Troubleshooting Hydraulic Pump Cavitation: Diagnosis, Root Causes and Solutions

1. Description of the problem and scope of application

Cavitation of hydraulic pumps is a critical malfunction manifested by the formation and destruction of steam or gas bubbles in the hydraulic fluid when the pressure in certain areas of the flow drops below the saturated vapor pressure of the fluid. This process leads to intense noise, vibration, erosion of the internal components of the pump, a significant reduction in efficiency and a reduction in the service life of the equipment. Prolonged cavitation causes the fluid to wear catalytically, increase its temperature, and eventually lead to complete failure of the hydraulic pump, often without warning.

Equipment at risk:

• Hydraulic systems of machine tools, presses, and injection molding machines in the metallurgical, machine-building, and food industries.
• Hydraulic drives in mobile machinery, construction equipment and agricultural machinery.
• Hydraulic control systems in aerospace and marine engineering.

Classification of severity:

• Critical: Immediate threat of damage to the pump and related system components. High risk of sudden equipment failure, which can lead to significant downtime and accidents. Needs immediate elimination.
• Significant: Noticeable noises (grinding, crackling), vibration, a noticeable decrease in pump performance or an increase in liquid temperature. If left untreated, rapidly progresses to critical wear.
• Minor: The initial stages of cavitation, manifested by a slight noise or a slight increase in temperature. Requires constant monitoring and preventive measures to prevent deterioration.

Applicable standards:

• DSTU EN ISO 4406: Purity of hydraulic fluids.
• DSTU ISO 2901: Hydraulic pumps and motors - Parameters and operating characteristics.
• DSTU EN ISO 3498: Hydraulic systems – Requirements for hydraulic fluids.

2. Safety measures

BEFORE BEGINNING ANY DIAGNOSTIC OR REPAIR WORK ON HYDRAULIC SYSTEMS, BE SURE TO OBSERVE THE FOLLOWING CRITICAL SAFETY RULES:

• Lockout and Tagout (LOTO): Always switch off the power supply to the hydraulic system and apply lockout and tagout procedures in accordance with internal instructions and DSTU EN ISO 14118. Ensure that the system cannot be accidentally activated.
• Residual Energy Discharge:Hydraulic systems can retain significant pressure in accumulators and pipelines even after power is turned off. Discharge all hydraulic accumulators and depressurize all hydraulic circuits to zero (0 bar) before disconnecting any components. Check the pressure with a manometer.
• Personal protective equipment (PPE): Use safety glasses or a face shield (CE EN 166), protective gloves (nitrile or chemical resistant EN 374), hearing protection (earmuffs or headphones EN 352) and appropriate work clothes.
• Hot surfaces and fluids: Hydraulic fluid and components can be very hot (up to 90°C and above). Allow the system to cool before operating. Use heat protective gloves if necessary.
• High Pressure Leaks: High pressure jets of hydraulic fluid (up to 400 bar or more) can penetrate the skin causing serious injury, infection or death. NEVER use your hands to check for leaks. Always use a piece of cardboard or a wooden stick. If liquid gets under the skin, seek medical attention immediately and tell the doctor about the type of liquid and the pressure.
• Lifting mechanisms: When working with heavy components (pumps, hydraulic motors), use appropriate lifting equipment certified by DSTU EN 13155 and observe safety rules when lifting.

3. Necessary diagnostic tools

Using the correct instruments with appropriate ranges and accuracy is critical for effective cavitation diagnosis.

4. Initial assessment checklist

Before beginning a systematic diagnosis, perform a thorough initial assessment. This will help narrow down potential problems and provide an understanding of the current state of the system.

5. Systematic diagnostic algorithm

This step-by-step algorithm will help locate the root cause of cavitation. Follow the sequence for efficient and safe diagnosis. Remember the safety measures before each intervention!

