential velocities between the forced and free vortex

areas, i.e. r1 is the radius at which the transition from forced to free vortex occurs.

The velocity head associated with the circulation will reduce the local hydrostatic pressure,

which will result in a localised lowering of the water surface. This drop in water surface will vary

from a surface dimple to a full air core vortex based on the strength of the circulation. The

classification of vortices is the subject of the next paragraph.

3.1.2 Vortex classification and strengths

Vortices are generally classified as (a) free surface vortices, starting from the free water surface,

or (b) sub-surface vortices, starting from the floor, side or back wall of the intake structure.

The free surface vortices, which might result in air being drawn in from the surface, could result

in unbalanced forces on the pump impeller, and vibrations that have a negative impact on the

pump performance.

The sub-surface vortices could introduce excessive swirls at the pump inlet, which could also

result in unbalanced forces on the pump impeller eye.

The Alden Research Laboratory (Hydraulic Institute, 1998) developed a visual classification

system to classify the strength of vortices. Figure 3.2 shows the strength classifications for free

surface and sub-surface vortices.

The focus of this study will be on free surface vortices, as the objective is to determine the

minimum water level required to prevent air entrainment.


Page | 16


Figure 3.2: Free surface and sub-surface vortex strength classification (Hydraulic Institute, 1998)

3.1.3 Acceptance criteria for pump intakes

The acceptance criteria for physical hydraulic model studies of pump intakes, as per the

Hydraulic Institute (1998), are:

Free surface vortices entering the pump must be less severe than Type 3, on condition that

they occur less than 10% of the time or only for infrequent pump operating conditions;

Sub-surface vortices entering the pump must be less severe than Type 2;

The average swirl angle must be less than five (5) degrees. Swirl angles of up to seven (7)

degrees will be accepted, but only if they occur less than 10% of the time; and

The time-averaged velocities at points in the throat of the bell inlet must be within 10% of the

average axial velocity in the throat of the bell inlet.


Page | 17


The above acceptance criteria do not explicitly address the air-by-water volumes that could be

tolerated by pumps, if any. The following information pertaining to air-by-water volumes was

obtained from various authors:

3% free air showed a drop in hydraulic efficiency of 15% for centrifugal pumps (Prosser,

1977);

3% air-by-water volume in a suction line can considerably reduce the head-discharge curves

of centrifugal pumps (Padmanabhan & Hecker, 1984);

3 to 4% free air may give rise to a small, but continuous, decrease in pump efficiency

(Knauss, 1987);

7 to 20% free air is required before pump operations are interrupted (Knauss, 1987);

The formation of air entraining vortices upstream of the pump intake must be prevented

(Sulzer Brothers Limited, 1987); and

3 to 5% air in the suction pipe can lower the pump efficiency (Karassik, Messina, Cooper, &

Heald, 2001).

Based on the above statements by Prosser and Sulzer Brothers Limited, as well as the

Hydraulic Institute’s acceptance criteria to not permit free surface vortices exceeding Type 3 in

strength, it is recommended that the critical submergence levels be determined in the physical

model as the level at which no air entrainment will take place.

3.1.4 Dimensionless parameters

In order to apply the results of the physical model to the prototype, dimensionless parameters

are used to define the complex interaction between the geometry of the intake structure, the

flow velocity and the liquid properties which all influence the critical submergence. The

following dimensionless parameters were proposed by different authors to describe the motion

of vortex flow:

Jain, Raju, & Garde (1978) proposed the following functional relationship for critical

submergence, which is defined as the water level required above the inlet of the bell to maintain

the circulation within acceptable limits:

Scr = f(W, D, Q, Γ, g, ρ, σ, ν) (3.3)

with Scr = critical submergence (m)


Page | 18


W = width of the inlet channel or diameter of the vortex tank (m)

D = diameter of the inlet bell (m)

Q = flow rate (m3/s)

Γ = circulation of flow (2πrVt) (m2/s)

g = gravitational acceleration (m/s2)

ρ = mass density of liquid (kg/m3)

σ = surface tension of liquid (N/m)

ⱱ = dynamic viscosity of liquid (m2/s).

A typical single pump intake configuration is shown in Figure 1.1 and includes the critical

parameters.

Equation 3.3 was re-written, by means of dimensional analysis and the re-grouping of

parameters, as:

√

(3.4)

With Q = πd2/4, it is possible to re-write Equation 3.4 as follows:

√

(3.5)

The first, third and fourth terms of Equation 3.5 represent the Reynolds, Froude and Weber

Numbers.

Anwar, Weller & Amphlett (1978) proposed the following relationship between the six

independent non-dimensional parameters that could be used to describe the formation of

vortices at horizontal pump intakes:

(

√

) (3.6)


Page | 19


with C = distance from the floor to the underside of the inlet bell (m)

A = cross-sectional area of the inlet bell (m2)

S = submergence depth above intake (m).

Equation 3.6 was re-written in the following form:

(

) (3.7)

with Cd = discharge coefficient

Re =

We = √

Knauss (1987) stated that critical submergence could be expressed by:

Scr = f(V, d, D, R, C, Γ, g, ρ, σ, ν) (3.8)

with V = average velocity at the inlet to the suction bell (m/s)

d = suction pipe diameter (m)

R = radius of rounding of the bellmouth entry (m).

Equation 3.8 was re-written, by means of dimensional analysis, as:

√ √

(3.9)

The three last terms of Equation 3.9 represent the Froude, Weber and Reynolds Numbers.

The Hydraulic Institute (1998) stated that, based on previous research, it can be concluded that

the flow conditions at a pump intake are represented by the following dimensionless

parameters:


Page | 20


√

(3.10)

It is evident that Froude, Reynolds and Weber Numbers need to be considered in physical

model studies of pump intakes. The approach flow pattern in the vicinity of the sump governs

the circulation, Γ. It can be accepted that circulation would be similar between the prototype

and scaled model provided that the approach flow patterns are similar. The relevance and

influence of these dimensionless parameters in physical model studies are considered in

Section 3.2.

3.2 Impacts of scale effects in physical models

3.2.1 Physical similarity

It is usually not financially viable to construct physical models of a pump intake at the full-scale

of the system. Physical models, constructed at a smaller scale than the full-scale system, are

therefore used to examine critical aspects that could influence the performance of the system.

The laws of similitude make it possible to relate the results obtained with the model to the

performance of the prototype. It is not possible, however, to satisfy all the laws of similitude

simultaneously, which results in discrepancies between the performance of the model and that

of the prototype. This is known as the “scale effect” (Webber, 1979).

“Physical similarity” is a generic term used to describe different types of similarity, e.g.

geometric, kinematic and dynamic similarity. These three types of similarity are discussed in

more detail below.

Geometric similarity

Geometric similarity is similarity of shape, which means that the model and prototype are

identical in shape but differ in size. The ratio of any two dimensions in the model therefore

corresponds to the ratio in the prototype, as shown in Equation 3.11.

(3.11)

with L = a linear dimension

m = model


Page | 21


p = prototype.

The “scale factor” is generally defined as the ratio between a linear dimension in the prototype

and the corresponding dimension in the model. If the linear scale of the model is 1:x, the scalar

relationship for area is 1:x2 and for volume it is 1:x3 (Webber, 1979).

It is not always possible to achieve complete geometric similarity, especially in respect of

surface roughness. The hydraulic behaviour arising from the boundary conditions is, however,

the more important factor and dissimilar geometry is therefore often acceptable (Massey, 1989).

Kinematic similarity

Kinematic similarity is similarity of motion, which implies similarity in geometry, time intervals,

velocity and acceleration (Massey, 1989), meaning that the following ratios will apply between

the model and the prototype (Webber, 1979):

and

(3.12)

with v = velocity

a = acceleration.

Fluid motions that are kinematically similar form streamlines that are geometrically similar at

corresponding times.

Dynamic similarity

Dynamic similarity is similarity of forces. If two systems are dynamically similar, the magnitude

of forces at similarly located points in the model and prototype systems must be in the same

ratio and act in the same direction. The following ratios apply between dynamically similar

models and prototypes:

(3.13)

with F = force


Page | 22


The forces in a system involving fluids could be due to pressure, gravity, viscosity or surface

tension. The conditions for dynamic similarity can be expressed as (Webber, 1979):

(3.14)

with Fp = force due to pressure

Fg = force due to gravity

Fν = force due to viscosity

Fst = force due to surface tension.

Perfect dynamic similarity implies that all the ratios between all the forces remain fixed. This is,

however, not the case, as the presence and significance of the various forces differ for different

hydraulic structures. It therefore is important to ensure dynamic similarity of the dominant

forces present in the model and prototype under consideration.

3.2.2 Similarity laws

The similarity laws that could be applicable to hydraulic model studies are discussed below.

Euler Law

The Euler number describes the relationship between pressure and velocity and can be

expressed as:

√

(3.15)

with E = Euler Number

V = velocity

∆p = change in pressure

ρ = density of liquid.


Page | 23


The Euler Law is relevant to enclosed fluid systems where the forces of gravity and surface

tension are absent.

Froude Law

The Froude Law is applicable to systems where gravity is the significant force that influences

the fluid motion. Systems with free surfaces, e.g. weirs, open channels, spillways and rivers,

are typical examples where gravity is the dominant force.

The Froude number is expressed as:

√ (3.16)

with Fr = Froude Number

V = velocity

g = gravitational acceleration

L = characteristic length, e.g. pipe diameter in the case of flow in a pipe.

For compliance with the Froude Law, the corresponding velocities must satisfy Equation 3.17.

(3.17)

with = scale factor.

Reynolds Law

The Reynolds Law is applicable to systems where only the forces of viscosity and inertia are

present. An example is a submarine submerged deep enough not to create any waves on the

surface.

The Reynolds Number is expressed as:

(3.18)


Page | 24


with Re = Reynolds Number

V = velocity

L = length

= viscosity of liquid.

For compliance with the Reynolds Law, the corresponding velocities must satisfy Equation 3.19:

(3.19)


Weber Law

The Weber Law describes the relationship between surface tension and velocity. Surface

tension is very seldom a significant force, but could become a factor where there is an air-water

interface in a structure with small dimensions.

The Weber number is expressed as:

√

(3.20)

with We = Weber Number

V = velocity

σ = surface tension of liquid

ρ = density of liquid

L = length.

Compliance with the Weber Law is achieved when velocities correspond to Equation 3.21:


Page | 25


(3.21)


Based on Equation 3.21 it can be derived that for the same fluid in the model and prototype, the

model velocity should be x0.5 times that in the prototype.

Mach Law

The Mach Law describes the relationship between elastic forces and velocity, and is applicable

to systems where the compressibility of the fluid is of importance (Massey, 1989).

The Mach number can be expressed as:

(3.22)

with = Mach Number

V = velocity

C = acoustic velocity in liquid medium

The velocity, flow and time scales for a model based on Froude scaling, can be calculated as

follows (Hydraulic Institute, 1998):

(3.23)

(3.24)

The model for the study of the Vlieëpoort pump intake is an open channel with a free surface.

Gravitational forces are the dominant forces and the Froude Law will be the criterion to be

satisfied. It will, however, be important to ensure sufficiently high Reynolds and Weber

Numbers to mitigate the potential scale effects due to viscosity and surface tension. The

Euler and Mach Laws are not relevant when performing physical model studies on pump

intake channels.


Page | 26


(3.25)

with Lr = geometric scale

m = model

p = prototype

V = velocity

Q = flow

T = time.

3.2.3 Modelling criteria

Minimum Reynolds and Weber Numbers

The following is a summary of minimum Reynolds and Weber Numbers, recommended by

various authors, to minimise the viscous and surface tension effects for physical models based

on Froude similarity:

Jain et al. (1978) indicated that Reynolds and Weber Numbers of 5 x 104 and 120 would be

required as minimum values;

Daggett and Keulegan (1974; cited in Padmanabhan & Hecker, 1984) found that the viscous

effects on vortex formation would be negligible if the Reynolds Number was greater than

3 x 104;

Zielinksi and Villemonte (1968; cited in Padmanabhan & Hecker, 1984) concluded on the

basis of experiments that viscous effects would be negligible for Reynolds Numbers greater

than 1 x 104;

Padmanabhan and Hecker (1984) concluded from their model testing that full-scale inlet

losses were predicted accurately by the reduced scale models for Reynolds Numbers of

1 x 105 or greater;

The Hydraulic Institute (1998) recommended that model studies be performed with Reynolds

and Weber Numbers of 6 x 104 and 240, respectively, which include a 2.0 factor of safety on

the Reynolds Number recommended by Daggett and Keulegan (1974; cited in

Padmanabhan & Hecker, 1984) and the Weber Number recommended by Jain (1978);


Page | 27


Jones, Sanks, Tchobanoglous and Bosserman (2008) recommend minimum Reynolds and

Weber Numbers of 3 x 104 and 120 respectively; and

Ahmad, Jain and Mittal (2011) selected the model geometric scale to ensure that minimum

Reynolds and Weber Numbers of 6 x 104 and 240 are achieved at the bell entrance.

The above Reynolds and Weber Numbers refer to those at the bell entrance.

Based on the above information, it is recommended that the minimum Reynolds and Weber

Numbers should be 3 x 104 and 120, respectively, but that where feasible, these Numbers

should be increased to 6 x 104 and 240.

