Why a Pipeline Pump Will Not Pump: Air Binding and Priming

What is air binding?

A centrifugal pump moves water by spinning it. Air is around 800 times lighter than water, so if the casing is full of air the impeller spins it without creating enough pressure difference to draw water in. The pump runs, warms up and delivers nothing.

How do I prime the pump?

1. With a flooded suction, open the suction valve and the vent on the pump until water comes out without bubbles.
2. With a suction lift, fit a foot valve with a strainer on the suction pipe and fill the pipe and casing with water before starting.
3. Close the vent, then start the pump with the discharge valve partly open.

Why does it lose prime later?

• Air leaking in at a suction joint or a worn seal
• A foot valve that leaks back when the pump stops
• The water level falling until a vortex draws air into the suction

Why should it not run dry?

The mechanical seal relies on the liquid to cool and lubricate its faces. Running without water can damage it in a short time.