Pump Loses Prime When Turned Off: Air Leak Detection

Solving Recurring Prime Loss in Lake-Fed Irrigation

A 1.5 HP pump draws from a lake for 6 sprinklers. Prime is lost immediately after shutdown unless a manual ball valve on the discharge is closed before power-off, then reopened after restart. Foot valve and discharge check valve installed but didn't help.

Diagnosis: Suction-Side Air Leak

The manual valve workaround confirms air is entering the suction line when the pump stops. The foot valve holds water in the suction line, but an air leak allows water to drain back and air to enter.

Systematic Leak Detection

1. Re-do all threaded connections with fresh PTFE tape (3-4 wraps)
2. Check pump casing bolts and gaskets for tightness and integrity
3. Inspect suction pipe for cracks, especially at fittings and glued joints
4. Verify foot valve seating—debris can prevent full closure
5. Pressure test the suction line (isolate and pressurize to 10-15 PSI with air, check for bubbles with soapy water)

Even tiny leaks (pinhole in glued joint, loose clamp) destroy prime. Methodical inspection usually reveals the culprit.