Scales of other physical model studies

The following is a summary of scales used in other physical model studies or recommended for

model studies:

Anwar, Weller and Amphlett (1978; cited in Knauss, 1987) suggested a model scale of not

less than 1:20 to reproduce vortex formation;

Prosser (1977) recommended scales varying from 1:4 to 1:25 for sump models;

A study by Dhillon (1979; cited in Hecker, 1981) indicated good model-prototype agreement

of vortex strength for a 1:20 scale model that was operated on Froude similarity;

A comparison between model-prototype vortex formation was undertaken by Hecker (1981)

on 22 projects with model scales varying from 1:12 to 1:120. It was found that, in 16

projects, the model and prototype vortices were essentially equal, whereas five (5) projects

indicated that the prototype vortices were stronger than that in the model, and only one (1)

study indicated prototype vortices that were weaker than in the model;

A model study by Arboleda and El-Fadel (1996) to evaluate the impact of approach flow

conditions on pump sump design was undertaken at a scale of 1:18;

Ansar and Nakato (2001) undertook a model study at a scale of 1:10 to investigate the

impacts of cross flow on pump intakes; and

Ahmad et al. (2011) used a 1:10 scale to model a pump sump with five cooling water pumps

and three auxiliary cooling water pumps.

It is evident that the above authors recommend scales not exceeding 1:20 for physical model

studies of pump intakes that are based on Froude similarity.


Page | 28


3.2.4 Scale of proposed physical model

The prototype dimensions of the Vlieëpoort pump intake are shown in Table 1.2. It was stated

by the TCTA that the design flow per sand trap canal might be as little 1.0 m3/s and as high as

2.5 m3/s, which will result in prototype bell inlet velocities of 0.88 m/s and 2.21 m/s respectively

for an inlet bell with a diameter of 1 200 mm.

It follows that the scale that has to be selected to model prototype bell inlet velocities ranging

from 0.9 m/s up to 2.2 m/s still has to meet the minimum Reynolds and Weber Numbers for

Froude based models. Table 3.1 summarises the prototype information and scale effects

based on a model with a scale of 1:10. The velocities and flows were scaled in accordance with

Equations 3.23 and 3.24.

It is evident from Table 3.1 that the minimum scaled Reynolds and Weber Numbers will be

3.4 x 104 and 133 respectively for a prototype bell inlet velocity of 0.9 m/s. The Reynolds and

Weber Numbers will increase to 9.1 x 104 and 947, respectively, for a prototype bell inlet

velocity of 2.4 m/s.

A scale of 1:10 therefore will satisfy the minimum recommended Reynolds and Weber

Numbers of 3 x 104 and 120 applicable to physical model studies based on Froude

similarity.


Page | 29


Table 3.1: Prototype information and scale effects

Description Prototype bell

inlet velocity =

0.9 m/s

Prototype bell

inlet velocity =

1.2 m/s

Prototype bell

inlet velocity =

1.5 m/s

Prototype bell

inlet velocity =

1.8 m/s

Prototype bell

inlet velocity =

2.1 m/s

Prototype bell

inlet velocity =

2.4 m/s

P M P M P M P M P M P M

Flow

(P = m3/s)

(M = ℓ/s)

1.02 3.22 1.36 4.29 1.70 5.36 2.04 6.44 2.37 7.51 2.71 8.58

Bell inlet

velocity (m/s) 0.90 0.28 1.20 0.38 1.50 0.47 1.80 0.57 2.10 0.66 2.40 0.76

Froude

number 0.262 0.262 0.350 0.350 0.437 0.437 0.524 0.524 0.612 0.612 0.699 0.699

Reynolds

number

1.1.E+06 3.4.E+04 1.4.E+06 4.6.E+04 1.8.E+06 5.7.E+04 2.2.E+06 6.8.E+04 2.5.E+06 8.0.E+04 2.9.E+06 9.1.E+04

Weber

number 13 318 133 23 665 237 36 966 370 53 221 532 72 429 724 94 661 947

P = prototype

M = model


Page | 30


3.3 Design guidelines to calculated critical submergence

Various design guidelines have been published for the calculation of critical submergence. This

paragraph summarises the critical submergence requirements as proposed by different authors.

Aspects such as approach flow velocities and suction pipework velocities are also addressed for

completeness. The various guidelines discussed in the sub-sections below are compared in

Figure 3.3.

3.3.1 The hydraulic design of pump sumps and intakes (Prosser, 1977)

Approach flow to inlet bell ≤ 0.3 m/s;

Critical submergence, S ≥ 1.5D (D = inlet bell diameter). The recommended mean velocity

at the inlet bell is 1.3 m/s; and

Velocity in suction pipework ≤ 4 m/s.

3.3.2 Centrifugal pump handbook (Sulzer Brothers Limited, 1987)


Critical submergence, S ≥ 1.5D (D = inlet bell diameter). The recommended maximum

velocity at the inlet bell is 1.3 m/s; and


3.3.3 Pump intake design (Hydraulic Institute, 1998)


Critical submergence, S = D(1 + 2.3 Fr). The acceptable inlet bell velocities for a pump flow

exceeding 1 260 ℓ/s ranges from 1.2 m/s to 2.1 m/s, with a recommended inlet bell velocity

of 1.7 m/s; and

Velocity in suction pipework ≤ 2.4 m/s.

3.3.4 Pump station design (Jones et al., 2008)


Critical submergence, S = D(1 + 2.3 Fr). The maximum allowable velocity at the inlet bell is

1.5 m/s, with 1.1 to 1.2 m/s recommended as the optimum bell intake velocity; and



Page | 31


3.3.5 Pump handbook (Karassik et al., 2001)


Critical submergence, S = D(1 + 2.3 Fr). It is recommended that inlet bell velocities be

related to the pumping head as follow:

o Vbell ≤ 0.76 m/s for pumping heads up to 5 m (i.e. 15 feet);

o Vbell ≤ 1.20 m/s for pumping heads up to 15 m (i.e. 50 feet);

o Vbell ≤ 1.70 m/s for pumping heads greater than 15 m; and


The reason for linking the pumping head to the maximum allowable bell inlet velocities is to

prevent the velocity head loss at the bell inlet to be a large percentage of the total pumping

head.

3.3.6 Swirling flow problems at intakes (Knauss, 1987)


Critical submergence:

o S = D(1.5 + 2.5 Fr) was proposed by Paterson and Noble (1982);

o S = D(1 + 2.3 Fr) was proposed by Hecker (Knauss, 1987);

o S = D(0.5 + 2.0 Fr) was proposed by Knauss (1987);

o S ≤ 1.5D was proposed by Prosser (1977); and


No recommendation was made by Knauss on the formula to be used for calculating critical

submergence, nor were any guidelines provided for allowable bell inlet velocities. Knauss did,

however, refer to the guidelines recommended by Prosser (1977) for approach and suction

pipework velocities.

3.3.7 Gorman-Rupp design guidelines (Strydom, 2010b)

It is proposed by Gorman-Rupp that critical submergence be determined as per the data

presented in Table 3.2.


Page | 32


Table 3.2: Critical submergence recommended by Gorman-Rupp

Bell inlet velocity (m/s) Critical submergence (m)

0.5 0.35

1.0 0.55

1.5 0.80

2.0 1.15

2.5 1.50

3.0 2.00

3.3.8 KSB design guidelines (Gouws, 2010)

KSB proposes that the critical submergence be determined as per the data presented in

Table 3.3. It should be noted that the critical submergence is for pump intakes with an open

feed in the vicinity of the pump intake.

Table 3.3: Critical submergence recommended by KSB


0.5 0.3

1.0 0.5

2.0 1.2

3.0 2.1

3.3.9 Collection and pumping of wastewater (Metcalf and Eddy, 1981, provided by

Strydom, 2010a)

It is proposed by Metcalf and Eddy that critical submergence is determined as per the data

presented in Table 3.4.


Page | 33


Table 3.4: Critical submergence recommended by Metcalf and Eddy (1981)


0.6 0.3

1.0 0.6

1.5 1.0

1.8 1.4

2.1 1.7

2.4 2.15

2.7 2.6

3.3.10 Design recommendations for pumping stations with dry installed submersible

pumps (Flygt, 2002)

Critical submergence, S = 1.7Fr on condition of this being larger than 1.75D. The

acceptable inlet bell velocities for a pump flow exceeding 1 200 ℓ/s range from 1.2 m/s to

2.1 m/s, with a recommended inlet bell velocity of 1.7 m/s.

3.3.11 Werth and Frizzell (2009)

Critical submergence, S = D(2.1 + 1.33Fr0.67)

This equation was developed for pump intakes situated at the end of large reservoirs, which

implies very low approach velocities, and for water levels where no Type 3 vortices (i.e.

coherent dye core) are present.

Figure 3.3 provides a graphical presentation on the equations proposed by the different authors

to calculate critical submergence for a 1.2 m diameter inlet bell and a range of inlet bell

velocities.

The critical submergence presented in Figure 3.3 was used to calculate approach velocities,

assuming a sand trap canal width of 2.4 m (i.e. 2D). The approach velocities are plotted against

the bell intake velocities and are presented in Figure 3.4.


Page | 34


Figure 3.3: Critical submergence versus bell inlet velocity

Figure 3.4: Approach velocities versus bell inlet velocity

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0.60 0.90 1.20 1.50 1.80 2.10 2.40 2.70 3.00

Cri

tica

l su

bm

erge

nce

(m

)

Bell inlet velocity (m/s)

Critical submergence versus bell inlet velocity

1. Prosser & Sulzer 2. Hydraulic institute & Jones 3. Peterson and Noble

4. Knauss 5. Gorman Rupp 6. KSB

7. Metcalf & Eddy 8. Flygt 9. D Werth

1

23

4 56

7

8

9

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.60 0.90 1.20 1.50 1.80 2.10 2.40 2.70 3.00

Ap

pro

ach

vel

oci

ty (

m/s

)


Approach velocity versus bell inlet velocity

1. Prosser & Sulzer 2. Hydraulic institute & Jones 3. Peterson and Noble

4. Knauss 5. Gorman Rupp 6. KSB

7. Metcalf & Eddy 8. Flygt 9. D Werth

1

23

4

56

7

8

9


Page | 35


It is evident from Figures 3.3 and 3.4 that:

Large variations exist between the critical submergence recommended by the different

authors. The critical submergence proposed by KSB, Flygt, and Metcalf and Eddy is much

lower compared to that proposed by the other authors (e.g. the critical submergence

required for a bell inlet velocity of 1.5 m/s is 1.0 m according to Metcalf and Eddy, compared

to 2.4 m recommended by the Hydraulic Institute);

The approach velocities, for the critical submergence requirements proposed by KSB, Flygt,

and Metcalf and Eddy exceed 0.5 m/s for all bell inlet velocities, yet they are proposing the

lowest critical submergence levels of all the authors. The high approach velocities are

possibly related to wastewater applications, which require higher approach velocities to keep

solids in suspension. Irrespective of the reason for the higher approach velocities, this

seems like a contradiction, as it would be expected that critical submergence would increase

as the approach flow velocity increases; and

All the authors, apart from KSB, Flygt, and Metcalf and Eddy, propose limiting approach

velocities:

o Prosser recommended an optimum bell inlet velocity of 1.3 m/s, which corresponds

to an approach velocity of 0.3 m/s;

o Sulzer Brothers Limited recommended a maximum bell inlet velocity of 1.3 m/s,

which corresponds to an approach velocity of 0.3 m/s;

o The Hydraulic Institute stated that the maximum inlet velocity for flows exceeding

1 260 ℓ/s should be limited to 2.1 m/s, which corresponds to an approach velocity of

less than 0.5 m/s; and

o The equation proposed by Knauss will result in an approach velocity of less than

0.5 m/s for bell inlet velocities lower than 2.4 m/s.

The experimental test results of the preferred suction bell configuration were compared with the

above published design guidelines for the calculation of minimum submergence levels, in order

to determine which equation or guideline should be used for raised pump intake configurations.


Page | 36


4. EXPERIMENTAL SET-UP AND TEST PROCEDURES

4.1 Experimental set-up

A 1:10 geometric scale was chosen for the physical model study as it satisfies the minimum

Reynolds (i.e. 3 x 104) and Weber (i.e. 120) Numbers to ensure that the effects of viscosity and

surface tension could be neglected for the range of inlet bell velocities to be tested.

Figures 4.1 and 4.2 show a plan and sectional view, respectively, of the scale model based on

the prototype dimensions of the Vlieëpoort sand trap canal (see Table 1.2).

Figure 4.1: Plan view of physical model

Figure 4.2: Sectional view of physical model

The model consisted of a stilling basin, an approach canal, a suction bell and pipework, a pump

and the delivery pipework.


Page | 37


The stilling basin, 750 mm wide x 750 mm long x 1 540 mm high, was connected to the 240 mm

wide approach canal with a 340 x 240 mm reducer, 200 mm long. Small pipes, 32 mm in

diameter and 300 mm long, were placed 3 000 mm away from the end of the canal as flow

straighteners.

The canal was 240 mm wide, 1 000 mm high and 4 000 mm long. The sides and bottom of the

canal were constructed from glass panels to allow visualisation of the vortex formation and flow

patterns. The front end of the canal was constructed as an adjustable steel sluice to raise and

lower the suction bell.

The inlet bell diameter was 120 mm. The diameter of the suction pipework was 100 mm, which

was reduced to 65 mm at the pump’s suction flange by means of a 200 mm long eccentric

reducer. The inlet bell and suction pipework were constructed from perspex to allow

visualisation of the vortex formation inside the suction bell and pipework.

