Symptom
The pump run status is on, the motor is turning, and the wet well or tank level does not respond. Flow reads zero or low, or the level rises with the pump running.
The short answer
Pump Runs But No Flow
A pump that runs without moving water is either unable to move water or unable to move it into the discharge: a rag-bound or worn impeller, an air-bound or unprimed pump, reverse rotation, a broken shaft or coupling, a closed or failed discharge valve, a stuck check valve, or a blocked suction. Motor current and discharge pressure together separate the causes: high current and low pressure is a clog, low current and low pressure is air or a broken drive train, normal current and high pressure is a closed discharge.
Key points
- Read the motor current first. It says more than any other single measurement.
- High current, no flow: something is in the impeller. Low current, no flow: the pump is moving air or nothing is connected to the motor.
- Normal current, high discharge pressure, no flow: the discharge is closed.
- Check rotation on any pump that was recently reconnected. Reverse rotation moves a little water badly.
- A check valve stuck shut looks exactly like a closed valve. A check valve stuck open sends the flow back.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Impeller clogged with rags or debris | Motor current high, often near or above full-load, discharge pressure low, vibration. Pull and clear. At a lift station this is the first suspect. |
| Impeller worn or wear ring clearance excessive | Current normal to low, flow gradually declining over months, drawdown rate slower than the record. Compare with the original drawdown test. |
| Pump air-bound or lost prime | Current well below normal, discharge pressure near zero, pump quiet or gurgling. Vent the volute; check for a leaking suction line or low submergence. |
| Reverse rotation | Current somewhat below normal, small flow, low pressure. Bump the motor and observe rotation, or check phase sequence after any reconnection. |
| Broken coupling, shaft, or impeller key | Motor current at no-load level, no pressure, no vibration change. Motor turns, pump does not. Lock out and inspect. |
| Discharge valve closed or failed | Current normal or slightly low, discharge pressure at shutoff head. Check valve position at the valve, not on the HMI. |
| Check valve stuck closed | Same signature as a closed discharge valve. Isolate and inspect the check. |
| Check valve stuck open on the other pump | Pump appears to run normally; the water recirculates through the idle pump back into the well. Feel the idle pump discharge for flow; watch the level. |
| Suction blocked or screen fouled | Current low, cavitation noise, low discharge pressure, suction gauge reading very low. |
| Force main or discharge line blocked | Pressure at shutoff head across all pumps on the line. A force main problem affects every station on it. |
Read the instruments before pulling anything
Two measurements, motor current and discharge pressure, identify the cause of most no-flow conditions from the panel. Motor current is proportional to the work the pump is doing. A pump moving water draws its normal current; a pump grinding on rags draws more; a pump spinning in air or disconnected from its motor draws less. Discharge pressure says whether the pump is producing head. A pump producing shutoff head into a closed discharge reads high; a pump producing nothing reads low.
| Motor current | Discharge pressure | Points toward |
|---|---|---|
| High | Low | Clogged impeller; something jammed in the pump |
| Low | Low or zero | Air-bound, lost prime, reverse rotation, or broken coupling |
| Normal or slightly low | High, at shutoff head | Closed discharge valve, stuck check valve, or blocked force main |
| Normal | Normal | Water is moving somewhere: recirculation through another check valve, a bypass open, or the level signal is wrong |
| Slowly declining over months | Slowly declining | Wear: impeller, wear rings, or a slowly closing valve |
Diagnostic procedure
- 1
Confirm the pump is actually running
Run confirmation from the starter auxiliary, and motor current above zero. An output that is on with no current is a pump that is not running, and a different page.
- 2
Compare current to the nameplate and to history
The nameplate full-load current is the ceiling. The trend from last month is the baseline. Note whether the current is high, normal, or low, and whether it is steady or fluctuating.
- 3
Read discharge pressure
At the gauge on the discharge, or the transmitter if there is one. Compare with the pump curve shutoff head and with normal operating pressure.
- 4
Check the level reading
Against a tape or a visual. If the level is actually falling, the pump is pumping and the measurement is the problem.
- 5
Check valve positions at the valve
Discharge isolation valve open, by the stem or the handwheel. Check valve arm or indicator, where fitted, showing open with the pump running.
- 6
Feel for recirculation
With one pump running, the idle pump discharge piping should be still and its check valve closed. Flow or vibration in the idle pump discharge means its check valve is passing and the running pump is pumping in a circle.
- 7
Check rotation
If the pump has been reconnected, replaced, or the panel has been worked on, bump the motor and observe rotation against the arrow, or check the phase sequence. Submersibles can be checked by the direction of kick at start.
- 8
Vent the pump
On a dry-pit or self-priming pump, open the vent and see whether air comes out. Check the suction line for leaks and the submergence of the intake.
- 9
Lock out and inspect
Pull the submersible or open the volute. Clear the impeller. Check the coupling, the shaft, and the impeller key. Measure wear ring clearance if the flow has been declining.
- 10
Check the force main
If more than one pump or station on the same line shows shutoff head, the line itself is blocked, air-locked at a high point, or a valve on it is closed. Check the air release valves.
After clearing a clog
A pump that clogged once will clog again. Record the event and the run time since the last clog. A station where one pump clogs repeatedly and the other does not usually has an impeller difference, a worn wear ring on one pump letting rags in, or a rotation problem. A station where both clog has a wipes problem in the collection system, and the answers are a different impeller design, a grinder or a screen, and public education about what goes down the toilet. Motor current trending with an alarm at a set percentage above normal catches the next clog before the well reaches high level.
Common mistakes
- Pulling the pump before reading the current. A closed valve and a clogged impeller are told apart from the panel.
- Trusting the HMI valve position. The indicator shows what the limit switch says, not where the disc is.
- Ignoring reverse rotation after a motor replacement. Every three-phase reconnection is a rotation check.
- Resetting a high-current trip and restarting into the same rags until the motor is damaged.
- Not checking the other pump for a passing check valve when one pump seems weak.
Frequently asked questions
- The pump moves water in HAND but not in AUTO. Is that a pump problem?
- No. If the pump moves water at all, the pump is fine. In AUTO it is running for a shorter time, at a different speed, or with a different valve state. Look at the drive speed reference in AUTO, the minimum run time, and any valve that is sequenced with the pump.
- How do I know whether the impeller is worn out?
- Compare the drawdown rate with the commissioning record at the same level band and the same number of pumps. A pump delivering 70 percent of its original flow at normal current and normal pressure has lost capacity to wear. Wear ring clearance measured at the next pull confirms it.
- Can a variable speed pump run without flow?
- Yes, if the speed is below the point where the pump develops enough head to open the check valve against the force main static head. A drive minimum speed set too low, or a speed reference stuck low, gives a running pump moving nothing. Raise the minimum speed above the point where flow begins, which is found from the curve or by test.
- What causes a station to reach high level with both pumps showing running?
- Both pumps clogged, which happens in a wipes event; a blocked or closed force main; or a level transmitter reading low with the pumps having shut off long ago. Motor current and discharge pressure on both pumps answer it.
Related topics
- 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.
- 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.
- Pump Will Not StartA pump that is called and does nothing. The order to check: permissives and mode, control power, the starter or drive, the motor circuit, and the pump itself, with the measurements that settle each one in minutes.
- Wet Well Level MeasurementHow lift station wet well level is measured, which technology suits which well, the failure modes that flood a station, and why the high level float must stay hardwired.
- 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.
- 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.
Direct contact
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