Diagnostic Guide: Hydraulic Pump Cavitation - Troubleshooting

1. Description of the problem and scope of application

Cavitation of a hydraulic pump is a critical malfunction, which is accompanied by the formation and subsequent destruction of steam bubbles in the flow of hydraulic fluid. This phenomenon occurs when the pressure in certain areas of the pump (usually the inlet) drops below the saturated vapor pressure of the liquid. The collapse of these bubbles generates localized high-intensity shock waves that cause significant damage to the internal surfaces of the pump and other hydraulic system components.

Symptoms of cavitation:

• Noise: A characteristic sound reminiscent of the crackling of gravel, metal grinding or rattling inside the pump.
• Vibration: Increased vibration of the pump housing and adjacent pipelines.
• Decrease in productivity: Drop in pressure and flow of liquid supplied by the pump, decrease in system efficiency.
• Overheating: Local temperature increase of liquid and pump components.
• Erosive damage: Pitting, potholes, metal erosion on the working surfaces of the impellers, gears, pistons and pump housing.

This manual applies to all types of hydraulic pumps used in industry (gear, vane, piston) and is designed to systematically identify and eliminate the root causes of cavitation.

Severity Classification:

• Critical: Rapid damage progression, risk of sudden pump failure and production shutdown. Requires immediate intervention.
• Significant: Noticeable decrease in efficiency, increased noise and vibration, accelerated wear. Requires planning of repair work.
• Minor: Initial symptoms (intermittent noise, slight vibration) indicating a potential problem. Needs monitoring and diagnostics.

2. Precautions

WARNING! Before starting any diagnostic or repair work on hydraulic equipment, strictly follow the safety rules:

• LOCKOUT/TAGOUT (LOTO): Always ensure complete de-energization and lockout/tagout of power sources (electrical, hydraulic, pneumatic) to prevent accidental starting of equipment.
• RELIEF OF RESIDUAL PRESSURE: Make sure all hydraulic system pressure is relieved before disassembling any components. Pressurized hydraulic fluid can cause serious injury.
• PERSONAL PROTECTIVE EQUIPMENT (PPE): Use appropriate PPE: safety glasses/shields, gloves, work clothes, safety shoes. Hot hydraulic fluid can cause burns and pressurized fluid can cause puncture wounds.
• HIGH TEMPERATURE SURFACES: Be careful as some hydraulic system components may be hot during or after operation.

3. Necessary diagnostic tools

4. Initial assessment checklist

Before starting detailed diagnostics, take the following steps to gather information and minimize downtime:

5. Systematic flow of diagnostics

The diagnosis of cavitation begins with the observation of symptoms and subsequent sequential exclusion of possible causes:

1. Is there a characteristic noise (cracking, grinding) and/or vibration from the pump?
   
   • YES: Continue diagnostics.
   • NO: The problem is probably not related to cavitation. Refer to other hydraulic system diagnostic manuals.
2. Checking the hydraulic fluid level in the tank.
   
   • IF level below the MIN mark:
     
     1. Root cause: Low fluid level.
     2. Action: Top up with hydraulic fluid of the appropriate type and purity class (eg ISO VG 46, purity class NAS 6-8).
     3. Check: Run the system and assess for cavitation symptoms.
   • IF level is normal: Continue.
3. Hydraulic fluid temperature check.
   
   • Measure the temperature with an infrared thermometer.
   • IF temperature is much lower than recommended (<20°C):
     
     1. Root cause: Fluid viscosity is too high.
     2. Action: Use a fluid heating system or allow the system to warm up. Consider using a fluid with a lower viscosity index for cold conditions (according to ISO 3448).
     3. Check: Start the system after warm-up and assess for symptoms.
   • IF temperature significantly higher than recommended (>70°C):
     
     1. Root cause: Fluid viscosity too low / System overheating.
     2. Action: Check the efficiency of the cooling system. Consider using a higher viscosity or cooling index fluid.
     3. Check: Start the system after the temperature stabilizes.
   • IF temperature is normal (40-60°C): Continue.
4. Measure pump inlet line pressure/vacuum.
   
   • Connect a pressure/vacuum gauge to a test point on the inlet line as close to the pump as possible.
   • IF vacuum exceeds permissible values ​​(< -0.4 bar):
     
     1. Probable causes: Air intake or suction restriction.
     2. Continue to check restrictions.
   • IF pressure is normal (0 ... -0.3 bar):
     
     1. Root cause: Unlikely cavitation through the intake line. Consider internal damage to the pump or other factors (such as pressure pulses).
5. Diagnoses intake line restrictions.
   