1. Detecting the initial symptoms of cavitation?
   • (Excessive noise - grinding, cracking, hissing, "gravel in the pump", vibration, reduced pump performance, increased fluid temperature, foam in the tank)
   • If YES: Go to step 2.
   • If NO: The problem is probably not related to cavitation. Refer to other troubleshooting guides.
2. Checking the hydraulic fluid level in the tank.
   • Visually check the fluid level by the indicator (slit glass, float) with the pump off and the fluid cooled.
   • Is the level below the minimum mark or is the fluid exposing the suction nozzle?
     • If YES (level is low):
       1. Top up the hydraulic fluid to the recommended level using fluid of the appropriate specification and purity (DSTU EN ISO 3498, DSTU EN ISO 4406).
       2. Check for external leaks throughout the system. Eliminate them.
       3. If there are no leaks, check if the pump is sucking air through the vortex funnels (this may be at a low, but still "acceptable" level).
       4. Start the system and repeat the cavitation test.
     • If NO (level is normal): Go to step 3.
3. Inspect suction line for restrictions.
   • Visual inspection: Check suction filter/strainer for contamination, inspect suction pipe/hose for kinks, constrictions, kinks, or partial overlap.
   • Measuring vacuum:
     1. Install a vacuum gauge on the suction line (as close as possible to the pump inlet).
     2. Start the system and record readings under normal operating load.
     3. Removal reading > -0.3 bar? (For example, -0.4 bar or -0.5 bar, or higher than the value recommended by the pump manufacturer).
   • If YES (excessive dilution):
     1. Wash or replace suction filter/mesh.
     2. Check the diameter of the suction line and its length: Too small a diameter or too long will increase resistance. Make sure that the diameter of the suction line is at least equal to the diameter of the pump inlet, preferably larger.
     3. Eliminate kinks, narrowing or partial overlaps of the pipeline.
     4. Check that there is nothing obstructing the free flow of liquid in the tank (for example, foreign objects or detached pieces of the filter).
     5. After elimination, repeat the vacuum measurement and the cavitation test.
   • If NO (thinning is normal): Go to step 4.
4. Check for air suction in the suction line.
   • Visual inspection: Turn on the pump and observe the fluid in the tank through the inspection window (if applicable) or directly at the fluid return: are there significant bubbles or foam? (This requires compliance with security).
   • Checking tightness (CAUTION! PPE!):
     1. Tighten all connections on the suction line: flanges, fittings, clamps.
     2. Inspect the pump shaft seal for fluid leaks (a sign of air entrainment) or air bubbles passing through.
     3. You can carefully apply a soapy solution to potential leaks (while the pump is running, being safe). The appearance of bubbles will confirm the suction of air.
   • Air suction detected?
     • If YES:
       1. Replace damaged seals, gaskets, or piping.
       2. Tighten all connections securely to the recommended torque.
       3. Make sure that all elements of the suction line (including welds) are sealed.
       4. Start the system and repeat the cavitation test.
     • If NO: Go to step 5.
5. Hydraulic fluid viscosity check.
   • Visual evaluation: At cold temperatures, the fluid may appear excessively "thick" or, on the contrary, too "liquid" at high temperatures.
   • Temperature measurement: Make sure the fluid has reached the recommended operating temperature (eg 40-60°C).
   • Fluid analysis: Take a sample of the liquid and send it for laboratory analysis for accurate determination of kinematic viscosity (DSTU ISO 3104) and viscosity index.
   • The fluid viscosity is outside the limits recommended for this pump (usually 10-100 cSt at operating temperature)?
     • If YES:
       1. Replace the hydraulic fluid with the one recommended by the pump and system manufacturer (according to DSTU EN ISO 3498) with the correct viscosity class (ISO VG).
       2. Check the operation of the fluid cooler/heater: Make sure the fluid is maintained in the optimal temperature range.
       3. Avoid excessively low starting temperatures, which increase viscosity. Ensure that the liquid is warmed up to working temperature.
       4. Start the system and repeat the cavitation test.
     • If NO: Go to step 6.
6. Checking the internal condition of the pump.
   • Decreased pump performance, despite all previous checks and troubleshooting? (Manifested as a slowdown in the drive, a decrease in maximum pressure, despite normal system parameters).
   • If YES:
     1. Probable cause is internal wear of the pump: damage to working elements (gear wheels, vanes, pistons), wear of end plates or seals.
     2. Perform a volumetric pump efficiency test. This can be done using a portable flow meter or by measuring the fill time of a calibrated container on the return line. A decrease in efficiency below 70-80% indicates significant wear.
     3. Schedule pump removal and inspection.
     4. Replace or repair the pump using original UNITEC spare parts.
   • If NO: Despite the performed diagnosis, the symptoms of cavitation are still present and all parameters are normal? Contact the equipment manufacturer or a qualified UNITEC technician for in-depth diagnosis, the cause may not be standard.

6. Matrix "Failure-Cause"

This matrix will help you quickly identify likely root causes of cavitation based on observed symptoms and diagnostic test results.

7. Root cause analysis of each malfunction

A thorough understanding of the root causes is key to effective repair and prevention of cavitation recurrence.

1. Restrictions on entry (restriction)

• Why this happens: Restricting the flow of hydraulic fluid to the pump inlet creates excessive vacuum (negative pressure) in the suction line. This rarefaction lowers the pressure of the liquid below its saturated vapor pressure, resulting in the formation of vapor bubbles. After entering the high-pressure zone of the pump, these bubbles suddenly collapse (implode), creating shock waves. The most common causes include: a clogged suction filter (mesh) or a filter with an excessively small micron rating, a kinked or partially blocked suction hose/pipe, a suction line diameter too small (for the flow being pumped), an excessive suction line length, too many elbows, fittings or valves in the suction line, a blocked tank inlet.
• How to confirm: Vacuum pressure gauge measurement directly at the pump inlet. A vacuum reading above -0.3 bar (or above the value recommended by the pump manufacturer) confirms the problem. In addition, a visual inspection of the filter and pipeline is carried out for visible damage or contamination.
• What damage is caused by: Constant erosion of the working surfaces of the pump due to implosion of bubbles. Destruction of hydraulic fluid (degradation), its overheating, increased noise and vibration, premature failure of the pump, which can lead to catalytic failure of the entire hydraulic system.