A Lowara FHS 450-160 centrifugal end-suction pump, operating at 1 450 rpm, and fitted with an

impeller with a diameter of 174 mm, was installed. A WEG variable speed drive was connected

to the 1.1 kW electric pump motor to alter the pump speed. It has been stated by Karassik et al.

(2001) that vortices are not generated by pumps or pump impellers and that vortices, whether

falling into clockwise or anticlockwise rotation, are not influenced by the pump rotation.

The discharge pipework was constructed from 50 mm diameter HDPE and PVC-U pipes. A

20 mm diameter off-take was located on the discharge pipework, which was used to drain the

water from the canal. A 50 mm diameter Endress & Hauser magnetic flow meter was installed

on the discharge pipeline. The 50 mm diameter discharge pipeline terminated in the stilling

basin situated at the head of the approach canal.

Photographs of the model are shown in Figures 4.3 and 4.4.

As stated in Section 2.2, four suction bell configurations were tested. The dimensions of the

four suction bell configurations are shown in Figure 4.5. The dimensions are in accordance

with the American Water Works Association (1983) guidelines entitled “Dimensions for

fabricated steel water pipe fittings”.


Page | 38


Figure 4.3: View of pump model

Figure 4.4: View of stilling basin and flow straightener pipes


Page | 39


Figure 4.5: Dimensions of suction bell configurations


Page | 40


Photographs of the manufactured suction bells are shown in Figures 4.6 and 4.7.

Figure 4.6: Rear view of suction bell configurations (from left to right: Type 1B, Type 1A, Type 2B, Type 2A)

Figure 4.7: Side view of suction bell configurations (from left to right: Type 2A, Type 2B, Type 1A, Type 1B)


Page | 41


4.2 Instrumentation

The following equipment was used to record flow, pump speed and velocities:

An Endress & Hauser magnetic flow meter was used to measure the flow rate in ℓ/s;

The frequency (in Hertz) was displayed on the WEG variable speed drive; and

Velocities in the approach canal were measured with a Vectrino 25 MHz acoustic Doppler

velocimeter (ADV). A downward-facing probe was used to measure the three-dimensional

velocities. The instrument was mounted on a free-standing frame to avoid any interference

that might be caused by the pump. Table 4.1 summarises the configuration set-up used for

this project.

Table 4.1: Configuration set-up of ADV instrument

Description Unit Setting

Sampling rate Hz 25

Nominal velocity range (1) m/s ± 0.3

Sampling volume mm 1.8

Transmit pulse length mm 7

(1) The nominal velocity range is set to cover the range of the velocities anticipated during the data collection. The

available nominal velocity range settings were 0.01 m/s, 0.1 m/s, 0.3 m/s, 1.0 m/s, 2.0 m/s and 4 m/s.

Photographs of the ADV support frame and the Vectrino instrument are shown in Figures 4.8

and 4.9, respectively.


Page | 42


Figure 4.8: ADV support frame

Figure 4.9: Vectrino instrument

4.3 Test procedure

4.3.1 Submergence for different types of vortices

This section provides a step-by-step description of the test procedures followed during the

laboratory testing.


Page | 43


The objective was to determine the minimum water level at which no air entrainment occurs,

which corresponds to a Type 5 vortex (refer to Figure 3.2 for the strength classification of

vortices). The acceptance criteria for physical hydraulic model studies of pump intakes

(Hydraulic Institute, 1998), however, requires that free surface vortices entering the pump must

be less severe than Type 3 and should occur less than 10% of the time.

Type 3 vortices are classified as “coherent dye core” vortices, which require the injection of dye

to identify their presence. Vortices, however, vary in strength and duration over time for a given

water level and flow, making it difficult to inject the dye at the correct position and time. It

therefore was decided to rather compare the submergence levels for the four different pump

intake configurations on the types of vortices that can be determined more easily by visual

assessment, and that dye injection should only be done for the preferred pump intake

configuration to determine the water level at which Type 3 vortices occur.

The types of vortices that are the easiest to observe visually are:

Type 2 (surface dimple);

Type 5 (vortex pulling air bubbles to intake); and

Type 6 (full air core to intake).

The water level at which Type 1 vortices (i.e. a surface swirl) occur was not recorded, as

surface swirls were often caused by break-away actions from the suction pipework, especially at

higher approach velocities. The Type 4 vortices (i.e. pulling floating debris to intake) were also

not measured, as no clear guidelines were available in the literature on what constitutes

“floating debris” in terms of size, weight and density. Furthermore, the addition of floating debris

might affect the surface tension. It therefore was decided to record the water levels of Type 2, 5

and 6 vortices as these would be the least subjective.

Figure 4.10 presents a flow diagram that graphically represents the test procedure used to

determine the water levels of the Type 2, 5 and 6 vortices.


Page | 44


Figure 4.10: Flow diagram of test procedure


Page | 45


It is shown in Figure 4.10 that the frequency of the different types of vortices was monitored for

10 minutes at a fixed water level. The 10 minutes represent approximately 32 minutes in a

prototype model, based on Equation 3.25. The minimum time over which observations should

be made in scaled models was recommended as five minutes and 10 minutes by Prosser

(1977) and the Hydraulic Institute (1998), respectively.

It was also decided to follow a methodology whereby the Type 2, 5 and 6 water levels were

determined for one velocity, after which the tests were repeated for the next velocity, compared

to starting at the highest velocity and determining all the Type 2 water levels for the different

velocities by reducing the pump speed and water level, followed by determining all the Type 5

water levels for the different velocities, etc. The main reasons for the approach shown in

Figure 4.10 were to:

Ensure that water levels were tested independently to record any possible anomalies, i.e.

the Type 2 water level at a flow of 4.29 ℓ/s might be higher than that for a flow of 6.44 ℓ/s,

which would be missed with the latter approach; and

Maintain a constant water temperature, i.e. testing started at the lowest velocity and needed

to be raised from the Type 6 level to 100 mm above the recorded Type 2 level, after which

testing for the next velocity commenced.

Once all bell inlet velocities had been tested for a specific suction bell configuration, the bell was

raised to the next level, after which all the tests were repeated.

4.3.2 Dye injection tests

The experimental results obtained for the four suction bell configurations were compared

against one another to determine which of the four bell configurations had the lowest critical

submergence requirements. Dye injection tests were then performed for the preferred suction

bell configuration to determine the water level at which Type 3 vortices occur.

The procedures followed with the dye injection tests are detailed below:

Step 1: Change the VSD to achieve a flow of 3.22 ℓ/s at the water level recorded for a

Type 2 vortex.

Step 2: Sprinkle cinnamon on the water surface while lowering the water level. Stop the

draining of water at the first sign of cinnamon being pulled below the water surface.


Page | 46


Step 3: Monitor the frequency of vortices that pull the cinnamon below the water surface. If

this occurs more than three (3) times in five (5) minutes, inject dye at the location where the

vortex forms. Repeat the dye injection each time a vortex forms for a duration of five (5)

minutes. If no coherent dye core vortex is visible during this time, lower the water level by

10 mm and repeat this step until a coherent dye core vortex becomes visible. Record the

water level at which the Type 3 vortex occurs.

Step 4: Fill the canal to the water level recorded for a Type 2 vortex at a flow of 4.29 ℓ/s.

Amend the pump speed on the VSD to achieve this higher flow and repeat Steps 1 to 3.

Repeat this procedure for all the other flows.

It was found that the above test procedure was not practical, as the cinnamon was only pulled

below the water surface at water levels slightly higher than those measured for Type 5 vortices.

It therefore is likely that the water levels at which the cinnamon was pulled below the water

surface represent Type 4 vortices. The test procedure was then amended as follows:

Step 1: Change the VSD to achieve a flow of 3.22 ℓ/s at the water level recorded for a

Type 2 vortex.

Step 2: Inject dye at the surface dimple when it forms. If no coherent dye core is visible,

lower the water level by 5 mm and repeat this until a coherent dye core vortex becomes

visible. Record the water level at which the Type 3 vortex occurs.

Step 3: Fill the canal to the water level recorded for a Type 2 vortex at a flow of 4.29 ℓ/s.

Amend the pump speed on the VSD to achieve this higher flow and repeat Steps 1 and 2.

Repeat this procedure for all the other flows.

This test procedure was also found to be impractical. At the Type 2 water levels, the dye would

disperse with no sign of any coherent dye core vortices. This phenomenon continued to occur

as the water level was lowered to a point where the dye was suddenly pulled from the water

surface into the suction bell, i.e. this occurred when injecting the dye just upstream of the

suction bell with no signs of surface dimples or depressions being present. This is

demonstrated by the series of photos shown in Figure 4.11, where dye was injected upstream

of the suction bell at the location where a Type 2 vortex was busy forming.


Page | 47


(a)

(b)

(c)

(d)

Figure 4.11: Path of dye injected upstream of suction bell


Page | 48


It was further noted that, when injecting dye at the location where the surface dimple started, a

vortex with a dye core was starting to form, but that the vortex moved to the back of the canal

(see Figure 4.12) (mainly caused by the approach velocity in the canal), with the dye core never

reaching the inlet to the suction bell, as the vortex strength reduced as it moved away from the

suction bell.

Figure 4.12: Evidence of a coherent dye core behind the suction bell

The results obtained from the dye injection were therefore inconclusive and it was decided that

these results would not be used for further analyses. It is expected that the use of dye injection

would be practical only if the tests are conducted with very low approach velocities (i.e.

prototype approach velocities of less than 0.2 to 0.3 m/s and where the pump intake is situated

in a large reservoir where the velocities at the inlet bell are distributed more evenly compared to

the distribution expected for a pump intake situated in a canal.

4.3.3 ADV measurements

It is also an acceptance criterion (Hydraulic Institute, 1998) that the time-averaged velocities at

points in the throat of the bell inlet must be within 10% of the average axial velocity. In order to

verify whether this criterion would be met for the Type 2 water levels, acoustic Doppler

velocimeter (ADV) readings were taken at the five positions indicated in Figure 4.13. The ADV

measurements were taken only for the preferred inlet bell configuration, located at 0.5D above

the canal floor, but for all six velocities.


Page | 49


Figure 4.13: Positions of ADV measurements at inlet bell

The ADV instrument was set to record samples at a frequency of 25 Hz, i.e. every 0.04

seconds. The accuracy of the ADV measurements was influenced by the amount of particles in

the water, which required the addition of fine sand particles to the potable water used in the

model. This influenced the sampling period, as the sand particles settled out at low approach

canal velocities, causing larger fluctuations in the measurements towards the end of the

sampling period. It was found that the sampling time varied from 30 seconds (i.e. 750 samples)

up to 60 seconds (i.e. 1 500 samples) before the measurements became unstable.


Page | 50


The ADV instrument recorded velocities in the X, Y and Z direction. Figure 4.14 shows the XYZ

coordinate system, with the X-axis being parallel to the approach canal. A positive Y-axis is to

the right when facing downstream, while a positive Z-axis is towards the electronics of the

instrument (i.e. upwards).

Figure 4.14: XYZ coordinates of ADV instrument (Nortek, 2004)

A further objective of the physical model study was to obtain sufficient information on velocity

distributions along the sand trap canal to assist with the calibration of the CFD model.

Figure 4.15 shows the positions and depths at which ADV measurements were taken for

calibration purposes. The ADV measurements were taken for the scenarios detailed in

Table 2.3.


Page | 51


Figure 4.15: Position of ADV measurements in sand trap canal


Page | 52


5. EXPERIMENTAL TEST RESULTS

5.1 Experimental test results

Tables 5.1 to 5.12 summarise the experimental test results for the various inlet bell

configurations. The prototype Type 2, 5 and 6 submergence levels are plotted against the

prototype bell inlet velocities in Figures 5.1 to 5.12 for the corresponding table numbers.

Table 5.1: Test results – inlet bell Type 1A, 0.5D above floor

Description Prototype bell inlet velocities (m/s)

0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.22 4.29 5.36 6.44 7.5 8.57

Pump speed (Hz) 23.0 27.4 32.2 37.4 42.8 48.8

Water temperature

(°C) 20.0 20.0 20.0 20.5 20.5 20.5

Type 2 vortex level

(mm) (1)

80 83 89 96 104 107

Type 5 vortex level

(mm) (1)

42 62 64 69 80 87

Type 6 vortex level

(mm) (1)

36 57 58 64 74 78

(1) The water levels represent the “S” dimension shown in Figures 2.1 and 2.2, and are also model scale levels.


