For steam, air and non-corrosive gases
This type of PRV incorporates two valves—a pilot valve and main valve—in one unit. The pilot valve has a design similar to that of the direct-acting valve. The discharge pressure from the pilot valve is transmitted on top of a piston, which opens the main valve. This design makes use of inlet pressure in opening a larger main valve than could otherwise be opened directly. As a result, there is greater capacity per line size and greater accuracy (+/-5%) than with the direct-acting valve.