Diagnosis and correction of cavitation in hydraulic pumps

1. Problem description & scope of application

Cavitation in hydraulic pumps is a critical operating problem that can result in significant wear, loss of performance and ultimately pump failure. It occurs when the static pressure in the pump suction line falls below the vapor pressure of the hydraulic fluid. This leads to the formation and subsequent collapse of vapor bubbles, creating pressure spikes and local temperature increases that cause material erosion.

This guide covers diagnosing and resolving cavitation caused by the following main factors:

• Suction side restrictions: Blocked filters, narrowed lines or inadequate sizing.
• Low reservoir level: Insufficient fluid supply to the pump.
• Insufficient fluid viscosity: Deviations from manufacturer specifications, often due to temperature issues.
• Air entering the suction line: Leaks at connections or seals.

Affected system types include all hydraulic systems with positive displacement pumps, such as gear pumps, vane pumps and piston pumps, which are widely used in the DACH manufacturing industry. The severity of cavitation is classified as follows:

• Critical: Clear noises (nails, crackling), strong vibrations, significant loss of power, immediate action required to avoid total failure.
• Major: Audible noise, moderate vibration, slight loss of power, timely diagnosis and resolution required.
• Minor: Occasional slight noise, minor vibration, long-term damage possible, preventive measures and monitoring recommended.

2. Safety precautions

ATTENTION: Hydraulic systems work under high pressure and with potentially hot fluids. Improper handling can result in serious injury or death. Carry out all work in accordance with the applicable safety regulations and standards (e.g. DIN EN ISO 4413, DGUV Rule 113-015). Please note the following critical security steps:

• Disconnecting, securing, reporting (LOTO): Before starting any work on the system, the power supply must be disconnected, secured against being switched on again and the system must be marked as "Do not switch on".
• Pressure Relief: Ensure all system pressures are fully relieved before opening any lines or components. Residual energy can still be present after switching off.
• Personal Protective Equipment (PPE): Always wear appropriate PPE, including safety glasses, gloves, safety shoes and, if necessary, hearing protection.
• Hot surfaces and fluids: Hydraulic fluids can reach high temperatures. Be careful when handling components and liquids.
• Handling hydraulic fluid: Avoid skin contact with hydraulic fluid. In case of eye contact, rinse immediately and seek medical advice. Dispose of used liquids in an environmentally friendly manner.

3. Required diagnostic tools

For a precise diagnosis of cavitation, the following tools and their correct use are essential:

4. Checklist for initial assessment

Before starting the detailed diagnosis, a systematic initial assessment of the system under operating conditions must be carried out. This provides valuable clues:

5. Systematic diagnostic flow chart

This decision tree guides the technician through the systematic diagnosis of cavitation problems:

1. Symptom: Loud noises (crackling, banging) and/or vibrations at the pump.
   1. Check 1: Reservoir level and fluid condition.
      1. IF Level below minimum THEN Probable Cause: Insufficient Container filling level. Go to 7.1.
      2. ELSE IF Fill level correct THEN continue to b.
   2. Check 2: Hydraulic fluid viscosity and temperature.
      1. Measure fluid temperature with IR thermometer. Check viscosity with viscometer.
      2. IF Viscosity too high (fluid too cold) THEN Probable Cause: Fluid viscosity too high (cold start). Go to 7.2.
      3. ELSE IF Viscosity too low (fluid overheated) THEN Probable Cause: Fluid viscosity too low (overheating). Go to 7.3.
      4. ELSE IF Viscosity and temperature in the target range THEN continue to c.
   3. Check 3: Suction pressure measurement and visual condition of the suction line.
      1. Install vacuum gauge on the suction side of the pump (according to manufacturer's instructions).
      2. IF Suction pressure below -0.2 bar (relative) or very close to the vapor pressure of the fluid THEN continue to d (suction side restriction or air inlet).
      3. ELSE IF Suction pressure in acceptable range THEN Probable Cause: Cavitation unlikely due to these factors. Check other causes (e.g. pump defect, pressure peaks).
   4. Check 4: Look for suction side restrictions or air inlet.
      1. IF Pressure gauge on suction line filter shows high pressure drop THEN Probable Cause: Clogged suction line filter. Go to 7.4.
      2. ELSE IF Inspect the suction line with an endoscope (blockages, deposits).
      3. ELSE IF Use an ultrasonic leak detector on all connections and seals in the suction line.
      4. IF Leaks detected THEN Probable Cause: Air entering the suction line. Go to 7.5.
      5. ELSE IF No leaks or filter problems THEN Probable Cause: Insufficiently dimensioned suction line or components (e.g. too small a diameter, too many bends). Go to 7.6.

6. Error-cause matrix

This matrix compares common symptoms of cavitation, likely causes, recommended diagnostic tests, and expected results.

7. Root cause analysis for each error

A deep understanding of the causes is crucial for sustainable solutions.

