The role of the filter in the hydraulic system is to keep the liquid clean. Since the purpose of keeping the fluid clean is to ensure the maximum service life of the system components, it is important to know that some filtration locations may have a negative impact, and the suction line is one of them
From a filtration point of view, the inlet of the pump is the ideal location for filtering media. In theory, there is no high-speed fluid to interfere with the trapped particles, and there is no high pressure drop on the filter element to promote particle separation, thus improving filtration efficiency. However, these benefits may be offset by the limitations caused by the filter element in the oil intake line and the negative impact on pump life
The pump inlet filter or suction filter usually takes the form of a 150 micron (100 mesh) filter that is screwed onto the pump inlet in the tank. The throttling effect caused by the suction filter increases at low fluid temperatures (high viscosity) and increases with the blockage of the filter element, thus increasing the chance of a partial vacuum at the pump inlet. Excessive vacuum at the pump inlet may cause cavitation and mechanical damage
Gas corrosion
When a local vacuum occurs in the inlet pipe path of the pump, the fluid may produce gas and/or bubbles if the absolute pressure drops. When these bubbles are under high pressure at the pump outlet, they rupture violently. The data table shows that when the bursting pressure is greater than 10000 bar, the temperature of the air/oil mixture after combustion can reach 1100 degrees
Foam corrodes when it bursts near the metal surface
Cavitation erosion occurs on hardened surfaces
Air corrosion can damage the surface of the core components, and wear particles can contaminate the hydraulic oil. Chronic cavitation can cause severe corrosion and lead to pump failure
Equipment damage
When a local vacuum occurs at the pump inlet, the mechanical force generated by the vacuum itself can cause a fatal failure. A vacuum is created in the cavity of the axial piston pump, and the piston head and shoe are in a stretched state. The ball end is not designed to withstand excessive tension, so the boot falls off the piston
The vacuum degree of the pump inlet is too high, and the piston is separated from the slide shoe
When the tension generated by the vacuum is large enough, it can occur immediately or after many hours of use
The main function of the piston retainer plate is to make the piston boot in contact with the inclined plate that must resist the force separating the piston from the boot
This load from the vacuum will accelerate the wear between the boot and the fixed plate, which may bend. In this way, the shoe loses contact with the rotating slant during the inlet, and then the slant can be hammered again when the pressurized fluid acts on the end of the piston at the flow outlet. The impact can damage the piston boot and the swivel plate, which can quickly cause fatal failure
In inclined shaft pump designs, the piston is better able to withstand the tension generated by vacuum. Piston construction is usually stronger. The piston ball is usually fixed to the shaft groove with a bolted plate. However, even under high vacuum conditions, failure of piston rod elongation and/or bending of fixed plate may continue to occur
In a vane pump design, the blade must protrude from a contracted position inside the rotor to the inlet. In this case, the fluid at the pump inlet fills the gap in the rotor created by the expanding blade. Pump inlet when there is too much vacuum for the bottom of the blade. This will cause the blade to lose contact with the CAM ring during the entrance, and then pressurize the fluid on the bottom of the blade during the exit, thus re-hammer the blade back into the CAM ring. Impact can damage the blade end and CAM ring, which can quickly cause fatal failure
Gear Pump on the machine. Least affected by vacuum forces. Despite these facts, product-induced oil absorber clogging reduces the life of external gear pumps by at least 50%. Given that suction filters can damage the pump, why use them? If the tank and fluid in the tank are initially clean and all air and fluid entering the tank is completely filtered out, the fluid in the tank does not include particles hard enough to be captured by the crude Oil Suction Filter. Obviously, the installation parameters of the suction filter need to be checked
Keep out debris
A popular view is that pumps must be fitted with suction filters to avoid getting into debris due to careless maintenance work. Nuts, bolts, tools, and similar debris pose minimal threat to the pump in a reasonable tank with a pump inlet at least 100 mm from the ground. In the case of the installation of suction filters, due to neglect of maintenance, even cavitation will eventually damage the pump
Nevertheless, the best solution to this problem is to take measures to prevent contaminants from entering the tank in the first place
Warranty problem
Another common misconception surrounding the oil suction mesh is that the pump manufacturer's warranty is void if the Oil Filter is not used. The nut or bolt enters the pump through the inlet, and when the pump fails, it can be expected that the manufacturer will refuse the warranty. The manufacturer can also deny warranty if the pump failure is caused by particles smaller than the filter body holes, or cavitation caused by filter clogging. Therefore, if the pump fails due to contamination or air corrosion, the manufacturer is unlikely to be covered without a suction filter or suction filter.
If an oil suction filter is installed, the housing for removing and discarding the suction filter is eye-catching. In most applications, the negative impact of these filters on pump life far outweighs the pollution control advantages these filters provide. Applications that need to be installed or that are manually disturbed to uninstall must take precautions to avoid damaging components
If an oil suction filter is installed, a filter outside the tank is preferable to an suction filter. The inconvenience of repairing the filter inside the tank is the general reason that the oil suction filter cannot be used before the pump fails. If using an suction filter, choose 60 amps (240 microns) instead of the more common 100 amps (150 microns). Even in the most adverse circumstances, the filter size should be increased substantially according to the pump flow rate in order to minimize the pressure drop. Regardless of which filter is used, a bypass valve must be installed to ensure that the pressure drop of the filter does not exceed the safe vacuum limit of the pump. In order to be able to continuously monitor the absolute pressure at the pump inlet, a gauge or sensor must also be installed downstream of the filter
