Pressure relief valve by structure: First, the role of pressure reducing valve, the most simple pressure reducing valve, direct-acting pressure reducing valve, with a flat diaphragm or bellows. Because it is a standalone structure, there is no need to install external sense wires downstream. It is the smallest of the three pressure reducing valves, the most economical one, designed for low and medium flow. The accuracy of the direct acting pressure reducing valve is typically +/- 10% of the downstream set point. Second, the piston-type valve: This type of valve set of two kinds of valves - pilot valve and the main valve - in one. The design of the pilot valve is similar to the direct-acting relief valve. Exhaust pressure from the pilot valve acts on the piston, causing the piston to open the main valve. If the main valve is large and can not be opened directly, this design will use the inlet pressure to open the main valve. As a result, this type of pressure relief valve offers higher capacity and accuracy (+/- 5%) at the same pipe size than direct-acting relief valves. As with the direct acting pressure reducing valve, pressure relief valves sense the pressure inside, eliminating the need for external sensing lines. Third, the film-type valve: In this type of valve, double diaphragm instead of the piston in the internal relief valve. This increased diaphragm area opens a larger main valve and has greater capacity than the internal pilot piston relief valve for the same pipe size. In addition, the diaphragm is more sensitive to pressure changes with an accuracy of +/- 1%. The higher accuracy is due to the positioning of the downstream sensing line (outside the valve) where turbulence of gas or liquid is less.