Page | 53


Figure 5.1: Type 1A bellmouth, 0.5D above floor – prototype type submergence



0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.21 4.3 5.37 6.42 7.53 8.58

Pump speed (Hz) 22.7 27.3 32.1 37.4 43.1 48.9

Water temperature

(°C) 20.5 20.0 20.0 20.0 20.5 20.5

Type 2 vortex level

(mm) (1)

73 87 92 95 104 106

Type 5 vortex level

(mm) (1)

45 62 67 72 77 77

Type 6 vortex level

(mm) (1)

34 51 60 67 70 72


0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)


Submergence: Type 1A bellmouth, 0.5D (60 mm) above floor

Type 2Type 5

Type 6


Page | 54





0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.22 4.28 5.36 6.45 7.52 8.57

Pump speed (Hz) 22.1 26.6 31.6 37.0 43 49

Water temperature

(°C) 20.0 20.0 20.0 20.0 20.0 20.0

Type 2 vortex level

(mm) (1)

154 169 187 218 226 244

Type 5 vortex level

(mm) (1)

47 74 122 146 181 209

Type 6 vortex level

(mm) (1)

29 60 75 111 163 187


0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Type2

Type 5

Type 6


Page | 55



Table 5.4: Test results – inlet bell Type 1B, 0.5D above floor


0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.22 4.29 5.34 6.44 7.5 8.55

Pump speed (Hz) 22.0 24.7 30.4 37.0 42.7 48.75

Water temperature

(°C) 18.5 18.5 19.0 19.5 19.5 19.5

Type 2 vortex level

(mm) (1)

109 123 144 151 156 166

Type 5 vortex level

(mm) (1)

37 60 66 74 78 85

Type 6 vortex level

(mm) (1)

19 51 54 56 62 73


0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Type 2

Type 5

Type 6


Page | 56


Figure 5.4: Type 1B bellmouth, 0.5D above floor – prototype type submergence



0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.21 4.29 5.36 6.44 7.51 8.58

Pump speed (Hz) 23.1 27.6 32.5 37.9 43.4 49.3

Water temperature

(°C) 20.0 20.0 20.0 20.0 20.0 19.5

Type 2 vortex level

(mm) (1)

69 91 100 131 143 188

Type 5 vortex level

(mm) (1)

21 46 55 70 81 86

Type 6 vortex level

(mm) (1)

12 29 48 63 72 78


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)


Submergence: Type 1B bellmouth, 0.5D (60 mm) above floor (Test No 1)

Type 2

Type 5

Type 6


Page | 57





0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.21 4.28 5.32 6.44 7.52 8.57

Pump speed (Hz) 22.0 26.6 31.5 37.0 42.9 49.2

Water temperature

(°C) 19.0 19.0 19.0 19.0 18.5 17.5

Type 2 vortex level

(mm) (1)

164 181 183 206 225 242

Type 5 vortex level

(mm) (1)

36 72 96 149 156 162

Type 6 vortex level

(mm) (1)

18 45 74 119 137 144


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)


Submergence: Type 1B bellmouth, 1.0D (120 mm) above floor

Type 2

Type 5

Type 6


Page | 58





0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.19 4.28 5.35 6.44 7.54 8.58

Pump speed (Hz) 22.8 27.3 32.1 37.3 42.8 49.1

Water temperature

(°C) 20.0 20.0 20.0 20.0 19.0 18.5

Type 2 vortex level

(mm) (1)

35 54 66 75 101 118

Type 5 vortex level

(mm) (1)

11 19 31 39 38 37

Type 6 vortex level

(mm) (1)

7 12 26 33 27 31


0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Type 2

Type 5

Type 6


Page | 59





0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.2 4.29 5.36 6.43 7.54 8.58

Pump speed (Hz) 22.0 26.7 31.6 36.7 42.7 48.7

Water temperature

(°C) 21.0 21.0 21.0 21.0 21.0 21.0

Type 2 vortex level

(mm) (1)

122 140 146 150 167 182

Type 5 vortex level

(mm) (1)

38 89 104 116 136 134

Type 6 vortex level

(mm) (1)

10 66 91 102 121 126


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Type 2

Type 5

Type 6


Page | 60





0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.2 4.31 5.36 6.44 7.54 8.6

Pump speed (Hz) 21.8 26.4 31.3 36.5 42.7 48.9

Water temperature

(°C) 21.0 21.0 21.0 21.0 21.0 21.0

Type 2 vortex level

(mm) (1)

98 134 162 180 189 203

Type 5 vortex level

(mm) (1)

51 83 94 112 119 152

Type 6 vortex level

(mm) (1)

17 65 81 97 109 141


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

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m)



Type 2

Type 5

Type 6


Page | 61





0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.21 4.28 5.35 6.42 7.51 8.59

Pump speed (Hz) 22.9 27.4 32.2 37.3 42.7 48.8

Water temperature

(°C) 20.0 20.0 20.0 20.0 20.0 20.0

Type 2 vortex level

(mm) (1)

52 53 62 94 116 159

Type 5 vortex level

(mm) (1)

29 37 40 71 56 44

Type 6 vortex level

(mm) (1)

23 32 37 66 51 41


0.0

0.5

1.0

1.5

2.0

2.5

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

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m)



Type 2

Type 5

Type 6


Page | 62





0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.22 4.28 5.36 6.44 7.52 8.56

Pump speed (Hz) 22.1 26.7 31.6 36.9 42.8 48.9

Water temperature

(°C) 18.5 18.5 18.5 15.0 16.0 16.0

Type 2 vortex level

(mm) (1)

136 146 167 209 218 221

Type 5 vortex level

(mm) (1)

57 82 121 126 130 139

Type 6 vortex level

(mm) (1)

12 42 101 106 115 132


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

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m)


Submergence: Type 2B bellmouth, 0.5D (60 mm) above floor (Test No 1)

Type 2

Type 5

Type 6


Page | 63





0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Actual measured

flow (ℓ/s) 3.23 4.32 5.36 6.44 7.51 8.59

Pump speed (Hz) 21.7 26.5 31.2 36.5 42.7 48.9

Water temperature

(°C) 17.0 18.0 18.0 18.0 19.0 19.0

Type 2 vortex level

(mm) (1)

153 197 220 224 249 248

Type 5 vortex level

(mm) (1)

36 104 118 127 147 160

Type 6 vortex level

(mm) (1)

19 79 94 112 123 132


0.0

0.5

1.0

1.5

2.0

2.5

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Type 2

Type 5

Type 6


Page | 64



The above experimental test results are analysed in more detail in Section 5.4.

5.2 Repeatability of tests

The water levels at which Types 2, 5 and 6 vortices occur are based on subjective visual

observations. In order to verify the accuracy and reliability of the water level measurements, the

tests for the two long radius inlet bells (i.e. Types 1B and 2B), situated 0.5D above the floor,

were repeated.

The results of the additional tests are summarised in Tables 5.13 to 5.19. The prototype

Type 2, 5 and 6 submergence levels for the three tests are plotted against the prototype bell

inlet velocities in Figures 5.13 to 5.19 for the corresponding table numbers.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Type 2

Type 5

Type 6


Page | 65


Table 5.13: Repeatability test results – inlet bell Type 1B, 0.5D above floor, Type 2 vortices


0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58

Type 2 vortex level –

Test No 1 (mm) (1)

109 123 144 151 156 166


Test No 2 (mm) (1)

108 120 147 155 157 170


Test No 3 (mm) (1)

118 126 153 148 158 172

Average Type 2

vortex level (mm) 112 123 148 151 157 169

Maximum deviation

from average Type 2

vortex level

6% 2% 3% 2% 1% 2%


Figure 5.13: Type 1B bellmouth, 0.5D above floor – prototype type submergence for Type 2 vortices

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)


Submergence: Type 1B bellmouth, 0.5D (60 mm) above floor, Type 2 vortex

Test 3

Test 1

Test 2


Page | 66




0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58


Test No 1 (mm) (1)

37 60 66 74 78 85


Test No 2 (mm) (1)

52 53 75 85 88 87


Test No 3 (mm) (1)

49 60 78 80 85 92

Average Type 5

vortex level (mm) 46 58 73 80 84 88

Maximum deviation

from average Type 5

vortex level

13% 4% 7% 7% 5% 5%



0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Test 3

Test 1

Test 2


Page | 67




0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58


Test No 1 (mm) (1)

19 51 54 56 62 73


Test No 2 (mm) (1)

44 46 59 75 76 74


Test No 3 (mm) (1)

44 51 48 62 73 74

Average Type 6

vortex level (mm) 36 49 54 64 70 74

Maximum deviation

from average Type 6

vortex level

23% 3% 10% 17% 8% 0%



0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Test 3 Test 1

Test 2


Page | 68




0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58


Test No 1 (mm) (1)

52 53 62 94 116 159


Test No 2 (mm) (1)

51 56 60 101 110 152


Test No 3 (mm) (1)

45 56 60 106 109 135

Average Type 2

vortex level (mm) 49 55 61 100 112 149

Maximum deviation

from average Type 2

vortex level

5% 2% 2% 6% 4% 7%



0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Test 3

Test 1

Test 2


Page | 69




0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58


Test No 1 (mm) (1)

29 37 40 71 56 44


Test No 2 (mm) (1)

36 40 45 67 45 40


Test No 3 (mm) (1)

28 35 36 80 50 42

Average Type 5

vortex level (mm) 31 37 40 73 50 42

Maximum deviation

from average Type 5

vortex level

16% 7% 12% 10% 11% 5%



0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

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m)



Test 3

Test 1Test 2


Page | 70




0.9 1.2 1.5 1.8 2.1 2.4

Theoretical scaled

model target flow

(ℓ/s)

3.22 4.29 5.36 6.44 7.51 8.58


Test No 1 (mm) (1)

23 32 37 66 51 41


Test No 2 (mm) (1)

19 26 35 62 42 40


Test No 3 (mm) (1)

15 30 31 72 44 37

Average Type 6

vortex level (mm) 19 29 34 67 46 39

Maximum deviation

from average Type 6

vortex level

21% 9% 8% 8% 12% 4%



0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



Test 3

Test 1

Test 2


Page | 71


It can be concluded from Table 5.13 to 5.18 that:

The maximum deviations from the average water levels for Type 2, 5 and 6 vortices were

7%, 16% and 23% respectively;

The average deviation, for all six velocities, from the average water levels for Type 2, 5 and

6 vortices were only 4%, 9% and 10% respectively; and

The water level measurements for the Type 2, 5 and 6 vortices can be repeated with a

reasonably high level of confidence.

The percentage deviations from the average water levels increased for the Type 5 and 6

vortices, but this was expected due to the higher turbulence caused by the much higher

approach velocities in the sand trap canal, which would result in more unstable conditions for

vortices to form.

It is also evident from Tables 5.13 and 5.16 that, for Type 2 vortices, the maximum difference

between the minimum and maximum water levels for all the flows is 18% (i.e. for a bell inlet

velocity of 2.4 m/s). This implies that, should the critical submergence for the prototype design

be accepted as the water level recorded for Type 2 vortices, a safety factor of 18% should be

allowed if only one water level measurement was taken, to cater for the possible inaccuracies

and subjective measurements associated with physical model studies.

5.3 Visual observations

Figure 3.2 shows the strength classification of the different types of vortices. Figures 5.19,

5.20 and 5.21 show photos of Type 2, 5 and 6 vortices as they were identified in the physical

model study.


Page | 72


Figure 5.19: Type 2 vortex (surface dimple)

Figure 5.20: Type 5 vortex (pulling air bubbles to intake)


Page | 73


Figure 5.21: Type 6 vortex (full air core to intake)

A summary of the visual observations made during the physical model testing is provided below:

General remarks applicable to all bell configurations

The formation of surface dimples (i.e. Type 2 vortices) could be influenced by break-away

actions when the water level is at a joint on the segmented suction bend, especially at

higher bell intake velocities, which also lead to higher approach canal velocities. This is

demonstrated graphically in Figure 5.22;

High approach canal velocities resulted in Type 5 vortices breaking up before full air core

entrainment took place, i.e. vortices broke up shortly after they formed; and

The differences in water levels at which Type 5 and 6 vortices formed were often very small,

i.e. 7 mm (which represents 70 mm in the prototype model) or less.

Type 1A bell configuration

Surface oscillations were visible at the water levels for the Type 5 and 6 vortices for a

prototype bell inlet velocity of 0.9 m/s and a bell height of 0.5D; and

The water surface was very turbulent at higher bell inlet velocities (i.e. 2.1 m/s and higher)

for a bell height of 0.5D, which made the identification of Type 2 vortices more difficult.


Page | 74


Type 1B bell configuration

Surface oscillations were visible at the water levels for the Type 5 and 6 vortices for

prototype bell inlet velocities of 0.9 m/s and 1.2 m/s, and a bell height of 1.0D. This resulted

in the vortices breaking up before full air core vortices could form; and

The required submergence for Type 2 vortices increased noticeably when the bell height

was changed from 1.0D to 1.5D.

Type 2A bell configuration

Minor surface oscillations were visible at the water levels for the Type 5 and 6 vortices for

prototype bell inlet velocities of 0.9 m/s and 1.2 m/s, and a bell height of 0.5D; and



Type 2B bell configuration

Minor surface oscillations were visible at the water levels for the Type 5 and 6 vortices for a

prototype bell inlet velocity of 0.9 m/s and a bell height of 0.5D; and



Figure 5.22: Break-away caused when water level is at joint on segmented bend


Page | 75


The break-away actions caused by the segmented suction bend, which could influence the

formation of Type 2 vortices, are perhaps one of the reasons why the majority of pump intake

model studies were performed using a long straight pipe as part of the suction configuration. It

has been demonstrated in Section 5.2, however, that the tests could be repeated with a

reasonably high level of confidence which implies that the joints on the segmented bends did

not have a significant impact on the test results.

5.4 Analysis of experimental test results

The motivation for the study is the question – what impact does the raising of the suction bell

have on the minimum submergence required to prevent air entrainment? In answer to this

question, the submergence value required for each of the measured vortex strengths was

plotted against bell inlet velocity for the different suction bell heights.

5.4.1 Comparison of submergence for different suction bell heights

Figures 5.23, 5.24 and 5.25 show the prototype submergence required for Type 2, 5 and 6

vortices for the Type 1A suction bell configuration situated at heights of 0.5D, 1.0D and 1.5D

above the canal floor level.

