pump station design manual

STORM SEWER PUMP

STATION SIZING

SPREADSHEET

Francis Mitchell, M.S., P.E. [email protected]

mailto:[email protected]

Francis Mitchell, M.S., P.E. PAGE 1

STORM SEWER PUMP STATION SIZING SPREADSHEET Abstract: This spreadsheet is a tool to help the drainage engineer designs a storm sewer pump station by performing a level pool analysis of the pumping system, and storage areas. The settings of the pumps “on” and “off” points, flows, system storage, the storm hydrograph, rainfall depth, and duration, are parameters that could be adjusted to optimize the size of a pump station. This spreadsheet can be downloaded through the link below,

https://www.dropbox.com/sh/jf8vsmhhq013mdd/AACJfnUjpiiCieTCusBS0JxEa?dl=0



STORM SEWER PUMP STATION SIZING SPREADSHEET Table of Contents Page

1.0 DESCRIPTION ................................................................................................................................ 3

2.0 INPUT DATA WORKSHEETS.......................................................................................................... 4

3.0 OUTPUT DATA WORKSHEETS ...................................................................................................... 8

4.0 CASE STUDY ................................................................................................................................ 16

5.0 AUTHOR ...................................................................................................................................... 33

6.0 REFERENCES ............................................................................................................................... 34


STORM SEWER PUMP STATION SIZING SPREADSHEET

1.0 DESCRIPTION

This spreadsheet was developed as a tool to perform the preliminary sizing of storm sewer pump

station and to complement the analysis performed by much advanced software which don’t have

the flexibility of quickly varying the input data. This spreadsheet could be used first to pre-size and

calibrate a pump station and then use the results for further analysis without having the burden of

performing countless trial and error analysis.

The analysis performed by this spreadsheet is a level pool flood routing of a pump station system

given its watershed area, the rainfall depth and duration, the unit hydrograph and peaking factor.

In addition the user can choose different storm hydrograph generating procedure. The pump

station is defined by its geometry, pumps capacities, “on” and “off” setting, and the storage

available in the storm sewer network.

This spreadsheet generates watershed inflow hydrograph output, outflow hydrograph output,

stage duration graph, and displays also the number of times a pump will turn “on” and “off”, as

well as the amount of working hours.


2.0 INPUT DATA WORKSHEETS


Design Unit

Design unit is either “Metric” or “English”.

Areas

Areas are entered as “Acres” or “Hectares”.

SCS CN

CN (curve number) depends on the type of land cover.

Time of Concentration

The time of concentration is entered in minutes.

Hydrologic Method

Three different hydrologic analysis method can be used.

SCS – Santa Barbara Method

SCS – Design Storm

SCS – Flood Hydrograph

Rainfall Depth

Rainfall depth for the selected storm is entered in inches or millimeters.

Routing Time

This is the duration of the storm in hours.

Flood Routing Time Step

The time step increment for the routing analysis to be performed.

Print Display Time step

The time step for a result to be printed. This option is currently disabled.

Peaking Factor

The SCS peaking factor, and the length of the receding leg of the hydrograph. This variable

is dependent on the hydrologic method selected.


Top Elevation and Top Area

The pump station top elevation and top area are entered in feet, and square feet, or meter,

and square meter.

Critical Water Level

The stage at which the water level reached a critical value is entered in feet or meter. This

input data is only for graphical representation of the design constraint.

Starting Water Level

The starting water level is the level of the permanent pool of water inside the pump station.

This data is entered in feet or meter. The stage of the starting water level cannot be higher

than the lowest pump starting elevation.

Bottom Elevation and Bottom Area

The pump station bottom elevation and bottom area are entered in feet, and square feet,

or meter, and square meter.

Pumps Settings

The pumps “on” and “off” elevations are entered in feet or meter. The CUMULATIVE flow

for all the pumps are entered at each row. The unit is either cfs, or cubic meter per second.

Additional Storage

The storage within the drainage system is entered at each stage elevation. The unit is feet,

and cubic feet, or meter, and cubic meter.


3.0 OUTPUT DATA WORKSHEETS


4.0 CASE STUDY

Project:

The City of Miami Beach, and the Florida Department of Transportation District Six, have decided

to jointly fund the construction of a major storm sewer pump station facility to offer relief from

recurrent flooding events along Indian Creek Drive from 25th Street to 41st Street. These floodings

are mainly caused by the ever increasing high tides, and deficient gravity drainage systems that

were built in the 1940s.

Design Criteria:

Design storm to satisfy FDOT critical storms duration as per Chapter 14-86.

Minimum elevation of +3.70 for new pavement.

Drainage for low lying side streets will need to be accommodated.

Design:

Total tributary area A=36.81 acres

SCS weighted curve number CN = 96

Critical storm is FDOT 1 Hour hydrograph with rainfall depth of 3.60 inches

Maximum watershed runoff flow is 132.23 cfs at 54.25 hours

Trunk line length is 4,600 feet divided into a south and north reach.

The south reach controls the design with a length of 2,490 feet.

The south reach has 30 manholes risers, and 1 terminal manhole.

The north reach is 2,110 feet.

The north reach has 25 manholes risers, and 1 terminal manhole.

Minimum existing inlet elevation +2.20 NAVD

Step 1

Find pump station preliminary total flow estimated at 70% of the watershed flow.

