The safety valve on my compressor is leaking air. I found plenty of places that have replacement valves and they look easy to replace. My question is how do I know which one to pick they all have a preset PSI. My compressor has an operating PSI of 115. I have found safety valves that are 100 and 125, I was assuming I needed one that was 115. A friend said that he'd expect I should go higher than the 115 so it wouldn't open the safety valve until higher than the operating pressure. Is this correct? What pressure should I be looking at?
Your friend is right. If the operating and safety pressures were the same (nominally, given tolerances), you'd likely have nuisance trips of the safety valve.
Enough safety factor is built into the tank that it's not a concern to be 10 psi higher with the safety valve. In fact, the tank is probably the same one used on higher-duty models, with the pressure being limited by the compressor motor's abilities.