Page 437 - APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition

Process Safety and Pressure-Relieving Devices 403

SPINDLE
SPINDLE ------
SEAT RETAINER

...._ __ NOZZLE

Figure 7-3A. Safety relief valve with rubber or plastic seats. By permission, Anderson, Greenwood and Co.

Definition of Pressure-Relief Terms (See Figures (7-3, Overpressure: pressure increase over the set pressure of
7-3A, 7-4, and Ref. [35J) the primary relieving device is overpressure. It is the same
as accumulation when the relieving device is set at the
maximum allowable working pressure of the vessel [ 10].
Set Pressure: the set pressure, in pounds per square inch
gauge, is the inlet pressure at which the safety or relief valve
is adjusted to open [10,13]. This pressure is set regardless Accumulation: pressure increase over the maximum
of any back ?ressure on the discharge of the valve, and is allowable working pressure of the vessel during discharge
not to be confused with a manufacturer's spring setting. through the safety or relief valve, expressed as a percent