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Design guidelines for alternative formed suction inlets

D. E. Werth PhD, PE and D. E. Cheek MS

Formed suction inlets are often used to improve approach

flow hydraulics to large vertical turbine pumps. Current

design guidelines require that the pump bell be removed

and the pump modified to allow for attachment of the

formed suction inlet. The present research study was

aimed at developing a dimensionless design procedure for

a formed suction inlet based on pump bell diameter,

which does not require removal of the bell, allowing for

greater flexibility and economic feasibility for use in

existing pump intakes. Model studies have shown this

type of inlet to be successful at alleviating adverse

hydraulic phenomena, but the results and design proce-

dures are typically private and proprietary and are not

readily available in the public domain. A formed suction

inlet which can be constructed outside the sump and con-

sists primarily of flat sides has been developed. The rela-

tively simple geometry should minimise construction

costs. In addition, the inlet is designed for use under

existing pumps and does not require pump removal ormodification.

NOTATION

BH inlet height at the back wall

D pump throat diameter

d pump bell diameter

EH inlet height at the entrance

IL overall inlet length

IW inlet width at entrance

W pump bay width

1. INTRODUCTION

Formed suction inlet (FSI) devices have often been used on

large vertical turbine pumps for a variety of reasons. They are

relatively insensitive to high cross-flow conditions, eliminate

sub-surface vortex activity, and may reduce the required

minimum pump submergence to minimise surface vortex

activity. The authors have previously presented a summary of

the advantages and disadvantages of FSIs and outlined the

preliminary findings of the study.1 This paper is intended to

expand upon the preliminary work and present the results of

the experimental study.

The most commonly accepted, and only readily available

performance and design guidance for this type of pump inlet

has been from the US Army Corps of Engineers Type 10 inlet.

This inlet has been proved to be effective in a wide variety of

pumping applications and is shown inFig. 1.

However, the relatively complex geometry can substantially

increase the cost, and the need to remove the pump bell often

limits its applicability as a corrective measure when retrofittingan existing pumping station. Therefore, it would be useful to

have additional options and design guidance available for

alternative formed inlets which are less costly and can be easily

used on existing pumps.

Private modelling laboratories have developed alternative

formed inlet designs in the past; however, this information is

often proprietary and not readily accessible for use by other

design engineers. The present research is, in part, a result of

numerous model studies which were conducted to develop

alternative FSIs for a variety of inlet configurations. Each of the

inlets was developed with the intention of utilising the existingpump bell. This paper presents the results of this study and

proposes a set of functional design guidelines which can be

used by engineers to develop an FSI based on pump bell

diameter.

2. EXISTING KNOWLEDGE

A commonly used type of FSI was developed by the United

States Army Corps of Engineers (USACE) and is known as the

Corps Type 10 inlet. The Type 10 design was originally

published by Fletcher and Oswalt2 and entitled Geometry

Limitations for the Formed Suction Intake. This document was

recently rescinded from the public domain for unknownreasons. This design is also used as the standard for the

Hydraulic Institute (HI) Standard Pump Intake Design manual.3

The Hydraulic Institutes Standard suggests that the FSI may be

a fix all for adverse sump pit hydraulics. However, the USACE

Type 10 inlet is often considered costly and difficult to build.

The FSI has the potential to be very beneficial for certain

pumps and pump pit designs, especially for those pump sumps

that are being retrofitted for a higher capacity or corrected for

existing hydraulic problems.

Antunes and Holman4 noted that the FSI has some tremendous

advantages including its decreased sensitivity to unstableapproach flows, and the ability to raise sump floors because

they require less submergence. This reduces the elevation of the

impeller and the excavation required for the pump sump. This

Proceedings of the Institution of

Civil Engineers

Water Management157September 2004 Issue WM3

Pages 151^158

Paper 13637Received 12/01/2004

Accepted 15/07/2004

Keywords:floods & floodworks/hydraulics &

hydrodynamics/models (physical)

David E. WerthAssistant Professor, Depart-ment of Civil Engineering,

Clemson University,

Clemson, SC, USA

Daniel E. CheekProject Engineer, Hodges,Harbin, Newberry &

Tribble Inc., Macon, GA,

USA

Water Management 157 Issue WM3 Design guidelines for alternative formed suction inlets Werth Cheek 151

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is especially helpful in areas

with a high water table or

when excavating in rock.

