Page 226 - Valve Selection Handbook

Pressure Relief Valves 213

Figure 5-44. Main Valve of a Pilot-Operated Pressure Relief Valve with Indirect
Acting Pilot, Designed to Open on Being Energized. (Courtesy of Bopp & Reuther.)

When the system pressure rises and reaches the set pressure, the pilot
opens and introduces the system pressure via a central tube to the top of
the piston. The pressure build-up above the piston causes the main valve
to open. Some of the pressure is allowed to bleed through a small hole to
the valve outlet.
When the excess pressure recedes and reaches the blowdown pressure,
the pilot closes and allows the pressure above the central piston to bleed
to the valve outlet. A spring above the closing member assists the valve
to close suddenly.
In both types of main valves, the closure members are located within
the system pressure zone and are exposed to the temperature of the sys-
tem pressure. In the case of high temperature service, the closure mem-
ber and guide are therefore not exposed to thermal shock. Also, solids
carried by the flowing fluid do not impact directly on the seating faces of
the closure member. This flow direction significantly reduces potential
seating damage.
Both types of pilot-operated pressure relief valve are widely employed
in conventional and nuclear power stations.