   • Visual inspection: Check intake filter/strainer for clogging (dust, dirt, deposits).
   • IF filter clogged:
     
     1. Root cause: Intake filter clogged.
     2. Action: Clean or replace the filter.
     3. Check: Start the system, monitor the intake pressure and the presence of noise.
   • Visual inspection: Check the intake pipe for kinks, deformations, excessive length, incorrect diameter.
   • IF detected deformations/bends:
     
     1. Root cause: Mechanical restriction of intake line.
     2. Action: Replace or repair the pipeline, make sure free flow.
     3. Check: Start the system, monitor the intake pressure and the presence of noise.
6. Air Intake Diagnostics.
   
   • Use a mechanic's stethoscope or an ultrasonic detector to listen to all connections, seals, and gaskets on the intake line (from tank to pump).
   • The 'soap water' method: Apply a soapy solution to the suspect connections and observe the formation of bubbles while the system is running.
   • IF detected air intake:
     
     1. Root cause: Intake line leak.
     2. Action: Tighten connections, replace damaged seals, hoses or flanges.
     3. Check: Recheck for leaks, start the system, and evaluate the symptoms.

6. Matrix of malfunctions and causes

7. Root cause analysis for each malfunction

7.1. Low level of hydraulic fluid in the tank

Why this happens: Insufficient volume of liquid in the tank leads to the fact that the pump begins to draw in air along with the liquid. This can be caused by leaks in the system, insufficient fluid topping up during service, or fluid evaporation at high temperatures. As the liquid level drops, the submergence of the suction line becomes insufficient and the surface film of the liquid is broken, allowing air to enter the pump.

How to confirm: Visual inspection of the liquid level indicator on the hydraulic tank. The level should be between the MIN and MAX marks when the system is turned off and cooled down (according to DSTU EN ISO 4413).

What damage it does:

• Accelerated wear of the pump due to hydraulic shocks and erosion of internal components.
• Foaming of hydraulic fluid, which leads to its oxidation and deterioration of lubricating properties.
• Overheating of the system due to a decrease in the heat exchange capacity of the liquid.
• Potential pump failure.

7.2. Clogging of the intake filter/mesh

Why this happens: Inlet filters or screens are designed to protect the pump from large mechanical particles. Over time, they can become clogged with deposits, dirt or wear products. A clogged filter creates a significant hydraulic resistance to the flow of liquid, which leads to a drop in the pressure at the pump inlet below the permissible level, provoking cavitation.

How to confirm:

• Measure the vacuum on the inlet line of the pump: a value < -0.4 bar indicates excessive resistance.
• Visual inspection of the intake filter after disassembly.

What damage it does:

• Increased rarefaction at the inlet, which directly causes cavitation.
• Deterioration of pump filling, reduction of its productivity and efficiency.
• Accelerated wear of internal parts of the pump.

7.3. Air suction on the intake line

Why this happens: Leaking connections, cracks in hoses or piping, damaged seals, or faulty pump shaft seals can allow atmospheric air to enter the suction line. Air bubbles entering the pump behave similarly to cavitation bubbles, causing noise, vibration and damage.

How to confirm:

• Using a mechanic's stethoscope or an ultrasound detector to detect hissing sounds at joints.
• Applying a soapy solution to potentially leaky areas of the intake line while the system is running - the formation of bubbles will confirm air suction.
• Visual inspection of the liquid in the tank: the presence of significant foaming.

What damage it does:

• Cavitation-like pump damage due to compression and expansion of air bubbles.
• Foaming of hydraulic fluid, which leads to its degradation and oxidation.
• Reduction of rigidity of the hydraulic system, imprecise operation of executive mechanisms.

7.4. Incorrect viscosity of hydraulic fluid

Why this happens: The viscosity of the hydraulic fluid is a critically important parameter. If the viscosity is too high (for example, at low temperatures), the liquid cannot fill the suction cavity of the pump quickly enough, creating rarefaction and cavitation. If the viscosity is too low (for example, with excessive overheating), the fluid does not provide sufficient lubrication and may form vapor bubbles more easily than at optimal viscosity.

How to confirm:

• Measurement of the actual temperature of the liquid in the system.
• Measurement of kinematic viscosity of a liquid using a portable viscometer or laboratory analysis.
• Comparison of the obtained values ​​with those recommended by the manufacturer of the pump and hydraulic fluid (according to ISO VG and DSTU EN ISO 3448).

What damage it does:

• At high viscosity: overloading of the pump drive, cavitation, reduced productivity.
• At low viscosity: insufficient lubrication, increased wear, overheating, reduced efficiency, possibility of formation of steam bubbles.