The main disadvantage of

FSIs is that, with the excep-

tion of the USACE Type 10

inlet, there are no standar-

dised design criteria in which

one set of dimensions may be

applied to any situation. This

has curbed its attractiveness

as an acceptable alternative

for solving hydraulic prob-

lems that occur in pump

sumps. There is a lack of

information on FSIs in scien-

tific literature. Much of the

work done to investigate FSIs

is model studies of actual

designs, not research studies.

However, often these studiesare typically applied to speci-

fic design criteria. As these

studies are usually part of the

design process, cost is always

a factor, which eliminates the

possibility of generalising the model study to develop sub-

stantial design data for the inlet itself. Tullis5 noted that for

these reasons, information regarding formed suction inlets

rarely becomes available to the engineering community and

results found during model studies are not shared among

researchers.

3. DESIGN DEVELOPMENT

To reach a starting point for the laboratory study, previous

designs for FSIs were considered. In some cases, researchers

described inlet dimensions in terms of the impeller or throat

diameters. The FSI has little available documented information

on its performance with reference to a particular design, and

could be considered more of a concept or idea rather than a

certain specific shape. In the past, researchers used this formed

inlet idea and applied it to their specific criteria, often with the

aid of a physical hydraulic model study.57 In order to directly

compare the dimensions between FSIs based on bell diameter

and those based on impeller diameter, it was assumed that the

impeller diameter is typically 60% of the bell diameter.

The range of values found during a review of literature for the

overall FSI inlet dimensions was as follows:

Overall inlet width (IW): 139d213d

Overall inlet length (IL): 205d228d

Overall entrance height (EH): 053d10d

Overall backwall height (BH): 024d081d

The overall length was measured from the entrance of the

formed suction inlet to the back wall, not the centreline of the

impeller shaft. Furthermore, not all of the examples reviewed inthe literature were designed with back walls.

The primary aim of the study was to develop an FSI which not

only provides uniform approach flow conditions at the pump,

but is also economical and easy to build. This could be

accomplished by developing an inlet which had primarily flat

panels, with the exception of a simple radius at the back wall of

the inlet.

A secondary aim was to develop an inlet which could easily be

used to retrofit an existing pumping station. The Type 10 inlet

replaces the typical pump bell. To use the Type 10 as a retrofit

device requires removal of the existing pump bell and

modifications to the pump itself. This is not always possible in

existing stations where it may not be feasible or ideal to shut

down the pump for an extended period of time. Therefore, it

was desirable to develop an inlet which could be constructed

outside the sump, then lowered into the sump and placed

directly under the existing pump bell. This would eliminate the

need to remove the pump or pump bell, and could be

implemented without emptying the sump. To accomplish these

aims, a series of model tests were conducted with a variety of

inlet geometries and approach flow conditions.

4. MODEL TESTING

The major components used in the experiments were the model

basins, the FSI and the model pump. Four separate basins, each

with unique approach flow geometries, were used in the design

development. The first basin, shown inFig. 2,was referred to as

sump configuration 1 and was used to optimise a preliminary

design that eliminated any undesirable hydraulic problems

located inside the FSI. Once an acceptable working design was

found, the scale and flow rate of sump configuration 1 were

varied in order to verify the inlets effectiveness over a range of

flow rates.

Eight FSI designs were constructed and tested in four different

inlet configurations during this study. The first five inlet

Fig. 1. USACE Type 10 inlet

52 Water Management 157 Issue WM3 Design guidelines for alternative formed suctioninlets Werth Cheek

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configurations were used to develop an acceptable working

design based on dimensionless parameters. The last three were

built using these parameters and tested in different approach

flow conditions. In all, over 60 tests were conducted at varying

flow rates and water levels.

