Page 176 - Valve Selection Handbook

Pressure Relief Valves 163

sure system. This pressure is also the maximum pressure setting of the
pressure-relief devices that protect the pressure system.

Accumulation: The pressure increase in the pressure system over the
maximum allowable working pressure, expressed in pressure units or
percent of the maximum allowable working pressure.

Maximum allowable accumulation: The maximum allowable pressure
increase in the fluid pressure over the maximum allowable working pres-
sure as established by applicable codes for operating and fire contingen-
cies, expressed in pressure units or percent of the maximum allowable
working pressure.

Overpressure: The pressure increase over the set pressure of the relieving
device, expressed as a percentage of the set pressure or in pressure units.

Device Characteristics

Actual coefficient of discharge Kj: ? The ratio of the measured relieving
capacity to the theoretical relieving capacity of a theoretical ideal nozzle.

2
Rated or certified coefficient of discharge K: The actual coefficient of
discharge derated by a factor of 0.9.

Actual discharge capacity: The calculated discharge capacity of a theoret-
ically perfect nozzle having a cross-sectional flow area equal to the dis-
charge area of the valve multiplied by the actual coefficient of discharge.

Rated or certified discharge capacity: The discharge capacity of a theo-
retically perfect nozzle having a cross-sectional flow area equal to the
flow area of the valve multiplied by the rated or certified coefficient of
discharge.

Sef pressure: The inlet pressure at which the pressure relief valve com-
mences to open under service conditions. 3

1
ASME Code (1992) Section VIII, Division 1, UG-131 (e)(2).
2
ASME Code (1992) Section VIII, Division 1, UG-131 (e)(3).
3
The Pressure Relief Device Certifications book of The National Board of Boiler and
Pressure Vessel Inspectors employs a variety of definitions of the term "set pressure,"
such as case 1: First leakage/initial discharge/simmer, case 2: First steady stream/first
measurable lift, case 3: pop. The definition of the term set pressure as used in this book
corresponds with the term "First measurable lift."