Pump Sump Modelling Report

Proposed60MGDWaterworksandAncillaryFacilitiesatLowerSeletarReport:Pumpsumpmodellingmembranefeedpumpstation147216SECM170012rev0PreparedforPUB

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EXECUTIVE SUMMARY

1. There is a requirement to carry out hydraulic sump model study for the membrane

feed pumping station at Lower Seletar Waterworks (LSWW).

2. The physical model of the pump sump is constructed at the model-to-prototype scale

of 1:15. Froudian modelling criteria are used for the flow conditions and dimensions

of model. The physical model system is setup in the Maritime Research Centre,

Nanyang Techanological University.

3. Based on the findings and observations, the hydraulic performance of the pump

sump is generally satisfactory at top water level but not acceptable at low water

level.

4. The influent flow is turbulent and fluctuant at low water level. If two or more pumps

on duty in one tank, the strong inflow cause splashing and bubbles. The bubbles can

be sucked into nearby pump intakes, such as A1, A2, and A5, which can lead to

severe pump problems.

5. No obvious bubbles are observed at top water level because the inflow is submerged.

6. Due to the asymmetrical inflow into the narrow tank, the approaching flows to

different pumps are not parallel and straight. Such asymmetrical flow pattern can

cause circulations in the tank and pre-rotation around pipe intakes, especially at low

water level.

7. At low water level, surface vortex can be generated and air can be sucked into

pumps, for example A3 in Tank 1 and A4 in Take 2.

8. At top water level, there is no surface vortex observed because the pipe intake

submergence in increased.

9. The submergence of pipe intakes at low water level is insufficient to avoid formation

of surface vortices.

10. The submergence of pipe intakes at top water level is sufficient to avoid formation of

surface vortices.

11. The pipe intake clearances from the floor and back wall are adequate to avoid

formation of vortices for all the pumps.

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12. The submergence of influent flow at low water level is insufficient to avoid

formation of strong surface perturbations and air bubbles.

13. The submergence of influent flow at top water level is sufficient to avoid formation

of air bubbles.

14. Do not operate the pumps at design low water level (118.5 mRL);

15. The minimum level should be high enough to have a sufficient submergence of

inflow pipes as well as intake pipes. The recommended LWL is 120.4 mRL;

16. Higher water level is always more favourable for hydraulic performance in the sump.

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CONTENTS

EXECUTIVE SUMMARY .............................................................................................. 1

1. INTRODUCTION ..................................................................................................... 3

2. SCOPE AND METHODOLOGY ............................................................................ 8

3. PUMP SUMP AND TEST SCENARIOS ................................................................ 9

4. FLOW PATTERNS ................................................................................................. 11

5. RESULTS AND DISCUSSIONS ............................................................................ 68

6. CONCLUSION ........................................................................................................ 70

1. INTRODUCTION

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There is a requirement to carry out hydraulic sump model study for the membrane

feed pumping station at Lower Seletar Waterworks (LSWW).

The membrane feed pumping station comprises of two equal tanks to allow one tank

to be taken out for cleaning without interrupting the flow to the membrane plant. The

inflow from the clarifiers is discharged by two DN 1600 mm pipes to the two tanks.

There will be seven membrane feed pumps to be installed in the two tanks, including two

small pumps of capacity 25 Ml/d and five big pumps of capacity 75 Ml/d. Three big

pumps and one small pump are to be installed in Tank 1. Two big pumps and one small

pump are to be installed in Tank 2. These pumps are of the double suction split casing

type with vertical spindle. The layout and dimensions of the tanks is shown in Figure 1.

Figure 1 Design drawings of the membrane feed pumping station at LSWW: Plan view

(left) and cross-sectional view (right)

The objective of this study is to determine the necessity of any baffle walls or any

other undesirable flow mitigation structures as a result of cross-flow from abstracting

water through the two interconnection pipes. The physical model of the existing and new

Tank 1

Tank 2

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pump sumps are constructed at the model-to-prototype scale of 1:15. Froudian modelling

criteria are used for the flow conditions and dimensions of model. The physical model

system is setup in the Maritime Research Centre, Nanyang Technological University, as

shown in Figure 2.

