Pump Cavitation: Symptoms, Causes and Prevention

Pump Cavitation: Symptoms, Causes and Prevention

Pump cavitation can cause unusual pump noise, unstable flow, excessive pump vibration and premature damage to impellers, bearings and mechanical seals. It often begins when the suction side of a pump cannot provide enough pressure for the liquid entering the impeller.

The problem is not always easy to identify. Cavitation symptoms can resemble bearing failure, misalignment, air leakage or a blocked suction line. This guide explains what pump cavitation means, how to diagnose it and what maintenance and design changes can help prevent cavitation in industrial pumping systems.

What Is Pump Cavitation?

Pump cavitation occurs when the local pressure of a liquid falls below its vapour pressure. Vapour bubbles form in the low-pressure area, usually near the impeller eye. As the liquid moves into a higher-pressure region, the bubbles collapse.

This repeated formation and collapse can create:

• Crackling or gravel-like pump noise
• Increased vibration
• Unstable flow and pressure
• Impeller pitting and erosion
• Mechanical seal leakage
• Reduced pump efficiency
• Premature bearing or shaft damage

Pump Cavitation Meaning in Simple Terms

The simplest pump cavitation meaning is that the pump is not receiving liquid at a sufficiently high pressure on its suction side.

It does not necessarily mean that the pump is out of liquid. A pump may still have liquid flowing through it while suffering from inadequate suction pressure because of high temperature, excessive pipe losses, a low tank level or an unsuitable operating point.

What Causes Cavitation in Pumps?

Cavitation in pumps is usually caused by insufficient suction pressure. Engineers assess this condition using NPSH, or Net Positive Suction Head.

Two values are important:

• NPSH available (NPSHₐ): the suction pressure provided by the system
• NPSH required (NPSHᵣ): the pressure margin required by the pump at a particular flow and speed

As a basic design principle:

NPSHa​>NPSHr​

A suitable margin should also be allowed for changes in flow, temperature, liquid level and system conditions. Operating too close to the minimum value can make the pump vulnerable to cavitation when the process changes.

Low Liquid Level

A low level in the suction tank reduces the static pressure available at the pump inlet. A pump may operate normally when the tank is full but begin to cavitate as the level falls.

Check:

• Minimum tank operating level
• Distance between the liquid surface and pump centreline
• Tank pressure
• Suction submergence
• Possible vortex formation

Excessive Suction-Pipe Losses

A suction pipe that is too small, too long or filled with restrictive fittings can create excessive pressure loss before the liquid reaches the pump.

Common causes include:

• Undersized suction piping
• Excessive elbows
• Blocked strainers
• Partially closed valves
• Poorly positioned reducers
• Internal deposits or obstruction

The pump nozzle size alone does not prove that the suction piping is adequate. The complete suction arrangement must be considered.

High Liquid Temperature

As liquid temperature increases, vapour pressure also increases. The liquid can therefore form vapour bubbles at a higher absolute pressure.

This is particularly relevant in:

• Hot-water systems
• Condensate transfer
• Boiler-feed applications
• Chemical processing
• Heated oil systems

If the process temperature has increased since the pump was selected, the original NPSH assessment may no longer be valid.

Air Entering the Suction System

Air entering through a loose flange, damaged gasket or poorly sealed connection can create symptoms similar to cavitation.

Air may also enter through:

• Vortexing in a tank
• Insufficient suction submergence
• Leaking valve stems
• Incorrect pipe routing
• Low liquid level

Air entrainment often produces irregular flow and noise. It should be investigated separately from vapour-related cavitation.

Operation Outside the Intended Range

A pump is selected for a particular range of flow and head. Operating far from that range can create internal recirculation, unstable hydraulic forces and increased NPSH requirements.

Problems may occur when:

• The pump is oversized for the current duty
• The discharge valve is used to force an unsuitable operating point
• Process demand changes frequently
• The impeller diameter is incorrect
• Pump speed has been increased
• Several pumps operate in parallel without proper control

A pump curve should be reviewed against the actual system operating point rather than relying only on the rated flow or motor power.

What Are the Main Cavitation Symptoms?

1. Crackling or Gravel-Like Pump Noise

The most familiar cavitation symptom is a harsh, irregular sound from the pump casing. Operators may describe it as:

• Gravel moving through the pump
• Crackling
• Rattling
• Popping
• Metallic knocking

Noise alone is not enough to confirm cavitation. Loose components, damaged bearings and air entering the suction line can produce similar sounds.

2. Increased Pump Vibration

Collapsing vapour bubbles create unstable hydraulic forces around the impeller. This can result in increased pump vibration at the casing, bearings, shaft and connected pipework.

