A scrap yard had a metal compactor that is fed by a large metal shredding machine driven by a 1500 HP electric motor. The compactor had a 3-bank series manifold with DO5 directional valves incorporating A & B meter-out flow controls. Circuit reliefs were used on all the A & B lines to protect the vane pumps and reduce energy consumption. Only one circuit at a time would be engaged during operation and each function would operate at a different pressure. The PLC program was checked and verified that all the interlocks would only let one function operate at a time. The lowest relief valve was set at 600 psi while the highest relieved at 2100 psi. The idea was to use meter-out flow controls, and set the various reliefs approximately 150 psi above the cylinder and motor requirements, so the various circuit reliefs would reduce the heat generated when the unit operated. They were instructed that flow controls cause heat and, if possible, keep reliefs set as low as possible, and flow controls as open as possible to get the required speeds.
They were having problems with the vane pumps not lasting more than 1 to 2 months and sometimes only a week or two. The repair shop determined that the failure was due to excessive pressures. The side plates were “smearing” the ends of the vanes and rotor, as well as destroying the brass coating on the end plates. However, there were no signs of cavitation or aeration, and there wasn’t any scaring on the inside face of the rings or tips of the vanes.
All of the relief valve pressures were verified, and none were found to be out of adjustment.
Any idea what the problem might be?
Reducing the pump pressure for each different circuit reducing the heat generated was a good target to shoot for. Everyone has different ideas on how to best design systems. However, this might have been a good application for load sensing a main relief and reducing the number of circuit reliefs. Unfortunately, the field technician often does not have the criteria necessitated in the original design, and there may have been a solid reason for a particular arrangement.
However, the problem with this design was the selection of the spools for the directional valves. The majority of sliding spool valves have five possible positions they could either stop in or stick in. In other words, as they start to shift, what happens while they are shifting? Three position valves have two transition positions. Normally, two position valves never stop in the center position, so they have three transition positions. During transition or crossover, are all of the ports blocked or open? The three position valves did have open transitions, but the two position valve shown had all the transition positions blocked. The valve they selected had a closed transition or crossover condition that caused the pump pressure to spike beyond the pressure rating of the pump. The simple solution was to change the two position directional valve with one that had open transitions; and to add a main system pressure relief set 150 psi above the highest circuit relief setting. Crossover positions, even if they are only milliseconds long, create a danger, especially when troubleshooting and manually shifting the valve through these positions.
Robert J. Sheaf, President CFC Industrial Training