Figure 5.23: Type 1A suction bell – submergence required for Type 2 vortices at bell heights of 0.5D, 1.0D and 1.5D

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)


Submergence: Type 1A bellmouth, 0.5D - 1.5D above floor, Type 2 vortex

1.5D

0.5D

1.0D


Page | 76




0.0

0.5

1.0

1.5

2.0

2.5

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D

1.0D

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D

1.0D


Page | 77


The following is evident from Figures 5.23 to 5.25:

The submergence required for suction bells located at 0.5D and 1.0D is almost identical for

the Type 2, 5 and 6 vortices;

The submergence required for Type 2 vortices, where the suction bell is located at 1.5 D, is

much higher than that required for suction bells located at 0.5D and 1.0D above the floor;

and

At bell inlet velocities of 1.2 m/s or lower, the submergence required for Type 5 and 6

vortices is very similar, irrespective of the height of the suction bell. At bell inlet velocities of

1.5 m/s and higher, the submergence required for Type 5 and 6 vortices for a suction bell

situated at 1.5D becomes substantially higher than that required for the 0.5D and 1.0D

suction bells.

The prototype submergence required for Type 2, 5 and 6 vortices for the Type 1B suction bell

configuration situated at heights of 0.5D, 1.0D and 1.5D above the canal floor level is shown in

Figures 5.26, 5.27 and 5.28 respectively.

Figure 5.26: Type 1B suction bell – submergence required for Type 2 vortices at bell heights of 0.5D, 1.0D and 1.5D

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)


Submergence: Type 1B bellmouth, 0.5D - 1.5D above floor, Type 2 vortex

1.5D

0.5D

1.0D


Page | 78




0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D

1.0D

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D

1.0D


Page | 79


It is evident from the results shown in Figures 5.26 to 5.28 that:

The submergence required for suction bells located at 0.5D and 1.0D is fairly similar for the

Type 2, 5 and 6 vortices;

The submergence required for Type 2 vortices, where the suction bell is located at 1.5 D, is

much higher than that required for suction bells located at 0.5D and 1.0D above the floor;

and

The submergence required for Type 5 and 6 vortices is very similar, irrespective of the

height of the suction bell, provided that the bell inlet velocities are 1.2 m/s or lower. At bell

inlet velocities of 1.5 m/s and higher, the submergence required for Type 5 and 6 vortices for

a suction bell situated at 1.5D becomes substantially higher than that required for the 0.5D

and 1.0D suction bells.

Figures 5.29, 5.30 and 5.31 show the prototype submergence required for Type 2, 5 and 6

vortices for the Type 2A suction bell configuration situated at heights of 0.5D, 1.0D and 1.5D

above the canal floor level.


0.0

0.5

1.0

1.5

2.0

2.5

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D

1.0D


Page | 80




0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D

1.0D

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D

1.0D


Page | 81


It is evident from Figures 5.29 to 5.31 that:

The submergence required for suction bells located at 1.0D and 1.5D is very similar for the

Type 2, 5 and 6 vortices; and

The submergence required for Type 2, 5 and 6 vortices, where the suction bell is located at

1.0D and 1.5 D, is much higher than that required for a suction bell located at 0.5D above

the floor.

The prototype submergence required for Type 2, 5 and 6 vortices for the Type 2B suction bell

configuration situated at heights of 0.5D, 1.0D and 1.5D above the canal floor level is shown in

Figures 5.32, 5.33 and 5.34 respectively.


0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D

1.0D


Page | 82




0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D1.0D

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)



1.5D

0.5D

1.0D


Page | 83


It is evident from the results shown in Figures 5.32 to 5.34 that:

The submergence required for suction bells located at 1.0D and 1.5D is fairly similar for the

Type 2, 5 and 6 vortices;

The submergence required for Type 2, 5 and 6 vortices, where the suction bell is located at

1.0D and 1.5 D, is much higher than that required for a suction bell located at 0.5D above

the floor; and

A peak in the submergence required for Type 5 and 6 vortices, for a suction bell located at

0.5 D above the canal floor, is evident for a bell inlet velocity of 1.8 m/s.

5.4.2 Explaining the increase in submergence when raising the pump intake

This section of the thesis focuses on explanations for the sudden increase in submergence

when raising the pump intake to above a certain level from the canal floor.

The following aspects were evaluated in more detail for each of the inlet bell configurations:

The Reynolds Number of the approach flow; and

The approach velocity expressed as a ratio of the bell inlet velocity.

Reynolds Number of approach flow

A Reynolds Number less than 2 000 represents laminar flow, whereas a Reynolds Number

greater than 4 000 represents turbulent flow. The area where Reynolds Numbers vary from

2 000 to 4 000 is referred to as the “transitional zone” (Featherstone & Nalluri, 1988). The

question is whether the sudden increase in submergence could be related to the flow changing

from turbulent to laminar as the approach velocity will reduce in respect to an increase in the

height of the suction bell.

The aspects that stood out from the analysis of the physical model test results are that

(a) the required submergence for flat suction bell configurations (i.e. Types 1A and 1B)

substantially increases when the suction bell is raised above 1.0D, and (b) the same

occurrence is evident for slanted suction bell configurations (i.e. Types 2A and 2B), but

for heights higher than 0.5D above the floor. This leads to the question – what is

causing this “break-away” or increased submergence when raising the pump intakes to

above a certain level?


Page | 84


The Reynolds Number for the approach flow was calculated using the following equation:

(5.1)

with Re = Reynolds Number at model scale

V = approach canal velocity (m/s)

L = hydraulic radius (m)

= viscosity of liquid (1 x 10-6 m2/s)

Table 5.19 summarises the model scale Reynolds Numbers calculated for all the suction bell

configurations, the different heights above the canal floor, the different vortex types and the

entire range of bell inlet velocities. The cells in Table 5.19 were highlighted green where the

Reynolds Number was less than 4 000.

Table 5.19: Reynolds Numbers for approach velocities

Bell

configuration

Height

above

floor

Vortex

type

Scale model Reynolds numbers for prototype bell

inlet velocities (m/s)

0.9 1.2 1.5 1.8 2.1 2.4

1A

0.5D

2 6 192 8 156 9 963 11 667 13 204 14 930

5 7 252 8 864 10 984 12 932 14 423 16 049

6 7 454 9 051 11 261 13 197 14 764 16 609

1.0D

2 5 128 6 575 8 087 9 582 10 945 12 399

5 5 632 7 119 8 746 10 288 11 877 13 533

6 5 858 7 388 8 950 10 456 12 145 13 750

1.5D

2 3 546 4 563 5 503 6 226 7 148 7 877

5 4 640 5 722 6 351 7 231 7 817 8 418

6 4 894 5 944 7 147 7 847 8 121 8 799

1B

0.5D

2 5 571 7 079 8 241 9 728 11 161 12 355

5 7 419 8 938 10 854 12 677 14 535 16 132

6 8 090 9 286 11 410 13 644 15 496 16 897

1.0D

2 5 194 6 480 7 882 8 679 9 804 10 023

5 6 149 7 500 9 085 10 387 11 698 13 160

6 6 369 7 974 9 306 10 627 12 035 13 491

1.5D

2 3 459 4 449 5 507 6 364 7 162 7 906

5 4 777 5 753 6 717 7 171 8 246 9 275

6 5 047 6 203 7 112 7 685 8 604 9 651


Page | 85


Bell

configuration

Height

above

floor

Vortex

type

Scale model Reynolds numbers for prototype bell

inlet velocities (m/s)

2A

0.5D

2 5 800 7 279 8 742 10 222 11 056 11 983

5 6 355 8 263 9 871 11 541 13 561 15 487

6 6 457 8 492 10 056 11 795 14 120 15 830

1.0D

2 3 791 4 875 6 009 7 144 8 073 8 900

5 4 734 5 514 6 634 7 728 8 647 9 885

6 5 161 5 861 6 854 7 998 8 955 10 070

1.5D

2 3 493 4 362 5 134 5 963 6 867 7 638

5 3 893 4 865 5 903 6 822 7 871 8 398

6 4 244 5 071 6 077 7 046 8 038 8 583

2B

0.5D

2 5 497 7 304 8 858 9 611 10 548 10 764

5 5 967 7 726 9 554 10 322 12 686 15 123

6 6 103 7 868 9 657 10 490 12 904 15 285

1.0D

2 3 693 4 798 5 739 6 326 7 259 8 215

5 4 510 5 602 6 366 7 559 8 744 9 749

6 5 160 6 257 6 683 7 931 9 060 9 907

1.5D

2 3 148 3 878 4 621 5 514 6 166 7 064

5 4 078 4 655 5 607 6 612 7 406 8 260

6 4 261 4 920 5 903 6 822 7 774 8 730

It is evident from Table 5.19 that the Reynolds Numbers for almost all the scenarios fall within

the turbulent zone. The sudden increase in submergence required for suction bell inlets raised

above a certain height from the floor was therefore not due to a change in flow conditions from

turbulent to laminar.

The approach velocity

It is evident from the review of acceptable approach velocities (refer to Section 3.3) that the

maximum prototype approach velocity should be 0.3 m/s or less, and that various authors also

recommend maximum bell inlet velocities ranging from 1.3 m/s (Prosser, 1977) to 2.1 m/s

(Hydraulic Institute, 1998). The approach velocity is linked to the bell inlet velocity and it was

therefore decided to plot prototype submergence against the dimensionless ratio of inlet bell

velocity/approach canal velocity to determine whether the sudden increase in submergence

could be related to this dimensionless ratio.

Figures 5.35, 5.36 and 5.37 show the submergence required for Type 2, 5 and 6 vortices

plotted against the equivalent bell inlet velocity/approach canal velocity ratio for a Type 1A

suction bell configuration.


Page | 86


Figure 5.35: Type 1A suction bell – submergence against bell/approach velocity ratios for Type 2 vortices


0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Sub

mer

gen

ce (

m)

Bell/approach velocity ratio

Type 1A bellmouth: Bell/approach velocity ratios for Type 2 vortices

0.5D 1.0D 1.5D

0.0

0.5

1.0

1.5

2.0

2.5

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

Sub

mer

gen

ce (

m)



0.5D 1.0D 1.5D


Page | 87



The ovals on Figures 5.35 to 5.37 represent the points where the submergence suddenly

increased, i.e. the minimum and maximum submergence required for the 0.5D (the blue

markers) and 1.0D (the red markers) are similar, but the submergence for some points of the

1.5D installation are much higher than for the 0.5D and 1.0D installations. These points are the

same points shown in Figures 5.23 to 5.25 where the submergence suddenly increased as the

bell height was increased.

It is interesting to note from Figures 5.35 to 5.37 that this sudden “jump” in all the graphs is

located near or above an inlet bell/approach velocity ratio of 6.0.

The submergence required for Type 2, 5 and 6 vortices, which is plotted against the equivalent

bell inlet velocity/approach canal velocity ratio for a Type 1B suction bell configuration, is shown

in Figures 5.38 to 5.40 respectively.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

Sub

mer

gen

ce (

m)



0.5D 1.0D 1.5D


Page | 88


Figure 5.38: Type 1B suction bell – submergence against bell/approach velocity ratios for Type 2 vortices


0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Sub

mer

gen

ce (

m)


Type 1B bellmouth: Bell/Approach velocity ratio against submergence for Type 2 vortices

0.5D 1.0D 1.5D

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Sub

mer

gen

ce (

m)



0.5D 1.0D 1.5D


Page | 89



It is evident from Figures 5.38 to 5.40 that the points inside the ovals, which represent the

points where the submergence suddenly increased, are all plotted near or above an inlet

bell/approach canal velocity ratio of 6.0, which is similar to what was seen in Figures 5.35 to

5.37 for the Type 1A bell configuration.

Figures 5.41 to 5.43 show the submergence required for Type 2, 5 and 6 vortices plotted

against the equivalent bell inlet velocity/approach canal velocity ratio for a Type 2A suction bell

configuration.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

Sub

mer

gen

ce (

m)



0.5D 1.0D 1.5D


Page | 90




0.0

0.5

1.0

1.5

2.0

2.5

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Sub

mer

gen

ce (

m)



0.5D 1.0D 1.5D

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

Sub

mer

gen

ce (

m)



0.5D 1.0D 1.5D


Page | 91



The following is evident from Figures 5.41 to 5.43:

Both the 1.0D and 1.5D bell heights plot inside the oval and represent points where a

sudden increase in the required submergence was observed. This is evident for the Type 2,

5 and 6 vortices. It should be noted that a 1.0D slanted bell is similar to a 1.5D flat bell, as

the submergence for the slanted bell is measured from the water level to the top of the bell,

i.e. in this specific application, the vertical distance from the bottom to the top of the slanted

bell is 60 mm, i.e. 0.5D, meaning that the top of the slanted bell is at a height of 1.5D above

the floor level; and

The points inside the oval are all located near or above a bell inlet velocity/approach canal

velocity ratio of 6.0, which was also the case for the Type 1A and 1B bell configurations.

