The below shop tip was originally featured in the October 2007 ESA Newsletter.
By: Bob Sheaf, CFC-Solar, Inc.
Pressure Compensated Pumps with Safety Relief Valves
There are 3 pressure levels to these types of systems:
1. Maximum Spike Pressure (highest PSI present)
Pressure compensated pumps de-stroke when Maximum System Pressure is reached. However, these types of pumps can’t instantaneously destroke. They need as much as 250 MS (1/4 of a second) to 750MS (3/4 of a second) to fully destroke to the position to maintain their pressure setting. This lag in time causes the pump to continue to produce unwanted volume into the system.
A 9-piston pump turning at 1800 RPM will produce 16,200 piston shots per minute, or 270 shots per second, or 27 piston shots per 100 milliseconds. This extra flow causes System spikes well above the pump compensator setting. 10 to 20 times the pump setting is NOT uncommon.
To prevent this unwanted spike, a “directly operated” relief valve should be installed. (Pilot operated reliefs are slower acting than direct acting type). This relief should be set approximately 150 to 200 PSI above the pump compensator setting.
Maximum System Pressure should ALWAYS be adjusted using the pressure compensator adjustment normally located on the pump.
2. Maximum System Pressure
Maximum System Pressure is the allowable normal maximum the hydraulic system is allowed to operate. This is always controlled by the pump compensator, NOT the safety relief valve.
3. System Operating Pressure System
Operating Pressure is the pressure required to move actuators and should be at least 150 to 200 PSI below the setting of the pump compensator.
Pressure Adjustment Set-up Procedures
Start by simultaneously bringing BOTH the pump compensator and safety relief valve up to Maximum System Pressure trying to keep the safety relief a little higher than the pump. If the relief is slightly higher, you should hear the pump quiet down since it is de-stroking.
Next, increase the relief about V4 to % turn in further. Now slowly increase the pump compensator until the Pressure is about 150 to 200 PSI above the desired Maximum System Pressure. (Further increase in relief may be needed to do this adjustment.)
Next, reduce, (lower), the relief valve setting until you hear the pump starting to stroke to full volume. (Noise level will increase). Lock the relief valve adjustment at this position. Then lower the pump compensator adjustment down to the desired Maximum System Pressure and lock it also.
After system has run for 15 to 20 minutes, feel the relief valve tank line and make sure it is cooler than the average system temperature. If it is warm or hot to the touch, then unlock the relief adjustment and increase the setting XA turn and recheck as outlined above.
System should now be set properly.
- Pump compensator above the operating system pressure.
- Safety relief set just high enough ABOVE the pump to only clip pressure spikes caused by the slow reaction time of the pump swash plate.