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Consider the following scenario: “We have been involved in designing and building a hydraulic machine. The system has three separate circuits, each with an axial-piston pump and a common reservoir. Case drain filtration was included to reduce the possibility of cross-contamination if a failure occured. The pump manufacturer led me to believe that although it isn’t the norm, if the pressure drop across the filter is kept to less than 30 PSI it will be all right. This forces filter maintenance. What filter micron and beta rating should be used?”
Filters in a hydraulic system maintain fluid cleanliness at a level that maximizes component life. The appropriate cleanliness level is based on factors such as operating pressure and the internal clearances of components within a system. Given that the objective of this process is to extend the service life of components in a system, it is imperative to understand that some filter locations can have the opposite effect.
The rationale for installing filters in piston pump and motor case drain lines is similar to the rationale for locating filtering media in the return line, meaning; if the reservoir and the fluid it contains start out clean and all air entering the reservoir and returning fluid is adequately filtered, fluid cleanliness will be maintained. The main disadvantage of installing filters on piston pump or motor case drain lines is that the back pressure created by the element can cause failures. Drain line filters can cause excessive case pressure, resulting in seal failure and mechanical damage.
High case pressure results in excessive load on the lip of the shaft seal. This causes the seal lip to wear a groove in the shaft, eventually resulting in leakage past the seal. If case pressure exceeds the shaft seal’s design limits, instantaneous failure can occur. The subsequent loss of oil from the case may result in damage through inadequate lubrication.
The effect of high case pressure on axial piston pumps is similar to excessive vacuum at the pump inlet. Both conditions put the piston-ball and slipper-pad socket in tension during inlet (Figure 1). This causes buckling of the piston retaining plate and/or separation of the slipper from the piston, resulting in catastrophic failure.
Figure 1. Effect of High Case Pressure on Axial Piston Design
High case pressure can cause the pistons of radial piston motors to be lifted off the cam. This occurs in operation during the outlet cycle. The pistons are then hammered back onto the cam during inlet, destroying the motor. If residual case pressure remains high when the motor is stopped, loss of contact between the pistons and cam allows the motor to freewheel, resulting in uncontrolled machine movement.
I recently witnessed a situation where these problems occurred. I was asked to locate a replacement for a radial piston motor no longer in production. The motor in question was powering a winch on a barge working in an offshore oilfield. The situation was urgent because the downtime was costing the company $40,000 per day. When I inquired about the nature of the failure of the original motor, he reported the following symptoms:
On-site personnel had torn down the motor and sent photos back to the company’s onshore office. The photos of the motor internals showed no obvious failure, confirming my suspicions. I explained that the symptoms described were consistent with high case pressure, most likely as a result of a blocked or restricted case drain line. I later learned that a technician, who had recently completed a contamination control course, connected the motor case drain line back to the tank through the system’s return filter.
To avoid these problems, piston pump and motor case drain lines should be returned to the reservoir through dedicated penetrations. These penetrations must be higher than the unit’s case port and be connected to a drop-pipe inside the reservoir that extends below minimum fluid level. Due to the reasons above, filters are not recommended on case drain lines. While this does allow a small percentage of fluid to return to the reservoir unfiltered, in most applications the contamination risk is low and can be effectively managed using oil analysis and other condition-based maintenance techniques.
Filter with Caution
The primary objective of contamination control is extending the service life of hydraulic components. Unfortunately, case drain filters can reduce the service life of piston pumps and motors, making their installation in pursuit of this objective a paradox. Effective contamination control is achievable using alternative filter locations that do not compromise component reliability. However, if case drain filters are included in a system, precautions must be taken to ensure that damage is not caused to the components they were installed to protect.
If a filter is fitted to a piston pump or motor drain line, I recommend a 125-micron screen, grossly oversized for the maximum expected flow rate to ensure that pressure drop is minimized, even under the most adverse conditions. The filter housing must incorporate a bypass valve with an opening pressure lower than the maximum allowable case pressure for the particular component (typically 5 to 15 PSIG). Installing a gauge or transducer upstream of the filter for monitoring case pressure is also advisable.