11 Simple Steps for Flushing a Hydraulic System

11 Simple Steps for Flushing a Hydraulic System

I was recently asked about a procedure for flushing hydraulic systems in order to change from one type of fluid to another. Among the ideas mentioned involved using brake cleaner, diesel fuel or some type of acid cleaning.

However, brake cleaner includes a number of chemicals such as acetone and tetrachloroethylene. These solvents are known to cause problems for nitrile, neoprene, millable polyurethane and silicone seals. Ethylene-propylene (EPDM) seals have a very poor petroleum oil and solvent resistance, and are not recommended for exposure to aromatic hydrocarbons or diesel oil.

Therefore, depending on the types of O-rings and seals in your hydraulic system, the solvents used in brake cleaner and diesel fuel can dry out or damage your system’s O-rings. There is also the issue of compatibility with the new type of fluid that has been chosen.

For these reasons, it's important to understand flushing properly or to use an experienced oil flushing service provider who can help you get the job done right.

In his article for Machinery Lubrication titled “Cleaning and Flushing Basics for Hydraulic Systems and Similar Machines,” Tom Odden outlines the procedure for thoroughly cleaning a hydraulic system. This would be the only “one-size-fits-all” solution and an example of best practices. It involves mechanical and chemical cleaning of both the components and the system.

Of course, not everyone is going to do a complete teardown along with a chemical and mechanical cleaning of each component and the system each time a fluid changeover is performed. So let’s examine what should be done at the bare minimum to clean a hydraulic system.

Step 1

While the fluid is at operating temperature, completely drain the system, paying attention to the reservoir, all lines, cylinders, accumulators, filter housings or any area of fluid accumulation. Also, replace the filters.

Step 2

With a lint-free rag, clean the reservoir of all sludge and deposits. Make sure the entire reservoir is free of any soft or loosened paint.

Step 3

Flush the system with a lower viscosity fluid that is similar to the fluid to be used. A Reynolds number between 2,000 and 4,000 should be selected to achieve enough turbulence to remove particles from the lines. Stroke valves frequently to ensure they are thoroughly flushed. The fluid should be filtered and the flushing should continue until reaching one level beyond the system’s target cleanliness levels. For example, if the target is ISO 15/13/11, continue to flush the system until ISO 14/12/10 is reached.

Step 4

Drain the flushing fluid as hot and as quickly as possible. Replace the filters and inspect/clean the reservoir again.

Step 5

Fill the system to approximately 75 percent with the fluid to be used. Bleed/vent the pump. If the pump has a pressure relief or bypass, it should be wide open. Run the pump for 15 seconds, then stop and let it sit for 45 seconds. Repeat this procedure a few times to prime the pump.

Step 6

Run the pump for a minute with the bypass or pressure relief open. Stop the pump and let it sit for a minute. Close the bypass and permit the pump to operate loaded for no more than five minutes. Allow the relief valve to lift to confirm that it is flushed as well. Do not operate the actuators at this time. Stop the pump and let the system sit for about five minutes.

Step 7

Start the pump and operate the actuators one at a time, allowing fluid to return to the reservoir before moving to the next actuator. After operating the final actuator, shut down the system. Keep an eye on the fluid level in the reservoir. If the level drops below 25 percent, add fluid and fill to 50 percent.

Step 8

Refill the reservoir to 75 percent and run the system in five-minute intervals. At each shutdown, bleed the air from the system. Pay close attention to the system sounds to determine if the pump is cavitating.

Step 9

Run the system for 30 minutes to bring it to normal operating temperature. Shut down the system and replace the filters. Inspect the reservoir for obvious signs of cross-contamination. If any indication of cross-contamination is present, drain and flush the system again.

Step 10

After six hours of operation, shut down the system, replace the filters and sample and test the fluid.

Step 11

The sampling frequency should be increased until you are confident that the system fluid is stable.

Some adherent machine deposits require tactics that are more aggressive than a high-velocity flush, so you must match the flushing tactic and strategy to the problem you are trying to resolve with the flush. Once you understand the problem within the machine that needs to be cleaned, you can then select the appropriate flushing tactic to remedy it. This issue was described in Jim Fitch’s three-part series on flushing for Machinery Lubrication, which can be read at 

At this point, it should be obvious that a fluid changeout is not just a drain-and-fill operation. Care must be taken to confirm that the system is as clean as possible prior to introducing the new fluid. Most changeover procedures suggest that some of the old fluid will need to be either drained off the bottom or skimmed off the top of the reservoir after a period of time.

Just because the changeover has been completed does not mean that you are “out of the woods.” Your system will need to be closely monitored for a while to make certain that the flushing was thorough. Taking the time to verify that the system is fully flushed and purged of the old fluid prior to introducing the new fluid will go a long way toward ensuring a healthier hydraulic system.