A centrifugal pump can look completely normal from the outside while destructive conditions are developing inside the casing.
Then the sound changes.
The pump begins to rattle.
Flow becomes unstable.
Discharge pressure falls.
Vibration increases.
Operators sometimes describe the noise as gravel moving through the pump.
One possible cause is cavitation.
Cavitation happens when pressure inside the pump falls low enough for part of the liquid to vaporize. Tiny vapor bubbles form, travel into a region of higher pressure, and collapse.
Those collapsing bubbles can damage the impeller, seals, bearings, and other pump components.
The important point is that cavitation is often not really a pump problem.
It is frequently a suction system problem.
A low tank level, dirty strainer, restricted suction valve, hot liquid, undersized piping, excessive pump speed, or poor piping layout can create conditions that cause a perfectly good pump to cavitate.
That is why replacing the pump without understanding the process can lead to the same failure again.
What Is Cavitation in a Centrifugal Pump?
Cavitation occurs when local liquid pressure falls close enough to the liquid vapor pressure that vapor bubbles begin forming.
A centrifugal pump creates a low pressure region near the center of the impeller.
Liquid enters this area through the suction side.
Under normal conditions, enough pressure remains to keep the fluid liquid.
If suction conditions become poor, pressure near the impeller can fall too far.
The liquid begins vaporizing.
Those vapor bubbles are then carried outward through the impeller where pressure rises.
They collapse rapidly.
That repeated formation and collapse is cavitation.
Why Do the Vapor Bubbles Collapse?
The bubbles form because local pressure has fallen sufficiently.
As the impeller moves the fluid outward, pressure increases.
The vapor inside the bubble can no longer remain in that form.
The surrounding liquid rushes inward.
The bubble collapses violently.
Each individual event is tiny, but thousands of repeated collapses near a metal surface can cause serious damage.
Over time, the affected metal can develop a rough, pitted appearance.
Why Does Cavitation Sound Like Gravel?
The repeated collapse of vapor bubbles creates pressure pulses and vibration.
To someone standing near the pump, the resulting sound can resemble gravel, marbles, or small rocks passing through the casing.
That sound is a useful warning.
However, operators should not diagnose cavitation based on sound alone.
Air entering the suction, bearing damage, loose components, mechanical contact, and other problems can also create unusual noise.
Use the sound as a clue, then check the process conditions.
What Is Vapor Pressure?
Vapor pressure describes how easily a liquid tends to vaporize at a particular temperature.
As liquid temperature increases, vapor pressure normally increases.
This matters because a hotter liquid can begin forming vapor at a higher pressure than the same liquid when cold.
Consider a pump that operates comfortably with cool water.
Increase the water temperature significantly without changing anything else.
The available margin before vaporization is now smaller.
A suction system that worked well when cold can begin cavitating when hot.
Why Does Low Suction Pressure Cause Cavitation?
The pump needs enough pressure at its inlet to keep the liquid from vaporizing as it enters the low pressure region of the impeller.
If suction pressure falls, the margin above vapor pressure becomes smaller.
Eventually the local pressure becomes low enough for bubbles to form.
This is why operators investigating cavitation should look upstream of the pump early in the troubleshooting process.
The question is not only whether the pump is running.
The better question is whether the pump is receiving liquid under acceptable suction conditions.
What Is NPSH?
NPSH means net positive suction head.
It is a way of describing the pressure energy available at the pump suction above the liquid vapor pressure.
Two terms matter.
NPSH available describes what the actual piping system provides.
NPSH required describes what the pump needs at a particular operating condition.
The pump manufacturer determines the required value through testing and provides it with pump performance information.
Reliable operation requires enough available suction head above the pump requirement, with suitable operating margin.
What Is NPSH Available?
NPSH available depends on the system.
Important factors include:
Pressure above the liquid source
Liquid level above or below the pump
Friction through suction piping
Liquid temperature
Suction line size
Strainer restriction
Valve restriction
Fluid properties
Atmospheric pressure when an open tank is involved
Anything that lowers suction pressure or raises vapor pressure can reduce the available margin.
What Is NPSH Required?
NPSH required is a characteristic of the pump.
It changes with operating flow.
