Understanding Pump Cavitation: Causes, Symptoms, and Solutions

If your pump sounds like it is pumping gravel — a rattling, crackling noise that comes and goes — you are almost certainly experiencing cavitation. Left unchecked, cavitation will destroy an impeller in weeks, damage mechanical seals, and eventually wreck the pump casing.

Understanding what cavitation is, why it happens, and how to stop it is essential knowledge for anyone responsible for process pumps.

Cavitation occurs when the local pressure in the pump falls below the vapour pressure of the liquid. At that point, the liquid flashes to vapour, forming tiny bubbles. When these bubbles travel into a higher-pressure region — typically at the impeller vanes — they collapse violently. Each collapse releases a micro-jet of liquid at extremely high velocity, which hammers the metal surface.

The result is a characteristic pitting pattern on the impeller suction face — small, rough craters that look like the surface of a golf ball. In severe cases, entire vane sections are eroded away within a few months of operation.

Net Positive Suction Head (NPSH) is the key parameter governing cavitation risk.

NPSHr (Required) is a property of the pump — the minimum suction head the pump needs to avoid cavitation. It is published on the pump's performance curve and increases with flow rate.

NPSHa (Available) is a property of the installation — the actual suction head available at the pump inlet, calculated from the system geometry and fluid properties.

The rule: NPSHa must always exceed NPSHr by a safety margin of at least 0.5–1.0 m. When NPSHa drops below NPSHr, cavitation begins.

NPSHa = (Absolute pressure at suction source) + (Static suction head) − (Friction losses in suction line) − (Vapour pressure of liquid)

Each term matters:

• Suction source pressure — a pump drawing from a vacuum vessel has much less NPSHa than one drawing from an atmospheric tank
• Static suction head — a pump located below the liquid level has positive suction head; one located above has negative suction head (suction lift)
• Friction losses — long suction lines, small pipe diameters, and excessive fittings all reduce NPSHa
• Vapour pressure — hot liquids have high vapour pressure, which dramatically reduces NPSHa

Locating the pump too high above the suction vessel is the most common cause of cavitation in field installations. Every metre of suction lift reduces NPSHa by one metre. For hot liquids or volatile chemicals, even modest suction lifts can cause problems.

Solution: Lower the pump, raise the suction vessel, or increase the suction pipe diameter.

A suction pipe that is too small creates high velocity and high friction losses, both of which reduce NPSHa. The suction pipe should always be at least one size larger than the pump suction nozzle.

Solution: Increase suction pipe diameter. Eliminate unnecessary bends, valves, and fittings in the suction line.

A throttled suction valve is a common cause of cavitation — and an entirely avoidable one. The suction valve should always be fully open during pump operation.

Solution: Open the suction valve fully. If flow control is required, throttle the discharge valve, not the suction valve.

Running a pump at flow rates significantly above its best efficiency point increases the velocity at the impeller eye, reducing local pressure and triggering cavitation even when NPSHa appears adequate.

Solution: Reduce flow rate by partially closing the discharge valve, or select a larger pump with a BEP closer to the required flow.

Liquids near their boiling point have high vapour pressure, which consumes most of the available NPSH margin. Chemical plants handling solvents, acids at elevated temperature, or liquids near their flash point are particularly vulnerable.

Solution: Increase system pressure, lower liquid temperature, or select a pump with a lower NPSHr (typically a double-suction or low-speed design).

Sound: A rattling, crackling, or gravel-like noise from the pump casing. The sound may be intermittent, varying with flow rate or suction conditions.

Vibration: Increased vibration levels, particularly at the pump casing and bearing housing.

Performance: Fluctuating flow rate and pressure. The pump may appear to "hunt" — flow rises and falls without any change in the system.

Inspection findings: Pitting on the impeller suction face, erosion at the leading edge of the vanes, and roughening of the casing volute near the impeller eye.

When cavitation is confirmed, work through this checklist in order:

1. Check the suction valve — is it fully open?
2. Check the suction strainer — is it blocked?
3. Measure suction pressure — compare against the calculated NPSHa
4. Check liquid temperature — has it increased since commissioning?
5. Check flow rate — is the pump operating to the right of BEP?
6. Inspect the suction piping — any air leaks, vortex formation, or undersized sections?

If the root cause cannot be resolved through operational changes, the engineering options are:

• Increase suction pipe diameter to reduce friction losses
• Lower the pump relative to the suction vessel
• Install an inducer — a low-NPSHr pre-impeller that boosts suction pressure
• Select a pump with lower NPSHr — typically a slower-speed or larger-eye impeller design

The best time to address cavitation risk is at the design stage, before the pump is installed. A proper NPSH calculation takes less than an hour and can prevent years of maintenance headaches.

If you are experiencing cavitation in an existing installation, our engineers can review your system and recommend the most cost-effective solution. Contact us at [email protected].