pumping stations awareness.doc

7/28/2019 PUMPING STATIONS AWARENESS.doc

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PUMPING STATIONS AWARENESS

BY

ANDY FINN

APRIL 2005


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1. PUMP TYPES

The types and sizes of pumps are dependent upon: Flows and heads Nature of liquid being pumped

Pump Type

C e n

t r i f u g a

l R a d

i a l F l o w

S c r e w

C e n

t r i f u g a

l

V o r

t e x

I m p e

l l e r

M i x e d

F l o w

V o l u t e

A x i a l

F l o w

P e r

i s t a t i c / D i a p h r a g m

P l u n g e r

P r o g r e s s i v e

C a v

i t y

A r c

h i m e d

i a n

S c r e w

C l e a n

W a t e r

M i x e d

F l o w

B o w

l

Raw Sewage, unscreened X X X XRaw, Sewage, screened X X X XStorm water with sewage X X X XStorm water (surface water only) X X X X X XPrimary treated sewage X X X X X XFinal effluent X X X X X X XPrimary sludge (>94% moisture) X X X X XPrimary sludge (


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relation to vortex impeller pumps. Advantages of vortex impeller pumps arereduced wear and improved freedom from choking.


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2. IMPELLERS


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3. PUMP MOTORS

Aspects for consideration in determining most suitable motor for a pumpinclude:

Explosion proof Operational temperature of the motor Seals Bearings

Motor protection devices include: Overheating Water intrusion Seal failure Bearing failure Winding insulation deterioration


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4. PUMP CONNECTIONS


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5. PUMP MATERIALS

The most suitable material for the pumps is dependent upon: Corrosion resistance Wearing resistance Abrasive liquid


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6. TYPES OF PUMPING STATIONS

a) WET WELL PUMPING STATION (also known as SubmersiblePumping Station)

The pumps are located in a single wet chamber below ground level and are fullysubmersible types (ie the pumps are submerged under the liquid) with guide railsand low level coupling systems. The guide rail and coupling system allow thepump to be installed or removed quickly and easily from the surface. The pumpsare fitted with permanent lifting chains attached to each pump with the top endattached to a bracket just inside the access cover, ready for use. A suitably ratedremovable lifting davit with winch or pulley block is used to withdraw the pumps.

A floor socket is located in the top slab to receive the davit.

Pumps are connected by flexible cables to a control panel mounted at ground

level which incorporates the starters and control equipment for the pumps. Thepanel is mounted within a weatherproof kiosk.

Isolating sluice valves and non-return valves are fitted within a separate valvechamber with access through a removable cover.

Control of the pumps is normally by ultrasonic level sensors with backup usingfloat type level regulators set to start and stop the pumps at high and low levelsrespectively.

For pumping stations handling a large variation in flows (eg Combined sewers

which may deal with only dry weather flow (DWF) at one time and heavy stormand dry weather flows at another time), this situation can be dealt with by usingtwo pumps (ie DWF pump + storm pump) or by one variable speed pump.


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6 TYPES OF PUMPING STATIONS

A) WET WELL PUMPING STATION (also known as Submersible Pumping Station)


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A) WET WELL PUMPING STATION (also known as SubmersiblePumping Station)


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Design Considerations:

i) Requirement to stop flows entering the wet well to enablemaintenance / repairs hence penstock on inflow

ii) The pumping station may breakdown allow for contingencies as appropriate. Eg

a) Power failure consider use of temporarygenerator

b) Pump failure switch over to standby pumpc) Inflow exceeds capacity of wet well and

pumps provide gravity overflowiii) Operate as much as possible from the surface to avoid

confined spaces. Preferably with a small cover over devices

such that it can be operated by a single person and there isno risk of falling in. eg Small covers over level devices,spindles, etc.