A pump does not have one universal required NPSH value for every condition.
As flow changes, the suction requirements can also change.
This is one reason the pump curve matters.
A pump selected for one operating point may behave poorly if the process later forces it to operate far from that condition.
Is NPSH Required the Exact Point Where Damage Begins?
Not necessarily.
The published required value is based on a defined pump testing method.
Cavitation can begin developing before a dramatic loss in pump performance becomes obvious.
Critical services therefore need appropriate margin rather than operating exactly at the published limit.
A pump that technically still produces pressure can already be experiencing harmful cavitation.
What Causes Pump Cavitation?
Several common problems can reduce suction pressure or increase the tendency of the liquid to vaporize.
The most common include:
Low liquid level in the suction vessel
Plugged suction strainers
Partially closed suction valves
Undersized suction piping
Long suction piping
Excessive flow
High liquid temperature
Excessive pump speed
Poor suction piping layout
High suction lift
Unexpected fluid composition
Air or gas entering the suction
Several of these conditions can occur at the same time.
How Does Low Tank Level Cause Cavitation?
Imagine a pump taking suction from a tank.
When the tank is full, the liquid above the pump creates additional static pressure at the suction.
As the tank level falls, that pressure decreases.
If the system was already close to its suction limit, a low tank level can push the pump into cavitation.
This is why operators may notice a pump runs normally with a full vessel but becomes noisy near the bottom of the operating range.
The pump did not suddenly change.
The suction conditions did.
Can a Tank Level Instrument Cause Confusion?
Yes.
An operator may see a normal tank level on the control system while the actual tank level is much lower because the instrument is stuck or inaccurate.
If a pump begins cavitating unexpectedly, physically verifying important process conditions can be worthwhile when safe to do so.
Do not automatically assume every displayed value is correct.
Instrumentation is part of the troubleshooting process too.
How Does a Plugged Suction Strainer Cause Cavitation?
A strainer creates some normal pressure loss.
As debris accumulates, that pressure loss increases.
Pressure upstream of the strainer may still look healthy while pressure at the pump suction becomes too low.
The pump then cavitates.
A differential pressure measurement across the strainer can help identify the restriction.
This is one of the simplest causes to check and one of the easiest to overlook.
What Can Plug a Suction Strainer?
Almost anything carried into the suction piping can contribute.
Possible examples include:
Rust
Scale
Sand
Sludge
Gasket material
Tank debris
Maintenance debris
Wax
Heavy deposits
The pump does not care why the strainer is restricted.
It only experiences the reduced pressure available at its inlet.
Can Closing the Suction Valve Cause Cavitation?
Yes.
Restricting the suction valve creates additional pressure loss before the pump.
That reduces suction pressure.
For a centrifugal pump, throttling the suction is generally a poor way to control flow unless the system has specifically been designed for that purpose.
If flow needs to be reduced, the discharge side is often the more appropriate control location for a conventional centrifugal pump.
Operating procedures and equipment design still take priority.
Why Is Throttling the Discharge Different?
Closing the discharge valve somewhat increases the resistance the pump works against.
The flow rate usually decreases.
Lower flow through the suction piping can reduce friction losses there.
This can improve suction conditions.
That does not mean every centrifugal pump should simply be operated with the discharge valve nearly closed.
Very low flow has its own risks.
The point is that suction throttling and discharge throttling affect the pump very differently.
Can Opening the Discharge Too Far Cause Cavitation?
It can contribute.
If system resistance becomes very low, the pump may move much more liquid.
Higher flow creates greater friction losses through the suction piping.
The pump may also require more NPSH at the higher flow rate.
A pump that has plenty of suction margin at moderate flow can lose that margin at excessive flow.
This is sometimes called running too far out on the pump curve.
What Is the Pump Curve?
A pump curve shows how a centrifugal pump is expected to perform under different flow conditions.
It commonly includes information about:
Flow
Developed head
Efficiency
Power
Required NPSH
The curve helps engineers determine whether the pump is operating near its intended region.
If actual pressure and flow are far from the expected curve, something deserves investigation.
Possible causes include the wrong pump speed, wrong impeller, worn internals, incorrect measurements, process restrictions, gas handling, or cavitation.
