Pump Losing Prime: Causes & Solutions

What is Losing Prime?

A pump is 'primed' when the suction casing and impeller are completely filled with liquid. When a pump 'loses prime', air or vapour enters the suction casing, displacing the liquid. The impeller spins in air/vapour — which it can't pump — and flow stops. If the pump continues running without liquid, it will overheat and destroy its mechanical seal and bearings within minutes.

Pumps on suction lift (where the liquid source is below the pump) are prone to losing prime. Pumps with flooded suction (liquid source above the pump) generally don't lose prime — unless there's a leak or the suction source runs dry.

Causes of Losing Prime

• Air leak in suction piping: The most common cause. Even a tiny air leak at a flange gasket, threaded joint, or valve packing will allow air to enter the suction line when the pump is running. The air accumulates in the pump casing and displaces the liquid.
• Foot valve leaking: A foot valve (non-return valve at the bottom of the suction pipe) prevents liquid from draining back to the source when the pump stops. If the foot valve leaks, the suction pipe drains and the pump loses prime on restart.
• Vortexing in the suction tank: If the liquid level in the suction tank is too low, a vortex forms at the suction inlet, drawing air into the pipe. Fix: maintain adequate liquid level or install a vortex breaker.
• Suction source runs dry: If the supply tank empties or the inlet gets blocked, the pump draws in air. This is a particular risk with automatic unattended pumps.
• Vapour release from the liquid: If the liquid is near its boiling point, dissolved gases come out of solution in the low-pressure suction zone, filling the pump casing with vapour.
• Incorrect piping geometry: If the suction piping has a high point (an upward loop), air can collect there and gradually migrate into the pump. Suction piping should slope upward toward the pump with no high points.
• Pump stopped for too long: If the pump sits idle for a long time, liquid may slowly drain back through a leaking foot valve or check valve. When the pump restarts, it's air-bound.

Diagnosis

1. Check for air leaks: With the pump running, spray soapy water on suction flanges, threaded joints, and valve packings. Bubbles indicate air ingress.
2. Test the foot valve: Stop the pump and observe the pressure gauge on the pump suction. If the pressure drops to zero over time, the foot valve is leaking.
3. Check the suction tank level: Verify the liquid level is above the minimum submergence required to prevent vortexing.
4. Inspect the suction piping: Look for high points, air pockets, or sections where air could accumulate. The pipe should slope continuously upward toward the pump.
5. Check the seal water supply: If the pump has an external seal flush, verify it's not introducing air into the pump casing.

Solutions

• Install or repair a foot valve at the bottom of the suction pipe to prevent drain-back
• Seal all suction-side leaks — use PTFE tape on threaded joints, replace damaged flange gaskets, tighten valve packings
• Install an automatic priming system (vacuum pump or self-priming pump) for difficult suction-lift applications
• Design suction piping with continuous upward slope toward the pump — no high points where air can collect
• Install a low-level alarm on the suction tank to prevent the source from running dry
• Install a vortex breaker in the suction tank if the level drops near the inlet
• Consider converting from suction lift to flooded suction by moving the pump below the liquid source
• Install a dry-running protection device that shuts down the pump when flow is lost