Hydraulic filters

Contaminated fluid causes most hydraulic system failures. Oil in a reservoir may look clean to the naked eye, but silt contamination particles too small to see can still wreck pumps, cause valves to stick, and erode cylinder bores. In many facilities, components may take the blame for problems in error, when contaminated fluid is the culprit. It is amazing that some plants will change pumps every six months (believing that is normal component life), when they could add a proper filtration system and get many times longer pump service life.

A well-filtered hydraulic system should not have particles in the fluid larger than 10 microns. (A micron is 0.000039 inches.) A contamination particle that measures 0.001 in. across is 25 microns. The smallest dirt particle that is visible to the naked eye is 40 microns. Simply looking at an oil sample is not a good way to tell if the filters are cleaning the fluid.

Nominal or absolute are common terms found in hydraulic filter micron ratings. A filter with a nominal rating takes out most of the particles that measure the same size or larger than the stated micron size. A filter with an absolute rating takes out all particles the same size or larger than the rated micron size. A newer filter-rating system called the beta ratio is replacing the old nominal and absolute designations.

The beta ratio indicates what size particles the filter removes, followed by the ratio of the number of this size particle in the fluid upstream from the filter, divided by number of particles that size in the fluid downstream from the filter. For example: a filter rating of beta 5 = 90 indicates the filter will remove 90 of every 100 particles of 5 micron or larger size from the fluid passing through it. The efficiency of this filter would be 98.9% -- or 100 - (100/90).

Most hydraulic filters employ a closely controlled paper fiber mat or a woven wire mesh element to trap particles. While woven wire is more expensive than paper, the ability to manufacture it with more precisely sized fluid flow openings makes it a better choice. Also, woven wire elements can withstand higher pressure drops without collapsing.

Figures 9-1, 9-2, and 9-3 show the symbols used in circuit diagrams for the common filter types. The hydraulic circuit diagrammed in Figure 9-4 has these filters in typical locations.

Suction strainers

Figure 9-4 shows a hydraulic circuit with filters in standard locations. Strainer is a common name for filters with openings of 75 microns or larger. Suction strainers usually are installed in the pump inlet line to protect the pump from large, damaging contamination particles that can cause catastrophic failure. The suction strainer also protects the pump from ingesting any start-up debris left in the tank and piping. In addition, the suction strainer traps large contamination particles introduced to the system from external sources or resulting from internal part failure.

Don’t use filters with low-micron ratings in suction lines because pumps without supercharged inlets can only tolerate a portion of one atmosphere pressure drop without affecting inlet flow. With this low-pressure drop (14.7 psi maximum, at sea level on an average day), a restriction such as a low-micron filter can cause the pump to cavitate. Cavitation will cause pump failure almost as fast as dirty oil, so avoid it in every situation.

Suction strainers are normally available with openings ranging from 75 to 150 microns. Some manufacturers offer inlet filters with ratings as low as 25 microns. These low-micron elements have large filtering surfaces.


















If the pump is force-fed by another pump (sometimes called a supercharge pump), use of a low-micron rated element is possible. The supercharging pump will force fluid through a very fine filter to the working pump without cavitation.

A suction strainer or filter should have a bypass relief valve. Set the bypass to open at a pressure of 1 to 3 psi when the strainer becomes clogged. The reasoning behind this is that the pump will run many hours on contaminated oil, but will fail in a few minutes with little or no oil.

Suction strainers can be located inside or outside the reservoir. Internal strainers are less expensive, but their condition is more difficult to monitor. External strainers are easy to service and often include an indicator to show when the filter starts bypassing. The indicator can be as simple as a vacuum gauge or it might be a vacuum-operated electrical output to a warning light or controller.

Many older circuits have nothing but a suction strainer for filtration. Retrofitting these systems with the off-line or kidney filters discussed later in this chapter is advisable.