Cavitation is a very common phenomenon in hydraulic pumps. It causes noise and vibration, and accelerates the wear of internal parts. If you do not solve this problem, it will shorten the hydraulic pump’s life. At the same time, the hydraulic system cannot operate stably and may even stop suddenly. This article will explain the phenomenon of hydraulic pump cavitation, formation causes and diagnostic ideas, and teach you how to prevent the risk of cavitation.
What Is Hydraulic Pump Cavitation？
If you want to know what cavitation means, you need to understand a basic common sense: at the same temperature, the higher the pressure, the more gas can be dissolved in the liquid. Once the pressure is lowered, the gas will separate from the liquid.
When the pressure at the suction port of the hydraulic pump is too low, the gas in the hydraulic oil will be precipitated from the hydraulic oil, forming a large number of tiny bubbles. These bubbles will enter the high-pressure area of the pump with the oil and then quickly rupture, producing a very strong impact and high-frequency vibration in a very short period of time. This process is called cavitation.
Hydraulic Pump Cavitation Symptoms
In practice, cavitation is often accompanied by multiple unusual phenomena.
Abnormal Noise
The noise produced by cavitation is different from the normal sound of mechanical engagement. It usually appears as an irregular crackling sound, gravel impact, or a low knocking sound. And this sound is more noticeable when the oil suction conditions get worse or when the system warms up.
Abnormal Vibration
Air bubbles repeatedly form and burst inside the pump. This triggers pressure fluctuations that cause abnormal vibrations in the pump body and adjacent piping. This vibration tends to occur intermittently and is exacerbated when the system is started up or the load becomes heavy.
Decreased Stability
When air bubbles enter the pump chamber, they take up space that should be filled by oil. So the actual amount of oil delivered will be reduced, the actuator will move more slowly, and the system operation will become unstable.
Abnormal Temperature Rise
Energy is also expended during the formation and collapse of air bubbles. The efficiency of the hydraulic pump will decrease. The oil temperature will also gradually increase, thus aggravating the risk of cavitation and creating a vicious circle.
It is important to note that cavitation rarely presents as only one symptom. Abnormal noise can come from mechanical wear, and flow fluctuations can be caused by the control valve. Cavitation can only be identified when multiple unusual phenomena recur under the same operating conditions.
Cavitation vs Aeration vs Mechanical Wear: Key Differences
In hydraulic systems, cavitation, aeration, and mechanical wear are often confused because they all present as noise, vibration, and performance degradation. But the causes and ways of determining them are quite different. It is easy to distinguish between the three phenomena if you can grasp the root cause of these problems.
Cavitation
What Triggers It？
Cavitation is not caused by damage to the hydraulic pump. It is because the suction conditions cannot keep up with the current speed.
When the pump chamber expands very quickly, and the oil is too late to replenish, the pressure at the suction port will drop rapidly, and the dissolved gas in the oil will precipitate to form bubbles. These bubbles enter the high-pressure area and break up, thus triggering cavitation.
How to Judge It？
Symptoms will vary with speed, load, and oil temperature.
Symptoms are significantly relieved when oil suction conditions improve (e.g., by reducing speed, increasing temperature, or decreasing resistance).
Decreased efficiency and increased oil temperature often occur at the same time.
Aeration
What Triggers It？
Aeration is caused by air entering the oil from outside the system, not by pressure changes. Common causes include air leaks in lines and fittings, low tank levels, and return oil hitting the oil level directly.
How to Judge It？
Visible bubbles or foam can be seen in the oil.
Actuator action is soft and slow to respond.
The problem has little to do with speed or load changes.
The situation usually improves quickly after fixing the leak or improving the oil return conditions.
Mechanical Wear
What Triggers It？
Mechanical wear is damage that occurs gradually over a long period of equipment operation and is not related to oil suction conditions or air intake problems. It usually comes from component fatigue, insufficient lubrication, or long-term operation at higher loads.
How to Judge It？
Noise and efficiency problems will continue to worsen with longer use.
