Daily care and cavitation prevention for centrifugal pumps

Prime and vent the pump properly. With air in the casing the impeller cannot build pressure, and a mechanical seal running dry can burn out in tens of seconds. Even with a self-priming device, confirm the liquid level is sufficient.

Jog the pump to confirm rotation direction. A reversed pump delivers little flow, vibrates heavily, and can loosen the impeller nut. On newly installed or overhauled pumps, always jog to check direction before the first start.

For centrifugal pumps, start against a closed discharge valve, then open slowly to the duty point; starting under load slams the motor current. Positive-displacement pumps (screw, gear) are the opposite — never start them against a closed valve.

Long-running at low flow is a pump's worst enemy. Below 30% of rated flow, liquid recirculates inside the pump and heats up, and both cavitation and vibration worsen. If the process really needs low flow, add a recirculation line or a VFD — do not throttle it down with the discharge valve.

Cavitation has a classic sound: like gravel running through the pump, noisy, with discharge pressure fluctuating and current swinging. When you hear it, check the suction side first — level high enough, suction pipe clear, medium not too hot.

Mechanical seal leakage has a standard: up to 3–5 drops per minute is normal for general duty. A sudden increase, or leakage running in a stream, means the seal faces are already damaged — plan the replacement, do not wait for the seal to fail completely.

Log bearing temperature and vibration monthly and compare with history. The two bearing housings of a pump should not differ by more than 10 °C.

On oil-lubricated pumps keep the level at the centre line of the sight glass — never full. Overfilled oil churns, heats up, and gets squeezed out at the seals. Change the oil after the first 200 hours on a new pump, then on interval (typically 2000–4000 hours depending on duty).

On grease-lubricated pumps fill to the manufacturer's quantity — usually one third to one half of the bearing cavity. Filling it solid is just as wrong.

For seals with cooling or flush water (Plan 11/21/23/52 and the like), confirm the flush lines are clear and the flow is normal. Lose the flush and seal life collapses from thousands of hours to tens.

Break down the lifetime repair cost of a centrifugal pump and a counterintuitive distribution appears: the "big items" - casing and impeller - rarely fail, while the money is consumed by small items: mechanical seals, bearings and couplings. And eighty percent of those failures come from installation quality and deviation from design duty, not from poor component quality. That distribution decides where maintenance resources should go: watching the running conditions of seals and bearings is far more worthwhile than watching the casing.

The three great triggers of mechanical seal failure are dry running, cavitation and axial float. Dry running mostly happens when the pump is not primed before start or the supply is cut off; cavitation shock waves shatter the seal faces directly; axial float comes from impeller unbalance or bearing wear. All three give warning: a sudden increase in seal leakage, cracking noises and vibration at the casing, or fluctuating current. Put these three into daily inspection and seal life can be extended several times over.

Bearing failure triggers concentrate on lubrication and alignment. On lubrication, oil level too high is as harmful as too low - too high causes churning heat and oil emulsification, too low leaves an inadequate oil film. On alignment, misalignment between pump and motor loads extra radial force onto the bearings, which is the common reason newly replaced bearings fail within months. An alignment check costs half an hour of labor; a bearing costs thousands of yuan plus downtime. Every pump room should do that arithmetic clearly.

The design principle for an inspection card is "completable in ten minutes, every item judgeable". We suggest four groups with three to four items each: running parameters, leakage condition, vibration and temperature, and lubrication condition. Running parameters: current, suction and discharge pressure, and flow, compared against design values. Leakage: drip rate at the seal - a mechanical seal allows slight weeping, but a streaming flow is abnormal. Vibration and temperature: by hand plus simple instruments, at comparable points. Lubrication: oil level, oil color, and any sign of emulsification.

Inspection frequency follows pump criticality: critical pumps in continuous service, once per shift; standby and intermittent pumps, once a week; pumps out of service long-term, once a month for appearance and turning. Turning is easily overlooked - rotating an idle pump by hand a few turns each month prevents bearing brinelling and seal sticking, and it is a zero-cost maintenance action.

Be clear about where inspection records go: anomalies reported within the shift, data rolled up monthly into a trend sheet, and the trend sheet reviewed at the equipment meeting. An inspection card that is filled in but never read does not survive three months - true for every equipment class, and especially visible in pump rooms, because pumps degrade fast and once the data gap widens the trend disappears.