Overheating of Press with Heated Platen
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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.
What could be causing the problem, and what would be the solution?
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.
By Robert Sheaf, CFPAI/AJPP, CFPE, CFPS, CFPECS, CFPMT, CFPMIP, CFPMMH, CFPMIH, CFPMM, CFC Industrial Training
My first thought is that the pump compensator and main relief probably weren’t/aren’t set properly and there’s interaction between them causing full pump flow to intermittently flow over the relief. Especially since they are periodically “flushing” the relief by lowering the pressure and resetting it.
Another possibility is that because during the clamped phase (assuming compensator is working correctly) the only flow would be case leakage and whatever is needed to maintain pressure. Assuming the reservoir is sized adequately there may not be enough cooling capacity. A separate cooling loop could be added just to constantly circulate reservoir oil through the heat exchanger and also flush the pump case.
Good lesson in what not to do in hydraulic circuit design…
The overheating is caused by relying on allowing case flow to take two possible. You hope it goes thru the filter and cooler but as soon as this path exceeds 5 psi it won’t and when that happens you have lost all cooling in the system when the pump is trying to maintain clamping pressure.
The pressure drop thru the filter is variable and increases as the element loads with contaminants. Likewise with the cooler, but this will decrease as oil gets warmer.
Call it the ‘hydraulic hope’ design cooling approach. Never a real good idea to install check valves in a case drain line especially when other components downstream components have variable pressure drops.
To fix this circuit properly. First off get rid of the pump with control that doesn’t do what you want it to do. Get a pump with a remote compensator control. Get rid of the check valves on the case drain lines. Also
Heated press clamping a piece for several hours is a perfect spot to apply an accumulator (downstream of PO check valves) and you must also put in a thermal relief valve on the cap side of the cylinder (presumably the clamp cylinder is clamping). A pressure switch installed on the cap side of the cylinder cycles the electric motor on/off if required and the accumulator maintains the clamping pressure for hours. The power unit is off probably 90% of the time during the clamping/mold process. That is the end of your overheating problem.
Add in also an accumulator bleed-down safety valve for maintenance. If you want to be an all in ‘keener’ also add a kidney loop fixed displacement pump and use it for filtration and cooling added on to thru drive of the new pump with remote compensator control.
I agree with both solutions. My first thought was to use an open center spool with a Kepner o-leak PO cross checks and a pressure switch to control an accumulator set for holding. This way all the flow goes to the filter and cooler at low pressure. I also agree with your “hydraulic hope” thought. I like that comment. When the compensation setting of the pump is not clearly set and you have 2 paths to go you just do not know for sure what is happening.
Jim Farrior