Pump troubleshooting: most pump faults are system faults
Pump troubleshooting starts with the system, not the pump.
Most of what presents as a pump fault began somewhere else: a starved suction, a closed valve, a duty the pump was never sized for. Work from the symptom to the mechanism and the fix is usually obvious. Replace it without the mechanism and the new pump fails the same way.
Start with the system, not the pump
The pump is the component that complains, so it gets the blame. But a pump is a simple machine sitting in a complicated system, and if you don't understand the system the pump sits in, that is where the trouble starts. Before touching the pump, work through what surrounds it.
Ask what changed
A pump that ran for years and now doesn't was changed by something. A new medium, a new duty, maintenance elsewhere on the line, a colder week. Find the change and you have usually found the fault.
Walk the valves
Confirm every isolation valve on the suction and discharge is fully open. Somebody shuts a valve for maintenance and forgets it is shut, the pump runs against it, and the result reads as a pump failure. It happens on well-run sites, and it is the first thing to eliminate because it costs nothing to check.
Check the suction conditions
Is there liquid at the level the pump needs? A fallen tank level, a blocked strainer or an air leak on a suction joint starves the pump, and a starved pump presents every symptom in this guide at once.
Read the gauges
Discharge pressure and motor current, against the commissioning baseline if one exists. A reading tells you which half of the system to suspect; a listen and a guess does not.
The symptom points at the mechanism
Each symptom has a short list of usual causes, and the list is ordered. Start with the cheapest check.
| Symptom | First suspect | Start at |
|---|---|---|
| No flow | Lost prime, air in the suction line, a closed valve | The suction side |
| Low flow | Blocked line or strainer, reverse rotation, a changed duty | The readings, against the curve |
| Noise like gravel | Cavitation | The NPSH margin |
| Vibration without the gravel sound | Misalignment, imbalance, worn bearings, pipe strain | The mechanical checks |
| Seal leaking again | Dry running, or a seal not specified for the medium | The suction conditions |
| Blocks repeatedly | Rag the pump was never specified for | Upstream of the pump |
The sections below take each in turn.
No flow, or not enough
A pump delivering nothing has usually lost its prime or its suction. Check for air in the casing, a leaking foot valve, an empty suction line, or an air leak pulled in through a suction-side joint or a worn seal. A centrifugal pump full of air pumps nothing and gives little outward sign of it.
A three-phase motor that has been disturbed can be running backwards. Reverse rotation on a centrifugal pump gives weak, not zero, flow, which is why it survives commissioning when nobody checks the shaft against the arrow.
Low flow with the pump otherwise healthy usually means the system curve has moved: a partly blocked line, a fouled strainer, or added pipework the pump was never sized for. Compare the readings against the performance curve before condemning the pump. If the duty has genuinely changed, the answer is re-selection, not a bigger impeller on hope.
Noise and vibration
A healthy pump is quiet and stays quiet. Noise that arrives with a change in duty is nearly always cavitation: the suction can no longer supply the pump, vapour bubbles form at the impeller eye and collapse against the metal, and it sounds like gravel passing through. Left alone it pits the impeller and shakes seals and bearings loose. The fix is on the suction side, and we cover it in how to prevent cavitation.
Vibration without the gravel sound points at mechanics: coupling misalignment, a bent shaft, worn bearings, or an impeller running out of balance because rag or a solid is caught in it. Pipe strain does the same job more slowly, loading the casing until bearings and seals give up.
Banging in the pipework when the pump stops or a valve closes is water hammer, and it is a pipework problem rather than a pump problem. It still breaks pumps, non-return valves and joints, so treat it as a fault, not a character trait of the system.
Seal leaks and dry running
The weak point in almost any pump is the seal, so the seal is where trouble shows first. A mechanical seal needs liquid to lubricate its faces. Take the liquid away and the seal goes; that is dry running, and it is the third of the three things that kill most pumps in service.
A seal that fails once may just be worn out. A seal that fails repeatedly is reporting a system problem: dry running from poor suction conditions, cavitation, pipe strain, or a medium the seal faces and elastomers were never specified for. Fit the same seal again without an answer and you have bought the next failure.
How bad dry running is depends on the pump type. On centrifugal and rotary lobe pumps the usual cost is the seal. On a progressive cavity pump it is catastrophic: the rubber stator runs on the rotor without lubrication, burns through, and takes the rotor's surface with it. If a pump can be starved, protect it with a float switch, pressure switch or dry-run relay rather than relying on nobody ever making a mistake.
Blockages and ragging
On wastewater duties, three causes account for most failures: rag, grit and dry running, in that order. Rag is wipes and fibre binding up the impeller, and a pump that blocks on rag repeatedly will not be cured by a like-for-like replacement. Either the pump needs a genuinely solids-handling wet end, or the rag needs dealing with upstream by a macerator or chopper pump before it reaches the pump at all. We have talked customers out of new pumps on exactly this fault; the right answer was rag removal, not a better pump.
Grit is slower but just as certain. If the grit removal on a works is not doing its job, the grit passes to the pumps, abrades the wet end and brings wear forward by years. Early, repeated impeller and wear-ring replacement across several pumps points at grit, not at the pumps.
When to repair, when to replace, when to call
Repair or replace is an economic question, but only once the fault is understood. Age, parts availability and energy use settle the economics; the mechanism settles everything else. If the cause was in the system, fix the system first: you probably don't need a new pump. And on an old, oversized pump, the failure is often the opportunity to fit the right size instead.
If the fault will not diagnose, send us the symptoms and the readings, plus the model and serial number from the nameplate. The serial number is what lets us or the manufacturer look up the exact build and quote the right service kit, and a photograph of the nameplate is the fastest way to capture it. We will tell you what the fault mechanism is likely to be, and whether it needs a repair, a different pump, or a change to the system around it.