8. Step-by-step troubleshooting procedures

8.1. Adjusting the hydraulic fluid level

1. Safety: Perform the LOCKOUT/TAG (LOTO) procedure and depressurize the system.
2. Checking the level: Examine the level indicator on the hydraulic tank.
3. Fluid topping up: Top up with hydraulic fluid that meets the pump manufacturer's specifications (eg DIN 51524 HLP 46, cleanliness class no worse than NAS 7 according to ISO 4406). Use a filter cart to avoid additional contamination. Bring the level to the upper MAX mark.
4. Important: Do not overfill the tank to avoid foaming.
5. Startup and test: Start the system, let it run for a few minutes without load, then under load. Check the absence of noise and vibration, as well as the stability of the fluid level.

8.2. Cleaning/replacing the intake filter/mesh

1. Safety: Execute LOTO and release the pressure.
2. Filter Access: Locate the inlet filter or screen (usually inside the tank, at the end of the suction line).
3. Disassembly: Carefully disassemble the filter. Be prepared to drain a small amount of liquid.
4. Condition assessment: Visually assess the degree of filter contamination.
5. Cleaning/Replacing:
• Cleaning: If the filter is reusable (mesh), wash it thoroughly with a suitable solvent and blow with compressed air. Make sure all dirt is removed.
• Replacement: If the filter is disposable or damaged, replace it with a new element that meets the original specifications (size, material, nominal filtration size, eg 125 µm).
6. Assembly: Install a cleaned or new filter, making sure all seals are in the correct position. Tighten the connection to the recommended torque (according to DSTU EN ISO 4759-1).
7. Start-up and check: Start the system, check the tightness of the connections, monitor the inlet pressure (should be in the range of 0 ... -0.2 bar) and the absence of cavitation symptoms.

8.3. Elimination of air suction

1. Safety: Execute LOTO and release the pressure.
2. Visual inspection: Carefully inspect the entire suction line from the hydraulic tank to the pump: hoses (for cracks, abrasions, kinks), metal pipes, flanged and threaded connections, pump shaft seals, filter covers.
3. The 'soap water' test: Apply a soapy solution to the suspected joints. Run the system (briefly if necessary) and watch for bubbles to indicate air suction.
4. Corrective actions:
• Loose connections: Tighten threaded or flanged connections to recommended torques.
• Damaged seals/gaskets: Replace with new ones made of suitable material (eg NBR for mineral based hydraulic oils).
• Damaged hoses/pipes: Replace deformed, cracked or frayed hoses/pipes. Make sure the new pipe has the correct diameter and minimum number of bends.
5. Check: After troubleshooting, recheck the tightness of the connections. Start the system and monitor the absence of noise, vibration and foaming of the liquid.

8.4. Adjusting the viscosity of the hydraulic fluid

1. Temperature check: Using an infrared thermometer, measure the temperature of the liquid in the tank and at the pump inlet. The optimal temperature range is usually 40-60°C.
2. If the temperature is too low (<20°C):
   
   • Reason: High viscosity of the liquid, which makes it difficult to flow.
   • Action: Use the stock fluid heater (if available) or allow the system to warm up without load. Consider replacing the fluid with one that has a lower viscosity index or better low-temperature characteristics, according to DSTU EN ISO 3448.
3. If the temperature is too high (>70°C):
   
   • Reason: Low viscosity of the liquid, risk of steam bubbles and deterioration of lubrication. Check the cooling system (radiators, fans).
   • Action: Restore cooling system efficiency. If the fluid has degraded due to overheating (laboratory analysis), replace it with a new one that meets the manufacturer's requirements. Consider using a higher viscosity index fluid for high temperature applications.
4. Startup and Check: After stabilizing the temperature and/or changing the fluid, start the system and make sure there are no symptoms of cavitation.

9. Preventive measures

10. Spare parts and components

For quick and efficient cavitation troubleshooting, it is recommended to always have the following spare parts in stock:

You can order all necessary components and liquids in the UNITEC-D catalog: www.unitecd.com/e-catalog/

11. Links

• DSTU EN ISO 4406: Liquid hydraulics. Codification of the level of pollution by solid particles.
• DSTU EN ISO 4413: Hydraulic systems. General safety rules and requirements for systems and their components.
• DSTU EN ISO 3448: Lubricants, industrial oils and related products. Viscosity classification (ISO VG).
• DSTU ISO 6789-1: Mounting tool for threaded connections. Manual dynamometric tool. Part 1: Requirements and test methods for structural compliance, compliance verification and recalibration.
• Operation and maintenance manuals from hydraulic pump manufacturers (eg Bosch Rexroth, Eaton, Parker).
• UNITEC-D internal manuals: "Fundamentals of maintenance of hydraulic systems", "Analysis of hydraulic fluids".