Hydraulic System Flushing Procedure: Comprehensive Guide for Professionals

Hydraulic systems are the lifeblood of many industrial and technological operations and need to be maintained to the highest level to perform and last. One of the key maintenance tasks is hydraulic system flushing which removes contaminants and prevents system failure. Companies like INVEXOIL have developed advanced “Oil Flushing Systems” and offer “Cleaning and Flushing a Hydraulic System” services, we set the standard. This guide goes into the details of hydraulic system flushing steps, so you have the knowledge at the professional level.

Hydraulic System Flushing Meaning

Hydraulic system flushing means cleaning hydraulic circuits to remove dirt, debris, water, oxidation byproducts, and sludge. These contaminants can come from component wear, fluid degradation, or external contamination and can affect system performance and reliability. Hydraulic system flushing is required to achieve ISO cleanliness standards, especially in precision-driven applications such as aerospace, robotics, and heavy machinery.

The process aims to bring the hydraulic fluid cleanliness to a level that meets ISO 4406:1999 standards, often to ISO 16/14/11 or better. This ensures optimal performance, reduces wear, and extends the life of critical components. Flushing can involve different methods, fluids, and equipment depending on the system requirements.

Flushing Steps of Hydraulic System:

1. Prepare and Assess the Hydraulic System
2. Drain the Old Hydraulic Fluid
3. Clean and Remove Contaminant
4. Flush the Hydraulic System
5. Apply Post-Hydraulic System Flushing Procedures
6. Conduct Test and Final Checks

Related Article: Oil Flushing Types: Applications and Properties

Step 1: Preparation and Initial Assessment of Hydraulic System

Before you start the flushing process you need to do a full assessment of the hydraulic system. Proper preparation ensures the flushing process is efficient and effective.

1.1. System Inspection

• Check for leaks: Check all hoses, fittings, seals and connections for signs of leakage.
• Check fluid levels: Check the current fluid level and condition. Look for signs of contamination, discoloration or odor.
• Identify contamination sources: Is the contamination internal wear, external ingress or fluid degradation.

1.2. Tools and Equipment Needed for Flushing Hydraulic System

• Flushing rig with adjustable flow rate and filtration capabilities
• Fluid sampling kit
• Clean hydraulic fluid (OEM approved)
• Filtration units (microfilters and bypass filters)
• Safety equipment (gloves, goggles, protective clothing)
• Digital pressure gauges
• Thermometer and flow meter

1.3. Safety Precautions

• Depressurize the system before you start.
• Wear PPE (Personal Protective Equipment).
• Disconnect power sources to prevent accidental system activation.
• Post warning signs to alert personnel of maintenance in progress.

Step 2: Drain the Old Hydraulic Fluid

Draining the old contaminated hydraulic fluid is a critical step before you can pour in new fluid.

2.1. System Shutdown

• Power off the hydraulic system and disconnect it from the power source.
• Let the system cool down if it was in operation to avoid burns.

2.2. Hyraulic Fluid Draining Procedure

• Open drain valves and collect the old fluid in a designated container for disposal.
• Drain all reservoirs, lines and components, make sure no fluid is left in dead zones.
• Use a vacuum pump to extract fluid from hard to reach areas.

2.3. Disposal Hyraulic Fluid

• Dispose of the old hydraulic fluid as per local regulations.
• Note down the volume and condition of the drained fluid.

Step 3: Hyraulic System Cleaning and Contaminant Removal

Cleaning the inside of the hydraulic system is key to removing sludge, varnish and particulate matter.

3.1. Mechanical Cleaning

• Remove components such as filters, pumps and valves.
• Clean internal surfaces with lint free wipes and approved cleaning agents.

3.2. Chemical Cleaning (Optional)

• For heavily contaminated systems use a chemical flushing agent.
• Make sure the chemical agent is compatible with system materials to avoid corrosion or damage.

Recommended Cleaning Agent Specifications:

• pH 7-9
• Flash point: >100°C (212°F)
• Non-corrosive and non-toxic

Step 4: Flushing the Hydraulic System

Flushing removes residual contaminants and prepares the system for new fluid.

4.1. Flushing Fluid Selection

• Use an OEM approved hydraulic fluid with a similar viscosity to the operating fluid.
• Make sure the fluid has high detergency to remove contaminants.

At the end of this article we provided flushing fluid types.

4.2. Hydraulic Flushing Procedure

• Connect the flushing rig to the system, ensuring all connections are tight to avoid leaks.
• Set the flushing rig’s flow rate to achieve turbulent flow (Reynolds number > 4000). This will remove contaminants effectively.