2. Low level of liquid in the tank

• Why this happens: Insufficient volume of hydraulic fluid in the tank leads to the fact that the suction pipe of the pump is partially exposed or reaches the level where vortex funnels are formed, sucking atmospheric air directly into the suction line. This can also result in insufficient settling time for the liquid to remove air and insufficient cooling of the liquid.
• How to confirm: Visual check of the liquid level by the indicator (slit glass, float). Checking the system for external leaks that could have caused the level to drop.
• What damage is caused by: Air entrainment (which is a form of cavitation), overheating of the hydraulic fluid, reduction of its lubricating properties, damage to the pump due to shock loads and increased wear.

3. Liquid viscosity is too high

• Why this happens: If the viscosity of the hydraulic fluid is excessively high (for example, due to using the wrong type of fluid, the ambient temperature is too low, especially during a cold start, or a malfunction of the fluid heating system), the fluid becomes too "thick" to flow freely through the suction line to the pump. This greatly increases the hydraulic resistance to flow, creating excessive vacuum at the pump inlet, leading to cavitation.
• How to confirm: Measure the liquid temperature in the tank and the return line. Laboratory analysis of liquid for accurate determination of kinematic viscosity (DSTU ISO 3104) and viscosity index. Comparison of the obtained values ​​with those recommended by the pump and hydraulic system manufacturer.
• What damage is caused by: Excessive load on the pump and drive motor, overheating of the fluid, reduced system efficiency, increased wear of internal pump components due to abrasive friction and shock loads from cavitation.

4. Air suction in the suction line

• Why this happens: Even with a normal fluid level and no significant restrictions, atmospheric air can enter the suction line through leaky connections (loose or damaged fittings, flanges, gaskets), damaged hoses, faulty pump shaft seals, cracks in piping or defective welds. Air bubbles, entering the pump, compress and expand, simulating cavitation, and also contribute to the oxidation of the hydraulic fluid.
• How to confirm: Visually inspect the fluid in the tank for significant persistent bubbles or foam. Checking the connections with a soapy solution or special detector sprays (the appearance of bubbles will confirm air suction). Thorough inspection of the pump shaft seal.
• What damage does: Noise, vibration, oxidation and degradation of hydraulic fluid, destruction of the lubricating layer on the internal components of the pump, increased wear, "soft" and unstable operation of the system due to the presence of compressed air.

5. Internal wear of the pump

• Why this happens: Over time, as a result of normal operational wear, abrasive fluid contamination (non-compliance with DSTU EN ISO 4406) or long-term operation in cavitation conditions, the internal components of the pump (gears, vanes, pistons, cylinders, end plates) wear out. This increases internal clearances, which leads to a drop in volumetric efficiency, increased internal leakage, and can create local low-pressure zones inside the pump, contributing to the development of cavitation.
• How to confirm: Pump volumetric efficiency test (decrease in efficiency below 70-80% indicates significant wear). Liquid analysis for wear particles (metals) using spectral analysis. Dismantling and thorough inspection of the pump.
• What damage does: Progressive reduction of performance and pressure, increased noise, vibration, overheating of the pump, complete failure of the pump due to damage to internal components.

8. Step-by-step troubleshooting procedures

CAUTION: BEFORE STARTING ANY WORK, ALWAYS ENSURE SAFETY: LOCKOUT/TAGOUT (LOTO), RELIEF OF RESIDUAL PRESSURE AND USE OF PPE!

1. Removal of entry restrictions:

1. Measure the vacuum: Connect a vacuum gauge (range -1 to 0 bar) to the test point on the suction line as close to the pump as possible. Start the system and record the readings. The normal vacuum range is from -0.1 to -0.3 bar. A reading of > -0.3 bar indicates a problem.
2. Checking and replacing the filter: Shut down the system (LOTO!). Remove and inspect the suction filter or screen. If it is clogged, clean or replace it with a new element with the recommended micron rating (eg 100-150 microns for suction).
3. Pipe Inspection: Thoroughly inspect suction hoses and metal piping for kinks, kinks, constrictions, peeling of the inner rubber layer of the hose, or damage. Replace the damaged areas with elements of the appropriate diameter and material that comply with DSTU EN ISO 1432.
4. Line diameter check: Make sure the inside diameter of the suction line matches or exceeds the diameter of the pump inlet. The optimal flow speed in the suction line should be within 0.6-1.2 m/s.
5. Drag Reduction: If possible, reduce the number of fittings, elbows or valves in the suction line. Reduce its length to a minimum.
6. Verification: After completing the work, start the system and measure the vacuum again. It should return to the acceptable range. Check for noise and vibration.