The studies were conducted according to the 1998 Hydraulic

Institute Standards.3 The HI Standards indicate several accep-

tance criteria that are to be used when evaluating this type of

structure. In particular, pre-swirl, velocity distributions, turbu-

lence levels, and vortex activity are evaluated. Each model was

constructed as an undistorted Froude scaled model with a

length scale sufficiently large to ensure that the Reynolds

number (Re) at the model pump bell exceeded 16105. A

summary of the different designs is shown inTable 1and more

complete details can be found in a masters thesis by Cheek.8

Each of the designs tested was constructed entirely of flat

panels, with the exception of a simple-radius, curved backwall.

Figs 3and 4 show two of the model inlets

To evaluate the effectiveness of each of the designs, a series of

measurements and observations were recorded during the

testing of each inlet. These measurements included vortex

activity, velocity distributions around the throat of the pump,

pre-swirl of flow entering the pump, and turbulence levels

within the pump. The inlet was deemed acceptable if pre-swirl

was less than 58, velocity and turbulence variations were less

than 10% and no vortices greater than a type 1 or weak type 2

were observed entering the inlet. Some designs were tested to

determine the optimum dimension to both minimise size yet

provide acceptable conditions. Inlets that failed to meet the

established acceptance criteria were modified until acceptable.

It should be noted that while care was taken to minimise scale

effects by ensuring fully turbulent flow in the model, vortex

formation may be slightly less intense in the model than in the

prototype, particularly regarding air entrainment. To overcome

this limitation, no vortex greater than a weak type 2 was

permitted in the model. Should the vortex be slightly stronger

in the prototype, such as a well-developed type 2 or very weak

type 3, it would still be far less intense than that required to

ingest or pull air out of solution.

As the inlets were designed using models of existing intake

structures, the inlets them-

selves were constructed at the

same scale as the intake

structures. A summary of the

prototype pump and Froude

scale model information for

each sump configuration is

shown inTable 2.

5. RESULTSThe first FSI, inlet design 1,

was constructed based on the

one principle that gives the

Flow straightening baffle

Control valve (typ)

Orifice flow meter

Flow

froml

abpump

Plan view

Piers (typ)

False walls

Trash screensto be located

here (typ)

Elbow

Flow

Flow

tolabpump

Baffle wallFSI

Control valve

Circulating water pump(typ of 3)

Fig. 2. Sump configuration 1

Inlet design Width Backwall height Entrance height Overall length

1 278d 05d 05d 242d2 278d 05d 092d 242d3 2d 05d 075d 2d4 2d 05d 075d 25d5 2d 05d 075d 225d6 2d 05d 075d 25d7 2d 05d 075d 25d

8 2d 05d 075d 25d

Table 1. Summary of model configurations


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FSI a distinct advantage over conventional wet-pit intakes: a

constantly decreasing cross-sectional area. This causes the flow

to accelerate, which helps to eliminate vortices and dampen

adverse flows caused by poor approach angles. The initial

geometry was chosen to fit within the pump bay of sump

configuration 1. It had an internal geometry consisting of a

backwall fillet, two sidewall fillets, and centre floor splitter, and

a vertical backwall extending from the backwall fillet vertically

to the top of the inlet. After testing this design in sump

configuration 1, it was noted

that type 4 surface vortices

were entering the inlet and

there was a large separation

zone occurring along the roof

of the formed inlet. However,

the pre-swirl angle and velo-

city data were within criteria.

Inlet design 2 was built

exactly as inlet design 1 with

two exceptions. Instead of the

entrance height being equal

to the backwall height, it was

increased to 092d. Four

turning vanes were also

added at the entrance to

straighten the flow entering

the inlet. Inlet design 2 was

then tested in sump config-

uration 1 and was found to

still have some flow separa-tion along the flat part of the

roof at the entrance where the

turning vanes were located.

Next, a 03dhalf-round piece

was added to the top front

edge of the entrance, which eliminated the separation at the

turning vanes. Type 3 surface vortices were harder to find and

broke up quickly, but were observed entering the inlet. In

addition, a vertical curtain wall was extended from the front of

the inlet to above the water surface. This eliminated surface

vortices and prevented them from entering the pump. Although

inlet design 2 met the acceptance criteria for vortex formation

most bay widths for pump sump pits are only 2d; the 278d

width of inlet 2 was less than ideal for retro-fit applications.

