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.

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.

28% of lubrication professionals say mechanical cleaning is the flushing method used most frequently at their plant, according to a recent poll at machinerylubrication.com

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.

Flushing Tactics

There are a lot of different ways to flush out a machine. You want to match the flushing method to the flushing condition. Following are common tactics for accomplishing this:

Drawdown Filtration/Separation — Contaminants or insoluble suspensions removed by filtration or separation technologies at normal flow rates.

High Turbulence, High Fluid Velocity, Low Oil Viscosity — Flushing is enhanced by high turbulence flushing conditions by lower flush oil viscosity and increasing oil flow rates.

High Flush Oil Temperature — This reduces viscosity, increases turbulence and increases oil solvency. Temperatures in the range of 175 to 195 degrees F are generally targeted.

Cycling Flush Oil Temperature — Using heat exchangers and coolers to change temperature during flushing across a 100 degree F range helps dislodge crusty surface deposits.

Pulsating Flush Oil Flow — Rapidly changing flow rates by pulsation help dislodge contaminants from nooks and crannies.

Pneumatic Vibrators and Hammers — Used to break loose debris from pipe walls and connectors.

Sparge Flush — Air or nitrogen is bubbled into the flush fluid to improve cleaning effectiveness.

Reverse Flush Oil Flow — By changing fluid flow direction, some contaminants and surface deposits can be dislodged and washed away.

Wand Flush Tool — Used for wet sumps, gearboxes and reservoirs with access hatches and clean-out ports. A wand on the end of a flushing hose is used to direct high-velocity oil flow to loosen deposits or for picking up bottom sediment.

Charged Particle (Electrostatic) Separators — Some suppliers have demonstrated success at removing varnish from machine surfaces and stripping out submicron soft contaminants that can contribute to varnish and sludge.

Solvent/Detergent Flush Fluid — Various solvents and detergents have been used with different degrees of success, including mineral spirits, diesel fuel, motor oils and detergent/dispersant packages.

Chemical Cleaning — These are chemically active compounds, typically caustics and acids, used to aid in the removal of organic sludge and oxide deposits.

Mechanical Cleaning — This involves the use of scrapers, brushes and abrasives, typically used with solvents and other chemicals, to remove hard adherent surface deposits.

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 www.machinerylubrication.com/Read/609/oil-flush, www.machinerylubrication.com/Read/634/oil-flushing-tactics and www.machinerylubrication.com/Read/657/flushing-oil.

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.

References

Odden, Tom. (2001). “Cleaning and Flushing Basics for Hydraulic Systems and Similar Machines.” Machinery Lubrication.

Ruble, Lyle. (2006). “A Guide to Converting Hydraulic Systems from Mineral Oil to Synthetic Hydraulic Fluid.” MRL Hydraulics..