Why does a marine centrifugal pump lose suction or stop pumping?

A centrifugal pump that stops delivering has almost always lost its prime or is swallowing air, because an impeller full of air simply churns without generating pressure. The gauges tell you a lot before you touch anything: fluttering or falling discharge pressure with high vacuum on the suction gauge points to a starved or blocked suction; low vacuum and no delivery points to air ingress or an empty source; a noise like gravel in the casing means cavitation.

Work through the causes in groups:

• Air getting in: a leaking suction flange joint, a scored shaft sleeve under a packed gland, a gland that has lost its sealing water, or a suction-side valve spindle drawing air. Any of these breaks the prime. Air also collects at the top of the casing when pumping from a nearly empty bilge.
• Suction starved: a choked strainer or mud box (the commonest cause of all, especially on bilge suctions), a partly closed or jammed suction valve, a blocked or collapsed line, or the tank, well or sea chest running dry. In ice or heavy weed, sea suctions choke too.
• Cavitation: suction lift too high, liquid too hot, or flow too fast for the line, so pressure at the impeller eye falls below the vapour pressure and the liquid boils. The pump rattles and vibrates, delivery falls away and the impeller is slowly eaten.
• Mechanical wear: excessive wear-ring clearance recirculating flow internally, a worn or damaged impeller, or reversed rotation after motor work, which gives a fraction of rated capacity.

Restoring suction is systematic: stop the pump, confirm the source has liquid and every suction valve is fully open, clean the strainer, refill and vent the casing, check gland sealing water, and restart with the air cock cracked until solid liquid appears. If it still will not hold prime, look for air leaks on the suction line; spraying water over joints while it runs under vacuum often finds them. For recurring cavitation, reduce the lift, cool the liquid, open up the suction line or slow the pump.

Log chronic cases and escalate them. A bilge pump that keeps losing suction on one well may be telling you about a blocked strum box or a fractured line under the tank top, and finding that out matters far more than re-priming yet again.