Hydraulic System Flushing

How To Get The Best Results When Flushing Your Hydraulic System

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Oil Sampling
The single most important preventive maintenance procedure is regular oil sampling and analysis. Over 96% of all hydraulic failures can be traced directly to oil that has been improperly cared for. The only way to tell for sure if the hydraulic oil needs attention is to have it analyzed. And just as a doctor can see signs of illness in a patient by the analysis of body fluids, a trained hydraulic technician can find signs of component wear by oil analysis.

But even when an oil analysis is returned with unsatisfactory results, it is not always necessary – or even desirable – to replace the oil. As long as the oil has not lost viscosity and has not been damaged by overheating, it can be reclaimed by flushing the system. When the oil in the reservoir is merely replaced, several gallons of bad oil are left in the lines and components – as much as half of the oil in some machines. As the new clean oil passes through the machine, it mixes with the old oil and absorbs contaminants and varnishes left on the surfaces of the components returning them to the reservoir and circulating them through the machine to cause even further damage. Often within a few weeks of operation, the next oil analysis is not much better than the one of the old oil.

Cleaning The Reservoir
The first step to flushing the system is to drain all of the old oil from the reservoir into separate container. The reservoir is then cleaned manually to remove all sludge and foreign matter. While the reservoir is being cleaned, the flushing machine is attached to the separate container to clean and reclaim the oil that came from the reservoir.


As shown in the picture on the right, what comes from the reservoir can tell a lot about the condition of the hydraulic system. It is during this step that evidence of component wear can be found and dealt with before causing a loss of production.

The Flushing Machine
There are several flushing machines on the market that will do an excellent job. But when we at GPM were looking for the best flushing machine, the features we looked for were:

  • Ease of use and maintenance
  • Durability and ability to withstand exposure to the elements
  • Flow rate
  • Level of filtration

Our machines can be left running outdoors without damage and delivers a good flow rate (a 5 GPM gear pump is used) to minimize the time that it must stay in service on a given machine. The filter elements used remove particles of contamination below one micron at a very high beta ratio. The result is oil that is ultra-clean and will act as a solvent to remove contaminants and varnishes in the downstream components of the system. These contaminants are returned to the reservoir and trapped by the filters of the flushing machine. Any water that is suspended in the oil is also trapped by the filters. In addition to cleaning hydraulic oil, there are filter elements available for this machine to clean diesel oil, transmission fluid, gear oil, water and anti-freeze.


One of the 1-micron Flushing Machine filters

Reclaiming the oil while refilling the reservoir.


Flushing The System
Once the reservoir has been cleaned and the oil that was removed has completed one pass through the flushing machine, the cleaned oil is replaced and the system is returned to service. The flushing machine is then installed to the reservoir to circulate oil through its filters while the system operates. The ultra-clean oil is sent through the system absorbing contaminants and varnishes while the system is in service. But instead of circulating through the system and causing damage, they are trapped by the flushing machine filter and removed from the system leaving only pristine oil and freshly cleaned hydraulic components. This avoids any loss of production as the hydraulic oil is reclaimed.

After The Flushing
The result is oil in the system that is much cleaner than new oil at a fraction of the cost. All of the oil in the system is reclaimed instead of replacing just the oil in the reservoir. The sample bottles on the right show (left to right) oil taken from the system before flushing, oil after one pass through the flushing machine, oil after four hours of flushing and oil after sixteen hours of flushing. These results were achieved on a system with a 500-gallon reservoir. In a little over half a day, all of the oil in this system was reclaimed and all of the components – cylinders, servo valves, pumps, directional valves, etc. – were cleaned to brand new condition.

Contact GPM right away to purchase your own machine and get these kind of results on your own equipment!


Oil samples taken from a 500-gallon reservoir with oil that was badly contaminated and had a high water content (left to right) before flushing, after one pass through flushing machine, after 4 hours of flushing and after 16 hours of flushing.