What Is the Best Efficiency Point?
The best efficiency point is the operating region where the pump converts mechanical energy into fluid movement most efficiently.
A centrifugal pump is generally designed to operate reasonably close to this region.
Operating far away from it can increase hydraulic forces, vibration, recirculation, and equipment wear.
The best efficiency point is not simply about electricity cost.
It is also related to pump reliability.
Can Operating at Too Little Flow Damage a Pump?
Yes.
People often associate cavitation only with insufficient suction pressure at high flow, but low flow can also create damaging hydraulic conditions.
At very low flow, internal recirculation can occur inside the pump.
Fluid can repeatedly circulate through parts of the impeller instead of moving smoothly through the casing.
Temperature can rise.
Vibration can increase.
Mechanical seals and bearings can suffer.
A pump is not automatically safe just because the discharge valve is mostly closed.
Can Hot Liquid Cause Cavitation?
Yes.
Hot liquid has higher vapor pressure.
That reduces the pressure margin available before vaporization begins.
This is particularly important in services involving:
Hot water
Hot glycol
Condensate
Boiler systems
Hot hydrocarbons
Process liquids near their boiling conditions
A pump that has acceptable suction pressure for cold liquid may cavitate badly after process temperature increases.
Always evaluate pressure together with temperature.
Why Can a Pump Cavitate After a Process Temperature Increase?
Nothing mechanical may have changed.
The same tank can be at the same level.
The same valves can be open.
The same pump can run at the same speed.
But hotter liquid is closer to vaporization.
The available NPSH has effectively decreased.
This is why process changes can cause pump problems even when maintenance history shows nothing unusual.
Does Increasing Suction Pressure Help?
Usually, yes.
Increasing the pressure available at the pump inlet increases the margin above vapor pressure.
Ways a system might accomplish this include:
Increasing liquid level
Raising vessel pressure
Reducing suction line losses
Reducing flow
Lowering liquid temperature
Moving the pump lower relative to the source
Using larger suction piping
Cleaning restrictions
The appropriate solution depends on why suction pressure is low.
Why Are Centrifugal Pumps Often Located Below Tanks?
Placing the pump below the normal liquid level creates positive static head at the suction.
Gravity helps feed the pump.
This improves suction conditions.
A pump located high above the liquid source has to operate with suction lift, which reduces available NPSH.
When designers have the option, providing flooded suction can improve reliability.
What Is Suction Lift?
Suction lift occurs when the pump is located above the liquid surface supplying it.
The pump must create sufficiently low pressure at its inlet to draw the liquid upward.
The elevation difference consumes part of the available suction head.
As the lift increases, cavitation risk increases.
There is a practical limit to how high a centrifugal pump can be placed above an open liquid source.
Why Does Suction Pipe Diameter Matter?
A smaller pipe produces greater liquid velocity for the same flow.
Higher velocity generally creates greater friction loss.
That means less pressure reaches the pump inlet.
Increasing suction pipe size can reduce friction and improve available NPSH.
This is why a pump can perform poorly even when the discharge piping seems perfectly acceptable.
The suction design matters enormously.
Can Too Many Elbows Cause Problems?
Yes.
Every elbow, valve, reducer, strainer, and fitting adds resistance.
One fitting alone may not matter much.
Combine several restrictions in a short suction line and the total loss can become significant.
Poorly arranged fittings close to the pump can also create uneven flow into the impeller.
Good suction piping aims to deliver stable liquid with minimal unnecessary pressure loss.
Why Is a Straight Suction Run Helpful?
The pump performs best when liquid approaches the impeller in a reasonably uniform way.
Strong swirling or distorted flow can create uneven loading.
A straight section of piping before the pump can help stabilize the velocity profile.
Actual piping requirements depend on pump design and system constraints.
The main idea is simple.
Do not make the liquid negotiate unnecessary disturbances immediately before entering the pump.
Can Air Entering the Suction Look Like Cavitation?
Yes.
Air entrainment can create similar symptoms.
The pump may become noisy.
Flow can fluctuate.
Discharge pressure can become unstable.
Vibration can increase.
The difference is that air has entered the system from somewhere else rather than being created by liquid vaporization inside the pump.
