The Local Knob Turner Strikes Again!

I was teaching the last day of our Machine Specific Hydraulic Troubleshooting course at a plant in Louisiana when it came time to go out and do the hands on exercises on the Stacker. The Stacker had a 29 GPM, pressure compensating pump which supplied oil to 4 different grade bins. A relief valve was used as a safety back up in the event the compensator did not shift to reduce the pump volume. Each bin had a cylinder for raising and lowering the bin and a hydraulic motor for driving the outfeed rolls when a full stack was accumulated. A flow control was connected in a meter in type arrangement to control the motor’s speed. There was also a normally closed manual valve connected off the main pressure line that was used as an air bleed valve on initial pump start up.

When we got to the hydraulic unit we found that the oil temperature was 300° F! Most all industrial hydraulic systems are designed to operate at below 140° F. Once the temperature exceeds 140° F the oil will start breaking down, reducing the life of the oil. The ideal oil temperature is usually 100° - 120° F. Since the oil temperature was so high I knew that one or more things had changed in the system. 

The first thing I looked at was the system cooler to verify that the fins were clean and there was good air flow through the unit. The next thing that was checked was the maximum pressure in the system. The manufacturer had designated that the compensator be set at 1500 PSI and the relief valve at 1800 PSI. When 1500 PSI is reached, the pump volume will be reduced and only supply the oil the system needs. In idle or when the Stacker was lowering, no pump volume was used so therefore, the output should have been near 0 GPM. The reading found on the gauge at the pump outlet port was 1800 PSI! One of the students then shot the tank line of the relief valve with an infrared camera which indicated 310° F. We then checked the outlet of the manual valve and it also was reading above 300° F. We found the manual valve was cracked partially open. I was later told that the valve was opened to reduce the noise of the pump! The third thing that was noticed was that the no. 3 bin outfeed rolls motor was driving the rolls at all times.

The first thing we did was attempt to set the compensator at 1500 PSI and the relief valve to 1800 PSI. This was successful meaning that the local knob turner had showed up at the stacker and set the compensator above the relief valve. Instead of the pump delivering a near 0 flow output when the volume was not needed, the output volume was dumping over the relief valve, generating heat. We then closed the manual valve to block the flow back to tank. We then found the limit switch that energized the directional valve to drive the outfeed rolls motor was broken. The electrician had changed the program so that the rolls rotated at all times. Flow controls cause a resistance in the system since the pressure at the inlet port is higher than at the outlet port. In this cause the pressure drop was very high since there was no load on the rolls. We had the limit switch replaced and the rolls operated only when needed.

After making these changes I returned to the unit 24 hours later and the oil temperature had dropped from 300° F to 132° F. The oil was changed and the system had to be flushed with Exxon’s System Cleaner to remove the varnish and sludge that had developed in the system.

Whenever a system is running too hot, there is a reason for it. I have seen many plants add additional coolers to the system instead of diagnosing the cause of the increased temperature. Any system will inherently develop heat over pump case drain lines, flow controls, proportional valves and pressure reducing valves. The system should be designed to remove this heat with an adequately sized heat exchanger. When troubleshooting heat issues, remember the phrase “hydraulic energy not used on the load turns to heat.” One of the best tools for troubleshooting heat problems is an infrared camera. I have seen several cases where cartridge valves were bypassing inside a manifold that was detected by an infrared camera.