Publisher Summary This chapter focuses on pressure relief valves, their types, and main functionalities. Pressure relief valves are designed to protect a pressure system against excessive normal or subnormal pressure in the event of positive or negative excursion of the system pressure. They are required to open at a predetermined system pressure, to discharge or let enter a specified amount of fluid so as to prevent the system pressure from exceeding a specified normal or subnormal pressure limit, and to reclose after the normal system pressure has been restored. The most commonly used pressure relief valve type is the direct-loaded pressure relief valve in which the fluid pressure acting on the valve disc in the opening direction is opposed by a direct mechanical-loading device such as a weight or spring. The second type of pressure relief valve is the pilot-operated pressure relief valve that consists of a main valve and a pilot. The main valve is the actual pressure-relieving device while the pilot positions the disc of the main valve disc in response to the system pressure.