Even if the operating conditions are adjusted or the oil suction conditions are improved, the situation does not get better.
More wear particles are often detected in the oil.
The table below will help you quickly distinguish between these three common problems.
| Comparison Dimension | Cavitation | Aeration | Mechanical Wear |
| What Triggers It | Suction port pressure is too low | Air is drawn into the oil | Gradual wear and tear of equipment over time |
| Whether it is related to the oil absorption conditions | Yes | No | No |
| Whether there is air entry | No | Yes | No |
| Relation to speed/load/oil temperature | The relationship is obvious | Relationships are not obvious | No Relationship |
| Typical behavior in fluids | You don’t always see bubbles at first | You can see a lot of bubbles or foam | Increased wear particles |
| Performance of actuators | Unstable movement and reduced efficiency | Weakness in movement and slow response | Output capacity slowly deteriorates, but the change is relatively smooth |
| Trends in the development of problems | The condition remains unchanged; it will recur | Repairing leaks can disappear quickly | Continuous aggravation with increasing running time |
How Cavitation Appears in Different Hydraulic Applications？
Construction and Agricultural Equipment
In construction machinery and agricultural equipment, hydraulic systems often operate at high speeds with frequent starts and stops. Cavitation usually produces abnormal noise and vibration at startup or acceleration, and the symptoms are reduced when the oil temperature rises.
When the working environment is noisy, this noise is easily dismissed as normal mechanical sound or hydraulic pump aging, and the problem of insufficient oil suction conditions is ignored.
Industrial Hydraulic Power Units
In industrial hydraulic stations, the system operates relatively stably. Cavitation usually presents as a loss of efficiency and an increase in oil temperature, rather than as a noticeable noise. The actuator moves erratically or fails to maintain its original rhythm.
These types of failures are easily dismissed as valve or load problems, and suction problems, such as clogged filters or too thick oil, are often overlooked.
Machine Tools and Precision Equipment
Machine tools and precision equipment require high-speed stability and accuracy. A slight amount of cavitation can make the action of the tool unstable and the quality of the machined surface degraded.
These phenomena are often mistaken for control system, servo regulation, or mechanical accuracy problems, and cavitation is often not the primary target for troubleshooting.
High-Pressure and High-Speed Systems
Cavitation is even more destructive in high-pressure or high-speed systems. It can erode the pump body in a short time and leave the pump with a noticeable drop in performance. Noise and vibration often appear suddenly and intensify rapidly.
These problems are easily and directly attributed to poor pump quality or wrong selection. However, the suction conditions have to follow suit when the speed is increased. This is often overlooked.
What Causes Cavitation in Pumps?
Hydraulic pump cavitation is usually caused by insufficient pressure on the suction side. Based on the different causes, the common causes can be categorized as follows:
Installation Location and System Structure
When the hydraulic pump is installed higher than the tank level, the oil needs to overcome gravity to enter the pump body, resulting in lower pressure at the suction port.
Long-term low tank level will lengthen the suction distance, making it difficult for the oil to fill the pump chamber in time.
When the pump displacement is too large, the amount of oil required per revolution exceeds the amount of oil that can be provided by the system, causing the pressure at the inlet to drop.
When the system is designed without checking the pressure margin on the suction side, cavitation is easily triggered when the temperature rises or the load changes.
Suction Line and Flow Resistance
When the diameter of the suction line is small, sufficient oil cannot be supplied at high speeds, and the pressure at the inlet will drop.
Too long suction lines will lead to excessive pressure loss, and the oil will be close to low pressure before it reaches the pump.
Elbows and three-way passes that are numerous and close to the pump inlet will create a low-pressure area at the oil inlet, resulting in gas precipitation.
Clogged or undersized suction filters or screens will restrict the flow of oil into the pump.
Aging, collapsing, or deformation of the inner wall of the suction hose will increase the resistance to oil flow and cause a pressure drop at the inlet.
Roughness of the inner wall of the line or improper assembly will continuously deplete the pressure margin on the suction side.
