Hydraulic power unit troubleshooting - symptoms, causes and fixes

Hydraulic power units (HPUs) have pumps for multiple functions. For example, a controlled crown roll HPU would need pumps for:

• High-pressure pumps to control shell deflection through hydrodynamic or hydrostatic load shoe bearings
• Lubrication pumps to lubricate shell load shoes, rotating bearings and to control temperature
• Scavenger pumps to return oil to the hydraulic unit

Pump failures will lead to product quality defects and production loss. There will typically be one or more backup pumps in order to minimize the risk of a pump failure affecting production.

Hydraulic power units comprise several elements, which may include: pumps, motors, oil tank, pressure filters, return filters, oil level switch, temperature switch, pressure switch, oil cooler/heater, thermostat valve, thermometer, electric terminal box and circuits for circulation/pressure/etc.

Press section hydraulic power unit, showing main elements

Troubleshooting

Symptom: The pump is not delivering fluid properly.

• The pump may be driven in the wrong direction of rotation. Stop the pump to prevent seizure. Check the rotation of the pump as indicated by the arrow on the pump body.
• The drive shaft is broken or the shaft key is sheared. Remove the pump from its mounting and inspect. Repair as necessary.
• Intake pipe is blocked or the oil viscosity is too heavy to prime. Drain the system. Add clean fluid of the proper viscosity. Filter the oil as recommended. Check the filter for cleanliness. Check the oil temperature in the reservoir.
• Intake has air leaks. Check the intake connections. Tighten securely.
• The pump is not priming. Loose connection in the outlet line. Bleed the air until hydraulic fluid flows.
• Fluid level is too low. The reservoir fluid level must be above the opening of the intake pipe. Check the system on start up to make sure that it is filled to the proper level.

Symptom: The hydraulic system pressure is too low.

• Relief valve is set too low. Use a pressure gauge to adjust the relief valve.
• Worn pump parts are causing extreme internal leakage. Replace the rotor sets and take the required corrective steps after examination of the pump parts.

Symptom: The pump is making excessive noise.

• The intake strainer/pipe is clogged or restricted. The pump must receive intake fluid freely or cavitation occurs. Drain the system, clean the pipes and clean or replace the strainer. Add new fluid and strain properly.
• The pump has defective bearings. Replace the bearings and inspect the pump shaft.
• Air leaks at the pump intake pipe joints or shaft seal. Pour fluid on the pipe joints and shaft seal while listening for a change in sound. Tighten the joints and replace seals as necessary. Check the shaft journal for scoring at the seals and replace as necessary.
• Drive for the pump is wrong. Make sure that the motor is running at the correct speed.
• The coupling is misaligned. Check the bearing or bushings and seals. Replace as necessary and align the shafts. Allowable misalignment is .005 inch.

Symptom: The pump shaft seal is leaking.

• The seal is worn/damaged. Replace the seals.
• There is excessive pressure on the seals. Check for restrictions or excessive length of line on externally drained pumps. Internal drain pumps should be drained to the inlet side with no more than 10-psi pressure.

Symptom: The pump has excessive heat (over 400°F) compared with the hydraulic lines.

• Too much thrust on the pump bearings. Pump shaft must not be out of line more than .005 inch with the motor shaft. Shaft end gap should be 1/8-inch minimum.
• The fluid temperature is too high. The pump should be shut down immediately after comparing pump, inlet line and reservoir temperatures. If the pump is excessively hot, it should be overhauled.

Notice: Maintenance on the hydraulic system and roll depend to a great extent on the timely attention that the filters receive.

The reservoir is usually the largest item on the unit and can make up part of the framework. The reservoir contains a cover plate, which should be kept in place except when inspecting or refilling. The life of any hydraulic system depends on keeping contaminants out of the oil. This is hard to do with the cover plate removed.

The reservoir for the unit must be large enough to handle all of the oil contained in the machinery (for example a CC roll) if there is a situation that requires the machinery be completely drained. The reservoir should be cleaned and inspected for corrosion and other damage yearly, or more frequently as needed.

For more information troubleshooting hydraulics, contact your Valmet representative.