Qtotal = 132.23 cfs x 70% of inflow = 92.56 cfs

A preliminary headloss – system curve analysis yields a pump station with three pumps

having a total capacity of 93.36 cfs. (refer to step 5)

Step 2

Find trunk line preliminary pipe diameter for a conveyance velocity of 3.0 ft/s to 3.5 ft/s

Diameter for 3.0 ft/s = (92.56 cfs x 4 / (3.0 ft/s x 3.14159))^.5 = 6.3 ft ( 6.0 feet = 72 inches)

Diameter for 3.5 ft/s = (92.56 cfs x 4 / (3.5 ft/s x 3.14159))^.5 = 5.8 ft ( 5.5 feet = 66 inches)

Step 3

Calculate head losses along trunk line


Head losses for 66 inches pipe – south branch

Ref: Hydraulic Calculator Software from HYDTRENCH

Manhole risers losses = 30 x 0.15 x ((3.93)^2 / 64.4) = 1.08 feet

Terminal manhole loss = 1 x 0.5 x ((3.93)^2 / 64.4) = 0.12 foot

Manhole 90 deg. Bend to pump station = 1 x 0.7 x ((3.93)^2 / 64.4) = 0.17 foot

Pipe exit losses to pump station = 1 x 1.0 x ((3.93)^2 / 64.4) = 0.24 foot

Losses along length of south pipe = 2,490 feet x 0.0659 / 100 = 1.64 feet

-------------

Total Losses 3.25 feet

Minimum crown elevation of pipe for HGL to be below existing inlet

+2.20 NAVD – 3.25 feet (head losses) = -1.05 NAVD


Head losses for 66 inches pipe – south branch

Ref: Hydraulic Calculator Software from HYDTRENCH

Manhole risers losses = 30 x 0.15 x ((3.30)^2 / 64.4) = 0.76 foot

Terminal manhole loss = 1 x 0.5 x ((3.30)^2 / 64.4) = 0.08 foot

Manhole 90 deg. Bend to pump station = 1 x 0.7 x ((3.30)^2 / 64.4) = 0.12 foot

Pipe exit losses to pump station = 1 x 1.0 x ((3.30)^2 / 64.4) = 0.17 foot

Losses along length of south pipe = 2,490 feet x 0.0414 / 100 = 1.03 feet

-------------

Total Losses 2.16 feet

Minimum crown elevation of pipe for HGL to be below existing inlet

+2.20 NAVD – 2.16 feet (head losses) = +0.04 NAVD


Step 4

Find crown elevation based on concrete manhole structures placed along new pavement

Frame and cover height 9.0 inches

Brick leveling (2 layers) 8.0 inches

Concrete slab thickness 8.0 inches

Minimum wall height above opening 6.0 inches

Minimum spaces around pipe 6.0 inches

Pipe thickness 7.0 inches

---------------

Total Cover 44 inches (3.67 feet)

Crown elevation based on structure type = 3.70 NAVD – 3.67 = +0.03 NAVD

In order to expedite construction the City has decided to use spiral reinforced HDPE pipe. This pipe

will be placed under water and requires a soil overburden atop the pipe to prevent floatation. The

minimum crown elevation needed to prevent uplift is -1.00 NAVD.

Although the 66 inches pipe is sufficient, the 72 inches pipe will be used with crown placed at -1.00

NAVD, invert set at -7.00 NAVD. The benefit of using the 72 inches instead of the 66 inches will be

less head loss (lower HGL), longer filling time which will translate into longer cycling time between

pumps start.


Typical project pump station layout

Drawing Sketch Credit: Ribbeck Engineering Inc.

The pumps used are “in tube” axial flow pump. This setting allows lower minimum

submergence depth. As a result the pump station needs not to be set very deep.

The spacing between pumps is also reduced.

Another requirement was to construct the station in a modular fashion. The modular

approach allowed the units to be pre-cast at the casting yard, and transported on

site for setting and installation.


Step 5 – Head loss at low water level

Pump flow at low water level (maximum head) is 29.57 cfs


Head loss at high water level

Pump flow at high water level (minimum head) is 32.67 cfs Average of pump flow for high and low water level is (32.67 cfs + 29.57 cfs)/2 = 31.12 cfs


Watershed input data


Pump station input data


Inflow Hydrograph


Outflow hydrograph


Inflow – outflow hydrograph


Stage duration


Cumulative inflow – outflow volume


Pumps running time


Pumps capacity and “on” “off” setting rating curve


System storage capacity rating curve


5.0 AUTHOR This spreadsheet can be ordered from the following address. Francis Mitchell, M.S., P.E. [email protected] Phone: (305) 979-6387 Or by accessing the link below,




6.0 REFERENCES

Federal Highway Administration, Highway Stormwater Pump Station Design Manual,

HEC-24, 2000. Formerly Highway Storm Water Pumping Stations, Volumes 1 and 2,

FHWA-IP-82-17.

Daugherty, Robert L., and Franzini, Joseph B.: “Fluid Mechanics with Engineering Applications,”

McGraw-Hill Book Co., New York, 1985.

Mc Cuen, Richard:”Hydrologic Analysis and Design”, Second Edition, Prentice Hall, New Jersey,

1998.

Reitz, and Jens:”Design of Urban Highway Drainage, the State of the Art”, FHWA-TS-79-225, Federal

Highway Administration, August 1979.

Shammas, Namir C.; “Mathematical Algorithms in Visual Basic for Scientists and Engineers,”


Soil Conservation Service:”National Engineering Handbook Section 4-Hydrology, (Part 1 of 2, 2 of

2), Engineering Division Soil Conservation Service, USDA, Washington, March 1985.

South Florida Water Management District:”Management and Storage of Surface Waters-Permit

Information Manual Volume IV”.Streeter, Victor L., and Wylie, Benjamin E.: “Fluid Mechanics,”


Mitchell, Francis : “HYDTRENCH, Exfiltration Trench manual”, Miami, 1999.

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