Pump cavitation happens when the liquid at the impeller inlet drops below its vapour pressure, forming tiny bubbles that collapse violently as they move into higher pressure. Left alone, it pits the impeller, wears bearings and seals, and can end in pump failure. The good news is that cavitation is usually caused by the system around the pump, and a few routine checks can prevent most of it.

What Is Actually Happening Inside the Pump?

As liquid enters the eye of the impeller, its pressure falls. If it falls far enough, the liquid flashes into vapour bubbles. Once those bubbles travel into the higher-pressure zone of the impeller, they implode against the metal surface.

One collapse is harmless. Thousands every second are not, and the repeated impacts erode the vane surfaces, often leaving them with a pitted, spongy appearance.

Why NPSH matters

Net positive suction head (NPSH) is how engineers describe the margin before this happens. NPSH available (NPSHa) is the pressure head at the pump suction minus the vapour pressure of the liquid. NPSH required (NPSHr) is the figure the pump manufacturer specifies to avoid cavitation. When NPSHa falls below NPSHr, cavitation begins. Allowing a healthy margin between the two gives the pump room to cope with real-world changes.

Two types worth knowing

Suction cavitation is the more common kind. It occurs when suction pressure is too low, and it damages the face of the impeller near the eye. Discharge cavitation occurs when the pump is forced to run against very high discharge pressure at low flow, so liquid circulates around the impeller at high velocity through tight clearances. It tends to wear the vane tips and the pump housing instead, and in severe cases it can contribute to shaft damage.

Common Causes of Pump Cavitation

     1. Suction-side problems

• Blocked strainers, filters or partly closed suction valves.
• Long, narrow or restrictive suction pipework with too many bends close to the pump.
• Low liquid level in the tank or sump, or a suction lift that is too deep.
  

  2. Process and operating conditions

• Liquid temperature higher than designed for, which raises vapour pressure and reduces the margin.
• Running at a flow rate well beyond the pump’s intended range, which increases the NPSH the pump needs.
• Very low flow with the discharge heavily restricted, which can cause discharge-side cavitation and damage to impeller vane tips and the casing.
  A cavitating pump is often blamed for a fault in the pump when the real cause is a change somewhere else in the system, such as a new pipe layout or a different duty.

  3. Warning Signs You Shouldn’t Ignore

• Noise that sounds like gravel or marbles rattling in the casing.
• Vibration that is new or has become noticeably worse.
• Reduced flow or pressure even though the pump is running normally.
• Premature bearing or seal failures on a pump that is otherwise well maintained.
• Pitted or eroded impeller vanes found during an inspection.
  Air getting into the suction line can sound similar, so it is worth checking for leaks at joints and shaft seals before assuming the cause. The two problems are fixed in different ways.

Pump Preventative Maintenance Checklist

Regular pump preventative maintenance will not change the physics of cavitation, but it catches the conditions that cause it. A practical routine includes:

1. Check suction conditions. Clean strainers, keep suction valves fully open and confirm the tank level is above the minimum.
2. Monitor temperature. Confirm the liquid is within the range the system was designed for.
3. Compare duty with design. If the pump is now moving a different flow than it was selected for, review whether it is still the right fit.
4. Listen and measure. Record vibration, noise, pressure and flow so changes show up early.
5. Inspect the impeller and wear parts. Look for pitting, erosion, clearance changes and damaged seals during planned shutdowns.
6. Keep the drive healthy. Lubrication, alignment and rotating balance all affect bearing life when a pump is already under stress.

What to do if you suspect cavitation

Common corrective steps include raising suction pressure where possible, cooling the liquid, reducing the flow rate, venting trapped gas and simplifying suction piping so that elbows and fittings are not crowded against the pump inlet. If damage has already occurred, the impeller will need repair or replacement, and the cause must be fixed or the damage will return.

It is also worth remembering that damage compounds. An eroded impeller changes the flow path and can upset rotating balance, which adds vibration to bearings that are already taking a beating. Treating the cause early protects the whole pump, not only the impeller.

How MNP Electrical Can Help

MNP Electrical provides industrial pump repair for centrifugal, positive displacement and submersible pumps, including on-site repairs, overhauls and preventative maintenance programs covering inspection, testing, lubrication and alignment checks. Where an impeller is involved, it can also be checked for balance through our dynamic balancing service.

Hear a Pump Rattling? Get It Looked At

A pump that sounds like it is pumping gravel is telling you something. Send us the details through our contact page and our team will help you work out whether it is cavitation, wear or something else, and what to do next.