Inlet designs 3 and 4 were

modified to reflect the bay

widths and bell clearances

recommended in the 1998 HI

standards. The turning vane

configuration and entrance

were modified to eliminate

the need for a radius at the

top of the inlet. Inlet 4 per-

formed very well and waswell within the established

criteria.Table 3presents a

summary of the results of the

final inlet 4 design when

placed in sump configuration

1. The water level was chosen

as the minimum suggested

level as indicated in the 1998

HI Standards. To verify the

need for the vertical wall at

the entrance to the inlet, tests

were conducted with andwithout the wall in place. At

the minimum recommended

water level, type 2 surface

Fig. 3. Formed suction inlet, test inlet No. 2

Fig. 4. Final formed suction inlet, test inlet No. 4


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vortices were observed with and without the wall in place.

However, these vortices tended to dissipate and break up as the

entered the inlet. It was also noted that at water levels 10%

below the recommended minimum submergence, much

stronger air-entraining surface vortices were observed when the

wall was removed, while vortex intensity was unchanged with

the wall in place. Due to the sensitivity of surface vortex

formation to water level, it is recommended that the vertical

wall be included.

The design development testing led to a relatively simple FSI

which can be easily constructed outside an existing sump and

installed by divers, without dewatering the pumping station,

and with minimum station down-time. An additional test was

conducted with inlet configuration 5 which shortened the inlet

slightly to 225d. Visual observations indicated that the flow

was less uniform within the inlet and that the slightly reduced

length was not beneficial. The dimensionless inlet design is

shown inFigs 5 to 7.

6. ADDITIONALVALIDATION

To further validate the final design, three additional inlets

(inlets 6, 7 and 8) were constructed based on the dimensionless

parameters developed during inlet 4 design. These inlets were

constructed based on the bell diameters of pumps for three

uniquely different sump configurations, including one with

significant cross-flow. Actual

model studies for these sumps

were conducted and the final

formed inlet was installed and

tested. The additional sumpswere referred to as sump

configurations 2, 3 and 4 and

are shown inFigs 8,9 and 10

respectively.

Sump configuration 2 was

tested for a variety of flow

rates. Again, the addition of a

vertical wall greatly reduced

the sensitivity of surface for-

mation to water levels. Pre-

swirl and turbulence levelswere well within criteria.

Several points around the

pump bell exceeded the

Sumpconfiguration

Prototype belldiameter: cm [in.]

Modelscale

Model flow, Q:l/s [ft3/s]

Prototype flow,Q:m3/h [gallons/min.]

Reynolds number(at bell entrance)

1 2896 [114] 157 146 [052] 51 330 [226000] 10 105

1 2362 [93] 128 161 [057] 34068 [150000] 11 105

1 1930 [76] 105 177 [063] 22712 [100000] 12 105

1 1219 [48] 66 224 [079] 9084 [40 000] 16 105

1 914 [36] 50 232 [082] 4996 [22 000] 16 105

2 1625 [64] 925 146 [051] 13 627 [60 000] 11 105

3 1880 [74] 945 174 [061] 17 170 [75 600] 11 105

4 1168 [46] 726 133 [047] 6814 [30 000] 11 105

Table 2. Model parameters

Sidewall

fillet Turning vane

Floor splitter

Backwall fillet

Vertical fillet

B

0.15D

0.33d

0.56d 0.42d IW = 2.0d

A

A

B

90

d

Fig. 5. Plan view of the recommended design

Prototypeflow rate: m3/h[gallons/min.]

Modelscale

Modelwater level:

cm [in.]

Pre-swirl:degrees

Surfacevortex

intensity

Sub-surfacevortex

intensity

Verticalwall

Velocitycriteria

met

Turbulencecriteria

met

4996 [22000] 50 526 [207] 12 Type 2 Type 1^2 No Yes Yes4996 [22000] 50 526 [207] 12 Type 2 Type 1^2 Yes Yes Yes9084 [40000] 66 49 [193] 08 Type 2 Type 1^2 No Yes Yes9084 [40000] 66 49 [193] 13 Type 2 Type 1^2 Yes Yes Yes

22 712 [100000] 105 429 [169] 15 Type 2 Type 1^2 No Yes Yes22 712 [100000] 105 429 [169] 10 Type 2 Type 1^2 Yes Yes Yes34 068 [150000] 128 409 [161] 10 Type 2 Type 1^2 No Yes Yes34 068 [150000] 128 409 [161] 16 Type 2 Type 1^2 Yes Yes Yes

Table 3. Summary of inlet 4 results


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maximum allowable velocity deviation, but it was later

determined that a misalignment of the pump bell above the

inlet was causing some flow

separation at the pump bell.

