Page 175 - Valve Selection Handbook

162 Valve Selection Handbook

Power-actuated pressure relief valve: A pressure relief valve consisting
of a main valve that is the actual pressure-relieving device and an exter-
nally powered piloting device that controls the opening and closing of
the main valve by either pressurizing or depressurizing the dome of the
main valve. 1

Dome: The chamber at the top of the piston-shaped closure member of
the main valve that is pressurized or depressurized to open or close the
main valve.

Dimensional Characteristics

Discharge area: The controlled minimum net area that determines the
flow through the valve.

Effective discharge area: The nominal or computed discharge area of a
pressure relief valve used in recognized flow formulae in conjunction with
the correlated effective coefficient of discharge to determine the size of the
valve. The effective discharge area is less than the actual discharge area.

Huddling chamber: The annular chamber located beyond the seat of
direct-loaded pressure relief valves formed by the lip around the disc for
the purpose of generating pop-opening action in gas or vapor service, and
raising the lifting force in both gas and liquid relief valves.

Secondary orifice: The annular opening at the outlet of the huddling
chamber.

System Characteristics

Operating pressure: The maximum pressure that is expected during nor-
mal operating conditions.

Operating pressure differential: The pressure differential between oper-
ating pressure and set pressure expressed as a percentage of the set pres-
sure or in pressure units.

Maximum allowable working pressure: The maximum pressure at
which the pressure system is permitted to operate under service condi-
tions in compliance with the applicable construction code for the pres-

1
U.S. Code of Practice: ASME, Sections I and III.
Draft European Standard: EN 1268-5.