Operating Conditions and Operating Parameters
When the pump speed is increased, the pump chamber expands faster than the oil replenishment, and the pressure at the inlet drops.
When the system load suddenly increases, the oil demand suddenly increases, and the suction system is often too late to provide sufficient flow.
When the system starts and stops frequently or accelerates rapidly, the suction side will enter the low-pressure state several times in a short period of time.
Fluid Condition and Temperature
The viscosity of the oil is too high during cold start, the fluidity is poor, and it is difficult for the oil to enter the pump chamber in time.
When the oil temperature changes, the available pressure margin at the suction port will be significantly reduced, and cavitation is more likely to be triggered.
There is more air dissolved in the oil, which can be easily precipitated to form bubbles in the low-pressure area.
Fuel Tank and Return Oil Arrangement
When the oil return port is not properly positioned, the oil return will directly stir the oil surface, allowing more air to be mixed into the oil.
When the flow inside the tank is not smooth, the oil is too late to release the air before it is sucked in again.
How to Calculate Cavitation Risk in Hydraulic Pumps？
Whether or not cavitation will occur depends on whether or not the pressure at the pump suction port is high enough to keep the oil in a liquid state. NPSH is the method used to quantify this pressure margin.
NPSHa（Available NPSH）
It represents the amount of suction margin that the system can actually provide. The formula is as follows:
NPSHa = Ptank/ρg + Hstatic − Hloss − Pvapor/ρg
What Do the Parameters Mean? Where Does the Data Come from?
Ptank: Tank Pressure
If you are using an open tank, Ptank ≈ atmospheric pressure
If you are using a closed or pressurized tank, Ptank = system design value
The higher the value, the easier it will be to suck in oil.
Hstatic: Hydrostatic Height
This value is the difference in height from the tank level to the center of the pump.
If the tank level is higher than the suction port of the pump, the value of Hstatic is positive, which is favorable for suction.
If the pump is installed above the tank level, the value of Hstatic is negative, which is unfavorable for oil suction.
Hloss: Pressure Loss on Suction Side
This value comes from all resistance in the suction line.
This includes resistance caused by small pipe diameters, long lines or elbows, clogged suction filters, and other factors.
This value can usually be estimated from experience or data given to you by the manufacturer.
Pvapor: Oil Vapor Pressure
Pvapor is related to oil temperature.
The higher the temperature, the more easily the oil vaporizes.
This value can be found from the oil data sheet.
ρ is the density of the oil, and g is the acceleration of gravity.
NPSHr（Required NPSH）
It indicates the minimum suction condition required for the pump to operate without significant cavitation at that condition.
This value is related to the speed and flow rate.
The higher the RPM and the higher the flow rate, the faster the pump chamber expands, the more oil is needed to replenish it, and the higher the value of NPSHr.
You can find this value in the charts provided by the pump manufacturer. The charts usually include a curve where you can find the NPSHr value for a specific flow rate and speed.
Quantitative Judgment Conditions of Cavitation
NPSHa ≥ NPSHr + Safety margin
When NPSHa < NPSHr, cavitation will almost certainly occur.
When NPSHa ≈ NPSHr, it indicates that the hydraulic pump is in the critical zone for cavitation.
When NPSHa ≥ NPSHr + 0.5-1.0 m, it indicates that the oil suction conditions are relatively safe.
Therefore, cavitation is likely to occur at high speed, cold start, or heavy load, and will be relieved when the working condition improves.
A Calculated Case
Known Conditions:
Open tank (Ptank = atmospheric pressure)
Pump mounted 1.2 m above tank level
Loss in suction line and filter: 0.6 m
Oil temperature 60°C, corresponding to a vapor pressure converted to 0.4 m
NPSHr as given in the pump catalogue: 2.8 m
Calculation of NPSHa
NPSHa = 0 – 1.2 – 0.6 – 0.4 = -2.2 m
Atmospheric and vapor pressures have been converted to the same unit (meter column).