This indicates that proper

alignment of the bell over the

opening in the inlet is essen-

tial.


tested to investigate the

impact of a sloping floorupstream of the inlet. Velo-

city, pre-swirl and turbulence

levels were well within the

established criteria and over-

all conditions were extremely

uniform within the sump.


tested to investigate the

impact of cross-flow directly

in front of the inlet. Velocity,

pre-swirl and turbulencelevels were well within the

established criteria and over-

all conditions were extremely

uniform within the intake.

Tests were conducted without

a vertical wall, effectively

simulating an intake without

dividing bay walls. This con-

figuration was not effective

and required the use of a

vertical wall above the inlet

similar to the configurations

that were tested with a

straight approach flow. The

vertical wall prevents flow

from travelling past the

entrance to the inlet, elimi-

nating the need for a dividing

bay wall between inlets.

However, some structural

support will probably be

required to support the verti-

cal wall and a short bay wall or pier could be placed between

inlets to provide this support. The inlets in sump configuration

4 were placed a prototype distance of 30 cm (12 in.) apart tofacilitate a vertical pier which was used to attach the vertical

wall above the inlet entrance. A prototype cross-flow of

078 m/s (25 ft/s) was present in front of the first inlet, which

was nearly twice the HI recommended maximum cross-flow of

25% of the bell velocity. It was found that a series of five

vertical vanes rather than three at the entrance resulted in less

flow separation at the inlet entrance. In addition, a vertical

fillet that was installed near the back of the pump improved

conditions during high cross-flow events, which agrees with

previous modelling experience. The introduction of the two

additional inlet vanes as well as the vertical backwall fillet are

relatively minor modifications from the design used with

straight approach flow conditions and rather than have two

different configurations, will be recommended regardless of the

approach flow conditions. Although these tests demonstrated

Floor splitter

Sidewall fillet

Section A

Backwall fillet

0.3d

2.0d

0.42d

0.22d

Vertical fillet

Fig. 7. End view of the recommended design

Vertical wall

Turning vane

Bell centred abovehole in top of inlet

Backwall fillet

Note: Inlet is bolted to floorMax clearance between belland inlet is 1.25 cm (0.5 in.)

Section B

BH = 0.5d

EH = 0.75d

IL = 2.5d

Sidewall fillet

Floor splitter

0.25d1.55d

1.75d

0.5d

45

Vertical fillet

Fig. 6. Elevation view of the recommended design

Plan view

Baffle wall

Sluice gate (full width)

Screen chamber

Sluice gate (full width)

Screen chamber

Flow

Flow

Control valve

From lab pump

Tolabpump



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the effectiveness with cross-flow velocities of nearly twice the

HI recommended value, further research is required to deter-

mine the upper limits of this value.

7. CONCLUSIONS

The purpose of this paper is to present an efficient and

economical FSI design that meets all of the 1998 HI acceptance

criteria. A FSI design which is based on the pump bell diameter

is proposed as a viable alternative for eliminating adverse flow

phenomena occurring in existing wet pit pump sumps. The final

design is applicable over a range of flow rates and approach

flow conditions and effectively meets the acceptance criteria

mentioned previously.

Finally, it was determined that the final FSI design is effective

at straightening cross-flow before it reaches the impeller, and isdesigned in an economical manner, using flat components, so

that it will be easily manufactured and assembled, resulting in

minimum down-time for the application the pump is serving.

Based on the semi-theoretical and empirical considerations for

the design of this FSI, as well as the laboratory experiments

conducted in a controlled environment, the following conclu-

sions and recommendations can be made.