Table 1. Prototype and Model Parameters

Item Prototype Scale Model

Sump

Dimensions

Sump width 4 m 1 : 15 0.266 m

Sump length 36.07 m 1 : 15 2.405 m

Sump depth 4.5 m 1 : 15 0.3 m

Pipe

Diameter

Diameter Influent pipe DN 1600

(ID 1529 mm)

1 : 15 100 mm

Diameter of suction pipe for

big pump

DN 900

(ID 899 mm)

1 : 15 60 mm

Diameter of suction pipe for

small pump

DN 500

(ID 495mm)

1 : 15 40 mm

Flow rate Maximum flow rate of big

pump

0.868 m3/s

(75Ml/d)

1 : 871.42 0.996 l/s

Maximum flow rate of small

pump

0.289 m3/s

(25Ml/d)

1 : 871.42 0.332 l/s

Maximum inflow rate in Tank

1

2.893 m3/s

(250 Ml/d)

1 : 871.42 3.32 l/s

Maximum inflow rate in Tank

2

2.025 m3/s

(175 Ml/d)

1 : 871.42 2.323 l/s

Velocity Maximum suction velocity for

big pump

1.368 m/s 1 : 3.873 0.353 m/s

Maximum suction velocity for

small pump

1.503 m/s 1 : 3.873 0.388 m/s

Maximum influent velocity

in Tank 1

1.576 m/s 1 : 3.873 0.407 m/s

Maximum influent velocity

in Tank 2

1.103 m/s 1 : 3.873 0.285 m/s

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Figure 2(a) Overview of the model setup

Figure 2(b) Side view of the exiting sump model

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This report presents the findings of the hydraulic performance of the sump with the

objectives to determine: (a) whether surface vortex occurs for different pumping

conditions, (b) whether there is swirl formation around pump for different pumping

conditions, and (c) the necessity of any baffle walls or any other measures to mitigate

undesirable flow patterns in the sump.

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2. SCOPE AND METHODOLOGY

This study include the following scope of work

1) Construction of a physical model of the pumping station including connection

pipes in a scale of 1:15;

2) The pumping system is modelled to reflect the equivalent flow system, and

the flow field in the sump.

3) Selected scenarios of pump operations are modeled to observe the flow

patterns and whether there are swirls, surface or submerged vortices and any

undesirable flow patterns with the sump.

4) Visualisation of the flow patterns is carried out with the aids of lighting and

dye. Relevant flow patterns are recorded with cameras; and

5) Identify and recommend possible modification or solution to avert any

undesirable flow conditions, if any.

Flow visualization is achieved by means of color dye injected through a small tube

and placed at different locations in the flow field. This technique is used extensively to

locate the possible origin of submerged vortices and to identify objectionable pump-

approach flow patterns. Flow patterns are photographed and videotaped, and selected

color photos are included herein.

The model flow rate is controlled by valve and the outflow rate of each suction pipe

is measured using volume-time method. Water surface elevations are determined and

monitored through the level marks to the tank sidewalls.

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3. PUMP SUMP AND TEST SCENARIOS

The pertinent dimensions of the sumps according to the data supplied by the client

are shown in Figure 1. Four pumps will be installed in Tank 1 and three pumps will be

installed in Tank 2. The schematic layout of the sumps and pumps and their names are

shown in Figure 3, where A1 A5 are big pumps and B1 B2 are small pumps.

Figure 3 Schematic layout of the sumps and pumps

The design capacity of the pumping station is 300 Ml/d, which can be achieved by

different pump combinations. The maximum pumping rate of Tank 1 is 250 Ml/d with 3

big pumps and 1 small pump, and the maximum pumping rate of Tank 2 is 175 Ml/d with

2 big pumps and 1 small pump. Although the critical operation conditions are generally

the case of the maximum pumping rate, other combinations of duty pumps are also

selected for test to observe the flow conditions.

The design lowest and highest water levels in the tanks are 118.5 mRL and 121

mRL, respectively. Normally the lowest water level is the worst condition. Therefore

most test runs are based on the lowest water level but the highest water level is also tested

for two selected cases. Besides, two test runs are conducted based on recommended

lowest water level. The test matrix is listed in Table 2.1 and 2.2.

Table 2.1 Test run matrix for the pump sump model

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No. Tank Pumps on duty Influent Flow Rate (Ml/d)

Tank Water level (m)

1

Tank 1

A1 25 118.5 (LWL)

2 A3 75 118.5

3 A2, A3 150 118.5

4 A1, A2, A3, 225 118.5

5 B1, A1, A2, A3 250 118.5

6 B1, A1, A2, A3 250 121.0 (HWL)

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Tank 2

B2 25 118.5 (LWL)

8 B2, A4 75 118.5

9 B2, A5 75 118.5

10 A4, A5 150 118.5

11 B2, A4, A5 175 118.5

12 B2, A4, A5 175 121.0 (HWL)

13 Tank 1 B1, A1, A2, A3 250 120.4 (R LWL)

14 Tank 1 B2, A4, A5 175 120.4 (R LWL)

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4. FLOW PATTERNS

This section shows the observed flow patterns for each case. Dye visualization

technique is used to enhance visualisation of the flow patterns in the sumps and around

the intakes of the suction pipes.