Possible sources include suction leaks, low tank level, vortexing, maintenance, and gas breakout.
What Is Vortexing?
Vortexing occurs when rotating flow develops above a pump suction point.
A visible example looks like the whirlpool that can form when water drains from a container.
If the vortex reaches the liquid surface, it can pull gas into the suction.
This becomes more likely when liquid level is low or withdrawal rate is high.
A pump may therefore appear to have adequate tank level while still ingesting gas because of poor suction geometry.
How Can Operators Reduce Vortexing?
Possible solutions depend on the equipment.
They can include:
Maintaining higher liquid level
Reducing withdrawal rate
Improving suction geometry
Installing suitable vortex prevention devices
Changing the location of the suction connection
The important point is to recognize that the problem may be inside the source vessel rather than inside the pump.
Can Gas Coming Out of Solution Cause Pump Problems?
Yes.
Hydrocarbon liquids can contain dissolved gas.
If pressure falls, some of that gas can leave the liquid.
This is especially important in crude oil, condensate, and other hydrocarbon services.
A pump suction may therefore contain a mixture of liquid and gas even though no external air leak exists.
Centrifugal pumps designed primarily for liquid can perform poorly when gas fraction becomes significant.
Is Gas Lock the Same as Cavitation?
No.
The symptoms can overlap, but the mechanisms are different.
Cavitation involves vapor forming because local pressure falls sufficiently.
Gas lock involves enough gas accumulating in the pump that normal liquid pumping becomes difficult or impossible.
An operator may hear noise and see unstable flow in both cases.
Understanding where the gas came from helps distinguish them.
How Can You Tell Whether a Pump Is Cavitating?
No single symptom proves cavitation.
Look for a combination of evidence.
Typical clues include:
Rattling or gravel like noise
Increased vibration
Unstable discharge pressure
Reduced flow
Lower than expected pump head
Low suction pressure
Damage near the impeller inlet
Changing symptoms as tank level changes
Changing symptoms as liquid temperature changes
High differential pressure across suction equipment
The strongest diagnosis connects the symptoms to actual process conditions.
What Does Cavitation Damage Look Like?
Cavitation erosion can produce pitting on pump surfaces exposed to bubble collapse.
The metal may look rough or eaten away.
Damage commonly appears in regions of the impeller where local pressure conditions and flow patterns promote cavitation.
Severe damage can eventually alter impeller geometry.
Once that happens, pump performance can deteriorate further.
Can Cavitation Damage a Mechanical Seal?
Yes.
Cavitation creates vibration and unstable hydraulic forces.
Mechanical seals depend on controlled alignment and operating conditions.
Persistent vibration can reduce seal life.
The pump may eventually begin leaking even though the original cause was poor suction conditions.
Replacing the seal without fixing the cavitation can result in repeated failures.
Can Cavitation Damage Bearings?
Yes.
Abnormal vibration and hydraulic loading can increase stress on bearings.
The cavitation itself happens in the liquid, but the effects are transmitted through the rotating equipment.
Repeated bearing failures can therefore be a symptom of a process problem rather than simply poor bearing quality.
Can Cavitation Reduce Pump Flow?
Yes.
As vapor occupies part of the impeller passages, the pump becomes less effective at moving liquid.
Developed head can fall.
Flow may become unstable.
Severe cavitation can produce a major loss of capacity.
A pump can therefore run continuously while delivering far less than expected.
Why Is Cavitation Sometimes Intermittent?
Because suction conditions can change.
Tank level falls.
Temperature rises.
Another pump starts.
A valve position changes.
Process flow increases.
A strainer gradually plugs.
Vessel pressure varies.
An intermittent problem is often especially useful for troubleshooting because the operator can compare conditions when the pump behaves normally with conditions when the noise appears.
Why Would Cavitation Happen Only at Low Tank Level?
Low tank level reduces static pressure at the pump suction.
It can also encourage vortex formation.
If the system has limited suction margin, cavitation may begin at a repeatable tank level.
That pattern is valuable evidence.
Instead of rebuilding the pump, investigate the suction system and vessel operating range.
Why Would Cavitation Happen Only During Hot Weather?