The submergence required for Type 2, 5 and 6 vortices, plotted against the equivalent bell inlet

velocity/approach canal velocity ratio for a Type 2B suction bell configuration, is shown in

Figures 5.44 to 5.46 respectively.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

Sub

mer

gen

ce (

m)



0.5D 1.0D 1.5D


Page | 92




0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.00 2.00 4.00 6.00 8.00 10.00 12.00

Sub

mer

gen

ce (

m)


Type 2B bellmouth: Bell/approach velocity ratios for Type 2 vortices

0.5D 1.0D 1.5D

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

Sub

mer

gen

ce (

m)



0.5D 1.0D 1.5D


Page | 93



It is clear from the results presented in Figures 5.44 to 5.46 that the points inside the ovals are

all plotting near or above a bell inlet velocity/approach canal velocity ratio of 6.0, as was seen

for the Type 1A, 1B, and 2A bell configurations.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00

Sub

mer

gen

ce (

m)



0.5D 1.0D 1.5D

It can be concluded that a “discontinuity” in the required submergence occurs for all the bell

configuration types when the bell inlet velocity/approach canal velocity ratio is approximately

6.0 or higher. It is further evident that the submergence required for pump intake

installations with bell inlet velocity/approach canal velocity ratios greater than 6.0 needs to

be much more than for those installations where this ratio is less than 6.0. This finding

applies to pump installations with the same geometry as that of the physical model. The

implications of this finding are further addressed in Section 6 of the thesis.


Page | 94


5.5 Comparison of test results of four bell intake configurations

It is recommended by Sulzer Brothers Limited (1987) that a slanted suction bell configuration be

used for pump intakes where the flow per pump exceeds 1 m3/s (refer to Figures 2.1 and 2.2).

Based on the physical hydraulic model testing, a decision is required on the preferred bell intake

configuration to be used when designing pump intake schemes such as Vlieëpoort and Lower

Thukela. This section describes a comparison between the results of the four bell intake

configurations that were tested and presents a recommendation on the preferred bell intake

configuration.

Figures 5.47, 5.48 and 5.49 show a comparison between the prototype submergence required

for Type 2, 5 and 6 vortices for Type 1A and 1B bell configurations (i.e. both are “flat” bell

types).

Figure 5.47: Comparison of submergence between Type 1A and 1B suction bell configurations for Type 2 vortices

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)


Comparison of submergence between Type 1A and 1B bellmouths for Type 2 vortices

0.5D Type 1B 1.0D Type 1B 1.5D Type 1B 0.5D Type 1A 1.0D Type 1A 1.5D Type 1A


Page | 95




0.0

0.5

1.0

1.5

2.0

2.5

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)




0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)





Page | 96


It is evident from the results presented in Figures 5.47 to 5.49 that:

The required submergence for Type 1A suction bells is slightly lower than that required for

Type 1B suction bells for Type 2 vortices. This difference might, however, be due to

measuring inaccuracies associated with the break-away action from the segmented suction

pipework;

The required submergence for Type 1A and 1B suction bells is similar for Type 5 and 6

vortices; and

The required submergence is not influenced by the length of the suction bend.

A comparison between the prototype submergence required for Type 2, 5 and 6 vortices for

Type 2A and 2B bell configurations (i.e. both are “slanted” bell types) is shown in Figures 5.50,

5.51 and 5.52 respectively.


0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)





Page | 97




0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)




0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)





Page | 98


It is evident from Figures 5.50 to 5.52 that:

The required submergence for Type 2 vortices is slightly lower for the Type 2A bell

configuration when compared to the Type 2B bell;

The required submergence for the Type 5 and 6 vortices is very similar for both bell

configurations; and

The radius of the suction bend does not influence the required submergence.

The submergence for the Type 1A and 1B bell configurations is very similar, as is the case

when comparing the Type 2A and 2B bell configurations. The submergence required for the

suction bells with the long radius bends seems to be slightly higher for Type 2 vortices and is

therefore regarded as the more conservative approach when determining critical submergence.

Figures 5.53, 5.54 and 5.55 show a comparison between the prototype submergence required

for Type 2, 5 and 6 vortices for Type 1B and 2B bell configurations (i.e. both are “long radius”

type suction bends). The submergence shown in Figures 5.50 to 5.52 for the slanted suction

bell was increased by 0.6 m to be comparable with the flat suction bell, i.e. the submergence is

measured from the same distance above the floor for both types of suction bells.

Figure 5.53: Comparison of submergence between Type 1B and 2B suction bell configurations for Type 2 vortices

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)


Comparison of submergence between Type 1B and 2B bellmouths for Type 2 vortices

0.5D Type 1B 1.0D Type 1B 1.5D Type 1B 0.5D Type 2B 1.0D Type 2B 1.5D Type 2B


Page | 99




0.0

0.5

1.0

1.5

2.0

2.5

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)




0.0

0.5

1.0

1.5

2.0

2.5

0.9 1.2 1.5 1.8 2.1 2.4

Sub

mer

gen

ce (

m)





Page | 100


It is evident, when comparing the results presented in Figures 5.53 to 5.55, that the

submergence required for the Type 1 B bell configuration is lower than that of the Type 2B bell

configurations for all types of vortices. The only exception to this is the submergence for a

Type 2 vortex for a 0.5D bell height, which is similar for the two suction bell configurations.

5.6 Acoustic Doppler velocimeter measurements

The acoustic Doppler velocimeter (ADV) measurements were taken for the recommended

suction bell configuration, i.e. a Type 1B suction bell.

The ADV measurements were taken to:

Determine the time-average velocity distribution near the edge of the suction bell; and

Obtain sufficient velocity data along the approach canal for the calibration of the CFD model.

5.6.1 ADV measurements at edge of suction bell

ADV measurements were taken at the five positions indicated in Figure 4.13. The focus points

of the ADV instrument were set up at a depth of 60 mm above the canal floor level, i.e. the

same level as the bottom of the suction bell. Table 5.20 summarises the water levels used for

each of the tests.

Table 5.20: Water levels for ADV tests at suction bell


0.9 1.2 1.5 1.8 2.1 2.4

Water level above

canal floor (mm) (1)

172 183 208 211 217 229

(1) The water levels were the average submergence levels determined for Type 2 vortices

It is recommended that the design of the Vlieëpoort and Lower Thukela pump intakes be

based on the conventional “flat” suction bell configuration as the submergence required is

less than that required for the “slanted” configuration. It is also recommended that the

required submergence be based on a long radius suction bend as the submergence

required for Type 2 vortices was slightly higher than that of a short radius bend, which is a

more conservative approach.


Page | 101


Tables 5.21 to 5.26 summarise the average X, Y and Z velocities measured at each of the

positions, as well as the calculated resultant vector, for prototype bell inlet velocities of 0.9 m/s,

1.2 m/s, 1.5 m/s, 1.8 m/s, 2.1 m/s and 2.4 m/s respectively. The resultant vector was calculated

as follows:

Vvector = [Vx2 + Vy

2 + Vz2]0.5 (5.1)

with Vvector = resultant velocity (m/s)

Vx = velocity in x-direction (m/s)

Vy = velocity in y-direction (m/s)

Vz = velocity in z-direction (m/s)

The velocities are presented in mm/s in Tables 5.21 to 5.26 due to the lower velocities

associated with the scaled physical model. The X, Y and Z coordinate system is based on the

following convention, facing downstream towards the pump intake (also refer to Figure 4.14):

Positive X-axis velocities represent flow towards the pump intake;

Positive Y-axis velocities represent flow from left to right, perpendicular to the X-axis; and

Positive Z-axis velocities represent flow towards the ADV instrument, i.e. upwards.

It was noted in Section 4.3.3 that fine sand particles had to be added to improve the accuracy

of the ADV measurements. Each recorded dataset was analysed to determine whether any

part(s) of it should be excluded when calculating the average velocities. Figure 5.56 shows the

recorded dataset at Position 2 for a prototype suction bell inlet velocity of 0.9 m/s. The outliers

represent inaccurate readings caused by insufficient suspended sand particles in the water.


Page | 102


Figure 5.56: X, Y and Z velocities for 0.9 m/s bell inlet velocity at Position 2

It is evident from Figure 5.56 that the average velocities should only be calculated for the

dataset from sample numbers 720 to 1 020.

Table 5.21: Summary of ADV measurements (model) at suction bell for prototype 0.9 m/s bell inlet velocity

Description Position 1 Position 2 Position 3 Position 4 Position 5

Average X-axis velocity

(mm/s) -9.9 76.9 94.3 97.2 50.6

Average Y-axis velocity

(mm/s) 29.5 -29.3 2.0 2.9 1.3

Average Z-axis velocity

(mm/s) -30.6 -76.7 -27.5 -32.3 -55.7

Average resultant

velocity (mm/s) 43.7 112.5 98.3 102.4 75.3

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

0 200 400 600 800 1000 1200

Vel

oci

ty (

m/s

)

Sample number

X, Y and Z velocities for 0.9 m/s bell inlet velocity at Position 2

X-velocities Y-velocities Z-velocities

Outliers in dataset

Outliers in dataset


Page | 103


The scaled average axial velocity at the entrance to the suction bell is 284.7 mm/s. It is evident

from Table 5.21 that the average resultant velocities are all substantially lower than the axial

velocity at the bell entrance. This is to be expected, as the ADV measurements were taken at

positions located 45 mm from the edge of the suction bell, which are still inside the canal and

not at the edge of the suction bell. It is clear from the results presented in Table 5.21, however,

that:

The resultant velocities are much higher at the positions located in front of the suction bell

(i.e. Positions 2, 3 and 4) compared to the positions behind the suction bell;

The average resultant velocities at Positions 2, 3 and 4 compare reasonably well with one

another;

The average x-axis velocity at Position 1 is negative, indicating some form of “reverse flow”

or swirl behind the suction bell; and

The 10% velocity distribution stated by the Hydraulic Institute (1998) as an acceptance

criterion is not met at the five positions measured.




(mm/s) -39.0 105.7 117.4 126.4 10.6


(mm/s) -16.3 -35.2 2.0 5.9 -23.9


(mm/s) -47.3 -85.2 -37.1 -46.8 -35.6

Average resultant

velocity (mm/s) 63.4 140.2 123.2 134.9 44.2

The same conclusions can be drawn for the results presented in Table 5.22 as those

summarised below Table 5.21, which were for the ADV velocities associated with a suction bell

prototype inlet velocity of 0.9 m/s.


Page | 104





(mm/s) -45.1 116.6 136.5 108.6 59.9


(mm/s) 59.6 -42.6 2.0 -5.6 -13.2


(mm/s) -27.2 -92.1 -45.9 -40.9 -65.2

Average resultant

velocity (mm/s) 79.5 154.6 144.1 116.2 89.5







(mm/s) 16.1 146.1 139.7 121.8 98.5


(mm/s) -76.0 -49.8 -10.0 -11.9 96.6


(mm/s) -44.6 -140.0 -56.8 -58.7 -121.1

Average resultant

velocity (mm/s) 89.5 208.4 151.1 135.8 183.6

It is evident from the results presented in Table 5.24 that:

All the velocities in the x-axis direction are now positive. This might be due to inaccuracies

in the ADV measurements caused by a lack of particles behind the suction bell; and

Larger variances are evident between the resultant velocities at the different positions, even

for Positions 2, 3 and 4, which are situated upstream of the suction bell. These larger

variations might also be due to a lack of particles, which influences the accuracy of the

measurements.


Page | 105





(mm/s) -55.6 166.2 189.8 173.8 83.0


(mm/s) 86.9 -47.8 3.0 -0.7 -19.1


(mm/s) -43.0 -130.9 -62.6 -78.1 -103.5

Average resultant

velocity (mm/s) 111.8 216.9 199.8 190.5 134.1







(mm/s) -59.9 191.9 223.6 184.8 77.0


(mm/s) 72.1 -20.0 -16.2 -3.9 -26.3


(mm/s) -72.4 -118.9 -58.3 -99.7 -93.6

Average resultant

velocity (mm/s) 118.4 226.6 231.7 210.0 124.0




Figure 5.57 shows the resulting velocities presented in Tables 5.21 to 5.26 in a plan view.


Page | 106


Figure 5.57: Plan view of resultant velocities at suction bell for prototype bell inlet velocities of 0.9 m/s to 2.4 m/s


Page | 107


Based on the ADV measurements taken near the edge of the suction bell for a range of suction

bell inlet velocities, it can be concluded that the 10% time-average velocity distribution,

recommended as an acceptance criterion by the Hydraulic Institute (1998), was not achieved for

this particular installation. It is likely that this acceptance criterion would be met for pump

intakes located in a large reservoir where the approach velocities from all sides would be more

evenly distributed, compared to a suction bell located at the end of a canal.

5.6.2 ADV measurements along approach canal

Figure 4.13 showed the positions and depths at which ADV measurements were taken for

calibration purposes. The ADV measurements were taken for the scenarios detailed in

Table 2.3. The water levels at which the ADV measurements were taken are summarised in

Table 5.27.

ADV measurements were taken at 27 positions for each set-up, i.e. a total of 108 points. The

average X, Y and Z velocities and the average resultant velocity for each of the positions are

summarised in Table 5.28.

Table 5.27: Water levels for ADV tests along approach canal

Prototype suction bell inlet

velocity (m/s)

Suction bell height Water level above

canal floor for Type 2

vortices (mm)

1.2 0.5D 183

1.2 1.0D 183 (1)

1.2 1.5D 361

1.8 0.5D 210

(1) The measured water level for the Type 2 vortex and a 1.0D bell height was 789 mm above the canal floor. The

water level was, however, kept the same for when the suction bell was 0.5D above the floor to allow a

comparison between these two set-ups, where the water level and suction bell inlet velocity are kept constant but

the bell height is varied.

The ADV datasets for each position had to be evaluated for “outliers”, as shown in Figure 5.58.

The average velocities were calculated only after these outliers had been eliminated from the

dataset.

Figure 5.59 shows the velocity distribution for the same dataset presented in Figure 5.58, but

only for sample numbers 195 to 295.