Judgment Results
NPSHa(−2.2) < NPSHr(2.8)
The conclusion is that the system suction conditions are seriously inadequate and cavitation is inevitable.
How Cavitation Damages Hydraulic Pumps Over Time？
Cavitation not only causes the pump to become noisier, but it also gradually damages internal components over time. Its effects can be categorized into short-term performance deterioration and long-term structural damage.
Short-Term Performance Effects
Air bubbles form and collapse in the pump chamber, causing a drop in pump output flow and fluctuations in pressure.
Bubble formation and collapse consume some of the energy that is not used to push the fluid, thus making the pump less efficient.
The energy loss and pressure fluctuations cause the oil temperature to rise, further increasing the risk of cavitation.
These phenomena are usually reversible, and if the oil suction conditions are improved, the symptoms will be significantly relieved.
Long-Term Structural Damage
The small impacts from the bursting of air bubbles can strike metal surfaces and accumulate over time to form pits and cavities.
The shock and vibration caused by the bursting of air bubbles will accelerate the wear of seals and may lead to leakage.
Long-term cavitation reduces the reliability and service life of the pump and may even lead to premature retirement of the pump.
This type of damage is irreversible. It cannot be repaired by improving suction conditions once it has formed.
Step-by-Step Guide to Diagnosing Hydraulic Pump Cavitation
Observe the Operating Phenomena
Check the pump and hydraulic system for abnormal noise, vibration, flow fluctuations, or elevated oil temperature.
Note if symptoms occur only under specific operating conditions, such as high RPM, heavy loads, or cold starts.
Record the time and conditions when the symptoms appear for subsequent analysis.
Determine the Type of Fault
Compare the observed symptoms with those that may result from cavitation, Aeration, and mechanical wear.
Refer to the previous “Quick Judgment Table” to determine if the symptoms are more like cavitation than other problems.
Special note: You cannot rely on a single symptom to determine what kind of problem is occurring, but must analyze it in combination with several phenomena.
Check System Parameters
Measure the pressure, oil temperature, and flow rate at the pump suction port and record the parameters under current operating conditions.
Check the tank level, the mounting height of the pump, the arrangement of the suction line, and the condition of the suction filter.
These factors have a direct influence on the suction pressure and NPSH of the pump and determine the risk of cavitation.
Calculate NPSHa and Compare with NPSHr
Calculate the value of NPSHa using the formula mentioned earlier.
Query the NPSHr of the pump at the current flow rate and speed.
Compare the values of NPSHa and NPSHr.
Determine whether cavitation is occurring under the current operating conditions.
Analyzing Potential Causes
If you judge that cavitation has occurred according to the previous steps, then the next step is to investigate the factors that may cause cavitation. For example:
The pump installation height is too high, or the tank level is too low.
Oil suction piping is too long, the pipe diameter is too small, or the local elbow is too large.
Suction filter clogging or filter resistance is too large.
Sudden increase of system load or frequent starting and stopping.
Change of oil temperature leads to an increase in the vapor pressure of oil.
Develop Corrective Measures
You should improve the suction conditions, such as lowering the pump mounting height, increasing the tank level, etc.
You can also optimize the system conditions, such as reducing the pump speed or decreasing the transient load.
If necessary, you also need to replace the pump or adjust the model to make sure that the NPSHa is higher than the NPSHr with a safety margin.
Verify the Effect
Run the system again after adjustment and observe changes in noise, vibration, flow rate, and oil temperature.
If the symptoms are significantly reduced or disappear, the cavitation problem is under control.
If the symptoms persist, you need to further inspect the pump for internal wear or other potential failures.
How to Avoid Cavitation without Replacing the Pump？
Without replacing the pump, cavitation can be eliminated by improving operating conditions, optimizing system details, and improving maintenance. The main measures are listed below:
Improve Oil Absorption Conditions
Lower the pump mounting height or increase the tank level to reduce the suction height.
Clean the oil suction filter element to make sure that the resistance of the filter element is not too large.
Optimize the oil suction pipeline, try to shorten the length, increase the diameter, and reduce the number of elbows.