Higher ambient temperature can warm stored liquids.
Cooling equipment may also become less effective.
If liquid temperature rises enough, vapor pressure rises.
The available cavitation margin decreases.
A pump that operates quietly during cold months may become noisy in summer.
This does not necessarily mean the pump suddenly wore out.
Why Would Two Identical Pumps Behave Differently?
The pumps may be identical while their systems are not.
One suction strainer may be dirtier.
One valve may not be fully open.
One pump may have different piping.
One impeller may be worn.
One pump may rotate at a different speed.
Instrumentation may also be misleading.
When comparing pumps, verify actual operating conditions before concluding the equipment behaves differently for no reason.
Can the Wrong Impeller Cause Poor Pump Performance?
Yes.
Centrifugal pump performance depends heavily on impeller diameter and design.
If the installed impeller differs from the one assumed by the pump curve, actual head and flow can differ significantly from expectations.
This can happen after equipment replacement or maintenance if records are incorrect.
Checking physical equipment against documentation can resolve confusing performance problems.
Can the Wrong Motor Speed Cause Problems?
Yes.
Pump speed has a major effect on centrifugal pump performance.
A motor or drive operating at a different speed than expected can change:
Flow
Developed head
Power demand
Required NPSH
A pump that appears far away from its published curve may actually be running at a different speed from the curve basis.
Variable speed drives make checking actual speed especially important.
Does Increasing Pump Speed Increase Cavitation Risk?
It can.
Higher speed usually increases pump capacity and changes the pressure conditions inside the impeller.
Required NPSH generally increases as operating demand rises.
Meanwhile the suction system may not be able to provide additional pressure.
Increasing speed can therefore turn a stable pump into a cavitating pump.
Operators should not treat speed as a free way to gain more production.
The whole system needs to support the increased rate.
Can a Variable Frequency Drive Help?
A variable frequency drive can allow pump speed to match actual process demand.
Reducing unnecessary speed can lower flow and improve suction margin in some situations.
It can also reduce energy consumption.
However, the drive does not fix every hydraulic problem.
A badly restricted suction line remains a badly restricted suction line.
Control strategy and pump limits still need to be understood.
Why Is Cavitation Common After Production Increases?
Facilities often change over time.
More wells are connected.
Tank turnover increases.
Transfer rates rise.
Operators speed up pumps.
What was once a generously sized suction system can become restrictive at the new flow.
Friction loss rises rapidly as liquid velocity increases.
The pump may then lose its original suction margin.
Production increased, but the piping did not.
Can Cleaning a Strainer Immediately Stop Cavitation?
If the strainer restriction was the cause, yes.
This is why checking simple process conditions before major mechanical work is so valuable.
A pump can sound terrible because of a heavily restricted suction strainer.
Clean the restriction and suction pressure returns.
The noise disappears.
The same pump operates normally again.
What Should an Operator Check First When a Pump Sounds Like It Is Cavitating?
Start with conditions that are easy to verify and directly affect the suction.
Check the source vessel level.
Check suction pressure.
Check liquid temperature.
Confirm the suction valve is fully open where it should be.
Review strainer differential pressure.
Check whether flow has recently increased.
Look for signs of gas entering the suction.
Review pump speed.
Then compare the operating point with the pump information if available.
This sequence often finds the problem faster than immediately dismantling the pump.
Should You Keep Running a Cavitating Pump?
Persistent cavitation should not be treated as normal operation.
The correct response depends on the severity, service, process consequences, and operating procedures.
Continuing to run can damage the pump.
However, shutting down critical equipment without understanding the process can also have consequences.
Operators should follow site procedures and involve the appropriate operations, maintenance, or engineering personnel.
Can Reducing Flow Stop Cavitation?
Often.
Reducing flow can lower friction losses in the suction piping and reduce the pump’s required NPSH.
If cavitation disappears when flow is reduced, suction limitation becomes a strong possibility.
That does not necessarily mean reduced production should become the permanent solution.
The long term fix may involve modifying piping, cleaning equipment, changing pump selection, or improving vessel conditions.
Can Lowering Liquid Temperature Stop Cavitation?
Yes, when high vapor pressure is a major contributor.
