Sizing wet wells for stormwater involves determining the appropriate capacity to accommodate peak inflows and ensure efficient pumping operations. Key factors include inflow rates, pump cycle times, and the need to minimize frequent pump cycling and excessive retention times. Use the volume of the incoming drainage piping as part of the wet well volume.
The design should consider the peak runoff rate from a specific design storm. The peak runoff rate is calculated using methods like the Rational Method, determines the volume of water entering the wet well. The pump station should be able to pump the peak runoff rate.
Wet well volume depends on the method of pump operation. If you use variable speed pumps and the pumps match the flowrate to the inflow, the wet well volume may be small. This time is normally less than 1 minute retention. That time will allow for the change in capacity with a pump is started or stopped before the next start or stop point is reached.
Sufficient storage volume is needed between pump start and stop elevations to meet minimum cycle time requirements. Frequent pump cycling can damage pumps. Too small of a wet well volume can lead to frequent cycling,. The maximum number of starts per hour for lift stations is typically 6. Pump cycle time equals 60 minutes divided by the starts per hour, so the maximum pump starts per hour of 6 equals a 10 minute cycle time