2. Correction of low liquid level in the tank:

1. Level Score: Shut down the system (LOTO!). Check the fluid level on the indicator. If it is lower than the minimum mark.
2. Search for leaks: Inspect the entire hydraulic system for external leaks: hoses, pipes, fittings, seals, cylinders. Repair leaks by replacing damaged components or tightening connections.
3. Topping up the fluid: Top up the hydraulic fluid to the recommended level (usually 2/3 - 3/4 of the height of the tank). Use only the liquid recommended by the manufacturer with the appropriate purity class (filtration at least 10 μm when filling). Do not mix different types of liquids.
4. Verification: Start the system. Make sure that the level has stabilized, there are no vortex funnels above the suction nozzle. Check the absence of cavitation noises.

3. Correction of hydraulic fluid viscosity:

1. Measure the temperature: Use the IR thermometer to measure the temperature of the liquid. If the temperature is below the recommended operating temperature (<40°C), turn on the heating system or allow the system to warm up at idle until the optimal temperature is reached.
2. Fluid Analysis: Collect a sample of the fluid and send it to the laboratory for kinematic viscosity at 40°C and 100°C. Compare the results with the pump manufacturer's specification.
3. Fluid replacement: If the analysis shows that the viscosity of the fluid does not meet the requirements or the fluid has degraded (the viscosity index is reduced, the content of water or impurities is high), completely drain the old fluid, flush the system and fill with a new one recommended by the manufacturer (purity class according to DSTU EN ISO 4406).
4. Verification: After replacing or reaching operating temperature, measure the vacuum at the pump inlet. It should be within acceptable limits. Check for stable operation and absence of cavitation.

4. Elimination of air suction:

1. Visual Inspection: Start the system and observe the liquid in the tank for bubbles. Pay special attention to the sealing of the pump shaft.
2. Leakage test (CAUTION! PPE!): Turn off the system (LOTO!). Tighten all bolts on flange connections and suction line fittings to the recommended torque. Start the pump and carefully apply a soapy solution to all suspicious joints and shaft seals. The appearance of foam or bubbles indicates a suction site.
3. Replacement of seals and components: If leaks are found, replace damaged gaskets, seals (including pump shaft seals), hoses or fittings. Use high-quality UNITEC components that meet CE and UkrSEPRO standards.
4. Verification: Start the system. Check for bubbles in the tank and cavitation noises.

5. Repair or replacement of a worn pump:

1. Efficiency test: If all the previous steps did not solve the problem, perform a pump volumetric efficiency test. Use a portable flow meter to measure actual flow. Compare it with the nominal flow of the pump at the given pressure. Volumetric efficiency below 70% indicates the need for repair or replacement.
2. Dismantling (LOTO!): Shut down system, provide LOTO, relieve pressure. Disconnect the pump from the hydraulic system and drive.
3. Defect: Perform a complete pump defect. Inspect working elements (gears, blades, pistons, cylinders, end plates) for wear, scratches, erosion. Check clearances with feelers.
4. Repair/Replacement: If wear is significant, replace the pump with a new one or overhaul using original UNITEC repair kits. Ensure the correct centering of the pump with the drive motor (allowable deviation <0.05 mm).
5. Verification: After installing the new/refurbished pump, fill it with fluid. Start the system, check for no cavitation, normal pressure and performance. Run an efficiency test.

9. Precautions

Prevention is the most effective way to avoid cavitation and ensure the longevity of the hydraulic system.

10. Spare parts and components

For quick and effective cavitation removal, it is important to have the appropriate spare parts on hand. UNITEC-D GmbH offers a wide range of high-quality hydraulic components certified by CE and UkrSEPRO.

To order and select the necessary spare parts, please visit our electronic catalog: www.unitecd.com/e-catalog/

11. Links

• DSTU EN ISO 4406: Fluid hydraulic energy. liquids The method of coding the level of pollution by solid particles.
• DSTU ISO 2901: Hydraulic pumps and motors. Parameters and operating characteristics.
• DSTU EN ISO 3498: Hydraulic systems. Requirements for hydraulic fluids.
• ISO 10816-1: The vibration is mechanical. Evaluation of machine vibration based on the results of measurements on stationary parts. General instructions.
• EN 837-1: Manometers. Part 1: Spring manometers.
• DSTU EN 61672-1: Electroacoustics. Sound meters. Part 1: Technical requirements.
• OEM (Original Equipment Manufacturer) equipment operation and maintenance manuals.
• UNITEC-D GmbH. Other manuals for maintenance of hydraulic systems.