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A small heated platen press for an aerospace parts manufacturer would close and hold pressure for 1 to 3 hours based on the part being molded. When the system was first installed, it developed trouble with the oil overheating. The employees made a design change, installing a 5-psi check in the pump case drain line back to tank and a ½-psi check going to the main return line. (See the circuit showing the final design.)
This modification worked just fine. When the press was in the clamped mode, the case drain would pass through the filter and heat exchanger, cooling the pump case drain during the holding phase. If the cylinder was cycled, it would cause flow and some back pressure in the main tank line and the back pressure would hold the ½-psi check closed, causing the pump case flow to divert directly to the tank over the 5-psi check. A 5-psi check was used since the shaft seal of the pump was limited to 10 psi.
Every so often, the press would start overheating during the clamped cycle. The employees would lower the pressure setting on the safety relief to flush any contamination that may cause it to leak and reset it back to 3000 psi. They would change the filter element, which looked dirty, and check the water connections on the heat exchanger for flow or corrosion.
See the Solution
When pressure-compensated pumps idle for long periods of time, the case flow can cause the reservoir oil to overheat. Using a 5 and ½ psi check valve circuit allowed the case flow to be cooled when the main system is static and the pump is compensated. When the designer installed the modification to cool the case drain, he should have connected the outlet of the ½-psi check to only flow through the cooler and not the 3-micron filter. The 15-gpm filter was undersized to begin with since the return flow from the cylinder retracting was 24½ gpm. As the dirt and contaminants accumulated in the filter, the pressure drop increased. As soon as the pressure drop exceeded 4½ psi, the case drain flow was returning to the reservoir through the 5-psi check heating up the oil.