PRV keeps blowing
by Don Box
(Reedley, Calif.)
I have a Central Pneumatic 21 gallon, vertical,oil lubricated air compressor. Model 94667. 4.9 SCFM at 90 PSI, 4.5 HP, runs at 120 volts with a single stage pump.The problem is that although the PRV is rated to pop at 125 PSI, it's releasing at aprox. 85 PSI!! At least thats what my input gauge says, and my output gauge is matching it as the pressure goes up. So it would seem that my PRV is bad, right? Well I changed the PRV out for a new one that only rated for 100 PSI, and it's releasing at the same 85 PSI as the old one!!!! Got any idea as to what the problem could be? I"m pretty sure that both my gauges dident go bad at the same time.
Bill says...
Hi Don.
Many PRV's work on "sticktion" (a term a friend came up with) basis. The parts of the PRV that react to the system air pressure are a close tolerance, and it requires a certain air pressure in the system to be able to push the internal piston of the PRV hard enough to overcome friction inside the valve. That's why, when a PRV pops off, you may need to manually push it back in.
Regardless of the way yours operates, the PRV is designed to open at the factory set pressure level. That it's opening sooner, on two valves, suggests to me that oil from the compressor lube or some kind of lubricant might be getting into the PRV, and reducing the friction, thereby allowing the PRV to open easier, and easier means at lower pressure.
That's the only thing I can think of, though it is possible, I suppose, that you've got two bum PRV's.
What I don't know is if the new PRV opened at a too low pressure the very first time you powered up the compressor after replacing it? If the PRV opening pressure is dropping over subsequent operations, that might support the lubricant theory.
Else, get a new PRV. Yours apparently isn't working right. :-)
Cheers,
Bill
