Pump Loses Prime Intermittently
Symptoms
Pump starts normally but discharge pressure drops suddenly during operation. Flow rate reduces to near zero. Motor current drops below normal. Cavitation noise (crackling or rattling) may be audible. Condition may self-correct temporarily then recur.
Likely Cause
Most commonly: air leak on suction side (loose fittings, worn mechanical seal on suction flange, cracked suction pipe), insufficient NPSH available (suction lift too high, fluid temperature too high, suction strainer blocked), or check valve failure allowing backflow.
Testing Procedure
1) Check suction pressure gauge — reading below pump NPSH required indicates cavitation condition.
2) Inspect all suction-side fittings, flanges, and mechanical seals visually for air bubbles or weeping.
3) Clean and inspect suction strainer — measure pressure drop across strainer.
4) Calculate available NPSH vs required NPSH from pump curve at operating flow rate.
5) Test check valve by closing isolation valve and monitoring pressure decay rate.
6) Perform vacuum test on suction piping to identify air ingestion points.
Tools Required
Pressure gauges, vacuum gauge, flow meter, ultrasonic leak detector, pipe inspection camera, NPSH calculation worksheet
Safety Precautions
Do not run pump dry — seal damage occurs within seconds. Isolate and de-pressurize before inspecting fittings or mechanical seal. Hot fluids: confirm system has cooled before opening any connections. Wear face shield when opening any pressurized connection.
Step-by-Step Solution
1. Stop pump and observe pressure gauge decay — rapid drop indicates check valve failure; replace or overhaul check valve.
2. Clean suction strainer and measure pressure drop again — replace element if differential exceeds specification.
3. Pressurize suction line to 1.5x operating pressure and perform soap-bubble or nitrogen leak test on all fittings.
4. Tighten or replace any leaking fittings, flanges, or gaskets.
5. Verify fluid temperature and suction lift against pump NPSH curve — if marginal, lower fluid temperature or reduce suction lift.
6. After repairs, reprime pump per manufacturer procedure and verify sustained discharge pressure.
Prevention
Inspect and clean suction strainer monthly. Check mechanical seal condition at each planned shutdown. Install low-suction-pressure switch with automatic shutdown on critical pumps.
Engineering Notes
Air ingestion at mechanical seals is often overlooked — a seal that leaks air inward under vacuum will show no external leakage but will destroy prime. Ultrasonic detection during operation is the most reliable non-invasive method for locating these leaks.