A new pump runs perfectly on the factory test bench. It arrives at your plant, gets piped in, and starts up. Within 10 minutes, it sounds like a bag of marbles rattling around inside the casing. Vibration spikes. Flow drops. The maintenance crew says "the pump is defective."

9 times out of 10, the pump is fine. The installation caused the cavitation. This guide walks through why new pumps cavitate after installation, and how to fix it.

What Is Cavitation, in Plain Terms

Liquid enters the pump suction at low pressure. If the pressure drops below the liquid's vapor pressure, the liquid boils — bubbles form inside the pump. These bubbles travel through the impeller to the high-pressure discharge side, where they collapse violently. Each collapse is a tiny explosion on the impeller surface.

After months of cavitation, the impeller looks like it has been hit by shotgun pellets — pitted and eroded.

The sound is unmistakable: gravel or marbles rattling inside the pump. If you hear this, the pump is cavitating. Stop it immediately — every minute of cavitation shortens pump life by hours.

Why a New Pump Cavitates Right After Installation

The pump passed the factory test. It has the performance curve you expected. Something about the site installation does not match the pump's NPSH requirement. Five reasons account for 90% of new-install cavitation.

1. Suction Lift Is Too High

The pump is installed above the liquid level. The pump must suck the fluid up from below. Every meter of suction lift reduces NPSHa by 1 m. If the pump needs NPSHr = 5 m and you have 3 m of suction lift, NPSHa is only 2 m. Cavitation starts immediately.

Fix: Lower the pump below the liquid level (flooded suction). If that is not possible, install a booster pump on the suction side. Or switch to a self-priming pump.

2. Suction Pipe Is Undersized

The suction pipe diameter is smaller than the pump suction flange. This increases friction loss in the suction line. High friction loss = low NPSHa at the pump.

Rule of thumb: suction pipe should be at least one size larger than the pump suction flange. For example, if the pump suction is 3", the suction pipe should be 4". This reduces velocity and friction loss.

Fix: Replace the suction pipe with a larger diameter. Add a reducer at the pump suction — eccentric reducer, flat side up, to prevent air pockets.

3. Suction Strainer Is Too Restrictive

The suction strainer has tiny holes — designed for clean water service. When pumping fluid with any solids, the strainer clogs quickly. A clogged strainer = high suction loss = cavitation.

Fix: Use a strainer with larger perforations (10 mm instead of 2 mm). Or install a larger suction screen. Clean the strainer regularly — add it to the maintenance checklist.

4. Air Leaks in Suction Piping

Air is sucked into the pump through flange gaskets, valve packing, or cracked pipe. The air mixes with the fluid, causing intermittent cavitation. The pump sounds fine at low flow, then rattles at high flow.

Fix: Pressurize the suction line with compressed air. Brush soapy water on all flanges and fittings. Bubbles = leak. Tighten flange bolts or replace gaskets.

5. Fluid Is Too Hot

NPSHr on the pump curve is published for water at 20°C. If you pump hot water (80°C+), the vapor pressure is much higher. NPSHa is effectively lower — hot water boils at lower pressure.

For example, water at 80°C has vapor pressure of 47 kPa. At sea level atmospheric pressure (101 kPa), the net positive suction head available is only ~5.5 m — even before accounting for suction losses. The pump may cavitate at 80°C water even though it worked fine with 20°C water.

Fix: Calculate NPSHa at actual fluid temperature. If NPSHa < NPSHr, increase suction static head (raise the tank), or reduce pump speed (VFD), or switch to a pump with lower NPSHr.

Diagnostic Steps: Find the Cavitation Source

When a new pump cavitates, work through these steps in order. The answer is usually in the first three.

Step 1: Measure actual flow vs. design flow. If the pump runs at 120% of rated flow, NPSHr is much higher than at BEP. The performance curve shows NPSHr rising sharply at high flow. The fix: throttle the discharge valve slightly to bring flow back toward BEP.

Step 2: Measure suction pressure (if gauge available). Convert suction pressure to NPSHa. Compare to NPSHr from the pump curve. If NPSHa < NPSHr, the suction system is the problem.

Step 3: Check fluid temperature. If the pump handles hot fluid, calculate NPSHa at actual temperature. Hot fluid vapor pressure eats into NPSHa.

Step 4: Inspect suction piping. Look for sharp elbows close to the pump (within 5 pipe diameters of suction flange). Look for high spots in the pipe where air can collect. Look for partially closed isolation valves.

Step 5: Check pump speed. If the pump is running faster than rated (wrong pulley size on belt drive, or VFD set too high), NPSHr increases. Confirm actual rpm with a tachometer.

Quick Fixes That Work

If you cannot redesign the suction system immediately, these temporary fixes buy you time:

  • Throttle the discharge valve. Reducing flow reduces NPSHr. The pump moves away from the cavitation zone. This wastes energy but stops the noise.
  • Slow down the pump (if VFD). Reducing speed lowers both flow and NPSHr. The pump may run smoothly at 80% speed even if it cavitates at 100%.
  • Pressurize the suction tank. Even 0.5 bar extra pressure on the suction side adds 5 m to NPSHa. This is a quick fix for hot fluid applications.

When to Accept It vs. Redesign

Minor cavitation (light rattling, no visible damage after 3 months) can be tolerated for low-demand pumps. The impeller will pit, but the pump still works. Plan for impeller replacement in 12–18 months.

Severe cavitation (loud rattling, visible pitting within weeks) means the pump is the wrong size for this installation. Do not run it like this — you will destroy the pump. Redesign the suction system or replace the pump with a lower-NPSHr model.

For NPSH calculation and cavitation diagnosis, the pump power calculator estimates NPSH requirements. For pumps with low-NPSH requirements, our multistage centrifugal pump range includes models optimized for difficult suction conditions. For cavitation troubleshooting, talk to our engineers with your pump model and suction conditions.