Page | 108


Table 5.28: Summary of ADV measurements (model) along approach canal

Position Average X-axis

velocity (mm/s)

Average Y-axis

velocity (mm/s)

Average Z-axis

velocity (mm/s)

Average resultant

velocity (mm/s)

Suction bell at height of 0.5D with bell inlet velocity of 1.2 m/s

1A 118.5 0.6 -0.8 118.5

1B 118.0 -0.5 -6.7 118.2

1C 105.1 -0.5 -3.7 105.2

2A 118.1 0.1 0.1 118.1

2B 121.0 0.5 -0.3 121.0

2C 116.1 -0.2 0.0 116.1

3A 119.3 1.3 0.1 119.3

3B 122.4 -1.6 1.6 122.4

3C 111.6 0.0 1.7 111.6

4A 114.6 -0.1 -0.6 114.6

4B 112.0 0.5 -6.7 113.2

4C 103.2 1.6 1.2 103.2

5A 118.1 1.9 0.9 118.1

5B 123.2 0.7 1.1 123.3

5C 115.9 3.1 2.5 116.0

6A 119.1 0.3 0.9 119.1

6B 120.6 -3.4 -1.2 120.6

6C 117.4 -1.1 1.3 117.4

7A 117.2 0.6 -3.2 117.2

7B 114.2 -0.6 -2.3 114.2

7C 112.2 -3.3 -2.8 112.3

8A 121.6 -0.2 -2.9 121.6

8B 119.0 0.6 -4.6 119.1

8C 113.8 1.3 -3.0 113.9

9A 120.3 2.6 -3.0 120.4

9B 116.4 -5.8 -6.5 116.7

9C 111.7 0.0 -2.1 111.7


1A 120.5 0.5 -2.0 120.6

1B 110.8 -2.4 -20.7 112.7

1C 111.3 0.3 -1.9 111.3

2A 121.7 -0.3 -0.1 121.7


Page | 109



velocity (mm/s)

Average Y-axis

velocity (mm/s)

Average Z-axis

velocity (mm/s)

Average resultant

velocity (mm/s)

2B 125.8 -0.1 -2.0 125.8

2C 120.5 0.4 0.9 120.5

3A 123.9 0.6 -1.2 123.9

3B 125.5 0.0 -0.6 125.5

3C 116.0 -0.3 -0.1 116.0

4A 119.3 -0.6 2.5 119.4

4B 119.4 2.5 -4.9 119.5

4C 112.8 1.2 1.5 112.8

5A 122.2 0.8 2.6 122.2

5B 126.1 0.5 0.6 126.1

5C 118.6 0.6 0.0 118.6

6A 122.2 0.2 0.6 122.2

6B 123.8 0.3 1.6 123.8

6C 118.9 -0.2 0.8 118.9

7A 121.6 -0.1 -2.8 121.6

7B 114.8 -4.0 -21.8 117.0

7C 111.6 1.3 -1.5 111.7

8A 123.6 -0.1 -1.2 123.6

8B 129.7 -0.1 -0.8 129.7

8C 120.4 0.5 -0.6 120.4

9A 125.5 2.8 -1.3 125.6

9B 123.3 -0.3 -1.9 123.3

9C 119.1 0.8 -1.2 119.1


1A 71.2 1.0 2.4 71.2

1B 72.6 -3.5 -20.4 75.5

1C 81.2 -0.5 8.7 81.7

2A 71.5 -0.4 2.9 71.6

2B 64.5 -0.4 -7.6 65.0

2C 98.7 0.2 10.8 99.3

3A 83.7 -2.6 0.9 83.7

3B 99.8 3.5 4.1 99.9

3C 84.5 1.5 4.6 84.7

4A 85.4 -3.4 2.3 85.5


Page | 110



velocity (mm/s)

Average Y-axis

velocity (mm/s)

Average Z-axis

velocity (mm/s)

Average resultant

velocity (mm/s)

4B 71.4 -1.8 -11.0 72.2

4C 77.9 0.1 10.3 78.6

5A 84.0 -0.6 2.2 84.0

5B 70.9 -0.4 0.8 70.9

5C 79.0 2.5 -0.1 79.0

6A 68.5 -1.0 0.4 68.6

6B 63.5 -1.9 -11.4 64.5

6C 79.5 -1.9 7.0 79.8

7A 88.8 -1.6 -4.5 88.9

7B 63.6 1.8 -33.1 71.7

7C 93.6 0.5 -3.4 93.7

8A 89.5 -2.0 -4.4 89.6

8B 79.2 -2.5 -3.4 79.3

8C 96.6 8.9 -5.4 97.1

9A 94.4 0.0 -5.6 94.6

9B 100.6 -4.2 -0.4 100.7

9C 82.0 0.6 -6.1 82.2


1A 138.4 2.1 -6.4 138.6

1B 130.9 -1.6 -5.6 131.1

1C 126.8 -0.5 -0.6 126.8

2A 150.0 1.5 -1.3 150.1

2B 145.2 1.8 -0.2 145.2

2C 133.2 -0.8 -2.3 133.2

3A 154.8 1.3 -0.4 154.8

3B 150.0 1.1 -0.7 150.0

3C 144.6 1.1 -4.5 144.7

4A 138.9 3.7 -1.7 139.0

4B 126.9 -6.6 -19.2 128.5

4C 126.7 -1.7 -4.7 126.8

5A 148.6 1.0 0.9 148.6

5B 145.0 1.5 -2.8 145.0

5C 135.1 -4.5 2.1 135.2

6A 157.2 2.0 -0.4 157.2


Page | 111



velocity (mm/s)

Average Y-axis

velocity (mm/s)

Average Z-axis

velocity (mm/s)

Average resultant

velocity (mm/s)

6B 155.3 3.0 0.2 155.3

6C 150.6 1.0 0.9 150.6

7A 141.1 2.9 -4.6 141.2

7B 132.2 1.6 -4.3 132.3

7C 126.4 -1.1 -4.1 126.4

8A 150.2 -0.3 -3.5 150.3

8B 139.4 -2.9 -13.8 140.2

8C 139.5 1.4 -3.9 139.6

9A 159.6 1.8 -4.9 159.7

9B 148.6 -0.5 -23.7 150.4

9C 140.9 1.1 -3.6 141.0

Figure 5.58: Position 1C: X, Y and Z velocities for 0.5D and 1.2 m/s inlet bell velocity

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

0 200 400 600 800 1000 1200 1400 1600

Vel

oci

ty (

m/s

)

Sample number

Position 1C: X, Y and Z velocities for 0.5D and 1.2 m/s inlet bell velocity

X velocities Y velocities Z velocities

Outliers in dataset

Outliers in dataset


Page | 112


Figure 5.59: Position 1C: X, Y and Z velocities for 0.5D and 1.2 m/s inlet bell velocity for sample numbers 195 to 295

It is evident from Figure 5.59 that the velocities fluctuate over time. The fluctuations are

generally within 20% of the calculated average velocities, e.g. the average X-axis velocity was

calculated as 0.105 m/s, with all of the samples except for 10 being between 0.08 m/s and

0.12 m/s.

Figure 5.60 shows the average resultant velocities along centrelines 1-4-7, 2-5-8 and 3-6-9 for

an inlet bell velocity of 1.2 m/s and a bell height of 0.5D, whereas Figure 5.61 shows a plan

view of the average resultant velocities for the same bell details at the different levels above the

floor level.

Based on the results presented in Figure 5.60, it appears that the average resultant velocities

are lowest at the highest water level, i.e. Level C.

It is evident from Figure 5.61 that the velocities for the points along the centreline of the canal

are higher than those at the sides of the canal.

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

180 200 220 240 260 280 300

Vel

oci

ty (

m/s

)

Sample number

Position 1C: X, Y and Z velocities for 0.5D and 1.2 m/s inlet bell velocity

X velocities Y velocities Z velocities


Page | 113


A further interesting observation, when comparing the Z-axis velocities between the points near

the inlet bell (refer to Tables 5.21 to 5.26) to those in the canal (refer to Table 5.28), is that the

average Z-velocities become highly negative as the water approaches the inlet bell, i.e. the

water suddenly “dives” down near the inlet bell.

Figure 5.60: Average resultant velocities along centrelines 1-4-7, 2-5-8, and 3-6-9


Page | 114


Figure 5.61: Average resultant velocities along Planes A, B and C

Figure 5.62 shows a series of photos taken of a particle approaching the inlet bell that

demonstrates this observation.

Further analyses of the ADV measurements, and the use thereof to calibrate the CFD model,

are covered in the thesis of Hundley (2012).


Page | 115


(a) (b)

(c) (d)

(e) (f)

(g) (h)

Figure 5.62: Path of particle approaching the inlet bell


Page | 116


6. COMPARISON OF TEST RESULTS AGAINST DESIGN GUIDELINES

It was concluded in Section 5.5 that the Type 1B suction bell configuration is the recommended

configuration for the design of pump intakes such as the Vlieëpoort and Lower Thukela

schemes.

The design guidelines that are available to calculate critical submergence were discussed in

Section 3.3. It was also evident from Figure 3.3 that large differences exist between the critical

submergence recommended by the various authors. In this section, a comparison is made

between the submergence measured in the physical model study, for the Type 2, 5 and 6

vortices, and the published design guidelines to determine which of the design guidelines best

represent the measured submergence.

Figures 6.1, 6.2 and 6.3 show the comparison between the critical submergence recommended

in the design guidelines and the measured Type 2, 5 and 6 submergence for bell heights of

0.5D, 1.0D and 1.5D respectively.

Figure 6.1: Comparison of submergence between design guidelines and measured Type 2, 5 and 6 vortices for a bell height of 0.5D

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0.60 0.90 1.20 1.50 1.80 2.10 2.40 2.70 3.00

Cri

tica

l su

bm

erge

nce

(m

)


Critical submergence versus bell inlet velocity for 0.5D bell height

1. Prosser & Sulzer 2. Hydraulic institute & Jones 3. Peterson and Noble 4. Knauss

5. Gorman Rupp 6. KSB 7. Metcalf & Eddy 8. Flygt

9. D Werth Type 2 vortex Type 5 vortex Type 6 vortex

1

23

4 56

7

8

9


Page | 117


It is evident from Figure 6.1 that:

The measured Type 2 submergence levels are slightly lower than the submergence levels

recommended by Knauss (1987); and

The measured Type 5 and 6 submergence levels are slightly lower than the design

guidelines recommended by KSB (Gouws, 2010) and Gorman-Rupp (Strydom, 2010b).


It can be seen from the results presented in Figure 6.2 that:

The Type 2 measured submergence closely follows the critical submergence recommended

by Metcalf and Eddy (1981, E-mail communication with Strydom, 2010a); and

The Type 5 and 6 measured submergence is less than any of the design recommendations.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0.60 0.90 1.20 1.50 1.80 2.10 2.40 2.70 3.00

Cri

tica

l su

bm

erge

nce

(m

)






1

23

4 56

7

8

9


Page | 118



It is evident from Figure 6.3 that:

The measured Type 2 submergence is now higher than the submergence recommended by

Knauss (1987), but lower than that recommended by the Hydraulic Institute (1998);

The submergence recommended by Metcalf and Eddy (1981, E-mail communication with

Strydom, 2010a) is similar to the measured Type 5 submergence; and

The measured Type 6 submergence is similar to that recommended by KSB (Gouws, 2010)

and Gorman Rupp (Strydom, 2010b).

The objective of this study was to determine the submergence at which air entrainment occur,

which is represented by the Type 6 water levels. The Type 5 water levels represent vortices

that pull air bubbles to the bell intake, but is was found with the physical model testing that the

Type 5 and 6 water levels were generally very similar, which poses a high risk of air entrainment

if the design is based on Type 5 water levels as being the critical submergence. The Hydraulic

Institute (1998) recommended that the critical submergence be based on Type 3 vortices (i.e.

coherent dye core), provided that they occur for less than 10% of the time. It was demonstrated

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0.60 0.90 1.20 1.50 1.80 2.10 2.40 2.70 3.00

Cri

tica

l su

bm

erge

nce

(m

)






1

23

4 56

7

8

9


Page | 119


in the physical model testing that Type 3 vortices cannot be determined accurately and that it

would be very difficult to determine whether they occur for less than 10% of the time. This

effectively means that the critical submergence should rather be based on the water levels at

which Type 2 vortices are detected.

It was concluded in Section 5.2, which evaluated the repeatability of the results obtained with

the physical model testing, that a safety factor of 18% should be allowed if only one water level

measurement is taken to cater for possible inaccuracies and subjective measurements.

Figure 6.4 therefore shows the measured Type 2 submergence levels that were increased by

20% in comparison with the available design guidelines.

Figure 6.4: Comparison of submergence between design guidelines and measured Type 2 vortices for bell heights of 0.5D, 1.0D and 1.5D

The following conclusions can be drawn from Figure 6.4:

The critical submergence proposed by Knauss (1987) should be used for pump intakes that

are located at 0.5D and 1.0D above the canal floor; and

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

0.60 0.90 1.20 1.50 1.80 2.10 2.40 2.70 3.00

Cri

tica

l su

bm

erge

nce

(m

)


Critical submergence versus bell inlet velocity for Type 2 vortices



9. D Werth Type 2 vortex (0.5D) Type 2 vortex (1.0D) Type 2 vortex (1.5D)

1

23

4 56

7

8

9


Page | 120


The critical submergence recommended by the Hydraulic Institute (1998) should be used

when raising the pump intake to 1.5D above the canal floor.

