Hydraulics At Work

How To Know WHEN To Change Your Hydraulic Filters

Mistake #2 in "6 Costly Hydraulic Mistakes... And How To Avoid Them!" is changing filters on hours in service. If you do this, you're either changing them too early or too late. If you change them early - before all their dirt holding capacity is used up, you're wasting money on unnecessary filter changes. If you change them late - after the filter has gone on bypass, the increase in particles in the oil quietly reduces the service life of every component in the hydraulic system--costing a lot more in the long run.

The solution is to change hydraulic filters when all their dirt-holding capacity is used up - but before the bypass valve opens. This requires a mechanism to monitor the restriction to flow (pressure drop) across the filter element and alert you when this point is reached. Plus, continuous monitoring of the filter elements in a hydraulic system can provide valuable clues to the performance of the filter and the condition of the system. Before I elaborate on this, let's consider some of the advantages and disadvantages of common filter locations in a hydraulic system.

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 positive pressure (in comparison to an intake line filter) available to force fluid through the media. Filter efficiency may 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 system operating 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 reservoir and the oil it contains start out clean and all air entering the reservoir and returning oil is adequately filtered, then fluid 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.

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 water absorbent filters and heat exchangers can be included in the circuit for total fluid conditioning. Off-line filtration has a high initial cost, although this can usually be justified on a life-of-machine cost basis.

Filter condition monitoring
Warning of filter-bypass is typically afforded by visual or electric clogging-indicators. These devices indicate when pressure drop (delta P) across the element is approaching the opening pressure of the bypass valve (where fitted). In the case of a return filter for example, if the bypass valve opens at a delta P of 3 Bar, the clogging indicator will typically switch at 2 Bar.

Advanced filter condition monitoring
Replacing standard clogging-indicators with differential pressure gauges or transducers enables continuous monitoring of the filter element's pressure drop. This permits trending of fluid cleanliness against filter element pressure-drop, which may be used to optimize oil sample and filter change intervals. For example, the optimal change for a return filter in a particular system could be higher or lower than the clogging indicator switching pressure of 2 Bar.

Continuous monitoring of filter pressure drop can also provide early warning of component failures and element rupture. For example, if the delta P across a pressure filter suddenly increased from 1 to 3 Bar (all other things equal), this could be an indication of an imminent failure of a component upstream. Similarly, a sudden decrease in delta P could indicate a rupture in the element, something that a standard clogging indicator will not warn of.

Bottom line: changing filters on hours in service is a costly mistake, and one that a lot of hydraulic equipment users make. There are 5 more to avoid, which I explain in "6 Costly Mistakes Most Hydraulics Users Make…" If you haven't read it yet, download your free copy here.

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