One of our members wrote to me recently regarding the following problem:
“Recently, we bought a used hydraulic power unit (15HP electric motor directly coupled to a vane pump). A high-pitched, clicking noise is generated when the unit runs. We have checked the following:
–We thought it was a motor bearing, so we detached pump from motor, no noise heard.
–Pressure line was connected to tank line (to simulate low pressure < 100 psi), very little noise heard.
–As pressure is increased, noise gets louder and louder, very intolerable.
–Measured current draw of motor – no overload.
What do you think could be causing the excessive noise?”
Given that the symptoms described above are consistent with a restriction at the pump inlet, I inquired if there was a suction filter in the circuit. Our reader replied:
“The system has a 40 micron suction filter but I haven’t checked it because I have to drain the oil and take off the access hatch to get to the filter.”
The restriction caused by a suction filter, which increases at low oil 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 causes cavitation erosion and mechanical damage.
When a partial vacuum develops in the pump intake line, the decrease in absolute pressure results in the formation of gas and/or vapor bubbles within the oil. When these bubbles are exposed to elevated pressures at the pump outlet they implode violently. When bubbles collapse in proximity to a metal surface, erosion occurs. Cavitation erosion contaminates the hydraulic oil and damages critical surfaces.
When a partial vacuum develops at the pump inlet, the mechanical forces induced by the vacuum itself can cause catastrophic failure. 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 (figure 1). The impact damages the vane tips and cam ring, leading rapidly to catastrophic failure.
Figure 1. Vane pump section (Bosch Rexroth Corp).
The intolerable noise our reader is referring to is symptomatic of cavitation bubble collapse and possibly, the vanes being hammered against the cam ring. Both of these conditions are intensified by increasing system pressure.
The solution to our reader’s problem is simple: replace the suction filter or better still, discard it completely. If suction filtration must be installed, follow these precautions to prevent pump damage:
A filter located outside of 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 after the pump fails.
–If a suction strainer is installed, opt for 250 microns rather than the more common 150 microns.
–The filter 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 be installed downstream of the filter to enable continuous monitoring of absolute pressure at the pump inlet.
But even with these safeguards in place, the harsh reality is installing any type of filter in a pump intake line is generally a mistake. For more causes of hydraulic pump failures and how to prevent them, read Preventing Hydraulic Failures