Continued vibration may contribute to:

• Bearing wear
• Coupling damage
• Shaft deflection
• Mechanical seal leakage
• Loose fasteners
• Pipe-support problems

3. Fluctuating Flow and Pressure

A cavitating pump may fail to maintain a stable discharge pressure. Flow may fluctuate even when the motor speed remains constant.

This can affect:

• Chemical dosing
• Cooling-water circulation
• Filtration
• Process transfer
• Boiler systems
• Spray systems

4. Reduced Head or Flow

Cavitation can prevent the pump from achieving its expected performance. Increasing speed may appear to restore flow temporarily, but it can also increase suction losses and make the problem worse.

5. Impeller Pitting

When vapour bubbles collapse close to the impeller surface, they can create small pits. With continued operation, the damaged area may become rough and eroded.

Inspect the impeller for:

• Pitting around the suction eye
• Roughened metal surfaces
• Uneven erosion
• Deposits or blockage
• Hydraulic imbalance

6. Mechanical Seal Leakage

Cavitation does not always damage the seal directly. However, the vibration and shaft movement it creates can disturb the seal faces and increase leakage.

If a pump repeatedly experiences seal failure, do not inspect only the seal. Review the suction conditions, operating point, alignment and vibration history as well.

What Is the Difference Between Cavitation and Air Entrainment?

Cavitation occurs when the liquid’s local pressure falls below its vapour pressure. Air entrainment occurs when air or another gas enters the pump with the liquid.

Both conditions can cause:

• Pump noise
• Vibration
• Unstable flow
• Pressure fluctuations
• Loss of performance

The investigation should check both possibilities. A suction leak may not always show liquid leakage because air can enter when the pump is under negative pressure.

How to Diagnose a Cavitating Pump

A reliable investigation combines operating data, inspection and observation.

Step 1: Record the Operating Conditions

Record the conditions when the noise or vibration is strongest:

• Flow rate
• Suction pressure
• Discharge pressure
• Pump speed
• Liquid temperature
• Tank level
• Valve positions
• Vibration readings

A single reading may not reveal the cause. Record how the readings change as the flow, tank level or pump speed changes.

Step 2: Inspect the Suction Side

Start with the suction system because many cavitation problems occur before the liquid reaches the pump.

Check:

• Suction strainer condition
• Valve position
• Pipe blockage
• Flange and gasket joints
• Tank level
• Vortexing
• Pipe diameter and layout
• Reducer orientation

Do not adjust process valves without following the plant’s operating and safety procedures.

Step 3: Compare the Duty Point with the Pump Curve

Confirm the actual flow and head. Then compare them with the pump curve and the pump’s NPSH requirement at that flow.

The pump may be operating at a different point because of:

• System modifications
• Increased process demand
• A changed discharge route
• Incorrect valve settings
• Impeller replacement
• Speed changes

For related guidance, see MK FLOW’s article on how to read a pump curve.

Step 4: Check the Impeller and Internal Components

If the pump has operated with severe noise or vibration, inspect the impeller and casing during a planned shutdown.

Look for:

• Pitting
• Erosion
• Blockage
• Uneven wear
• Deposits
• Signs of solids damage

The damage pattern should be assessed alongside the operating history. Impeller erosion may also result from abrasive solids or corrosion, not only cavitation.

Step 5: Rule Out Mechanical Faults

The following problems can produce symptoms that resemble cavitation:

• Bearing damage
• Misalignment
• Coupling wear
• Shaft runout
• Pipe strain
• Loose foundation bolts
• Impeller imbalance

A pump may also have more than one problem at the same time.

How Can You Prevent Cavitation?

Select the Pump Using Complete Application Data

Pump selection should include more than flow rate. Provide:

• Required flow
• Total dynamic head
• Liquid name
• Liquid temperature
• Viscosity
• Vapour pressure, if available
• Solids concentration and size
• Suction arrangement
• Operating speed
• Duty cycle
• Expected process variation

This information helps determine whether a centrifugal pump, chemical process pump, slurry pump or positive-displacement pump is appropriate.

MK FLOW’s guide to industrial pump types and their applications provides a useful starting point when comparing pump technologies.

Reduce Suction-Side Pressure Loss

Where the system allows, consider:

• Increasing suction-pipe diameter
• Reducing unnecessary elbows
• Keeping the suction route short
• Cleaning strainers
• Removing avoidable restrictions
• Using suitable reducers
• Preventing air leakage

Any piping change should be checked against the pump manufacturer’s recommendations and the complete system design.

Maintain Adequate Tank Level and Submergence

Maintain the minimum liquid level required to prevent excessive pressure loss and vortexing. A vortex can draw air into the suction pipe even when the pump inlet remains submerged.

