Filter Elements Containment Function

by Joe Gordon, Chief Engineer

Fluid power applications, both hydraulic and pneumatic, are vulnerable to contamination. This fact leads to the success and growth of the filter business even in recent recessionary environments. It is critical to maintain the cleanliness of these systems as the filter elements continuously collect particulates.

A typical filter begins with an open area which causes very little resistance to flow. As the system is used, system generated particulates accumulate on the surface of the element and reduce the element open area. Perhaps the term effective surface of the element should be used because there are some very ingenious filter media on the market today which employ depth or a three dimensional matrix to trap particles. If the system flow rate is constant, the rate is constant with respect to fluid velocity. As the open area is reduced in the element, the fluid velocity increases and the rate of contamination increases proportionately with velocity. This type of rapid failure is call and “avalanche breakdown”.

To speak of this condition as a failure raises some interesting questions. The filter has not really failed; in fact, filters usually exhibit better performance as they clog because the contaminants help to filter the fluid. The ability of the filter to allow only clean fluid to pass improves. Of course, the problem is the filter element causes a parasitic pressure drop in the system which wastes energy and cause sluggishness in the fluid power system

Looking at this process mathematically is both interesting and informative. Starting with the energy conservation equation:

The graph shows that the true useful life of a filter is about 80% of its total life and the energy loss increase is very rapid at this 80% point.

This formula works fairly will for both gasses and liquids in fluid power because typical air systems, where P1 = 125 psi and P2 = 115 psi, exhibit very little change in density of the fluid, hence compressibility is not a factor. The gas acts like a liquid at these pressure ratios.

It is evident that a filter condition monitor or a differential pressure gauge which shows the condition of the filter media could be a significant energy saver and optimize the economies of the filter element replacement.

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