[chadr03]

NPT port diameter refers to the tapped thread that you screw a pipe or tubing fitting in. There are many standard sizes 1/8, 1/4, 1/2.... The tricky part with them is that the tapped hole will actually be larger than the size listed. A 1/8" NPT tapped hole is around 3/8" in diameter. The 1/8" is the approximate inner diameter or the pipe that will thread into that hole. You can find tables of standard dimensions online or in a Machinery’s Handbook.

Cv is a flow factor or flow coefficient. It is based on the inner geometry of the valve and how well something can flow through it. It is used to calculate flow capacity of valves and such.

The pressure relief valves have come pre-calibrated in the past. I would hook it up to your system very near the compressor and try to pressurize up to 120 psi and see if it starts venting. The valve has a spring that basically closes an exhaust port. When the pressure inside the system overcomes the spring force the port will open. If your valve does not open then it can be adjusted by loosening the jam nut (thin nut) then by turning the top cap with the hole in the end. If I remember right clockwise will increase pressure, counterclockwise will reduce the pressure.