The required submergence depends on factors such as:

• Suction velocity
• Tank geometry
• Pump flow
• Liquid properties
• Distance from the tank wall and floor

Control Liquid Temperature

When process temperature rises, reassess the vapour pressure and available NPSH. This is especially important when:

• A new heated process is introduced
• Seasonal conditions change
• Condensate temperature varies
• A chemical concentration changes
• The pump is moved to a hotter service

Operate Near the Appropriate Duty Point

The best operating point is not determined by efficiency alone. The pump should operate within a stable range that suits the system’s flow and head requirements.

If the duty varies substantially, a solution may involve:

• Variable-speed control
• Multiple pumps
• A bypass arrangement
• Different impeller sizing
• A different pump model
• Revised control logic

The appropriate option depends on the pump curve and process conditions.

When Should You Contact a Pump Manufacturer?

Ask a pump manufacturer or application engineer to review the system when:

• Cavitation continues after suction checks
• The pump repeatedly damages impellers
• Flow or head requirements have changed
• Liquid temperature has increased
• The process fluid has changed
• Solids or gas have entered the fluid
• The pump has been relocated
• Suction piping has been modified
• Bearings or mechanical seals fail repeatedly

A replacement pump should not be selected only by matching the old model number. The original pump may have been selected for different conditions or may now be operating outside its intended range.

For more information, see MK FLOW’s resources on NPSH in centrifugal pumps and centrifugal pump applications.

Pump Cavitation Checklist

Use this checklist during an initial investigation:

Check	Why it matters
Is the pump making crackling or gravel-like noise?	A common warning sign, although not conclusive
Has vibration increased?	May indicate hydraulic instability or mechanical damage
Is the flow fluctuating?	Can indicate unstable suction conditions
Is the suction valve fully open?	A restriction can reduce NPSH available
Is the suction strainer clean?	Blockage increases suction losses
Is the tank level adequate?	Low level reduces static suction pressure
Is air entering the suction line?	Air entrainment can mimic cavitation
Has liquid temperature increased?	Higher temperature raises vapour pressure
Is the pump operating near its intended range?	Extreme operation can increase instability
Does the pump curve match the actual duty?	Confirms whether the selected pump suits the system
Does the impeller show pitting?	May indicate prolonged bubble collapse
Are alignment and bearings satisfactory?	Mechanical faults can create similar symptoms

Frequently Asked Questions

What is pump cavitation?

Pump cavitation is the formation and collapse of vapour bubbles inside a pump when local liquid pressure falls below vapour pressure.

What are the main pump cavitation symptoms?

The main symptoms are crackling pump noise, increased pump vibration, unstable flow, fluctuating discharge pressure, reduced performance and impeller pitting.

Can cavitation damage a centrifugal pump?

Yes. Continued cavitation can damage the impeller, casing, bearings, shaft and mechanical seal. It can also reduce pump efficiency and increase maintenance requirements.

Is pump noise always caused by cavitation?

No. Air entering the suction line, damaged bearings, misalignment, pipe vibration and loose components can produce similar sounds. Measurements and inspection are needed.

How can I prevent cavitation in pumps?

To prevent cavitation, maintain adequate NPSH, reduce suction-side losses, keep strainers clean, prevent air entry, maintain sufficient tank level and operate the pump within a suitable flow range.

Does reducing pump speed eliminate cavitation?

Reducing speed may reduce flow, suction losses and the pump’s NPSH requirement. However, it is not a universal solution. The pump curve and process duty should be reviewed before changing speed.

How MK FLOW Can Help

MK FLOW is an industrial pump manufacturer and supplier serving applications such as chemical processing, water treatment, slurry handling, high-pressure transfer and industrial fluid circulation.

Its product range includes:

• Centrifugal pumps
• Chemical process pumps
• Slurry pumps
• AODD pumps
• Twin-screw pumps
• Lobe pumps
• High-pressure pumps

When requesting pump-selection support, provide the flow, head, fluid, temperature, solids content, suction arrangement and operating schedule. These details allow the pump application to be assessed against the actual process conditions.

For broader pump maintenance guidance, visit MK FLOW’s pump maintenance tips and chemical pump maintenance resources.

For recognised industry guidance on pump systems, NPSH and pump application practices, consult the Hydraulic Institute, a leading professional organisation for pump and pumping-system information.

Conclusion

Pump cavitation is usually a warning that the pump is not receiving suitable suction conditions for stable operation. Low tank level, restricted suction piping, high liquid temperature, air entrainment, incorrect pump selection and operation outside the intended range can all contribute to the problem.

Start by checking the suction system and recording the actual operating data. Then compare the duty point with the pump curve, review NPSH and inspect the pump for hydraulic and mechanical damage. If the problem continues, contact an application-focused manufacturer before replacing the pump or its components.

For help selecting a pump for changing or demanding process conditions, contact MK FLOW, an industrial pump manufacturer and supplier. A correct application review is the most reliable way to prevent cavitation and improve long-term pump performance.