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Calculating Active Volume for Pump and Lift Stations

A Graphic Displaying the Active Volume for A Pump Station with Float Level Sensors

Romtec Utilities designs and supplies pump and lift stations for all types of applications. One key aspect of the pump station engineering is determining the appropriate “active volume” for the pumping system. The active volume describes the volume of water in the well where the pumps are actively pumping. This volume is located between two elevations monitored by level sensors. The top level sensor tells the control panel to cycle a pump on while the lower level sensor tells the control panel to cycle the pump off. This active volume is an important measurement to establish correctly during the pump station design.

The philosophy of this calculation is to ensure that the pump motor will not overheat due to repeated start and stop cycles. When the active volume is too small, the pump cycles occur too frequently, causing potentially dangerous or destructive heat build-up. In systems where multiple pumps are available, the pumps alternate operation cycles, extending the time between individual pump starts. For the purposes of determining an appropriate active volume, we focus on a single pump operating to account for the worst case scenario.

Wet Well Drop Bowl

The formula used to determine the active volume is V = T x Q / 4

Where V is the active volume, T is the cycle time, and Q is the pumping rate. The pumping rate is typically determined by Romtec Utilities when finding a pump that will meet the total dynamic head and that will meet or exceed the peak inflow for the system. The cycle time is also determined by Romtec Utilities. Pump manufacturers specify the maximum starts per hour for their pumps. Romtec Utilities uses this number to conservatively find an appropriate cycle time for the pumps. In this calculation, it is better to lean on the conservative side because the only change required for the system is changing the elevations of the level sensors.

Being too conservative is not always appropriate either. When pumping wastewater, for example, if the active volume is too large, the sewage will sit for long periods of time between cycles. This can lead to the water becoming septic, which causes odors and corrosive conditions to form. In more applications like stormwater, long periods between cycles can allow solids to settle out of the water and collect at the bottom of the well. This can create a clogging concern. These scenarios can lead to pump maintenance issues or even premature pump failure.

Interior of a Lined Wet Well

The best approach when determining the active volume for a pump station is to trend slightly toward the conservative side while not going overboard. This creates an optimal scenario for the pumps to not build up heat that can wear out the pump motors while still pumping the water at frequent, regular intervals. The active volume can be easily changed even after the pump or lift station is installed by adjusting the level sensors as needed. This can account for changes or seasonal differences with inflow to the station.

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  1. Harper Campbell

    It’s interesting to know that when it comes to installing a pump and a lift station, that there is a calculation that should be taken into consideration. I like how you pointed out that being too conservative is not always the best option to go with when it comes to this. It will be nice to be able to know for sure that we are doing the best we can to prevent any type of clogging or backups in the system.

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