Figures 4 to 17 show the snapshots of the flow patterns for Cases 1 to 14 with self-

explanatory captions. More photographs and video clips are also provided with the

attached CD.

Figure 4(a) Overview of flow pattern for Case 1

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Figure 4(b) Flow pattern of influent flow for Case 1

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Figure 4(c) Flow pattern around A1 intake for Case 1

Figure 4(d) Dye visualization of influent flow for Case 1

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Figure 4(e) Dye visualization around A1intake for Case 1

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Figure 5(c) Flow pattern around A3 intake for Case 2

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Figure 5(e) Dye visualization around A3 intake for Case 2

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Figure 6(a) Overview of flow pattern of Case 3


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Figure 6(c) Flow pattern around A2 and A3 intakes for Case 3

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Figure 6(d) Dye visualization around A2 intake for Case 3

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Figure 7(c) Flow pattern around A1, A2 and A3 intakes for Case 4

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Figure 7(f) Dye visualization around A2 intake for Case 4

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Figure 7(g) Dye visualization around A3 intake for Case 4

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Figure 8(c) Flow pattern around A1, A2, A3 and B1 intakes for Case 5

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Figure 8(g) Dye visualization around B1 intake for Case 5

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Figure 9(e) Dye visualization around A2intake for Case 6

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Figure 9(f) Dye visualization around A3intake for Case 6

Figure 9(g) Dye visualization around B1intake for Case 6


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Figure 10(c) Flow pattern of B2 intake for Case 7

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Figure 10(e) Dye visualization around B1 intake for Case 7

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Figure 11(c) Flow pattern around A4 and B2 intakes for Case 8


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Figure 11(f) Dye visualization around B2 intake for Case 8



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Figure 12(c) Flow pattern around A5 and B2 intakes for Case 9


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Figure 13(c) Flow pattern around A4 and A5 intakes for Case 10

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Figure 14(c) Flow pattern around A4, A5 and B2 intakes for Case 11

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Figure 16(d) Dye visualization around A1 intake for Case13

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Figure 16(e) Dye visualization around A2 intake for Case13

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Figure 16(f) Dye visualization around A3 intake for Case13

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Figure 16(g) Dye visualization around B1 intake for Case13


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Figure 17(d) Dye visualization of flow around A4 intake for Case 14

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Figure 17(e) Dye visualization of flow around A5 intake for Case 14

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Figure 17(f) Dye visualization of flow around B2 intake for Case 14

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5. RESULTS AND DISCUSSIONS

The previous section shows the flow patterns in the pump sumps for various

scenarios of different flow conditions and pump duties. The major observations of flow

patterns pertinent to the study objectives include the followings:

The hydraulic performance of the pump sump is generally satisfactory at top water

level but not acceptable at low water level.

The influent flow is turbulent and fluctuant at low water level. If two or more

pumps on duty in one tank, the strong inflow cause splashing and bubbles. The

bubbles can be sucked into nearby pump intakes, such as A1, A2, and A5, which

can lead to severe pump problems.

No obvious bubbles are observed at top water level because the inflow is

submerged.

Due to the asymmetrical inflow into the narrow tank, the approaching flows to

different pumps are not parallel and straight. Such asymmetrical inflow can cause

circulations in the tank and pre-rotation around pipe intakes, especially at low

water level.

At low water level, surface vortex can be generated and air can be sucked into

pumps, for example A3 in Tank 1 and A4 in Take 2.

At top water level, there is no surface vortex observed because the intake

submergence in increased.

Based on the above observations and findings, we arrived at the following

conclusions:

The submergence of pipe intake at low water level is insufficient to avoid

formation of surface vortices;

The submergence of pipe intake at top water level is sufficient to avoid formation

of surface vortices;

The pipe intake clearances from the floor and back wall are adequate to avoid

formation of vortices for all the pumps;

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The submergence of influent flow at low water level is insufficient to avoid

formation of strong surface perturbations and air bubbles;

The submergence of influent flow at top water level is sufficient to avoid

formation of air bubbles.

Based on the conclusions and the considerations of sump space constraints, we

recommend:

Do not operate the pumps at design low water level (118.5 mRL);

The minimum level should be high enough to have a sufficient submergence of

inflow pipes as well as pipe intakes. The recommended LWL is 120.4 mRL;

Higher water level is always more favourable for hydraulic performance in the

sump.

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6. CONCLUSION

It is concluded that the hydraulic performance of the pump sump is generally

satisfactory at top water level but not acceptable at low water level. Air bubbles and

surface vortex are observed for the cases at low water level. It is not recommended to

operate at low water level and the minimum operation water level should be at least 20.4

mRL.

Pump Sump Modelling Report

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