The function of filters in hydraulic systems is to keep the fluid clean. Since maintaining fluid cleanliness is crucial to ensuring the maximum life of system components, it’s important to understand that some filter locations can have negative impacts, including the suction line.
From a filtration perspective, the pump inlet is the ideal location for filter media. In theory, the absence of high-velocity fluid to disrupt captured particles and the lack of high-pressure drop across the filter element promote particle separation, leading to improved filtration efficiency. However, these advantages may be offset by the restriction imposed by the filter element in the inlet line, negatively impacting pump life.
The pump’s inlet filter or suction strainer typically takes the form of a 150 micron (100 mesh) filter that screws onto the pump’s suction port inside the reservoir. The throttling effect caused by the suction strainer increases at low fluid temperatures (high viscosity) and as the filter element becomes clogged, thus increasing the chance of a partial vacuum at the pump’s inlet. Excessive vacuum at the pump’s inlet can cause cavitation and mechanical damage.
1. Gas Corrosion
When a partial vacuum occurs in the pump inlet piping, the absolute pressure drops, potentially generating gas and/or bubbles in the fluid. These bubbles, exposed to high pressure at the pump outlet, violently collapse. Datasheets indicate that when the burst pressure exceeds 10,000 bar, the air/oil mixture can reach temperatures as high as 1,100°C after combustion.
Corrosion occurs when bubbles collapse near metal surfaces.
Cavitation erosion occurs on hardened surfaces.
Gas corrosion damages the surface of core components, and wear particles contaminate the hydraulic fluid. Chronic cavitation can cause severe corrosion and lead to pump failure.
2. Equipment damage
(1) When a partial vacuum occurs at the pump inlet, the mechanical force generated by the vacuum itself can cause a fatal failure. A vacuum is generated in the chamber of an axial piston pump, and the piston head and the slipper are in a tensile state. The ball end is not designed to withstand excessive tension, so the slipper falls off the piston.
(2) This load generated by the vacuum accelerates the wear between the slipper and the fixed plate, and the fixed plate may bend. In this way, the slipper loses contact with the rotating inclined plate during the inlet, and then hammers the inclined plate again when the pressurized fluid at the outlet acts on the piston end. The impact damages the piston slipper and the rotating inclined plate, which can quickly cause a fatal failure.
(3) In the inclined axis pump design, the piston is more resistant to the tension generated by the vacuum. The piston structure is usually more robust. The piston ball is usually fixed to the shaft groove with a bolted plate. However, even under high vacuum conditions, tensile failure of the piston rod and/or bending of the fixed plate may continue to occur.
(4) In the vane pump design, the vanes must protrude from the contraction position in the rotor to the inlet. In this situation, fluid from the pump inlet fills the gaps in the rotor created by the expanding vanes. When there’s too much vacuum at the pump inlet, it’s applied to the base of the vanes. This causes the vanes to lose contact with the cam ring during inlet. Then, during outlet, pressurized fluid acts on the base of the vanes, hammering them back against the cam ring. This impact damages the blade tips and cam ring, quickly leading to fatal failure.
The gear pump is on the machine. It is least affected by vacuum forces. Despite these facts, product-induced clogging of the suction fluid can reduce the life of an external gear pump by at least 50%. Considering that a suction filter can damage the pump, why use one? If the oil tank and the fluid in the tank are initially clean, and all air and fluid entering the tank are completely filtered out, then the fluid in the tank will not contain particles hard enough to be captured by the coarse suction filter. Obviously, the installation parameters of the suction filter need to be checked.
3. Prevent debris from entering
A popular opinion is that pumps must be fitted with suction filters to prevent debris from entering due to careless maintenance work. Nuts, bolts, tools and similar debris pose the least threat to the pump if they are placed in a suitable tank with the pump inlet at least 100 mm above the ground. With suction filters fitted, even cavitation can eventually damage the pump due to neglected maintenance.
4. Warranty issues
Another common misconception surrounding suction filters is that the pump manufacturer’s warranty is void if an oil filter is not used. If a nut or bolt enters the pump through the inlet and the pump fails, the manufacturer can be expected to deny the warranty. The manufacturer can also deny the warranty if the pump failure is caused by particles smaller than the filter body pores or cavitation caused by a clogged filter. Therefore, if a pump fails due to contamination or airborne corrosion, it is unlikely that the manufacturer will cover the warranty without or with a suction filter.
