Procedure for flushing hydraulic systems: removal of impurities, replacement of filters and analysis of lubricant

1. Scope and purpose

This technical regulation defines a mandatory procedure for washing industrial hydraulic systems, removing solid and liquid contaminants, replacing filter elements and taking lubricant samples for laboratory analysis. The procedure was developed for technical specialists of enterprises in the machine-building, metallurgical and chemical industries of Ukraine. The purpose of the regulation is to restore the cleanliness of the working environment in accordance with the ISO 4406 standard to a level no worse than 16/14/11, to prevent premature wear of pumps, hydraulic distributors and executive cylinders.

Flushing and replacement of the working fluid is performed in the following cases:

• Scheduled oil change after reaching 4000-6000 hours of operation (according to OEM regulations).
• Exceeding the allowable indicators of pollution according to the results of laboratory analysis of the lubricant.
• After overhauling the hydraulic drive (replacing the pump, hydraulic cylinders, main pipes).
• A sharp drop in pressure in the system, overheating of the working fluid above 75 °C or activation of filter clogging indicators.
• Accidental destruction of system components (for example, the destruction of a pump bearing or burrs of plungers).

2. Safety measures

WARNING! Work with hydraulic systems involves high pressure (from 50 to 350 bar), hot liquids (working temperature up to 70 °C) and dangerous chemicals (mineral and synthetic oils).

1. Power Lockout (LOTO): Complete Lockout/Tagout procedure before starting work. Turn off the pump station's main automatic power switch, lock it with a padlock and post a warning sign "DO NOT TURN ON! People are working".

2. Residual pressure reset: Make sure the pressure accumulators are discharged. Open the drain valves of the high pressure line. Remember that a jet of oil under a pressure of more than 20 bar can pierce the human skin and cause blood infection (sepsis).

3. Personal protective equipment (PPE): Use safety glasses with side protection (according to EN 166), oil-resistant nitrile gloves, long-sleeved protective clothing and metal-toed safety shoes (Class S3).

4. Fire safety: Work should be carried out in a well-ventilated room. Smoking and open flames are prohibited. A real powder or carbon dioxide fire extinguisher (min. 6 kg) must be located near the work area.

3. Necessary tools and materials

4. Checklist before carrying out work

5. Step-by-step procedure for performing works

1. Preparing the system for drainage
   • Action: Run the hydrostation for 10 minutes at idle speed to warm the working fluid to a temperature of 40-45 °C to reduce viscosity and lift suspended sediment. Stop the pump and perform the LOTO procedure in step 2.
   • Specific values: Oil temperature — 42 °C (±3 °C).
   • Visual indicator: The indicator on the regular thermometer of the hydraulic tank.
   • Typical mistake: Draining cold oil (below 20 °C), which leads to leaving a large part of dirt and sludge on the walls of the tank.
2. Draining the old working fluid
   • Action: Connect the drain hose to the lower ball valve of the hydraulic tank. Open the faucet and pour the used oil into the prepared container. At the same time, open the top cover (hatch) of the hydraulic tank to ensure air flow and speed up draining.
   • Specific values: A complete drain should last at least 30 minutes.
   • Visual indicator: Complete lack of flow from the drain nozzle.
   • Typical mistake: Closing the faucet before the tank is completely empty or leaving fragments of the old seal inside the tank.
3. Cleaning the inner surface of the hydraulic tank
   • Action: Using lint-free technical wipes and a special cleaning solution (recommended to be compatible with mineral lubricants), thoroughly wash the inner walls of the tank, bottom and partitions. Remove all sediment, varnish deposits and metal shavings. Check the condition of the magnetic traps (plugs).
   • Specific values: The torque of the drain plug with magnet is 45 Nm.
   • Visual indicator: The metal surface is clean, without oil deposits and abrasive particles; magnets cleaned to bare metal.
   • Typical mistake: Using a rag with lint, which will clog the microfilters after filling the system.
4. Dismantling the old filters and cleaning the housings
   • Action: Dismantle the old pressure, drain and breather filter elements. Clean the internal cavities of the filter housings from remaining dirt. Check the sealing rings of the housings for cracks.
   • Specific values: Filter cover tightening torque — 35–40 Nm (according to the housing manufacturer's instructions).
   • Visual indicator: Absence of metal inclusions in the folds of old filter elements when they are visually cut.
   • Typical mistake: Installing new filters without first cleaning the bowls of the filter housings.
5. Connecting the external washing station (filter trolley)
   • Action: Connect the suction hose of the filter trolley to the drain fitting of the hydraulic tank, and the discharge hose to the pressure line of the system via technological adapters. Create a closed tank-filter-cart-tank loop, bypassing sensitive servo valves and pumps (use by-pass lines / bypasses).
   • Specific values: Fineness of filtration of the external cart — 3 μm absolute (Beta 3 >= 200).
   • Visual indicator: The pressure on the manometer of the filter cart does not exceed 2.5 bar.
   • Typical mistake: Flushing through the main pump of the machine without pre-filtering the oil, which leads to abrasive wear of the plungers.
6. Circulation flushing
   • Action: Fill the hydraulic tank with clean flushing oil (or fresh working fluid) to the minimum working level. Turn on the washing station. The time of continuous circulation should be at least 4 hours. Every 30 minutes, switch the manual distributors to flush all dead-end branches of the hydraulic system.
   • Specific values: The oil flow rate in the pipelines must ensure a turbulent regime (Reynolds number Re > 4000).
   • Visual indicator: Stable operation of the trolley pump without cavitation noise.
   • Typical error: Laminar flow regime (low velocity) in which wall deposits and heavy particles remain inside the pipes.
7. Taking a sample of lubricant for analysis
   • Action: During the operation of the washing station (or immediately after its stop), take an average sample of the lubricant in a special clean 100 ml bottle directly from the active zone of the flow using a sampling valve.
   • Specific values: Sample volume — 100 ml. Fill the bottle to 80% (leave the air cushion).
   • Visual indicator: The sample is transparent, without suspended particles and air bubbles.
   • Typical error: Sampling from the bottom of the tank or through an open lid, giving a false-positive high level of contamination.
8. Final filling and commissioning
   • Action: Drain the flushing lubricant (if a special process lubricant was used), fill the tank with fresh working fluid to the nominal level using the filling filter unit. Install new main filters. Remove the LOTO lock, run the main pump in discharge mode for 15 minutes to remove residual air.
   • Specific values: The oil level on the sight glass is in the middle at a temperature of 40 °C. The tightening torque of the hydraulic distributor flange is 28 Nm (for M8 bolts of class 10.9).
   • Visual indicator: Smooth movement of executive cylinders without jerks and extraneous sounds (suction noise).
   • Typical error: Starting an unventilated pump, which leads to burring of the rolling surface and failure during the first minutes of operation.

6. Verification checklist after completion of work

7. Troubleshooting guide

8. Recommended maintenance schedule

9. Reference for spare parts

To order certified components, filters and consumables according to European standards, visit the UNITEC-D electronic catalog: https://www.unitecd.com/e-catalog/.

10. References and regulatory framework

• ISO 4406: Hydraulic fluid. The method of coding the level of pollution by solid particles.
• ISO 2943: Liquid hydraulics. Filter elements. Verification of structural resistance to pressure drop.
• DSTU EN ISO 4413: Power hydraulics. General rules and safety requirements for systems and their components.
• ISO 6743-4: Lubricants, industrial oils and related products (Class H). Hydraulic lubrication systems.
• OEM Technical Manuals for Industrial Hydraulic Power Units (Bosch Rexroth, Parker Hannifin, Eaton Vickers).