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Section 3: Pump Station Hydrology

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Methods for Design

In order to design a pump station effectively, the inflow hydrology must be known. The hydrology developed for the associated storm drain system usually will not serve as a firm basis for discharge determination into the pump station. A hydrograph is required because the time component is critical in understanding the inflow which governs the sizing of the wet well. The designer needs to know not only the peak inflow, but the timing and volume. The difference between the input and the output hydrographs is the storage requirement of the pump station wet well. The hydrograph should consider the storage abilities of the storm drain system, which may reduce the required size of the wet well. Governmental regulations or the physical limitations of the receiving waters determine the output discharge from the pump station.

The storm drain system associated with the pump station may have a design basis of less than 2% AEP. However, TxDOT recommends at least a 2% AEP flood design because the pump station is generally used when drainage by gravity from a low point is inadequate or impractical.

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Procedure to Determine Mass Inflow

A mass inflow curve represents the cumulative inflow volume with respect to time. In order to determine a mass inflow curve, the hydraulic designer must first develop an inflow hydrograph based on a design storm. The most typical design method is the NRCS Dimensionless Unit Hydrograph, discussed in detail in Chapter 4. For the following procedure taken from FHWA Hydraulic Engineering Circular 24 ( HEC-24) example, the hydrograph data in Table 4-31 of this manual will be used.

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  1. Evaluate the time base of the hydrograph and select a time increment, usually the same time increment as that used for developing the inflow hydrograph.
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  3. Develop a table with columns for time, time increment, inflow rate, average inflow rate, incremental inflow rate, cumulative inflow volume, cumulative outflow volume, and storage difference as shown in Table 11-1.
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  5. At each time step, extract the inflow rate from the computed inflow hydrograph. (For this example, use Table 4-31, column Qu).
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  7. Compute and tabulate the average inflow rate as half of the current and of the previous inflow rates for each time step. (i.e. time step 30: 188/2 cfs + 350/2 cfs = 269 cfs).
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  9. Compute the incremental volume for each time step as the average inflow rate multiplied by the time step in seconds.
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  11. Compute the cumulative inflow as the sum of each time step and the previous time step.
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  13. Plot a curve of cumulative volume versus time. The result is a mass inflow curve, shown as Figure 11-1.
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  15. Determine the allowable discharge to the receiving waters. The pump flow rate must be at or below the allowable discharge rate. For this example, assume the allowable discharge rate is 100 cfs. Notice that the pumping did not start until a sufficient volume was in the wet well.
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  17. Multiply the allowable discharge by the time step for the pump flow. Notice that the pumping cannot start until the inflow has developed. The greatest difference between inflow and pump flow is the required storage of the facility. The greatest difference in this example is at time step 80, which is about 691,200 cubic feet. The negative numbers at time steps 230 and 240 indicate that regular pumping should have stopped at about time step 220. The Pump Flow line is also plotted with the inflow curve in Figure 11-1.
Anchor: #i1059481Table 11-1: Mass Inflow Computation Table

1

2

3

4

5

6

7

8

Time (minutes)

Time Increment (seconds)

Inflow Rate Qu (cfs)

Average Inflow (cfs)

Incremental Inflow (cubic feet)

Cumulative Inflow (cubic feet)

Cumulative Outflow (pump flow in cubic feet)

Storage Difference (cubic feet)

0

 

0

0

0

0

0

0

10

600

58

29.0

17,400

17,400

0

17,400

20

600

188

123.0

73,800

91,200

60,000

31,200

30

600

350

269.0

161,400

252,600

120,000

132,600

40

600

400

375.0

225,000

477,600

180,000

297,600

50

600

358

379.0

227,400

705,000

240,000

465,000

60

600

272

315.0

189,000

894,000

300,000

594,000

70

600

170

221.0

132,600

1,026,600

360,000

666,600

80

600

112

141.0

84,600

1,111,200

420,000

691,200

90

600

77

94.5

56,700

1,167,900

480,000

687,900

100

600

51

64.0

38,400

1,206,300

540,000

666,300

110

600

34

42.5

25,500

1,231,800

600,000

631,800

120

600

22

28.0

16,800

1,248,600

660,000

588,600

130

600

15

18.5

11,100

1,259,700

720,000

539,700

140

600

10

12.5

7,500

1,267,200

780,000

487,200

150

600

7

8.5

5,100

1,272,300

840,000

432,300

160

600

4

5.5

3,300

1,275,600

900,000

375,600

170

600

3

3.5

2,100

1,277,700

960,000

317,700

180

600

2

2.5

1,500

1,279,200

1,020,000

259,200

190

600

1

1.5

900

1,280,100

1,080,000

200,100

200

600

0

0.5

300

1,280,400

1,140,000

140,400

210

600

0

0.0

0

1,280,400

1,200,000

80,400

220

600

0

0.0

0

1,280,400

1,260,000

20,400

230

600

0

0.0

0

1,280,400

1,320,000

-39,600

240

600

0

0.0

0

1,280,400

1,380,000

-99,600



 Inflow versus Pump Flow (click in image to see full-size image) Anchor: #EJGLHFNIgrtop

Figure 11-1. Inflow versus Pump Flow

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