Lift Station Wet-Well Pump Cycling and a Force Main Air Pocket
A duplex submersible lift station serves a subdivision of 420 homes. Each pump is rated 350 gpm at 60 feet TDH. The wet well is 8 feet in diameter with a 5-foot operating band between lead-on and lag-off. The force main is 4,800 feet of 6-inch PVC with two high points that have no air-release valves. Over the past month, operators report the lead pump short-cycling (about 40 starts per hour) during dry weather, a sewage odor at the wet well, and occasional low-flow alarms at the receiving plant. The lag pump runs only during storms. The station's check valves are 12 years old and the wet-well level floats were replaced last year. The receiving plant reports the station's average daily flow is about 210,000 gallons per day.
1. Compute the wet-well cycling volume and estimate the expected pump run time per cycle during average dry-weather flow. What does this tell you about the short-cycling complaint?
Read the worked answer
Wet-well volume in the operating band:
- Area = π × (4 ft)² = 50.3 ft²
- Volume = 50.3 ft² × 5 ft = 251 ft³
- Convert: 251 ft³ × 7.48 gal/ft³ ≈ 1,880 gallons per cycle
Average dry-weather inflow: 210,000 gpd ÷ 1,440 min/day ≈ 146 gpm.
Pump-out rate net of inflow: 350 − 146 ≈ 204 gpm. Run time per cycle = 1,880 gal ÷ 204 gpm ≈ 9.2 minutes.
A 9-minute run time is normal - it should NOT produce 40 starts per hour (which would be a cycle every 1.5 minutes). So the short-cycling is not explained by the design volume and average inflow. The likely causes are: a check valve leaking back into the wet well (pump re-starts as the well refills), a float/level-control calibration error, or a partially plugged pump causing reduced output. The 12-year-old check valves are the prime suspect - a leaking check valve lets the discharge column drain back and refills the wet well quickly, mimicking high inflow.
2. What do the odor and the receiving plant's low-flow alarms suggest about the force main, and what would you inspect or install?
Read the worked answer
The force main has two high points with no air-release valves. Air trapped at high points creates an air pocket that:
- Reduces the effective flow area and raises head loss, so the pump runs farther out on its curve at lower flow;
- Causes surge and check-valve slam when the pocket releases;
- Allows septic sewage to sit and generate H2S, which explains the odor at the wet well and the low-flow alarms at the plant (flow arrives in surges rather than steadily).
Inspect/install:
- Add combination air-release/air-vacuum valves at both high points;
- Verify the force main is not partially blocked by roots or debris (CCTV or pigging);
- Check the pump curves against actual discharge pressure - a rising head with falling flow supports the air-pocket theory;
- Confirm check valves are seating (links to the short-cycling above).
Document the changes and re-measure flow at the receiving plant to confirm the fix.