Given that removing contaminants from hydraulic oil to extend hydraulic component life is the primary objective of filtration, it is a paradox for filters to be located where they reduce the service life of components they were installed to protect.

So when considering the possible locations for filters in a hydraulic system, the overarching principle should be: first, do no harm. In other words, the cure must not be worse than the disease. With this in mind, let us consider the pros and cons of the various hydraulic filter locations:

Pressure Filtration – Locating filtering media in the pressure line provides maximum protection for components located immediately downstream. Filtration rates of two microns or less are possible, due to the pressure available to force fluid through the media. But filter efficiency can be reduced by the presence of high flow velocities, and pressure and flow transients, which can disturb trapped particles. The major disadvantage of pressure filtration is economic. Because the housings and elements (high-collapse type) must be designed to withstand peak system pressure, pressure filtration has the highest initial and ongoing cost.

Return Filtration – The rationale for locating filtering media in the return line is this: if the tank and the fluid it contains start out clean and all air entering the tank and returning oil is adequately filtered, then oil cleanliness will be maintained. The other advantage of the return line as a filter location is that sufficient pressure is available to force fluid through fine media (typically 10 microns), but pressure is not high enough to complicate filter or housing design. This combined with relatively low flow velocity, means that a high degree of filtering efficiency can be achieved at an economical cost. For these reasons, return filtration is a feature of most hydraulic systems. The main disadvantage of return line filtration is that the back pressure created by the filter element can adversely affect the operation of and/or damage some components.

Off-Line Filtration – Off-line filtration enables continuous, multi-pass filtration at a controlled flow velocity and pressure drop, which results in high filtering efficiency. Filtration rates of two microns or less are possible, and polymeric (water absorbent) filters and heat exchangers can be included in the circuit for total fluid conditioning. The main disadvantage of off-line filtration is its high initial cost, although this can usually be justified on a life-of-machine cost basis.

Inlet (Suction) Filtration – From a filtration perspective, the pump intake is an ideal location for filtering media. Filter efficiency is increased by the absence of both high fluid velocity, which can disturb trapped particles, and high pressure-drop across the element, which can force migration of particles through the media. But these advantages are outweighed by the restriction the element creates in the intake line and the negative effect this can have on pump life.

A restriction at the pump inlet can cause cavitation erosion and mechanical damage. And while cavitation erosion contaminates the hydraulic fluid and damages critical surfaces, the mechanical effect of vacuum-induced forces has a more detrimental impact on pump life.

Pump inlet or suction filters usually take the form of a 150-micron (100-mesh) strainer, which is screwed onto the pump intake penetration inside the tank. In the 20+ years I’ve actively campaigned against their use – for the reasons outlined above – I’m sure I’ve heard all the counter arguments. And most arguments for the use of suction strainers are premised on bad design, bad maintenance, or a combination of both.

The argument that suction strainers are needed to protect the pump from debris which enters the tank as a result of poor maintenance practices is a popular one. The reality is nuts, bolts, tools and similar debris pose minimal threat to the pump in a properly designed reservoir, where the pump intake penetration is located a minimum of 4 inches off the bottom. Of course, the proper solution is to prevent contaminants from entering the tank in the first place.

A similar argument asserts that suction strainers are needed to prevent cross contamination where two or more pumps share a common inlet manifold. Here again, if suction strainers are necessary in this situation, then it is only due to bad design: the manifold has been located below the pumps’ intakes. If properly designed, there should be a head of oil above the inlet manifold and the inlet manifold should be above the pumps’ inlet ports. For cross contamination to occur in this arrangement, debris would have to travel uphill – against gravity and a positive head of oil. Something which is highly unlikely.

But even in situations where a suction strainer is mandated – for whatever reason, the problem is: the cure can actually be worse than the disease. This can make it a costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.