Extend the end of the oil return pipe below the liquid level to avoid the return oil directly impacting the oil surface to produce disturbances.
Optimize System Conditions
Reduce the speed of the pump so that the pump suction has enough time to replenish the oil.
Avoid situations where the load is suddenly too large or frequent starts and stops to prevent a sudden drop in suction port pressure.
Keep the oil temperature moderate to prevent air bubbles in the oil.
Maintenance and Monitoring
Regularly check the oil suction line, joints, and seals to avoid leakage.
Monitor the status of the oil and replace the aging or seriously contaminated hydraulic oil in time.
Pay attention to the noise, vibration, and flow rate changes of the pump, and adjust the operation status in time when abnormalities are found.
When to Replace a Hydraulic Pump Due to Cavitation？
Cavitation just appeared does not necessarily need to change the pump, but once the injury to the internal structure of the pump, only adjusting the working conditions or improving the suction conditions can not restore the performance, then you need to replace the hydraulic pump.
What Indicates Cavitation is Irreversible?
Noise and vibration persist after improving suction conditions with no significant relief.
The flow rate, pressure, or efficiency keeps dropping and cannot be restored to the original level even if the working conditions are stabilized.
Internal parts of the pump show signs of erosion, such as pits on the surface of the pump body, impeller, or flow distribution disk.
Wear particles in the fluid increase significantly, indicating that the metal parts have been seriously damaged.
Seal frequent failure or leakage aggravation, indicating that the internal structure has been damaged.
When the above situation occurs, cavitation is no longer just a running problem, but structural damage; continued use will only accelerate the failure.
Why Can’t It Continue to Run at This Time?
Erosion caused by cavitation is gradual and cumulative and does not stop on its own.
Efficincy decreases more and more rapidly as internal clearances increase.
The risk of sudden failure increases significantly, which can lead to system downtime and even damage to other components.
At this stage, it is better to replace the pump in time rather than repeatedly adjusting the working conditions.
Key Points that Must Be Attended to When Replacing A Hydraulic Pump
You have to account for NPSH to make sure that NPSHa is higher than NPSHr at the actual speed and maximum flow rate with a safety margin.
The displacement of the hydraulic pump should match the suction capacity of the system to avoid the pump needing more oil than can be supplied from the suction side.
Selection should be based on real working conditions. You should focus on the most demanding situations, such as cold starts, high loads, and frequent starts and stops.
When replacing a pump, you should also check the suction system design, including mounting height, pipe diameter, filter, and tank construction.
How to Prevent Cavitation through System Design？
Verify NPSHa from the Design Stage
Before finalizing the system solution, you should calculate the NPSHa available on the suction side and check whether there is still enough margin for the most extreme operating conditions.
This is the only way to avoid running the system on the verge of cavitation at cold start, high rpm, or high loads.
Properly Place the Pump and Tank
Where conditions permit, you should try to install the pump below the tank level to utilize the hydrostatic pressure generated by the level to assist suction, thus reducing the pump’s dependence on suction side pressure.
If the pump must be arranged at a higher level, this should be used as a critical design constraint and compensated for by raising the level, optimizing the piping, or reducing the speed.
Keep Suction Lines Simple
The suction line should be of sufficient pipe diameter, the shortest path, as straight as possible, and with a reduced number of elbows to minimize pressure loss along the route and locally.
Excessive elbows, too small a pipe diameter, or too long a suction line will significantly reduce the pressure margin on the suction side, thus increasing the risk of cavitation occurring.
Suction Filters Prioritize Low Resistance
The filter on the suction side should be of a model with low resistance and designed to take into account what happens when the filter gets dirty.
The problem of not filtering clean enough can still be remedied on the return side, but once the resistance on the suction side is too high, the pump will lack oil, and cavitation can easily occur immediately.
The Internal Structure of the Tank Should Facilitate Oil Venting
You can set up a spacer and deflector structure inside the tank so that the return oil flows slowly through the tank first, releasing the air before flowing to the suction port.