iv) Access covers over pumps require to be sufficiently sized toenable removal of the pumps. In sizing the covers, consider the obstruction from the guide rails. Also consider themaximum weight of a cover (generally this should notexceed 25kg for a single person). A safety grid should beprovided under the cover such that when the main cover isopened, personnel cannot fall into the wet well. Also thesafety grid should be self closing such that when the pump

has been pulled through it, it closes under its own weight.v) For ultrasonic levels, consider the spread of signal. ie Itneeds to pick up the water level and not get a reading fromthe benching, pipework or pumps as it will affect operationof the pumps

vi) There is some dead space under the pump inletvii) Get the benching arrangement checked by the pump

manufacturer in order that the pump manufacturer in thefuture cannot state that failure of the pumps is down to thebenching design

viii) Ducts for cables require to be sealed to prevent flammablegasses or water or vermin passing through them.(Remember electrics may be involved and therefore gasescan explode, whilst vermin can eat through the cables)

ix) A check may be required in relation to the possibility of thewet well and valve chamber floating. The wet well should beconsidered empty for the check

x) Depending upon the location of the wet well, odour mitigation may be required

xi) If there is a risk of differential movement between the wetwell and valve chamber, consider having the two structuresseparate otherwise reinforcement will require to bedesigned to allow for settlement.


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xii) Depending upon the size of inflow and the height of fall of the inflow, consideration should be given to whether a bafflearrangement is required on the end of the inlet pipe to thewet well.

xiii) Due to the pipework arrangement and the land available,consideration may have to be given to the use of arectangular shaped valve chamber rather than manholerings.

xiv) In determining the valve chamber arrangement, consider access for operation of the gate valves

xv) Water may end up in the valve chamber (ie liquid fromremoval of pipe fittings, rainwater, etc), thereforeconsideration must be given to its removal. Options include:a sump in the base of the valve chamber to enable pumping

out, a drain from the valve chamber to the wet well with apenstock or flap valve on it to prevent liquid from the wetwell backing up into the valve chamber if the pumps fail)

xvi) For large access openings, removable support beams maybe required. If they are required, check to see if there issufficient space for valves under them

xvii) Any surfaces in contact with foul or combined sewagerequire to be sulphate resisting. This can either be doneusing sulphate resisting concrete or to use a granolithic(high strength concrete) layer over the concrete.

xviii) Provide bauer connections to enable the hose from a tanker

to be connected. For ease of operation, it is preferable for the bauer to be horizontal and above ground to avoid entryinto a confined space.

xix) On the emergency overflow, consider the requirements of the environmental authority and the need for a screen


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Typical Site Layouts

MCC CABINET

WASH DOWN SET


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15m 15m 15m

TYPICAL ARRANGEMENT FOR ACCESS OFF LOW CLASSIFIED ROAD

MCC CABINET

WASH DOWN SET


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TYPICAL ARRANGEMENT FOR ACCESS OFF UNCLASSIFIED OR ESTATE ROAD


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Design Considerations for Site Arrangement

i) Consider the types of vehicles (eg tankers, vans, lorries) andwhere they require to get to on site (eg to enable removal of the pumps from the wet well)

ii) Provide surfaced footpaths to items requiring regular maintenance or inspection. eg MCC Control, valve chamber,wet well.

iii) Maximise use of concrete and gravel (say 14mm or 20mmsingle sized clean stone) on filter fabric surfacing in order tominimise maintenance rather than having the area grassed.

Also consider the consequence of contamination of the surface

eg when pumps are removed or leakage from hoses connectedto tankersiv) In determining the sizes of kiosks, make doors large enough to

enable removal of equipment, eg MCC. Check to see whether a surge vessel and compressors are required.

v) Normally an electricity meter will be required on the site.Consider its location and provision of a separate compartmentin order that the electricity company can read it without therequirement for the water authority personnel to be present. ieEnsure that the meter reader cannot have access to thepumping station controls.

vi) The control cabinet needs to be watertight, a flexiblemembrane may be required on the underside of the base to thesuperstructure and on top of the concrete.

vii) The control cabinet requires to be free of dust, thereforeconsider the need to seal the concrete

viii) Ensure an emergency stop is close to the wet well (normally itis sufficient for the stop to be incorporated in the control cabinetbut in some cases an external stop may be required

ix) If a washwater system is to be provided to clean down, saypumps, consider where the contaminated water is going to endup. Eg Consider the provision of bunding (using road kerbs)and the drainage of the bunded area back into the wet well.

