Wet Well Drawdown Test: A Reliable Method for Evaluating Pump Efficiency

In today's era of budget constraints and economic responsibility, it is crucial for utility providers to ensure that their lift stations are operating at peak efficiency. One effective way to achieve this is by conducting a wet well drawdown test. This test involves measuring the rise rate with and without the pump running, allowing operators to calculate the pump's efficiency and identify potential issues.

The proposed method for interpreting transient step-drawdown test data involves taking the derivative of the drawdown with respect to time for the entire pumping test period. This eliminates time-independent well-loss terms and provides a more accurate representation of the pump's performance.

To conduct a wet well drawdown test, operators need to collect specific data, including the size of the lift station (diameter 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. This information can be obtained from as-built plans or by conducting site visits.

The test itself involves measuring the water level in the lift station over a set period with the pump turned off (influent rate) and then turning the pump on and measuring the water level again. The difference between these two measurements represents the rise rate due to the pump. By repeating this process, operators can collect data on the pump's performance and calculate its efficiency.

In this example, let's say we want to evaluate the efficiency of a pump rated at 250 gallons per minute (gpm) in a lift station with a diameter of 10 feet and a depth of 25 feet. The influent flow rate rises from 8 feet in 10 minutes without the pump running to 5 feet in 10 minutes with the pump running.

To calculate the pump's efficiency, we need to determine its actual flow rate in gpm. First, we need to convert the rise rates from feet per minute to cubic feet per minute (cfm) and then to gallons per minute. Let's assume a stopwatch measurement of 5 minutes yields a rise of 10 inches over that period. This translates to 0.83 feet or approximately 1.26 cfm.

With the pump running, we can calculate the actual flow rate as follows:

Gpm = (1.26 cfm x 7.48 gal/ft3) / 5 minutes = approximately 149 gpm

Now, let's calculate the pump's efficiency:

Efficiency (%) = (Actual flow rate in gpm / Rated capacity of the pump in gpm) x 100

= (149 gpm / 250 gpm) x 100 ≈ 59.6%

In conclusion, a wet well drawdown test is a reliable method for evaluating pump efficiency in lift stations. By following this procedure and collecting the necessary data, operators can identify potential issues with their pumps and optimize their performance to achieve maximum efficiency.

Wet well drawdown test