Viewpoint: the pressure is on to filter out contamination

Selection of inadequate filters or poor maintenance procedures can cause excessive contamination levels that may result in the unreliable operation and breakdown of hydraulic components.

Filtration systems should be designed such that the fluid cleanliness level is better than that specified by the component manufacturers.

Loaded contact regions are typically where metal particles generated are in pumps and also where particles in the incoming fluid will accelerate the wear process.

Clearances in pumps, motors and valves are of the order of a few microns. It is essential that the particles in the fluid are maintained at a size that is appropriate for the prevention of wear in the clearances.

Filtration systems should be designed such that the fluid cleanliness level is better than that specified by the component manufacturers

Filters clearly have to have a rating that protects the smallest clearance in the system.

As fluid passes radially inward through the element of a high-pressure filter, contaminant is trapped in the material. With time, pressure drop across the filter will increase at a rate that is dependent on fluid condition. Eventually, this will cause the bypass valve to open, thus passing contaminated fluid directly into the system.

However, pressure drop can be monitored either mechanically or by electronic methods to give an early warning of bypassing. This aspect is an important feature in a properly maintained system.

A major problem associated with filtration is that its effect cannot be seen because of the small size of particles that can cause poor system reliability and component failure. So, it is important that monitoring of the filter condition is carried out on a regular basis.

Sampling techniques and the measurement of contaminant concentration provide an improved basis for monitoring the condition of the hydraulic system. Online monitoring is the most cost-effective method.

Performance of a filter is based on its ability to trap particles, defined by its beta ratio, β, that is obtained from an internationally accepted laboratorybased test method.

Filter performance has an immediate effect on the cleanliness level of the fluid downstream of the filter – the higher the beta ratio, the cleaner the fluid

The filtration media of most modern hydraulic filters consist of fibrous material, usually filaments of glass, and this gives the filter a different performance with size, which demonstrates what happens to the different grades of media when tested using the Multi-pass test.

Filter rating is obtained from this data and is the micron size (x) where a stated β value is attained, usually β x = 200 or β x = 1,000. Note that calling the rating ‘absolute’ for these filters has been discredited due to the statistical nature of particle removal.

Filter performance has an immediate effect on the cleanliness level of the fluid downstream of the filter – the higher the beta ratio, the cleaner the fluid.

The filters are selected on the basis of achieving desired contamination levels and having sufficient contaminantholding capacity to maintain the required contamination levels under the worst envisaged circumstances.

Various selection methods are available from different filter manufacturers. For consistency, the process developed by the British Fluid Power Association (BFPA) has been developed into two ISO Standards.

Contaminant levels are denoted by an ISO code that is related to the number of particles of sizes greater than four, six and 14 microns, respectively; if the analysis method is using an automatic particle counter, but if it is by microscope, the ISO 4406 Code is at -/5/15 microns.

• Professor Peter Chapple is chairman of the British Fluid Power Association/TC8 product testing committee. The above article has been edited from his book Principles of Hydraulic Systems Design by Peter Chapple