(a) The optimum overall design dimensions for this FSI design,

based on bell diameter, d, are: width, 2d; length, 25d; back

wall height, 05d; entrance height, 075d; distance from

back wall to where the inlet should begin to rise up to theentrance height, 15d; and number of turning vanes equally

spaced across the front entrance, 5. Furthermore, an

internal geometry consisting of a centre floor splitter,

backwall fillet, and side-

wall fillets should be

included.Figs 5, 6 and 7

illustrate these dimen-

sions.

(b) This formed suction inlet

design was not affected

by severe cross-flow

conditions, as demon-

strated with sump con-

figuration 4, and

maximum pre-swirl

values were well within

the acceptance criteria.

The ability of the FSI to

straighten the incoming

approach flow between

the time it enters the inlet

and reaches the bell is a

tremendous advantage

over conventional wet pit

pump intakes.(c) The fact that this inlet

design is completely

composed of straight

pieces with the exception

of the curved back wall

makes it very advanta-

geous for construction

purposes and therefore

costs. The material used

to construct this FSI will most likely be concrete or steel

but should be specified by the design engineer for the

specific application. Furthermore, the ability to prefabricate

the prototype FSI and simply lower it into place will

greatly reduce the down-time for correcting the hydraulic

problems occurring in the pump sump, which, in turn,

could result in tremendous savings. The inlet is bolted or

fixed directly to the floor beneath the pump bell. The

clearance between the pump bell and the hole in the top of

the inlet should be minimised with a maximum space of

125 cm (05 in) as shown in Fig. 3.

(d) This formed suction inlet design was model tested for flows

ranging from 4996 m3/h (22 000 gallons/min) (prototype

flow) to 51330 m3/h (226000 gallons/min). Consideration

of a model study should be made when applying this

design to flows out of this range or in configurations thatmay not be representative of those investigated during this

study.

(e) Surface vortices were highly dependent on water levels,

and the recommended design is based on water levels equal

to or greater than suggested by the 1998 HI Standards.

Although it may be possible to significantly reduce the

water level to below the minimum submergence suggested

in the 1998 HI Standards, further verification may be

required for these cases.

REFERENCES

1. WERTHERTHD. E. and CHEEKHEEKD. E. An alternate formed suctioninlet design for large vertical turbine pumps. Proceedings

of FEDSM03 4th ASMEJSME Joint Fluids Engineering

Conference, Hawaii, 2003.

Model baffles

Flow

Control valve

To lab pump

Fr

oml

abpump

Orifice flowmeter


Flow

Influent pipes Formed InletsVertical wall



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2. FLETCHERLETCHERB. P. and OSWALTSWALTR. Geometry Limitations for the

Formed Suction Intake. US Army Corps of Engineers,

Washington, DC, USA, 1992, Engineering Technical Letter

No. 1110-2-327.

3. HYDRAULICYDRAULICINSTITUTENSTITUTE(HI). American National Standard for

Pump Intake Design, ANSI/HI 9.81998. Hydraulics Insti-

tute, Parsippany, NJ, USA, 1998.

4. ANTUNESNTUNES F. F. and HOLMANOLMAN W. L. Formed suction inlets

on large high specific speed pumps. Proceedings of

the 3rd Joint ASCE/ASME Mechanics Conference

Pumping Machinery, University of California, 1989, 137

140.

5. TULLISULLISJ. P. Modeling in design of pumping pits. Journal

of the Hydraulic Division, ASCE, 1979, 105, No. 9, 1053

1063.

6. LEECHEECHJ. R. Model study of new Madrid pumping

station. Proceedings of the 1989 National Conference on

Hydraulic Engineering, New Orleans, ASCE, 1989, pp. 875

880.

7. LEHREHRV., WERTHERTH D. E., DEMLOWEMLOWT. C. and CORNMANORNMAN R. E.

Optimizing the design of a formed suction intake for large

flood relief pumps. Proceedings of the ASCE International

Water Resources Conference, Seattle, 1999.

8. CHEEKHEEKD. Alternate Formed Suction Inlet Design. Masters

Thesis, Department of Civil Engineering, Clemson Univer-

sity, USA, 2002.

Please email, fax or post your discussion contributions to the secretary by 1 March 2005: email: [email protected];

fax: +44 (0)20 7665 2294; or post to Emma Holder, Journals Department, Institution of Civil Engineers, 1^7 Great George Street,

London SW1P 3AA.


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