Hydraulic Pump Cavitation Diagnosis and Elimination: Troubleshooting Guide

1. Problem Description and Area of Application

Cavitation in industrial hydraulic systems is a critical destructive process that occurs due to the formation and instantaneous collapse of vapor bubbles in a liquid flow as a result of pressure drop below the saturated vapor pressure of the working medium. This manual covers the diagnosis and elimination of cavitation processes in gear, vane, and axial piston pumps used in presses, casting machines, metallurgical equipment, and power plants of Ukrainian industrial enterprises.

Severity Classification: Critical Condition (Severity Level 1). Ignoring cavitation symptoms leads to catastrophic damage to pump components (erosion of metal plungers, plates, housings), sharp drop in volumetric efficiency, oil overheating and complete seizing of the unit within 50–200 hours of operation.

2. Precautionary Measures and Safety

WARNING! HIGH PRESSURE AND HOT ENVIRONMENT:

• Lockout/Tagout (LOTO): Before performing any mechanical work or inspecting the suction line, completely disconnect the motor power supply and lock out the main switch.
• Зняття залишкового тиску: Переконайтеся у відсутності залишкового тиску в гідросистемі, відкривши запобіжні клапани або зливні магістралі. Робочий тиск у системі може досягати 350 bar.
• Засоби індивідуального захисту (ЗІЗ): Обов'язкове використання захисних окулярів, маслостіклих рукавиць, захисного взуття з металевим носком та термостійкого одягу. Робоча температура оливи у баку може досягати +80 °C.
• Небезпека інжекції рідини: Ніколи не перевіряйте герметичність всмоктувальних та напірних рукавів рукою. Тонкий струмінь рідини під високим тиском здатний пробити шкіру та спричинити важку інтоксикацію організму.

3. Required diagnostic tools

4. Initial Inspection Checklist

5. Systematic Diagnostic Scheme (Decision Tree)

1. Symptom: Occurrence of a characteristic metallic noise ("grinding noise" or crackling) in the pump area.
   • Check the oil level in the hydraulic reservoir during peak loads.
     • IF the level is below the critical minimum → Cause: Insufficient oil level or formation of aeration above the return port → Actions: Stop the system, add certified hydraulic oil according to ISO VG.
     • If level is normal → Proceed to step 2.
2. Check suction line resistance (filters, valves).
   • Install a vacuum gauge on the suction port of the pump.
     • IF vacuum exceeds -0.25 bar (absolute pressure < 0.75 bar) → Cause: Clogged suction filter or closed shut-off valve → Actions: Clean/replace filter element, fully open the ball valve on the tank.
     • If the vacuum is within the normal range (-0.05 ... -0.2 bar) → Proceed to step 3.
3. Checking the viscosity of the working fluid and temperature.
   • Measure the oil temperature and calculate the kinematic viscosity.
     • IF temperature is higher than +75 °C or viscosity is lower than 10 cSt → Cause: Overheating of the system, degradation of the additive package, too thin oil → Actions: Check the operation of the heat exchanger (cooler), replace the oil with a higher viscosity grade (for example, ISO VG 46 instead of ISO VG 32).
     • IF temperature is below +15 °C or viscosity is above 150 cSt → Cause: Cold start, too viscous oil → Actions: Switch on the oil preheating system until the operating temperature reaches a minimum of +25 °C before starting the pump.
     • If viscosity is within specification (16–100 cSt) → Proceed to item 4.
4. Air suction leakage diagnosis on the suction line.
   • Use an ultrasonic leak detector or the technical alcohol spray method on the pipe connections.
     • IF a drop in noise or change in tone is detected when spraying alcohol on the connection/flange → Cause: Suction hose leakage, wear of flange seal rings or pump shaft seal → Actions: Tighten threaded connections, replace rubber seals or seal.
     • If no leakage is detected through the connection → Proceed to item 5.
5. Checking of rotational speed and condition of the drive motor.
   • Measure the actual rotational speed of the pump shaft with a tachometer.
     • IF the rotational speed exceeds the pump's nominal characteristic by more than 5% → Cause: Mismatch between the frequency converter frequency or gear ratio → Actions: Reduce the drive speed to the nominal value.
     • If the rotation is normal, but cavitation persists → Cause: Internal pump defect (damaged distributor, wear of the pumping unit) → Actions: Disassemble the pump for inspection.

6. Fault Matrix and Causes

7. Root Cause Analysis

Cause A: Inlet Restriction

Why it occurs: Clogging of mesh filters on the suction line due to wear products of the system, excessively long suction line, presence of unnecessary elbows and constrictions, or partially closed ball valve between the tank and the pump. According to standard ISO 4406, exceeding the oil cleanliness class quickly clogs the fine and coarse filtration elements.

Як підтвердити: Встановлення вакуумного манометрта безпосередньо перед вхідним фланцем насоса. Якщо показник вакууму перевищує -0.25 bar (для мінеральних олив), створюються умови для виділення розчиненого повітря та закипання легких фракцій оливи.

Наслідки: Руйнування торців лопатей пластинчастих насосів, сколи плунжерів аксіально-поршневих машин, кавітаційна втома матеріалу корпуса навколо вікон розподільної плити (утворення каверн глибиною до кількох міліметрів).

Cause B: Air Leaks through the suction line

Why it occurs: Loss of elasticity of rubber seals on flange connections, loosening of clamps on suction hoses, cracks in the welded joints of the suction pipe below the oil level, or a defect in the pump drive shaft seal (if the suction chamber is under vacuum relative to the atmosphere).