Cooling the liquid reduces vapor pressure and increases the margin before vapor formation.
Whether cooling is practical depends on the process.
Sometimes temperature is fixed by upstream operations, so another solution is needed.
Can Raising the Source Vessel Pressure Help?
Yes.
If a closed vessel supplies the pump, increasing vessel pressure can increase pressure at the pump suction.
That can improve available NPSH.
Any pressure change must remain within process and equipment limits.
Changing vessel pressure can also affect upstream and downstream systems.
It is an engineering decision, not simply a pump adjustment.
Can Moving the Pump Lower Help?
Yes.
Lowering the pump relative to the liquid source increases static head at the suction.
This can significantly improve suction conditions.
It is a common design strategy when pumping liquids that are close to their vaporization conditions.
Obviously, relocating installed equipment can be expensive, which is why suction design matters during the original project.
Can Installing a Larger Pump Fix Cavitation?
Not automatically.
A larger pump may actually demand more flow and worsen suction conditions.
The problem needs to be defined first.
If the existing pump is poorly matched to the required service, replacement may be appropriate.
But cavitation caused by a blocked suction line will not be fixed by installing a more powerful pump.
Can Installing a Smaller Pump Help?
Sometimes.
An oversized pump may operate far from its intended region or move more liquid than the suction system can reliably supply.
A better matched pump can operate closer to the desired flow with improved reliability.
Pump selection should consider the system curve, expected operating range, fluid properties, and available suction conditions.
What Is a System Curve?
The system curve represents the pressure or head required to move different flow rates through the actual piping system.
As flow increases, friction losses increase.
The operating point occurs where the pump performance and system requirements intersect.
Changing a valve, pipe size, tank level, or downstream pressure changes the system behavior.
The pump and process therefore cannot be analyzed separately.
Why Is Cavitation Often Misdiagnosed?
Because the symptoms are not unique.
Noise could come from bearings.
Low flow could come from a plugged discharge.
Vibration could come from alignment.
Unstable pressure could come from gas.
A damaged seal could result from many causes.
Good troubleshooting avoids jumping from one symptom directly to a conclusion.
Instead, ask whether the process conditions support the suspected failure mechanism.
What Is the Difference Between Cavitation and Bearing Failure?
Cavitation noise is hydraulic and often changes when process conditions change.
For example, reducing flow or raising tank level may reduce the noise.
A damaged bearing may continue making noise regardless of those hydraulic changes.
Vibration analysis, temperature, sound location, and mechanical inspection can help separate the two.
Sometimes both exist because prolonged cavitation eventually damages mechanical components.
What Is the Difference Between Cavitation and a Plugged Discharge Line?
A plugged discharge can reduce flow, but it does not automatically create poor suction conditions.
In fact, lower flow can sometimes improve suction margin.
A heavily restricted discharge may instead push the pump toward very low flow operation.
Pressure readings on both sides of the pump help distinguish these situations.
Do not diagnose based only on low flow.
Why Are Suction Pressure Gauges So Useful?
A suction pressure reading tells you what the pump is actually receiving.
Without it, troubleshooting can become guesswork.
Tank level may look normal while piping losses are excessive.
A valve may appear open but be damaged internally.
A strainer may be plugged.
A reliable suction pressure measurement provides direct evidence.
For difficult pump problems, one good pressure reading can be more useful than several assumptions.
Why Should Operators Record Normal Pump Conditions?
A pump is easier to troubleshoot when normal values are known.
Useful baseline information includes:
Suction pressure
Discharge pressure
Flow rate
Liquid temperature
Pump speed
Motor current
Vibration
Source tank level
Strainer differential pressure
When something changes later, the operator can compare it with a known healthy condition.
Without baseline data, abnormal operation is harder to recognize.
Why Does Cavitation Sometimes Return After a Pump Rebuild?
Because the rebuild repaired the damage without correcting the cause.
The new impeller goes back into the same system.
The same suction restriction remains.
The same hot liquid returns.
The same low tank level is allowed.
The same excessive flow continues.
Eventually the new pump develops similar damage.
Repeated pump failures should always raise the question of whether the surrounding process is responsible.
Is Cavitation Always Obvious?
No.
Severe cavitation can be loud.