7.1. Insufficient tank level

Why it happens: A low fluid level in the hydraulic reservoir means that the pump cannot suck in enough fluid. The inlet of the suction line may be exposed, allowing direct air entry, or the static pressure across the inlet may be too low to compensate for the pressure loss in the suction line.

How to confirm: Check the fluid level on the sight glass or level indicator. The fluid level is often sufficient when stationary, but during operation it drops below the critical minimum due to volume displacement in cylinders and accumulators. Measure the suction pressure during operation. If the filling level is too low, the suction pressure will be very low.

What damage it causes: Direct air entry and cavitation lead to rapid wear of the pump and the entire hydraulic system due to material erosion and damage to seals. Increased system temperature due to air in the fluid. Reduced pump and component lifespan.

7.2. Fluid viscosity too high (cold start)

Why it happens: At low temperatures, the viscosity of the hydraulic fluid is very high. This significantly increases the flow resistance in the suction line, causing a large pressure drop. The absolute pressure in the pump can fall below the vapor pressure of the cold fluid, even if the system is otherwise correctly sized.

How to confirm: Does cavitation mainly occur after system startup at low ambient temperatures? Measure the fluid temperature in the container and compare the viscosity (with a viscometer) to the manufacturer's specifications for that temperature. A suction pressure below -0.2 bar (relative) with cold fluid is an indicator.

What damage it causes: In addition to increased wear caused by cavitation, excessive viscosity can lead to overloading of the drive motor, sluggish response of the system and poor lubricant film formation.

7.3. Fluid viscosity too low (overheating)

Why it happens: If the operating temperature of the hydraulic fluid is too high, it drastically reduces its viscosity. This lowers the vapor pressure of the fluid, increasing the likelihood of cavitation at normal suction pressures. In addition, the lubricity of the fluid deteriorates.

How to confirm: Measure the fluid temperature in the container during operation. Temperatures above the recommended maximum (e.g. >60-70 °C, depending on the fluid type according to DIN 51524) are critical. Check the viscosity of the fluid with a viscometer and compare it to the manufacturer's specifications.

What damage it causes: Cavitation and inadequate lubrication lead to rapid wear of the pump and valves. In addition, overheated fluid can lead to oxidation and aging of the oil, which promotes the formation of acids and sludge and shortens the life of the entire system.

7.4. Clogged suction line filter

Why it happens: The suction line filter is used to protect the pump from particles. As contamination progresses, the flow resistance in the filter increases. This results in a significant pressure drop in the suction line downstream of the filter element, causing the absolute pressure upstream of the pump to fall below the vapor pressure of the fluid.

How to confirm: Check the pressure gauge on the suction line filter (if equipped) for an increased pressure drop (>0.2 bar). A heavily dirty filter can often be seen visually or is an indicator that the filter needs to be changed. Measure the suction pressure before and after the filter.

What damage it causes: Cavitation causes pitting on the pump components. The reduced flow rate and pressure loss affect system performance. In addition, a collapsed filter can lose its protective function and allow unfiltered particles to enter the pump, causing further wear.

7.5. Air entering the suction line

Why it happens: Leaks in the suction line (e.g. loose screw connections, damaged seals, porous hoses) allow the ambient air to be sucked into the line. This air is sucked in, compressed and expanded by the pump, causing effects similar to cavitation (bubble collapse) and leading to loud noises. Technically this is aeration, which is often confused with cavitation and causes similar damage.

How to confirm: Visually inspect the suction line, connections and seals for leaks or sweating areas. Foam formation on the fluid surface in the container is a strong indicator. An ultrasonic leak detector can identify the smallest leaks. Bubbles moving in the suction line sight glass (if present) confirm air entry.

What damage it causes: Air in the fluid leads to irregular operation, increased wear due to compression/decompression of air bubbles, overheating and faster oxidation of the hydraulic fluid. It reduces lubricity and can lead to piston seizure or bearing damage.

7.6. Insufficiently sized suction line or components

Why it happens: A suction line that is too small in diameter, too long, has too many bends or unsuitable fittings creates excessive flow resistance. This results in a permanently high pressure loss and a critically low absolute pressure on the pump suction side, even under normal operating conditions.

How to confirm: Measure the suction pressure at rated operation. A constantly low suction pressure (below -0.2 bar relative) indicates this. Check the dimensions of the suction line, the suction strainer and the filter in accordance with the pump manufacturer's recommendations and the standards (e.g. DIN EN ISO 4413). Calculate the pressure loss in the suction line (e.g. using the Darcy-Weisbach equation or nomograms).

What damage it causes: Permanent cavitation and the associated wear on the pump and other components. Increased energy consumption due to the unnecessarily high pressure loss. System performance is permanently limited.

8. Step-by-step fix procedure

The fix requires precise action and compliance with technical specifications.

