Introduction
The function of a pressure relief valve is to protect pressure vessels, piping systems, and other equipment from pressures exceeding their design pressure by more that a fixed predetermined amount.  The permissible amount of overpressure is covered by various codes and is a function of the type of equipment and the conditions causing the overpressure.
It is not the purpose of a pressure relief valve to control or regulate the pressure in the vessel or system that the valve protects, and it does not take the place of a control or regulating valve.
The aim of safety systems in processing plants is to prevent damage to equipment, avoid injury to
personnel and to eliminate any risks of compromising the welfare of the community at large and the
environment.  Proper sizing, selection, manufacture, assembly, test, installation, and maintenance of a
pressure relief valve are critical to obtaining maximum protection.
Types, Design, and Construction
A pressure relief valve must be capable of operating at all times, especially during a period of power
failure; therefore, the sole source of power for the pressure relief valve is the process fluid.
The pressure relief valve must open at a predetermined set pressure, flow a rated capacity at a specified overpressure, and close when the system pressure has returned to a safe level.  Pressure relief valves must be designed with materials compatible with many process fluids from simple air and water to the most corrosive media.  They must also be designed to operate in a consistently smooth manner on a variety of fluids and fluid phases.  These design parameters lead to the wide array of pressure relief valve products available in the market today

LINK DOWNLOAD