One of our readers recently wrote to me about the following questions:

　　

“Recently, we purchased a second-hand hydraulic power unit (15HP motor directly connected to the vane pump). When the unit is running, it produces high-pitched, humming sounds. We checked the following:

　　

We thought this was a motor bearing, so we removed the pump from the motor and heard no noise.

　　

The pressure line is connected to the tank line (simulated low pressure <100 psi) and the noise is small.

　　

As the pressure increases, the noise is getting louder and louder and very unbearable.

　　

Measured motor current consumption - no overload.

　　

Do you think it may cause excessive noise? "

　　

Since the above symptoms are consistent with the limits at the pump inlet, I asked if there is a suction filter in the circuit. Our readers replied:

　　

“The system has a 40 micron suction filter, but I didn't check it because I had to drain the oil and then removed the access hatch to the filter.”

　　

The limitations imposed by the suction filter increase at low fluid temperatures (high viscosity) and blockage of the components, increasing the chance of creating a partial vacuum at the pump inlet. Excessive vacuum at the pump inlet can cause cavitation and mechanical damage.

　　

Cavitation

　　

When a partial vacuum is created in the pump suction line, a decrease in absolute pressure results in the formation of gas and/or vapor bubbles within the fluid. When these bubbles are exposed to high pressure at the pump outlet, they violently implosion. Corrosion occurs when bubbles collapse near the metal surface. Cavitation corrosion can contaminate hydraulic oil and damage critical surfaces.

　　

Mechanical damage

　　

When a partial vacuum is created at the pump inlet, mechanical forces caused by the vacuum itself can cause catastrophic failure. In a vane pump design, the vanes must extend from their retracted position in the rotor during the inlet. When this occurs, the fluid from the pump inlet fills the void in the rotor created by the extended vanes. If there is too much vacuum at the pump inlet - it will act on the bottom of the blade. This causes the blades to lose contact with the cam ring during the inlet and then hammer the hammer back onto the cam ring as it acts on the base of the blade during the exit. Impact damage to the blade tip and cam ring quickly leads to catastrophic failure.

　　

The unbearable noise that our readers refer to is the cavitation bubble collapse and the blade hitting the cam ring. These two conditions are enhanced by increasing system pressure.

　　

The solution to our reader's problem is simple: replace the suction filter or better, completely discard. If a suction filter must be installed, follow these precautions to prevent damage to the pump:

　　

The filter located outside the reservoir is superior to the suction filter. The inconvenience of repairing the filter located in the fuel tank is a common cause of the water filter being unserviced - until the pump fails.

　　

If a suction filter is installed, choose 250 microns instead of the more common 150 microns.

　　

For pump flow, the filter should be too large to ensure minimal pressure drop even under the toughest conditions.

　　

Regardless of the type of filter used, it must be equipped with a bypass valve to prevent the component from producing a pressure drop that exceeds the safe vacuum limit of the pump.

　　

A pressure gauge or sensor should be installed downstream of the filter to continuously monitor the absolute pressure at the pump inlet.

　　

For more information on vane pump failures and how to prevent them, please pay attention to the