Figure 1. Design Spring Loaded at Crosby Pressure Relief Valve
Figure 2 is a simple sketch that shows the disc is held in a position closed by a spring. When the system pressure reaches the desired opening pressure, the pressure force acting in Area A1 is the same as the spring strength, and the disc will lift and allow the liquid to flow out through the valve. When the pressure in the system returns to a safe level, the valve will return to the closed position.
When the Pressure Relief Valve starts to lift, the spring force increases. Thus the system pressure must increase if the appointment will continue. For this reason, Pressure Relief Valve is allowed to reduce full pressure to achieve full lift. This allowable excess pressure is generally 10% for valves on unwanted systems. This margin is relatively small and a number of ways must be provided to help with the appointment.
Figure 2. Pressure Relief Valve Trim Areas Diagram
Therefore, most Pressure Relief Valves have a secondary control room or clustered space to increase lift. A typical configuration is shown in Figure 3. When the disc starts to lift, the fluid enters the control room which exposes a larger area of the disc (Figure 2) to system pressure. This causes an additional power change that compensates for the increase in spring force and causes the valve to open at high speed. At the same time, the direction of the fluid flow is reversed and the momentum effect resulting from changes in flow direction increases the lift. These effects combine to allow the valve to reach maximum lift and maximum flow within the allowable excess pressure limit. Because a larger disk area A2 (Figure 2) is subject to system pressure after the valve has been lifted, the valve will not close until the system pressure is reduced to a number of levels below the set pressure. The control chamber design determines where the closing point will occur.
Figure 3. Pressure Relief Valve Control Chamber
The control design or huddling chamber involves a series of design sacrifices. If the design maximizes the lifting effort then the blowdown will be long. If the design goal is to minimize blowdown, the lifting effort will decrease. Therefore, many Pressure Relief Valves are equipped with a nozzle ring that can be adjusted to vary the control room geometry to meet certain system operating requirements (Figures 2 and 3)
Source : 1997. Pressure relief Valve Engineering Handbook: Crosby Valve Inc.