A conventional pressure relief valve is designed to open when the differential pressure across the valve reaches the net set pressure. The valve will reseat when the differential pressure drops below the net set pressure minus the blowdown.
Figure 1200-12 shows a schematic diagram of a typical conventional pressure relief valve. Because the fluid on the discharge side of the valve is in contact with the full area of the upper surface of the valve’s disk, the set pressure of this valve is the sum of the net set pressure (the resistance to opening supplied by the spring) and any superimposed back pressure at the valve’s outlet; see Figure 1200-13. When specifying a conventional valve for an application with superimposed back pressure, it is important to clearly specify the set pressure (i.e., the desired opening gauge pressure), the back pressure in service, and the net set pressure.
Most conventional pressure relief valves have internals (trim) designed specifically for either liquid or vapor service. Consideration should be given to all potential phases of flow through the relief valve when selecting the trim.
The conventional pressure relief valve is the most commonly used pressure relief device in the process industries. While it has some disadvantages relative to the rupture disk (see below), its ability to reclose represents an overwhelming advantage in most applications. By opening and then reclosing in response to the appearance and removal of a cause of overpressure, a pressure relief valve relieves the excess pressure without venting an entire vessel or a continuous process. Due to the simplicity of its design, a conventional valve is preferred to the other types of pressure relief valves in terms of reliability, cost, and maintenance. This also provides for future expansion and additional relief loads. Other types should be selected only when service conditions (as discussed below) or regulations require them.