Як підтвердити: Використання ультразвукового течешукача або обробка підозрілих стиків мильним розчином чи технічним спиртом. При попаданні спирту в мікротріщину під дією розרідження обороти або звук двигуна миттєво змінюються.

Наслідки: Дизельний ефект (мікровибухи стисненого повітря у циліндрах під високим тиском), обугорювання ущільнень, деградація оливи, термічне пошкодження робочих поверхонь через втрату змащувальної здатності піни.

Cause B: Fluid Viscosity Issues

Why it occurs: Operation of the system at temperatures below permissible limits (cold start with heavy oil having a viscosity > 150 cSt) or operation with overheated oil (above +75 °C) resulting in critical viscosity drop below 10 cSt, which disrupts the hydrodynamic oil wedge in friction pairs.

Як підтвердити: Зняття показників датчика температури оливи у баку та зіставлення їх із графіком залежності в'язкості марки оливи (наприклад, HLPD або HLP згідно DIN 51524).

Наслідки: Задири на плунжерах, знос блоку циліндрів, повне заклинювання насоса при пуску в зимовий період.

8. Step-by-step troubleshooting procedures

Procedure 1: Removal of restrictions in the suction line and filter replacement

1. Perform the LOTO procedure: de-energize the pump motor, install warning tags.
2. Override the manual shut-off valve on the suction line between the hydraulic reservoir and the pump (if it is installed below the oil level — prepare a container for collecting residual liquid).
3. Remove the suction filter or filter element. Conduct a visual and microscopic inspection of the contamination.
4. Clean the filter housing and install a new filter element with a filtration rating according to the pump manufacturer's recommendations (usually 10–25 microns for the discharge line, and a metal mesh 100–140 mesh for the suction line).
5. Check the internal diameter of the suction hose: it should not be smaller than the diameter of the pump inlet pipe. The length of the pipeline should be minimal.
6. Fully open the shut-off valve.
7. Fill the pump with clean working fluid through the drain or upper pressure port before the first start-up (dry start is strictly prohibited!).
8. Start the drive in coast-to-coast mode (short-term pulse starts) for 10-15 seconds to purge air from the system, then switch to operating mode.
9. Check the vacuum at suction: the nominal value in steady state should not exceed -0.15 ... -0.2 bar.

Procedure 2: Air Inlet Elimination and Seal Replacement

1. De-energize the hydraulic station (LOTO).
2. Drain the working fluid from the suction section or all the oil from the tank if the level is above the repair point.
3. Disassemble the flanged connections of the suction line. Clean the sealing surfaces from residue of sealant and dirt.
4. Replace all rubber sealing rings (recommended material FKM / Viton for resistance to lubricants and temperature) with new ones according to the O-ring specification.
5. Check the condition of the rubber suction hose for the presence of micro-cracks or delamination of the internal reinforcing layer. Replace the hose with a reliable oil-resistant reinforced hose if necessary according to ISO 3862.
6. Tighten the flange bolts with even crosswise torque using a torque wrench (for M8 thread, torque is 25 Nm, for M10 — 49 Nm).
7. Check the pump shaft seal. If oil marks are found on the front pump cover when the unit is stopped or air suction occurs during operation, remove the seal and install a new original seal.
8. Perform a control test using an ultrasonic flow meter.

9. Preventive Measures

Primary Cause	Preventive strategy	Monitoring method	Recommended interval
Suction limitations (filter clogging)	Regular pressure drop monitoring and planned filter element replacement	Differential contamination indicator / pressure sensor ΔP	Every 500 hours or when the indicator activates
Air leakage through the connection	Torque control of flanges, inspection of hose integrity	Visual inspection, ultrasonic testing	Every 1000 hours (or once every 6 months)
Viscosity and temperature mismatch	Use of oil heating systems in winter and powerful coolers in summer	Thermostat, temperature sensor control in the tank	Continuous monitoring (SCADA / controller)
Low level of oil in the tank	Engineered Start Lockout at Critical Level Drop	Oil Level Minimum Float Sensor	Daily check before start-up

10. Spare Parts and Components

Detail Description	Specification	When to replace	Category UNITEC
Suction filter element	Filter mesh fineness 100–125 µm, strong metal mesh	When ΔP > 0.3 bar or every 2000 hours	Hydraulic filters
Set of shaft seals and pump flanges	Material FKM (Viton), resistant to -20...+120 °C	At each major overhaul or appearance of suction signs	Hydraulic seals
Suction armored hose	Oil-resistant, with internal spiral, ISO 3862	When microcracks, delamination or every 4 years appear	Hydraulic hoses and fittings
Working hydraulic oil	HLP / HVLP according to DIN 51524, class ISO VG 32 / 46	According to the results of the laboratory analysis of the oil (usually every 4000-5000 hours)	Industrial oils and greases

Full range of certified components for hydraulic system repair is available in our catalog: https://www.unitecd.com/e-catalog/

11. References and Regulatory Basis

• DSTU ISO 4406:2003 — Hydraulic fluid. Method of coding the level of solid particle cleanliness.
• DSTU EN ISO 4413:2014 — Hydraulic fluid power. General rules and safety requirements for systems and their components.
• DIN 51524 Частини 1-3 — Гідравлічні рідини. Характеристики та вимоги до мастил типу HL, HLP, HVLP.
• ISO 3019-1/2 — Hydraulic pumps and motors. Coupling flanges and shaft ends.
• Technical documentation and operation manuals of leading OEM manufacturers of hydraulic equipment (Bosch Rexroth, Parker Hannifin, Danfoss Hydraulics).
• Related Guides UNITEC-D: "Hydraulic System Overheating Diagnosis and Oil Viscosity Selection", "Prevention of Failures in Axial Piston Pumps".