Calculating a Draw Down Test at a Lift Station

During these times of budget cuts and economic responsibility, we all have to find ways to determine if we are getting the biggest bang for our buck. One of the ways to determine if your lift stations are operating at peak efficiency is to do a drawn down test to determine the efficiency of the pumps in the station. This is accomplished by first collecting the data needed.

To collect the data, you will need to know the size of the lift station (diameter in feet or length, width and depth), design size and rated capacity of the pump, influent rate with no pump running and rise rate with one pump running.

How do I find this data? For the size requirement, you should check the as-built plans to determine the diameter or length, width and depth of the lift station. What about the influent rate? Does it change all the time? You have to measure the influent rate with a rod or pipe marked in feet and inches to determine the cubic footage over a set time period. So with your marked rod, a stopwatch and the pump turned off measure the water rise in a set time to determine the cubic feet per minute and multiply it times 7.48 gal/ft3 to get gallons per minute influent. Next, turn the pump on and measure the rise in water over a set period with the pump in operation. This is the rise rate with the pump running. Calculate the cubic feet per minute and gallons per minute, and then subtract the influent rate from the rise rate and determine the gallons per minute pump rate.

To determine the efficiency of the pump, divide the pump rate in gallons per minute by the rated capacity of the pump in gallons per minute and multiply that times 100 to get a percentage of efficiency.

The following is an example:

You want to check the flow rate of a pump in a lift station rated at 250 gpm to determine its efficiency as compared to its rated capacity. The lift station has a diameter of 10 feet and a depth of 25 feet. The influent flow to the lift station rises, with no pump running, at a rate of 8 feet in 10 minutes and with the pump running, the rise rate is 5 feet in 10 minutes.

a. What is the influent rate in gpm?
b. What is the rise rate with a pump running in gpm?
c. What is the pump rate in gpm?
d. How efficient is this pump in percentage?

Lift Station

Check out the Test Your Knowledge – Calculating a Draw Down Test at a Lift Station post for an additional practice problem.

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