Mild cavitation may be much harder to recognize.
The pump can continue operating while gradual erosion occurs.
Vibration trends, inspection history, pump performance, and repeated seal or impeller damage may reveal a problem that operators cannot clearly hear.
That is why reliable operation should not depend entirely on listening for noise.
Can Cavitation Be Completely Eliminated?
A well designed and properly operated system can greatly reduce damaging cavitation.
The goal is to provide adequate suction margin throughout the expected operating range.
That requires considering:
Lowest vessel level
Highest liquid temperature
Maximum flow
Minimum source pressure
Piping losses
Pump speed
Fluid composition
Normal equipment fouling
A system designed only for perfect conditions can become unreliable as soon as real operations deviate from the design point.
What Should New Operators Remember About Pump Cavitation?
Remember one idea.
A centrifugal pump cannot pull unlimited liquid through a poor suction system.
If the pump does not receive sufficient pressure at its inlet, the liquid can begin vaporizing inside the impeller.
The result may sound like a failing pump.
But before blaming the pump itself, look upstream.
Check the tank.
Check the temperature.
Check the valves.
Check the strainer.
Check the suction pressure.
Check the actual flow.
Very often, the source of the problem is somewhere before the pump.
Frequently Asked Questions
What causes cavitation in a centrifugal pump?
Cavitation occurs when pressure inside the pump falls low enough for the liquid to form vapor bubbles. These bubbles later collapse in higher pressure regions of the pump.
What does pump cavitation sound like?
It is often described as a rattling sound similar to gravel or small rocks moving through the pump.
Can a plugged suction strainer cause cavitation?
Yes. A plugged strainer creates additional pressure loss and can reduce pump suction pressure enough to cause cavitation.
Can a partially closed suction valve cause cavitation?
Yes. Restricting the suction valve reduces pressure available at the pump inlet.
Can low tank level cause cavitation?
Yes. Lower tank level reduces static pressure at the pump suction and can also increase the chance of vortexing.
Can hot liquid make a pump cavitate?
Yes. Higher temperature increases liquid vapor pressure, reducing the margin before vaporization begins.
What does NPSH mean?
NPSH means net positive suction head. It describes the pressure energy available above the liquid vapor pressure at the pump suction.
What is the difference between NPSH available and NPSH required?
NPSH available comes from the actual piping and process system. NPSH required is determined by the pump design and operating point.
Does cavitation damage the impeller?
Yes. Repeated vapor bubble collapse can cause pitting and erosion of impeller surfaces.
Can cavitation damage seals and bearings?
Yes. The vibration and unstable hydraulic forces created by cavitation can shorten seal and bearing life.
Can excessive pump speed cause cavitation?
Yes. Increasing speed can increase flow and suction demand, reducing the available operating margin.
Can reducing pump flow stop cavitation?
It can. Lower flow often reduces suction piping losses and the pump’s required NPSH.
Is air in the suction the same as cavitation?
No. Air entrainment and cavitation can create similar symptoms, but air enters from another source while cavitation produces vapor inside the liquid because of low pressure.
What is vortexing?
Vortexing is rotating flow near a suction connection that can draw gas from the liquid surface into the pump.
Why would a pump cavitate only when a tank is nearly empty?
The lower liquid level provides less static pressure at the pump suction and may also allow a vortex to form.
Why would a pump cavitate only when the liquid is hot?
Hot liquid has higher vapor pressure and therefore requires better suction conditions to remain completely liquid.
Should a centrifugal pump be throttled on the suction side?
Generally, suction restrictions should be avoided because they reduce pressure at the pump inlet. Flow control is commonly performed on the discharge side or through pump speed control when appropriate for the system.
How can an operator quickly investigate suspected cavitation?
Check suction pressure, source tank level, liquid temperature, suction valve position, strainer differential pressure, pump flow, pump speed, and signs of gas entering the suction.
Why does replacing a cavitating pump sometimes fail to solve the problem?
Because the original cause may be the piping or process system rather than the pump. If poor suction conditions remain, the replacement pump can cavitate too.
What is the most useful thing to remember about cavitation?
Do not look only at the pump. Cavitation frequently begins with conditions upstream of it.