Symptom
The pump runs with normal or low current and no flow; the discharge pressure is near zero; the suction gauge shows high vacuum or bounces; the pump pumps for minutes and then flow falls to nothing; on the next start it takes minutes to prime, or never does; a self-priming pump gurgles.
The short answer
Pump Loses Prime
A pump loses prime when air gets into the suction side faster than the pump can move it out: through a leaking joint, a valve packing, or a foot valve on the suction line; through a foot or check valve that leaks back and lets the suction line drain while the pump is off; through a suction lift that is too high for the water temperature and the pump, so that the water vaporizes at the impeller; through a vortex at a low wet well level or an inflow that carries air into the suction; or, on a self-priming pump, through a leak that drains the priming chamber. It is found by a vacuum test on the suction line, by watching the wet well for a vortex, and by inspecting the foot valve, and the controller detects it by a flow switch, a discharge pressure, or a motor current that stays low after the start, stops the pump before it runs dry, tries to reprime a limited number of times, and alarms.
Key points
- Air is getting in somewhere the water should be; the suction line is the suspect.
- A vacuum test on the isolated suction line finds a leak; a line that will not hold vacuum will not hold prime.
- A foot valve or a check valve that leaks back drains the line between runs; the next start is dry.
- A vortex at low level or an aerated inflow feeds air to the suction; a baffle or a higher stop level fixes it.
- The controller must detect loss of prime and stop the pump; a dry-running pump destroys its seal in minutes.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Air leak in the suction piping or fittings | Isolate the suction line, pull a vacuum with the pump or a hand pump, and watch the gauge; a falling vacuum is a leak. Soap the joints under vacuum, or pressurize and listen. |
| Suction valve packing or a gasket drawing air | Tighten or repack the valve; a valve on the suction side leaks air in rather than water out. |
| Foot valve or check valve leaking back | The suction line drains between runs; the pump starts dry. Fill the line, wait, and see if it holds; inspect the foot valve for debris on the seat. |
| Suction lift too high for the conditions | The lift, the pipe friction, and the water temperature against the pump net positive suction head required; warm water and a long suction line reduce the available lift. |
| Vortex at the suction | Watch the wet well at low level; a swirl at the surface feeds air. Raise the stop level or add a baffle or an anti-vortex plate. |
| Aerated inflow | An inflow that cascades into the well near the suction carries air. Redirect the inflow or move the suction. |
| Self-priming pump casing not holding its charge | The casing drains through a leak, a worn wear plate, or a missing check; the pump cannot reprime. Inspect the priming chamber and the check. |
| Clogged suction strainer or partly closed suction valve | High vacuum on the suction gauge; the pump cavitates and loses prime. Clean the strainer; open the valve. |
| Impeller worn or clearance opened | A self-priming pump depends on close clearances to prime; worn clearances prime slowly or not at all. Check the clearance against the manual. |
Finding the air
- 1
Watch a start
Discharge pressure, suction vacuum, and motor current through a start. Prime that never builds is a dry line or a big leak; prime that builds and then fails is a slow leak, a vortex, or cavitation.
- 2
Vacuum test the suction line
Close the discharge, fill the suction, isolate it, and pull a vacuum; watch it over minutes. A held vacuum clears the piping; a falling one is a leak to find joint by joint.
- 3
Check the foot or check valve
Fill the suction line and leave it; if it drains, the valve is leaking back. Pull the foot valve and look at the seat.
- 4
Look at the well
At the level where prime is lost: a vortex, an inflow splashing near the suction, the suction bell too close to the surface.
- 5
Check the lift
Measure the vertical lift and estimate the suction friction; compare with the pump curve for the water temperature. A pump at the edge of its lift loses prime on a warm day.
- 6
Fix and confirm
Repair the leak or the valve, adjust the level or the baffle, and watch several complete cycles.
What the gauges say
| Suction vacuum | Discharge pressure | Current | Meaning |
|---|---|---|---|
| Low and steady | Zero | Low | Pump is dry; the line drained or never filled |
| High and steady | Low | Low | Suction restricted: strainer, valve, or too much lift |
| Bouncing | Bouncing | Fluctuating | Air entering: a leak, a vortex, or an aerated inflow |
| Normal, then rising | Normal, then falling | Falling | Prime being lost progressively: a slow leak or cavitation |
| Normal | Normal | Normal | Prime is fine; look elsewhere |
What the controller should do
A pump that has lost prime is running dry, and its mechanical seal, which depends on the pumped liquid for cooling and lubrication, fails within minutes. The controller proves flow after every start: a flow switch, a discharge pressure above a minimum, or a motor current above the dry-running value, within a time that allows for priming. If flow is not proven, the pump stops and the controller may retry a limited number of times with a wait between, then locks out and alarms. Loss of flow while running is treated the same way. The setpoints and the retry count are in the functional description, and the alarm says which pump and why.
Frequently asked questions
- The pump primes if I fill it by hand, then loses prime again the next day.
- The suction line is draining between runs: a foot valve or a check valve leaking back, or an air leak high on the suction line that lets the water fall. Fill the line, wait, and check the level; then the foot valve.
- Why does it happen only in summer?
- Warm water vaporizes more readily, so the available suction lift falls with temperature, and a pump that is marginal on lift cavitates and loses prime on hot days. Reduce the lift, shorten the suction line, or lower the pump.
- Can a submersible pump lose prime?
- A submersible does not need priming; it can become air-bound if it starts in a well full of air, or draw air through a vortex at a low stop level, with the same result of no flow. An air release on the discharge and a higher stop level are the answers.
- How long should the controller allow for priming?
- As long as the pump takes on a good day with margin, from the pump manual and a timed test at commissioning, commonly one to three minutes for a self-priming pump. Longer than that is a dry pump, and the timer stops it.
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 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 ControlThe measurement that runs a lift station: choosing setpoints, protecting the pumps, validating the signal, and the failures that flood a station.
- Interlocks and PermissivesA permissive lets you start. An interlock stops you. Confusing the two produces equipment that either will not run or will not protect itself.
- Well Pump ControlControlling a groundwater well: submersible and line-shaft turbine pumps, the pump-to-waste start, drawdown and the low-water cutoff, cycling limits from the well and the aquifer, and what the controller monitors to protect a pump that cannot be seen.
- Seal Failure AlarmThe moisture probe in a submersible pump has tripped its relay: a seal that is leaking, or moisture in the cable, the junction box, or the probe circuit, a relay that does not match the pump, or a failed relay. What the alarm should do and what to check.
Direct contact
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