Formula for Calculating Reynolds Number:

Re=ρ⋅v⋅Dμ Re = \\frac{\rho \cdot v \cdot D}{\mu}

Where:

• ReRe = Reynolds number
• ρ\rho = Fluid density (kg/m³)
• vv = Fluid velocity (m/s)
• DD = Hydraulic diameter (m)
• μ\mu = Dynamic viscosity (Pa·s)

4.3. Monitoring During Flushing

• Use pressure gauges, flow meters and thermometers to monitor system parameters.
• Check filtration units frequently and replace filters as required.

Recommended Flow Rate:

• 2-3 times normal operating flow rate.

Temperature:

• Keep fluid temperature between 40°C and 60°C (104°F to 140°F) to remove contaminants without damaging system components.

Step 5: Post-Hydraulic System Flushing Procedures

After flushing, make sure the system is clean and ready to run.

5.1. Fluid Sampling and Testing

• Take fluid samples from multiple points in the system.
• Test samples for particulate contamination using ISO 4406 cleanliness code.
• Acceptable cleanliness levels are typically ISO 18/16/13 to 20/18/15 depending on system requirements.

5.2. Reassembly

• Put back all cleaned components.
• Replace seals, gaskets and filters as required.

5.3. Fill with New Hydraulic Fluid

• Use new, clean hydraulic fluid approved by the equipment manufacturer.
• Fill the system slowly to avoid air entrainment.

Step 6: Hydraulic System Testing and Final Checks

Once the system is reassembled and filled with new fluid, perform the following tests to ensure it’s running correctly.

6.1. Air Bleeding

• Bleed air from the system to prevent cavitation and smooth operation.
• Open bleed valves at high points and cycle the system components to remove air.

6.2. Pressure and Performance Testing

• Run the system at normal pressure and check for leaks.
• Make sure all components are working as expected.

6.3. Final Cleanliness Check

• Take a final fluid sample to ensure cleanliness targets are met.
• Record all system parameters: pressure, temperature, flow rate.

Types of Hydraulic System Flushing Methods

There are many types of flushing methods, each suited for different conditions and contamination levels. These include:

1. High-Velocity Flushing: This method uses turbulent fluid flow to dislodge contaminants. By running fluid at 3-5 meters per second you get Reynolds numbers over 4000, so it’s great for systems with heavy sludge or embedded particles.
2. Chemical Flushing: Chemicals are introduced to dissolve sludge, varnish or oxidation residues. Common chemicals are alkaline cleaners, solvents or surfactant-based solutions. Compatibility between the chemical and system materials is critical to prevent corrosion or damage.
3. Pulsation Flushing: Pulsation techniques apply high pressure pulses to dislodge stubborn contaminants, especially in dead zones and system crevices. This method simulates operational stress to clean hard to reach areas.
4. Filter Based Flushing: This method uses external filtration units with fine filters (β 200 efficiency for 10-micron particles or smaller). Multi-pass filtration cleans the fluid without disassembling the system.
5. Fluid Circulation Flushing: Continuous fluid circulation, heated to operational temperature (usually 50°C to 65°C) to remove free and dissolved contaminants. This method is less aggressive but suitable for systems that require minimal downtime.

Hydraulic System Flushing Fluids and Their Selection

The selection of flushing fluids depends on the system’s operational conditions, material compatibility and contaminant type. Commonly used fluids are:

• Mineral Based Hydraulic Oils: Compatible with most systems, moderate cleaning efficiency.
• Synthetic Fluids (PAOs or Esters): Superior thermal stability and cleaning performance, great for high temperature or high-pressure systems.
• Water-Glycol Mixtures: Used for fire resistant systems, not so good for sludge removal.
• Specialized Flushing Fluids: Designed with detergents and dispersants to enhance cleaning efficiency and compatible with the system’s operating fluid.

Hydraulic System Flushing Key Parameters and Best Practices

Hydraulic system flushing success depends on adhering to key parameters and best practices:

Table: Hydraulic System Flushing Key Parameters

Parameter	Recommended Value	Notes
Flow Velocity	3-5 m/s	Ensures turbulent flow for contaminant removal.
Temperature Range	50°C to 65°C	Prevents thermal degradation of flushing fluid.
Filtration Efficiency	β 200 for 10-micron particles	Ensures fine particulate removal.
Pulsation Frequency	10-30 Hz	Optimized for dislodging stubborn contaminants.
Pressure Range	5-15 bar	Safe for system components.

Best practices of hydraulic system flushing are:

• Flushing equipment material compatibility to prevent chemical reactions.
• Using a dedicated filtration system to prevent cross contamination.
• Pre-flush inspection to identify areas that need special attention.
• Documenting cleanliness levels before, during and after flushing.

Conclusion

Hydraulic system flushing is a complex but necessary process to maintain hydraulic system efficiency and reliability. INVEXOIL’s Oil Flushing System and services show the level of sophistication required for cleaning and maintenance. By understanding and applying advanced techniques and best practices, engineers can have their systems running at peak performance. With this knowledge, hydraulic system flushing goes beyond maintenance, it’s the foundation of operational excellence and reliability.