B) DRY WELL PUMPING STATION

The pumps are located in a dry chamber and draw from an adjacent wet sumpthrough suction pipes. Usually pumps are vertical with extended flexibly jointedshaft drives to electric motors mounted at ground level; thus avoiding thedangers of motors becoming flooded. A building is usually constructed aboveground to house electrical switchgear and pump motors.

With dry well pumping installations, pumps can be inspected easily andserviced with regular checks on bearings, glands, etc.


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C) ON-LINE PUMPING STATION


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DESIGN NOTES

1) The flooring shall be manufactured in sections suitable for manuallifting by one person and shall be suitably clamped in position.

2) The drawings above are examples only and may require amendingto suit detailed design of pipework / pumps and the utilitysrequirements. Notwithstanding this, the station may need to beextended if surge equipment is required.

3) The access door should be large enough to enable removal of equipment including the MCC.

4) The drawing depicts a bottom entry MCC cabinet with cable ducts inthe floor. As an alternative, the use of top entry MCC cabinets andwall mounted cable trays may be used. (A cable tray is the

steelwork used to support cables see below)5) The noise levels of the pumps require checking and if necessary thepumps may require to be placed in an acoustic enclosure.

6) An opening with small cover shall be provided in the wall to enablea cable from an emergency generator to be fitted from theoutside.

7) Louvres shall be incorporated in the walls and sized to suit thepumping units in order to assist with cooling the pumps. Thelouvres (and building) should be designed such that it iswatertight. Conversely heaters (and insulation) may be required inthe building to keep it warm in cold weather when the pumps are

not operating.


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PHOTOGRAPHS OF VARIOUS ELEMENTS OF A PUMPING STATION

Cable Tray

MCC


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Float Switch

Pump and Guide Rail


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Standby Generator

Davit


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Bauer Connection

Pumping Station Site


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6. PUMP SUMP

It is useful to obtain the agreement of the pump manufacturer regarding the sump arrangement. The following comments are for

guidance only.

The functions of a pump sump or wet well are: To receive or hold incoming sewage A control for the pumps (ie its volume will dictate the

frequency which the pumps will be operated). The maximumnumber of starts that can be tolerated by electric motors isdependent upon size and type of motor but a reasonable ruleof thumb is 10 times per hour.

The start level is normally below the invert level of the lowestincoming sewer with some freeboard. This is to prevent sewagebacking up the sewer system.

The stop level is dependent upon the required capacity of the wetwell, size of pump and frequency of operation of pump.

The sumps should have benching to direct solid matter towards thepump suctions and prevent solids build up. Minimum slope of benching > 45 0.

The positions of the suction bellmouths in sumps can affect the pumpperformance. Normally allow the distance between the floor of thesump and the opening of the bellmouth to be D / 2 where D =diameter at the opening of the bellmouth (ie not the pipe diameter) >80mm.

The distance between bellmouths = between 2D and 3D.

Incoming sewers should be positioned so that they distribute flowevenly to the sump. Large drops should be avoided as they will causesevere aeration of the liquid and possible cavitation in the pumps.

7. PUMPING HEAD

FRICTION HEAD Amount of head required to overcome the frictioncaused by the liquid passing through the pipes and valves to the pointof delivery

STATIC HEAD The vertical distance between the water level on thesuction side of the pump to the highest level on the delivery system.


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Max water level, B1

Level, CMin water level

Pump casing level, D

Max water level, A1

Mean water level, A2

Min water level, A3

Mean static suction lift = D A2Max static suction lift = D A1Min static suction lift = D A1

Total suction lift = static suction lift + total friction head

< suction capability of the pump = Nett Positive Suction Head

PUMP WORKING ON SUCTION LIFT


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