It was also shown in Figures 5.38 to 5.40 that the submergence required for the 0.5D and 1.0D

bell heights plotted below a bell inlet velocity/approach canal velocity ratio of 6.0, whereas the

1.5D bell installation plotted above a ratio of 6.0.

An important question to answer for the above recommendation, is whether the bell inlet

velocity/approach canal velocity ratio of 6.0 should be calculated with the equation published by

Knauss (1987) or by the equation published by the Hydraulic Institute (1998). The following

factors should be considered in answering this question:

1) The bell inlet velocity/approach canal velocity ratio of 6.0 was determined based on the

actual results obtained in the physical model study;

2) The water levels determined in the physical model were for Type 2 vortices, whereas the

equations developed by the Hydraulic Institute (1998) and Knauss (1987) were based on

Type 3 vortices that should not occur less than 10% of the time;

3) Figures 6.1 and 6.2, where the bell inlet velocity/approach canal velocity ratios were less

than 6.0, show that the Type 2 submergence levels measured in the physical model study

were lower than the submergence levels calculated with the equation published by

It therefore is recommended that, for pump intakes with a similar geometry to that tested with

the physical hydraulic model, critical submergence be calculated with the equation published

by Knauss (1987), i.e. S = D(0.5 + 2.0Fr), where the bell inlet velocity/approach canal velocity

ratio is less than 6.0, and that the equation published by the Hydraulic Institute (1998), i.e.

S = D(1 + 2.3Fr), be used where the ratio exceeds 6.0.

The Hydraulic Institute recommended inlet bell velocities of up to 2.1 m/s, with the optimum

inlet velocity being 1.7 m/s. It was, however, recommended by Prosser (1977) and Sulzer

Brothers Limited (1987) that inlet bell velocities be limited to 1.3 m/s, whereas Jones (2008)

recommended 1.5 m/s as the maximum inlet bell velocity. The water surface became

turbulent in the physical model study when the bell inlet velocities were increased above

1.8 m/s, which supports the recommendations by Prosser, Sulzer Brothers Limited and

Jones. It is recommended that bell inlet velocities therefore be limited to 1.5 m/s.


Page | 121


Knauss (1987). Similarly, Figure 6.3 shows that the Type 2 submergence levels measured

in the physical model study were lower than the submergence levels calculated with the

equation published by the Hydraulic Institute (1998), for bell inlet velocity/approach canal

velocity ratios exceeding 6.0. Therefore, calculating the bell inlet velocity/approach canal

velocity ratio with the equations published by Knauss (1987) and the Hydraulic Institute

(1998) will result in higher bell inlet velocity/approach canal velocity ratios compared to that

measured in the physical model study.

Based on the above factors, it would appear over-conservative to calculate the bell inlet

velocity/approach canal velocity ratio of 6.0 with the equation published by the Hydraulic

Institute (1998).

Tables 6.1 to 6.3 show the bell inlet velocity/approach canal velocity ratios calculated with both

equations, as well as the Type 2 submergence levels measured in the physical model study, as

further demonstration on which equation to use to calculate the bell inlet velocity/approach canal

velocity ratio of 6.0.

Table 6.1: Bell inlet velocity/approach canal velocity ratios for Type 1B suction bell located 0.6 m (prototype) above canal floor

Flow, Q (m3/s) 1.02 1.36 1.7 2.04 2.37

Assumed bell inlet velocity, V (m/s) 0.9 1.2 1.5 1.8 2.1

Calculated suction bell diameter (m) 1.20 1.20 1.20 1.20 1.20

Actual (rounded) suction bell diameter, D (m) 1.20 1.20 1.20 1.20 1.20

Actual bell inlet velocity, Vb (m/s) 0.90 1.20 1.50 1.80 2.10

Canal width, W (=2D) (m) 2.4 2.4 2.4 2.4 2.4

Required bell height above floor, C (m) 0.6 0.6 0.6 0.6 0.6

Froude Number, Fr 0.26 0.35 0.44 0.53 0.61

Knauss submergence, S = D(0.5+2Fr) (m) 1.23 1.44 1.65 1.86 2.07

Hydraulic Institute submergence, S = D(1+2.3Fr) (m) 1.93 2.17 2.41 2.65 2.89

Knauss approach velocity, Va (m/s) 0.23 0.28 0.31 0.35 0.37

Hydraulic Institute approach velocity, Va (m/s) 0.17 0.20 0.24 0.26 0.28

Ratio Vb/Va (Knauss) 3.89 4.33 4.78 5.22 5.66

Ratio Vb/Va (Hydraulic Institute) 5.36 5.87 6.39 6.90 7.40

Measured Type 2 submergence in physical model

for Type 1B suction bell (m) 1.12 1.23 1.48 1.51 1.57


Page | 122


It is evident from Table 6.1 that:

The inlet bell velocity/approach canal velocity ratios calculated with the equation published

by Knauss (1987) are all less than 6.0;


by the Hydraulic Institute (1998) vary from 5.36 to 7.40;

The Type 2 submergence levels measured in the physical model study correspond

reasonably well with the submergence levels calculated with the equation published by

Knauss (1987) for all bell inlet velocities; and

Calculating the submergence levels with the equation published by the Hydraulic Institute

(1998) for inlet bell velocity/approach canal velocity ratios exceeding 6.0, would result in

very conservative submergence levels when compared to those measured in the physical

model study.


Flow, Q (m3/s) 1.02 1.36 1.7 2.04 2.37





Canal width, W (=2D) (m) 2.4 2.4 2.4 2.4 2.4


Froude Number, Fr 0.26 0.35 0.44 0.53 0.61








for Type 1B suction bell (m) (1)

1.12 1.23 1.48 1.51 1.57

(1) The Type 2 submergence levels measured in the physical model are the maximum submergence values for suction bells

located at 0.6 m and 1.2 m above the floor, as some of the submergence levels for the 1.2 m suction bell installation were

lower than expected.


Page | 123




by Knauss (1987) vary from 5.16 to 6.93;


by the Hydraulic Institute (1998) all exceed 6.0;



Knauss (1987) for all bell inlet velocities; and

Calculating the submergence levels with the equation published by the Hydraulic Institute

(1998) for inlet bell velocity/approach canal velocity ratios exceeding 6.0, would result in

very conservative submergence levels when compared to those measured in the physical

model study. Similarly, calculating the submergence levels with the equation published by

Knauss (1987) for bell inlet velocities up to 1.5 m/s (i.e. bell inlet velocity/approach canal

velocity ratios of less than 6.0), and using the Hydraulic Institute’s equation for bell inlet

velocities above 1.5 m/s would also have resulted in conservative submergence levels. This

demonstrates the fact that the submergence levels calculated with the equation published by

Knauss (1987) will result in slightly higher bell inlet velocity/approach canal velocity ratios

compared to those determined from the results of the physical model study.


Page | 124



Flow, Q (m3/s) 1.02 1.36 1.7 2.04 2.37





Canal width, W (=2D) (m) 2.4 2.4 2.4 2.4 2.4


Froude Number, Fr 0.26 0.35 0.44 0.53 0.61








for Type 1B suction bell (m) 1.64 1.81 1.83 2.06 2.25



by Knauss (1987) all exceed 6.0;


by the Hydraulic Institute (1998) all exceed 6.0;



Hydraulic Institute (1998) for all bell inlet velocities; and

Calculating the submergence levels with the equation published by Knauss (1987) for inlet

bell velocity/approach canal velocity ratios exceeding 6.0, would result in inadequate

submergence levels.

It is evident from Tables 6.1 to 6.3 that the initial submergence should be calculated with the

equation published by Knauss (1987) and that, where the bell inlet velocity/approach canal

velocity ratio exceeds 6.0, the critical submergence be calculated with the equation published by

the Hydraulic Institute (1998).


Page | 125


The two examples presented below demonstrate the application of the two different formulas

recommended to calculate critical submergence for pump intakes.

Example 1 (Vlieëpoort abstraction works)

Flow, Q = 2.0 m3/s

Assumed bell inlet velocity, V = 1.2 m/s

Calculated suction bell diameter, D = 1.5 m

Actual bell inlet velocity, Vb = 1.13 m/s

Canal width, W = 3.0 m (i.e. 2D)

Required bell height above floor for flushing purposes, C = 1.5 m

Submergence, S = D(0.5 + 2.0Fr)

= 1.64 m

Approach canal velocity, Va = Q / (W x (C + S))

= 0.21 m/s

Ratio of Vb/Va = 5.3, which is less than 6.0, i.e. use the equation

published by Knauss

Final submergence, S = 1.64 m

Example 2 (Lower Thukela abstraction works)

Flow, Q = 0.467 m3/s

Assumed bell inlet velocity, V = 1.2 m/s

Calculated suction bell diameter, D = 0.7 m

Actual bell inlet velocity, Vb = 0.71 m/s

Canal width, W = 1.4 m (i.e. 2D)


Page | 126


Required bell height above floor for flushing purposes, C = 1.5 m

Submergence, S = D(0.5 + 2.0Fr)

= 1.00 m

Approach canal velocity, Va = Q / (W x (C + S))

= 0.13 m/s

Ratio of Vb/Va = 9.1, which is greater than 6.0, i.e. use the equation

published by the Hydraulic Institute (1998)

Final submergence, S = D(1 + 2.3Fr)

= 1.45 m


Page | 127


7. CONCLUSIONS AND RECOMMENDATIONS

7.1 Conclusions

A physical hydraulic model study, at a 1:10 scale, was undertaken to evaluate the impact on

critical submergence when raising the pump intake to above the recommended norm of 0.5

times the diameter of the inlet bell from the floor. Four different configurations of pump intakes

were tested. The following conclusions ensued from the results of the physical hydraulic model

investigation and associated literature review:

Various equations have been published to calculate critical submergence for pump intakes.

The submergence values calculated with these equations vary substantially, which posed

questions as to their validity.

The water level measurements at which Type 2, 5 and 6 vortices occurred in the physical

hydraulic model could be repeated with a reasonably high level of confidence. The

maximum deviations from the average water levels for Type 2, 5 and 6 vortices were 7%,

16% and 23% respectively. The maximum difference between the minimum and maximum

water levels for Type 2 vortices was 18%.

The submergence required to prevent Type 2, 5 and 6 vortex formation increased markedly

when the pump intake was raised above a certain height. The marked increase in

submergence occurred when the ratio, inlet bell velocity to approach canal velocity,

exceeded 6.0.

The submergence required for slanted suction bells was higher than for conventional flat

suction bells. The submergence required was similar for suction bells with short (i.e. 1 x

suction pipe diameter) and long radius (i.e. 2 x suction pipe diameter) bends, with the

recommendation to design based on long radius bends which had marginally higher

submergence requirements.

Critical submergence should be based on the water levels at which Type 2 (i.e. surface

dimples) vortices occur due to the difficulty associated with identifying Type 3 (i.e. coherent

dye core) vortices in the physical hydraulic model and given the Hydraulic Institute (1998)

guideline that Type 3 vortices should occur less than 10% of the time.

The Type 2 water levels, for different prototype bell inlet velocities and a prototype inlet bell

velocity/approach canal velocity ratio of less than 6.0, closely followed the submergence

levels calculated with the equation published by Knauss (1987), i.e. S = D(0.5 + 2.0Fr). The


Page | 128


Type 2 water levels, where the prototype bell inlet velocity/approach canal velocity ratio

exceeded 6.0, closely followed the submergence levels calculated with the equation

published by the Hydraulic Institute (1998), i.e. S = D(1 + 2.3Fr).

Prototype bell inlet velocities should be limited to 1.5 m/s, as the water surface became

turbulent in the physical hydraulic model when the bell inlet velocities were increased above

1.8 m/s. This was also reported by various authors referenced in the literature review.

7.2 Recommendations

The following recommendations are made for the design of pump intakes with a similar

geometry to that tested in the physical hydraulic model:

Conventional flat bottom suction bells can be used for flows of up to 2.5 m3/s per pump.

Prototype bell inlet velocities should be limited to 1.5 m/s.

The use of short or long radius suction bends should be evaluated against other criteria

such as net positive suction head requirements of the pumps and the overall height of the

pump installation.

The equation published by Knauss (1987), i.e. S = D(0.5 + 2.0Fr), can be used to calculate

critical submergence where the bell inlet velocity/approach canal velocity ratio, as

determined with Knauss’ equation, is less than 6.0. The equation published by the Hydraulic

Institute (1998), i.e. S = D(1 + 2.3Fr), can be used where the ratio exceeds 6.0.

The need may arise in future designs of sand trap canals to increase the width to more than the

recommended pump bay entrance width of 2D in order to reduce the approach velocity

sufficiently to trap sand particles upstream of the pump inlet. It is recommended that further

physical hydraulic model testing be performed in which the width of the sand trap canal is varied

to determine the bell inlet velocity/approach canal velocity ratio where the marked increase in

submergence occurs.


Page | 129


REFERENCE LIST

Ahmad, Z., Jain, B., & Mittal, M. (2011). Rational design of a pump-sump and its model testing.

Journal of Pipeline Systems Engineering and Practice, Vol 2 (p53-63).

American Water Works Association. (1983). C208: Dimensions for Fabricated Steel Water Pipe

Fittings. Denver, USA.

Ansar, M., & Nakato, T. (2001). Experimental study of 3D pump-intake flows with and without

cross flow. Journal of Hydraulic Engineering, Vol 127 (p825-834).

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