Symptom
Lead and lag both run for hours or continuously; the wet well level stays above the lag start and may rise toward high level; the drawdown with two pumps is slow or zero; run hours on both pumps climb; high level alarms follow.
The short answer
Both Pumps Running Constantly
Two pumps running continuously means the station cannot pump down: either the inflow exceeds what the pumps deliver, or the pumps deliver less than they should, or the level the controller sees is not the level in the well. Inflow is checked against the design and the weather; delivery is checked by timing the drawdown with one pump and with two and comparing with the pump curve, by reading the motor amps against the nameplate, and by reading the discharge pressure against the expected head; the level is checked by a tape against the transmitter and the floats. The common finds are wet weather inflow, a ragged or worn impeller, a check valve stuck or a discharge valve partly closed, a force main restriction or air lock, a pump running backward or air-bound, a level transmitter reading high, and a lag start setpoint set at or below the lead stop.
Key points
- Three possibilities: too much coming in, too little going out, or the level is wrong. The drawdown test separates them.
- Time the drawdown with one pump and with two; compare with the curve. That is the delivered flow.
- Amps and discharge pressure say whether a pump is pumping: low amps and low pressure is a pump moving nothing.
- A check valve stuck, a discharge valve partly closed, or a force main restriction shows as high head and low flow on both pumps.
- Check the setpoints: a lag start at or below the lead stop runs both forever.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Inflow exceeds the capacity of one pump | Wet weather, infiltration, or a new connection. The drawdown with both pumps is positive but slow; the inflow computed from the fill rate with pumps off exceeds one pump. Compare with the design inflow. |
| Impeller ragged, clogged, or worn | One pump delivers far less than its curve; its amps are low or erratic; the drawdown with that pump alone is slow. Pull and inspect. |
| Check valve stuck or partly closed | High discharge pressure and low flow on that pump; the valve arm does not move on start. Inspect the check. |
| Discharge or isolation valve not fully open | A valve left throttled after maintenance. Check every valve position on the discharge. |
| Force main restricted or air-locked | Both pumps show high pressure and low flow; a high point in the main with a failed air release. Check the air release valves and the downstream station. |
| Pump running backward | Low flow, noisy, low amps after a motor or cable change. Check rotation. |
| Pump air-bound | Runs with no flow after a dry start; discharge pressure zero. Vent the volute; check the air release. |
| Level transmitter reading high | The tape shows the well low while the transmitter shows high; the pumps run on a false level. Compare and correct; check the floats. |
| Setpoints wrong | The lag start at or below the lead stop, or the stop levels above the start levels after an edit. Read the setpoints against the functional description. |
| Downstream station or treatment plant backing up | Discharge pressure high on both pumps with no local restriction; the force main discharge is surcharged. |
The drawdown test
- 1
Measure the well
The wet well area from the drawings, and the level by tape against the transmitter.
- 2
Time the fill
With both pumps off for a few minutes, the rise in level over time; times the area, that is the inflow.
- 3
Time the drawdown with one pump
The fall in level over time with the lead alone; times the area, plus the inflow, that is the pump delivery. Repeat with the other pump alone.
- 4
Time it with both
The same with both running; two pumps in parallel deliver less than twice one, and the amount less says how much head the force main adds.
- 5
Compare
Each pump delivery against its curve at the measured head; both against the inflow. The comparison says whether the problem is the inflow, one pump, both pumps, or the discharge.
Q_pump = A × (Δh / Δt) + Q_inflow
- Q_pump = the delivered flow of the pump under test
- A = the wet well area over the level range used
- Δh / Δt = the rate of level fall with the pump running
- Q_inflow = the inflow, from the rate of rise with the pumps off, over the same period
What the readings say
| Amps | Discharge pressure | Drawdown | Meaning |
|---|---|---|---|
| Normal | Normal | Slow with both | Inflow exceeds the station; wet weather or a new source |
| Low | Low | None for that pump | That pump is not pumping: air-bound, running backward, or the impeller is gone |
| Low or erratic | Low | Slow for that pump | Ragged or worn impeller |
| High | High | Slow for both | Discharge restriction: check valve, throttled valve, force main, or downstream surcharge |
| Normal | Normal | Normal by tape but the HMI level stays high | Level transmitter reading high; the controller is pumping a well that is empty |
| Normal | Normal | Normal, and the pumps still cycle both | Setpoints: the lag start below the lead stop |
The level and the setpoints
Before anyone pulls a pump, the level is checked by tape and the setpoints are read. A transmitter fouled or drifted high tells the controller the well is full when it is not, and both pumps run against a dry well until the low-level cutoff, if there is one, stops them; the floats, if they are wired as backups, may disagree with the transmitter and the HMI will show it. A setpoint edit that put the lag start at or below the lead stop makes the lag start every cycle and never stop first, which looks like both pumps running continuously, and the functional description says what the levels should be.
Frequently asked questions
- The pumps are new and both still run all the time.
- Then the inflow is the problem, or the discharge is. Do the drawdown test: new pumps that deliver their curve into an inflow that exceeds them means the station is undersized or the inflow has grown, usually from infiltration; new pumps that deliver far less than their curve means the discharge is restricted or the force main is surcharged.
- How do I tell a ragged impeller from a worn one?
- A ragged impeller shows erratic amps and often clears partially when the pump is reversed or bumped; a worn impeller shows steadily low delivery with normal amps for the reduced flow. Both are confirmed by pulling the pump; rags on the impeller are obvious, and wear is measured at the clearances.
- Can both pumps running damage anything?
- Not by itself; pumps are made to run. The risk is the overflow when they cannot keep up, and the wear on both when one pump has failed and the other is carrying the station alone. A run hours alarm and a both-pumps-running alarm after a set time bring the condition to someone before the high level does.
- The force main has an air release valve. Could it still be air-locked?
- If the valve is stuck, plugged, or isolated, yes. An air release valve that has not been maintained does not release, and a high point in the main holds a pocket of air that both pumps push against. Check the valve; a main that is quiet when the pumps run and then surges when the air moves has a pocket.
Related topics
- Pump Runs But No FlowThe motor turns and nothing moves. How to tell a clogged impeller from an air-bound pump, a closed valve, a stuck check, a broken coupling, or reverse rotation, using motor current and discharge pressure before pulling anything.
- Pump Short CyclesA pump that starts and stops far more often than it should: the control band, the level signal, a leaking check valve, a waterlogged pressure tank, a lag pump doing the lead job, and the starts-per-hour limit that makes short cycling a motor killer.
- Lift Station Lead/Lag ControlHow a duplex or triplex station decides when one pump is enough and when it is not: the setpoint ladder, cycle volume, the lag call, parallel pumping, and what to do when the lead pump fails.
- Lift Station High LevelThe alarm that stands between a lift station and an overflow: where the high-level float goes, what it must do on its own, how much response time the well provides, and how to diagnose a high level that should not be happening.
- Transmitter Reads Wrong ValueThe reading is steady, plausible, and wrong. How to prove it against a reference, then separate a scaling mismatch, a range change, a zero shift, an installation effect, and a genuine transmitter fault, with the order of checks that finds it fastest.
- Duplex Lift Station ControlsThe two-pump station that makes up most of a collection system: what is in the panel, how the sequence works, what is monitored, and where these stations actually fail.
Direct contact
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