Part 5: Placements of filters in the system

hydraulic-filtersMost owners and operators of hydraulic equipment will not get the opportunity to choose where their filters are placed in a system. Nevertheless, a knowledge of the advantages and disadvantages of various filter placements can allow operators to make better decisions, upgrade their systems and ultimately to reduce down time and costs.


Common filter placements


Most hydraulic systems use return line filtration. Return line filtration relies on the oil in the reservoir starting out clean and being kept clean. This requires all air and all oil entering the reservoir to be filtered.

An advantage of the return line as a filter location is that sufficient pressure is available to force oil through fine media (typically 10 microns), however pressure is not high enough to complicate filter or housing design. This advantage, combined with relatively low flow velocity, means that a high degree of filtering efficiency can be achieved at an economical cost. The main disadvantage of return line filtration is that the back pressure created by the element can adversely affect the operation of and/or damage some components.


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.

However, these advantages are outweighed by the restriction the element creates in the intake line and the negative effect this can have on pump life. Almost all hydraulic pumps are sensitive to restrictions at the pump inlet. Any restriction is the suction line can cause cavitation erosion and mechanical damage. For this reason if a suction filter is required, these filters are mostly mesh strainers normally 50 -150 microns in size, and rated at a very low pressure drop.


Pressure filters are located downstream from the system pump. They are designed to handle the system pressure and are sized for the specific flow rate in the pressure line where they are located. Pressure filters are especially suited for protecting sensitive components directly downstream from the filter, such as servo valves. Located just downstream from the system pump, they also help protect the entire system from pump generated contamination. 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.

Air Filter

This forgotten system filter is an important, integral part of the contamination control package. Any breathing system (one which allows air to be drawn into and expelled from the fluid reservoir during system operation) will require an air filter. A good quality air filter, sized to meet the needs of the system components will be an effective barrier against the ingress of contaminants. Furthermore, system filters will be much more effective if they are only required to take out contaminants generated from within the system itself, rather than those introduced from outside as well.

Off-line filtration

Also referred to as recirculating, kidney loop, or auxiliary filtration, this filtration system is totally independent of a machine’s main hydraulic system. An independent system pumps oil out of the reservoir, through the filter, and back to the reservoir in a continuous fashion. With this “polishing” effect, off-line filtration is able to maintain a fluid at a constant contamination level.

Off-line filtration is also useful in cleaning a system which has had a spike in contamination.

Best Practice

Owners/ operators of hydraulic equipment should examine their equipment and the operations manuals so that they are aware of the types and positions of the filters in their system. They should always make sure that anything (0il or air) that enters a hydraulic system is clean.

If you are unsure of the placement or type of filter or you would like to improve the system, or would like to add a filter to your transfer pump, speak to your hydraulic specialist. Email: or phone +61 7 4978 6099



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