Lift Station: Low Pump Capacity and Impossible Cycle Counts

Drawdown calculation:

• Volume removed: 113 sq ft × 2.0 ft = 226 cu ft
• In gallons: 226 × 7.48 = 1,691 gal
• Rate: 1,691 gal ÷ 3.5 min = 483 gpm

That is about 69% of the 700 gpm design capacity.

What it does tell you: pump 1, as tested, is delivering roughly 483 gpm against its actual system head. That is a real loss and it explains the doubled run time - the station is running longer to move the same volume.

What it does not tell you:

• It understates true capacity. Inflow continued entering the well during the test, so the pump was actually moving 483 gpm plus the inflow. If inflow was near 120 gpm, true capacity is closer to 600 gpm, still short of design but not as bad as the raw number.
• It tests only one pump. Without a pump 2 drawdown and a two-pump test, you cannot tell a pump problem from a system problem. If both pumps are low, suspect the force main, a partially closed valve, or backwater from a high interceptor.
• It says nothing about the cause - worn wear rings, a rag or debris on the impeller, an air leak at a suction flange, or a vortex at low level all produce the same result.
• Measurement error is possible from level-sensor calibration, vortexing at low level, or timing; verify the wet well dimensions and the level reading before trusting the number.

Cycle volume implied by the floats:

• 113 sq ft × (8.0 − 4.0) ft = 452 cu ft = 3,381 gal

Expected cycle time at 483 gpm and 120 gpm inflow:

• Pump-out time: 3,381 ÷ (483 − 120) ≈ 9.3 min
• Fill time: 3,381 ÷ 120 = 28 min
• Total cycle: ≈ 37 min → about 1.6 starts/hour

Even with the pump at reduced capacity, the setting should produce fewer than 2 starts per hour. Twenty starts per hour requires a cycle of roughly 3 minutes, meaning a usable volume of about 1,256 gal - only 168 cu ft, or a level band of about 1.5 ft instead of 4 ft.

Implication: there is a second, separate fault. The candidates are a slipped or mis-scaled off float, a bouncing or fouled level sensor, a leaking check valve that lets the force main drain back into the well and refill it instantly, or a control/alternation problem. The station has two problems, not one, and the low-capacity finding cannot explain the cycling.

Raising the off float by a foot would mask the true level band and, if a check valve is leaking, would keep the pumps cycling against backflow until the motors or the valve fail.

Priority actions

1. Verify the level measurement against reality. Tape the well manually and compare with telemetry and float positions. Everything downstream depends on this being right.
2. Test pump 2 and both pumps together. Pump 1 alone low points to a pump problem - worn wear rings, a partially blocked impeller, or a suction air leak. Both pumps low points to a system problem - a partly closed discharge valve, force main restriction, or a high interceptor backing water up the main.
3. Check the check valves. After shutdown, watch whether the well level rises without inflow; listen for reverse flow. A leaking check valve explains both the cycling and part of the apparent capacity loss.
4. Do not raise the off float to hide the problem. After the faults are corrected, set the floats for a cycle volume that gives 3–6 starts per hour and verify by test.
5. Compare the station totalizer against the plant flow meter to confirm the capacity loss is real and to quantify how much bypasses or lags.

Protecting the station tonight

• Set the high-level alarm, test the dialer end-to-end, and confirm the on-call list is current.
• Station an operator with a portable pump and a spare check valve, or arrange an emergency pumper truck.
• Confirm the standby generator starts and carries both pumps.
• Log every action with times; if the station overflows, the record determines the reporting outcome.