8.1. Correction of the container filling level

1. Shut down and secure system: According to LOTO procedures (see Section 2).
2. Refill fluid: Only use the hydraulic fluid recommended by the manufacturer (specification e.g. ISO VG 46, class - DIN 51524 part 2 HLP, purity class according to ISO 4406: 18/16/13). Fill the container to the upper level marker.
3. Check fluid quality: If necessary, take a fluid sample and check for purity (ISO 4406) and water content.
4. Ventilate the system: Start up the pumps slowly and open all venting points on the system until bubble-free fluid emerges.
5. Functional test: Operate the system under load and check the fill level during operation. Refill if necessary.

8.2. Elimination of excessive fluid viscosity (cold start)

1. Preheating the fluid: Is the system designed for low temperatures? Install a container heating element (e.g. 2 kW with thermostat control at 30-40 °C).
2. Fluid selection: If operating temperatures are permanently low, check whether a hydraulic fluid with better viscosity-temperature behavior (e.g. HLP-D, HVLP according to DIN 51524 Part 3) can be used.
3. Slow startup: After starting, only operate the system without load until the fluid temperature has reached the minimum operating value.
4. Check filter dimensioning: If necessary, select suction line filters with a larger surface area or lower pressure loss during cold start.

8.3. Elimination of fluid viscosity that is too low (overheating)

1. Checking the cooling systems: Clean the heat exchangers and radiators from dirt. Check the operation of cooling fans or radiator pumps.
2. Maintain maximum temperature: Make sure that the fluid temperature does not exceed 60-70 °C (observe manufacturer's specifications).
3. Fluid change: If the fluid quality is severely degraded (dark color, odor, high acid number), a complete fluid change is required.
4. Hydraulic System Efficiency: Check the entire system for leaks or inefficient operating conditions that result in excessive heat generation.

8.4. Elimination of clogged suction line filters

1. Shut down and secure the system: According to LOTO procedures.
2. Change filter element: Replace the dirty filter element with a new, suitable original spare part (e.g. UNT-SF-100, 10 µm nominal, 150 l/min flow).
3. Check filter health indicator: Verify that the filter health indicator (if present) is functioning correctly and is reset.
4. Bleeding the system: After changing the filter, carefully bleed the suction line and pump.
5. Fluid analysis: If the filter was heavily contaminated, a fluid analysis makes sense to identify the cause of the contamination and check the purity of the fluid (ISO 4406).

8.5. Elimination of air ingress into the suction line

1. Shut down and secure the system: According to LOTO procedures.
2. Leak Detection: Identify the exact leak location using visual inspection, ultrasonic leak detector or by applying leak detection spray.
3. Fix leakage:
• Loose connections: Tighten screw connections (e.g. according to DIN 2353, torque according to manufacturer's table), tighten hose clamps (e.g. DIN 3017).
• Defective seals: Replace damaged O-rings (material NBR or FKM, hardness 70 Shore A), flat gaskets or shaft seals (e.g. DIN 3760).
• Damaged hoses/lines: Replace porous or cracked hoses (e.g. according to DIN EN 853) with new, appropriately sized and pressure-resistant lines.
4. Bleeding the system: After repairs, carefully bleed the suction line and pump.
5. Functional test: Operate the system under load and monitor for foam formation in the container or new leakage noises.

8.6. Elimination of inadequately sized suction lines

1. Shut down and secure the system: According to LOTO procedures.
2. Recalculation and Sizing: Perform a detailed recalculation of the suction line based on the maximum pump flow rate, fluid viscosity and line length. The goal is a suction pressure of at least -0.1 bar (relative) in normal operation. Consider the guidelines according to VDI 2242.
3. Replacing components: Replace inadequately sized suction lines, suction strainers or filters with components with a larger diameter or lower pressure loss. Reduce the number of arcs and use radii as large as possible.
4. Checking the assembly: Make sure that the suction line is installed without tension and with little vibration.
5. Bleeding and testing the system: After the conversion, carefully bleed the system and measure the suction pressure again under nominal load.

9. Preventive measures

Preventive maintenance is the key to preventing cavitation and maximizing the life of hydraulic systems.

10. Spare Parts & Components

The availability of original spare parts is crucial for quick and lasting repairs. All components listed below can be found in our UNITEC-D e-catalog.

Visit our UNITEC-D E-Catalog for a comprehensive selection of hydraulic components and spare parts.

11. References

• DIN EN ISO 4413: Hydraulic fluid technology - general rules and safety requirements for hydraulic systems and their components.
• DIN EN ISO 4406: Hydraulic fluid technology - Code for determining the degree of contamination of solid particles in fluids.
• DIN 51524: Hydraulic fluids – HLP and HLPD oils.
• VDI 2242: Avoidance of cavitation in fluid energy machines.
• VDI 3834: Measurement and assessment of machine vibrations - piston machines.
• DGUV Rule 113-015: Hydraulic systems.
• Manufacturer-specific documentation for hydraulic pumps and systems.