Should Filter Be Before Or After Pump

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

And then when considering the possible locations for filters in a hydraulic arrangement, the overarching principle must exist: first, do no impairment. In other words, "the cure must non be worse than the disease".

With this in listen, allow us consider the pros and cons of the diverse hydraulic filter locations:

Force per unit area 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 menstruation velocities and pressure level and flow transients, which tin can disturb trapped particles.

The major disadvantage of pressure filtration is economical. Because the housings and elements (high-collapse type) must be designed to withstand peak organization pressure, pressure filtration has the highest initial and ongoing toll.

Return filtration: The rationale for locating filtering media in the return line is this – if the reservoir and the fluid it contains start out clean, and all air entering the reservoir and returning fluid is adequately filtered, then fluid cleanliness will be maintained. The other advantage of the return line every bit a filter location is that sufficient pressure is available to force fluid through fine media (typically 10 microns), simply pressure is not high plenty to complicate filter or housing pattern.

This, combined with relatively low flow velocity, means that a high degree of filtering efficiency can be accomplished at an economic toll. For these reasons, return filtration is a feature of most hydraulic systems. The primary disadvantage of return line filtration is that the back pressure created by the chemical element tin 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 loftier 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 loftier initial cost, although this unremarkably can be justified on a life-of-car cost basis.

Suction filtration: From a filtration perspective, the pump intake is an ideal location for filtering media. Filter efficiency is increased past the absence of both loftier fluid velocity, which can disturb trapped particles, and high force per unit area driblet across the element, which can force migration of particles through the media. These advantages are outweighed by the brake the chemical element creates in the intake line and the negative effect this tin can have on pump life.

Figure one. The Effect of Tensile Forces Acting on Axial Piston Blueprint

Battling Vacuum-induced Forces

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

The creation of a vacuum in the pumping chambers of an axial pump puts the piston ball and slipper-pad socket in tension. This joint is not designed to withstand excessive tensile force; and as a issue, the slipper becomes discrete from the piston (Figure ane). This can occur either instantaneously, if the vacuum-induced tensile strength is pregnant enough, or over many hours of service as the ball articulation is repetitively put in tension during inlet.

The piston retaining plate, the principal function of which is to keep the piston slippers in contact with the swash plate, must resist the forces that human action to separate the piston from its slipper. This vacuum-induced load accelerates wear between the slipper and retaining plate and tin cause the retaining plate to buckle.

This allows the slipper to lose contact with the swash plate during inlet, and it is then hammered back onto the swash plate when pressurized fluid acts on the stop of the piston during outlet. The impact damages the piston slippers and swash plate, leading rapidly to catastrophic failure.

In aptitude axis pump designs, the piston is ameliorate able to withstand vacuum-induced tensile forces. Piston construction is generally more than rugged, and the piston ball usually is held in its shaft socket past a bolted retaining plate. Yet, tensile failure of the piston stem and/or buckling of the retaining plate all the same can occur under high vacuum conditions.

In vane pump designs, the vanes must extend from their retracted position in the rotor during inlet. As this happens, fluid from the pump inlet fills the void in the rotor created past the extending vane. If excessive vacuum exists at the pump inlet, it will act at the base of the vane.

This causes the vanes to lose contact with the cam ring during inlet; they are then hammered back onto the cam ring as pressurized fluid acts on the base of the vane during outlet. The impact damages the vane tips and cam ring, leading chop-chop to catastrophic failure.

Gear pumps are mechanically the least susceptible to vacuum-induced forces. Despite this fact, research has shown that a restricted intake tin reduce the service life of an external gear pump by at to the lowest degree 50 percent1.

The Facts on Suction Strainers

Pump inlet or suction filters ordinarily take the course of a 150-micron (100-mesh) strainer, which is screwed onto the pump intake penetration within the reservoir. In the 10 years I've actively campaigned against their employ (for reasons outlined earlier in this cavalcade), I'g sure I've heard all of the counter-arguments. About 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 reservoir as a result of poor maintenance practices is a popular ane. Nuts, bolts, tools and similar debris pose minimal threat to the pump in a properly designed reservoir, where the pump intake is located a minimum of 4 inches off the bottom. Of grade, the proper solution is to prevent contaminants from entering the reservoir 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. Hither once more, if suction strainers are necessary in this situation, then it is only due to bad design; the manifold must exist beneath the pumps' intakes.

If properly designed, there should be a caput of oil above the inlet manifold, and the inlet manifold should be in a higher place the pumps' intakes. For cross-contamination to occur in this arrangement, debris would accept to travel uphill – against gravity and a positive head of oil. That would be highly unlikely.

Simply fifty-fifty in situations where a suction strainer is mandated, for whatever reason, the problem is: The cure can be worse than the disease.

Reference:

Ingvast, H., "Diagnosing Tyrone Gear Pump Failures", The 3rd Scandinavian International Conference on Fluid Power, Vol. 2, 1993, pages 535-546.

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