Such a tank design can make the oil near the suction port purer, reduce air and air bubbles, and reduce the risk of cavitation from the source.
Designed for the Toughest Conditions
When designing a system, you should not just look at the data during normal operation; you should also focus on the extreme conditions of cold starts, high loads, rapid acceleration, and frequent starts and stops.
Many cavitation problems don’t exist all the time, but are triggered right at these very short, but most stressful moments.
Allow Sufficient Safety Margins for the System
The system should be designed to allow margins for suction conditions, pipe diameter, and pump selection.
A reasonable safety margin is not wasteful, but is intended to allow for more stable system operation.
What Information to Prepare Before Contacting a Hydraulic Pump Supplier？
Before contacting a hydraulic pump supplier, you should prepare the following information:
Actual Operating Conditions
Operating speed range (minimum, maximum, common)
System pressure (rated pressure, peak pressure)
Flow demand (steady flow or frequent fluctuations)
Description of Extreme Conditions
Oil temperature at cold start
Operating condition at maximum load
Whether there are frequent start-stop or rapid acceleration.
Information about Oil Suction Systems
The installation height difference between the pump and the tank level
Suction pipe diameter, length, and number of elbows
Suction filter model and rated pressure drop
Normal range of oil tank level
Fluid Related Parameters
Oil type and viscosity grade
Cold start and normal operation oil temperature range
Whether the fluid has a foaming or emulsification phenomenon.
Characterization of Current Problems
Is there abnormal noise or vibration？
Whether the problem occurs during startup, acceleration, or stable operation.
Does the problem vary significantly with speed, load, and oil temperature?
Can the problem be relieved by improving the working conditions?
Information about Existing Pumps
Pump model, displacement, and rated speed
NPSHr data from the manufacturer
Hours of use and maintenance history
FAQ
The hydraulic pump has developed minor cavitation, but the system is still working properly. How long is this acceptable?
Slight cavitation does not usually cause immediate problems in a short period of time. However, if it occurs frequently during normal operation, internal damage will gradually accumulate. You’d better deal with it as early as possible, and don’t let the hydraulic pump work in this state for a long time.
Does a short period of cavitation cause substantial damage to a hydraulic pump?
Occasional, short-lived cavitation generally does not immediately damage a hydraulic pump. However, if cavitation occurs frequently, even if it does not last long each time, it will gradually erode the pump components and eventually cause irreparable damage.
Why do some systems with NPSH calculations that appear to be adequate still experience cavitation in actual operation?
Many NPSH calculations are based on ideal, stable operating conditions. However, in actual operation, cold starts, increased speeds, slowly clogging filters, and increased line resistance can cause the actual available suction pressure to be lower than the calculated value, which can lead to cavitation.
How different is the risk of cavitation for the same pump, using different fluids?
The difference can be significant. The viscosity, vapor pressure, and temperature characteristics of the oil all affect the suction pressure. Cavitation is more likely to occur with oils that have a high vapor pressure or low temperature and high viscosity.
Are different types of hydraulic pumps differently sensitive to cavitation?
Yes, different pump types are sensitive to cavitation to different degrees. Pumps with high rotational speeds, small structural clearances, and rapid changes in the pump chamber require higher suction conditions and are more prone to cavitation.
When retrofitting a system, is it better to improve the suction system or to reduce the speed?
It is usually more effective to improve the suction system first. Reducing the speed can temporarily relieve cavitation, but if the suction conditions are not good, the problem will still occur under other operating conditions.
Final Thoughts
Hydraulic pump cavitation can affect system stability and shorten pump life. Understanding the causes, symptoms, diagnostics, and prevention of cavitation can help you identify the problem and minimize the risk, ensuring the long-term stability of your hydraulic system.Target Hydraulics has over 10 years of experience in hydraulic system design and pump applications, specializing in solving cavitation problems in hydraulic pumps. We can provide pump selection advice, suction system optimization, and on-site diagnostic services to help you stay safe from cavitation. Feel free to contact us to make your hydraulic system more reliable!
