Why has my centrifugal pump suddenly become noisy?
A centrifugal pump that sounds like it is pumping gravel is commonly cavitating. Cavitation means the pressure at the impeller inlet has dropped far enough for vapour bubbles to form and then collapse. Air in the suction line and operation far from the best efficiency point (BEP) can sound similar. Bearing faults and looseness are also easy to confuse by ear.
Noise alone cannot confirm the cause. Compare the sound with normal operation, then check flow, suction pressure, discharge pressure, motor current and vibration.
This article covers centrifugal pumps. Other pump types develop abnormal noises too, but the causes and checks are different.
What different pump noises can mean
If you work around a pump every day, you know what it normally sounds like. A new rattle, crackle, knock or whine means something has changed in the pump or the system. It does not mean the pump is about to fail, but it is worth investigating.
Use the sound to decide where to start looking, then confirm the cause with operating data and inspection. The table lists the most common and easiest checks first.
| What you hear | Possible cause | What to check |
|---|---|---|
| Gravel, crackling or rattling | Cavitation | Tank level, suction valves, strainers, suction pressure, fluid temperature, flow rate and NPSH |
| Intermittent crackling, surging or irregular hydraulic noise | Air entering the pump | Low tank level, vortexing at the suction inlet, leaking suction connections and gas in the process liquid |
| Knocking or clattering | Loose parts or coupling problems | Coupling, mounting bolts, baseplate, pipe supports and internal components |
| Rumbling or unstable noise at low flow | Internal recirculation, or operation far from the duty point | Actual flow, pump curve, control valve position and minimum flow requirement |
| Grinding or rough mechanical noise | Bearing damage or internal contact | Bearings, lubrication, shaft condition, clearances and rotating parts |
| High-pitched whine | Bearing, motor or hydraulic issue | Bearings, motor, speed, operating point and vibration |
| New noise with increased vibration | Mechanical or hydraulic fault | Alignment, bearings, cavitation, operating point, pipe strain and foundations |
Several faults produce similar sounds, and noise travels through pipework and structures from other equipment. Treat the table as a starting point only.
For help separating hydraulic and mechanical causes, see our guide to centrifugal pump vibration.
Why a centrifugal pump sounds like it is pumping gravel
Cavitation occurs when the local pressure in the liquid falls low enough for vapour cavities to form. When those cavities move into a higher-pressure region, they collapse. This produces the crackling or rattling sound. Under damaging conditions, it also causes erosion, vibration and loss of performance.
Look at the suction conditions before opening the pump. Check for:
- Low liquid level
- Excessive suction lift
- Blocked strainers or filters
- Partially closed suction valves
- Long or undersized suction pipework
- High suction pipe velocity
- Liquid temperature higher than normal
- Increased pump speed or flow
- A change in the pumped liquid
For prevention steps, see our guide on how to prevent pump cavitation.
NPSHa and NPSHr
Two terms matter when you investigate suction-side cavitation.
Net positive suction head available (NPSHa) is the pressure margin above the liquid’s vapour pressure at the pump inlet. The system sets it. Liquid level, suction vessel pressure, atmospheric pressure, fluid temperature and suction pipe losses all affect it.
Net positive suction head required (NPSHr) is a characteristic of the pump. The manufacturer publishes it on the pump curve, which plots the head the pump produces across its flow range. NPSHr is normally quoted at the point where pump head drops by 3 per cent. NPSHr generally rises as flow increases, so a higher flow can trigger cavitation in a pump that was fine before.
Both are expressed in metres of liquid. Cavitation risk rises as NPSHa falls toward NPSHr, so NPSHa must exceed NPSHr with a margin. Cavitation can begin before the 3 per cent head drop is reached. For more detail, see Net Positive Suction Head (NPSH).
Air in the suction can sound like cavitation
Air entering the pump can cause noise, unstable flow and vibration that people mistake for cavitation.
Air gets in through a vortex at a poorly submerged suction inlet, through leaking suction-side connections, or from gas already carried in the process liquid. Suction-side connections under vacuum are a common entry point.
Watch the suction vessel while the pump runs. Check whether a vortex forms as the level falls, whether the operating level has changed, and whether any suction fittings could draw in air.
Low flow and internal recirculation
Internal recirculation is a condition where some of the liquid flows backwards at the impeller inlet or outlet instead of moving through to the discharge. It becomes more likely when a centrifugal pump runs well below the flow it was designed for. It causes pressure pulsations, noise and vibration even when suction pressure is adequate.
If the noise started after someone throttled a valve, changed a variable frequency drive (VFD) setting or altered the process flow, check where the pump now sits on its curve. Compare that point with the best efficiency point (BEP), the flow at which the pump runs most efficiently. The pump may be mechanically sound but working in a region where the hydraulics are unstable.
For more on pump curves and BEP, see our centrifugal pump guide.
What to check before pulling the pump apart
Establish what changed before assuming the pump needs a rebuild.
- Compare current readings with the normal baseline. Check suction pressure, discharge pressure, flow rate, motor current, vibration, bearing temperature and pump speed where you can measure them.
- Walk the system. Check valve positions, tank levels, strainers, filters and suction pipework.
- Ask operators whether the process, VFD settings or pipework have changed recently.
A pump becomes noisier when its operating point moves, even if nothing inside it has failed. Operation away from the design region increases flow separation, vibration and noise.
For routine inspections, record sound alongside vibration, bearing temperature, pressures, flow and motor current, and trend the readings over time. A simple note such as “pump noticeably louder than last inspection” gives maintenance something to compare against. Noting what else changed when the sound did often points straight to the cause.
When to stop and isolate the pump
Do not keep running a pump to see whether an abnormal noise gets worse. Follow your site’s operating and safety procedures.
Stop and isolate the pump if the noise is severe or worsening quickly, or if it comes with any of these:
- Sudden or excessive vibration
- Rapidly rising bearing temperature
- Smoke or a burning smell
- Significant leakage
- Loss of flow or pressure
- Motor overload or repeated trips
- Suspected internal contact
- Loose rotating parts or coupling components
Before you inspect or open the pump, isolate and lock out the energy sources, depressurise the system and make the pumped fluid safe. Follow your site procedures and the manufacturer’s instructions. Never remove guards, loosen pipework or attempt an internal inspection while the pump is running or pressurised.
For routine checks, see the pump start-up and shut-down checklist.
Key takeaways
- A change in centrifugal pump noise tells you to investigate. It does not identify the cause.
- A gravel-like or crackling sound is commonly cavitation, but confirm it by checking the system.
- Air entering the suction and internal recirculation at low flow can produce similar noise.
- Cavitation risk rises as NPSHa approaches NPSHr, and NPSHr increases with flow.
- Check flow, pressures, vibration, temperature and operating point before assuming the pump has failed.
- Severe noise with vibration, heat, leakage or loss of performance justifies stopping and isolating the pump.
