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Some compressors come equipped to run either off the pressure switch, or off a pneumatic head unloader. Sometimes, as with Curtis, there is a valve that lets you choose to run off the pressure switch, or off the unloader. When it is running off the PS, it will come up to pressure and cut out. If it is running off the unloader, it will come up to pressure, unload, but keep running. Running like this is more economical if the number of motor stops and starts per hour starts to get excessive (i.e. you are running near the maximum capacity of the compressor (common when sandblasting)). Also, the pneumatic unloader can provide a safety backup if the PS ever fails to open. Instead of pressure building up to the pop-off setting of the safety valve, the unloaders will kick in and unload the head. Of course, this only works if they are set to a higher pressure than the PS, otherwise, they will unload the compressor first. Incidentally, the PS will also act as a safety backup if the unloaders fail as well, as pressure will rise and cause the switch to cut out and stop the machine. So, the unloader valve is basically a way to provide dual mode running: stop start for the PS, unloaded running with the unloader valve. BTW, there should always be a safety relief valve in every compressor tank, just in case the compressor does not shut off for any reason. There should also be a 60-70 psi safety valve in the cylinder head of your Quincy. If you are using your Quincy in strictly stop-start mode, you can ditch the unloader valve, and run a line directly from your tank, through the hydraulic unloader valve, to the head unloader towers. You will lose the dual-mode function however. I have cross-referenced rings from a Hastings piston ring catalog for some obsolete compressors once in awhile, so it can be done. For the 230 though, I would head to a local compressor repair shop and ask them to order up some ICC parts. They are (usually) not expensive and work very well.
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