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The function of filters in a hydraulic system is to maintain fluid cleanliness. Given that the objective of maintaining fluid cleanliness is to gain maximum service life from the system components, it is imperative to understand that some filter locations can have the opposite effect, the suction line is among them.
From a filtration perspective, the pump intake is an ideal location for filtering media. The absence of both high fluid velocity, which disturbs trapped particles, and high pressure-drop across the element, which forces migration of particles through the media, increases filter efficiency. However, these advantages may be outweighed by the flow restriction the element creates in the intake line and the negative effect this has on pump life.
Pump inlet or suction filters usually take the form of a 150-micron (100-mesh) strainer, which is screwed onto the pump intake penetration inside the reservoir. The restriction caused by a suction strainer, which increases at low fluid temperatures (high viscosity) and as the element clogs, increases the chances of a partial vacuum developing at the pump inlet. Excessive vacuum at the pump inlet may cause cavitation erosion and mechanical damage.
When a partial vacuum develops in the pump intake line, the decrease in absolute pressure can result in the formation of gas and/or vapor bubbles within the fluid. When these bubbles are exposed to elevated pressures at the pump outlet, they implode violently. Collapse pressures greater than 145,000 PSI have been recorded and if microdieseling occurs (combustion of air/oil mixture) temperatures as high as 2,012ºF are possible. When bubbles collapse in close proximity to a metal surface, erosion occurs (Figure 1).
Figure 1. Cavitation Erosion Damage to
Case-hardened Valve Plate
Cavitation erosion damages critical component surfaces and contaminates the hydraulic fluid with wear particles. Chronic cavitation can cause significant erosion and lead to pump failure.
When a partial vacuum develops at the pump inlet, the mechanical forces induced by the vacuum itself can cause catastrophic failure. 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 consequence, the slipper becomes detached from the piston (Figure 2).
Figure 2. Slipper Separated from its Piston as
a Result of Excessive Vacuum at the Pump Inlet
This can occur either instantaneously, if the vacuum-induced tensile force is great enough, or over many hours of service as the ball joint is repetitively put in tension during inlet.
The piston retaining plate, the primary function of which is to keep the piston slippers in contact with the swash plate, must resist the forces that act to separate the piston from its slipper. This vacuum-induced load accelerates wear between the slipper and retaining plate and can 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 end of the piston during outlet. The impact damages the piston slippers and swash plate, leading rapidly to catastrophic failure.
In bent axis pump designs, the piston is better able to withstand vacuum-induced tensile forces. Piston construction is generally more rugged and the piston ball is usually held in its shaft socket by a bolted retaining plate. However, tensile failure of the piston stem and/or buckling of the retaining plate can still 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 by 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, and 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 rapidly to catastrophic failure.
Gear pumps are mechanically the least susceptible to vacuum-induced forces. Despite this fact, research has shown that suction strainer clogging caused by resinous, oil oxidation by-products can reduce the service life of an external gear pump by at least 50 percent.
Given the potential for suction strainers to damage the pump, why use them at all? This question becomes more curious when you consider that if the reservoir and the fluid it contains starts out clean and all air and fluid entering the reservoir is adequately filtered, the fluid in the reservoir will not contain hard particles large enough to be captured by a coarse mesh strainer. Clearly, examination of the arguments for installing suction strainers is required.
The argument that suction strainers should be fitted to protect the pump from debris that enters the reservoir as a result of careless maintenance practices, is a popular one. 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 four inches off the bottom. When anecdotal evidence is presented that debris, which entered the tank through careless maintenance, did cause a pump failure, its weight is diminished on the basis that if a suction strainer had been fitted, the same neglect of its maintenance would have eventually resulted in the same outcome - premature pump failure. Notwithstanding the above, the preferred solution to this problem is to take action to prevent contaminants from entering the reservoir in the first place.
Another popular misconception surrounding suction strainers is that their absence voids the pump manufacturers’ warranty. If a nut or bolt enters the pump through its intake causing it to fail, it is reasonable to expect that the manufacturer will deny warranty. It is also reasonable to expect the manufacturer to deny warranty if a pump failure is caused by particles smaller than the mesh of a strainer or by cavitation as a result of a clogged strainer. So if a pump fails through either contamination or cavitation, the manufacturer is unlikely to accept warranty - suction strainer or no suction strainer.
Where suction filters are fitted, the case for removing and discarding them is compelling. In most applications, the contamination control benefits these filters offer are strongly outweighed by the negative impact they can have on pump service life. In applications that demand their installation or where human barriers prevent their removal, precautions must be taken to prevent component damage.
If suction filtration is installed, a filter located outside the reservoir is preferable to a suction strainer. The inconvenience of servicing a filter located inside the reservoir is a common reason why suction strainers go unserviced - until the pump fails. If a suction strainer is used, opt for 60-mesh (240-microns) rather than the more common 100-mesh (150-microns). The strainer should be grossly oversized for the pump’s flow rate to ensure that pressure drop is minimized, even under the most adverse conditions. Regardless of the type of filter employed, it must incorporate a bypass valve to prevent the element from creating a pressure drop that exceeds the safe vacuum limit of the pump. A gauge or transducer should also be installed downstream of the filter to enable continuous monitoring of absolute pressure at the pump inlet.