Page 178 - Valve Selection Handbook

Pressure Relief Valves 165

Blowdown: The difference between the set pressure and the pressure at
reseating of the pressure relief valve, expressed as a percentage of the set
pressure. 1

Valve blowdown setting: The difference between the set pressure and
the valve inlet pressure at the commencement of rapid valve closing,
being the sum of the blowdown of the seated valve and the valve inlet
pressure loss at the commencement of rapid valve closing, expressed as a
percentage of the set pressure.

Blowdown pressure: See "reseating pressure."

Flutter: Abnormal, rapid reciprocating motion of the movable parts of the
pressure relief valve in which the disc does not contact the nozzle.

Chatter: Abnormal, reciprocating motion of the movable parts of the
pressure relief valve in which the disc contacts the nozzle.

Crawl: The gradual decreasing of the set pressure of spring-loaded pres-
sure relief valves from below to normal after the temperature of the
spring has been raised by the fluid just discharged.

Simmer: The audible or visible escape of compressible fluid between the
seat and the disc at no measurable capacity as the set pressure is
approached.

DIRECT-LOADED PRESSURE RELIEF VALVES

Review

Early pressure relief valves were of the direct-loaded type in which the
disc was loaded by a weight. The shape of the disc resembled that of a
globe valve and was commonly either tapered or flat. When rising fluid
pressure raised the disc of its seat, the escaping fluid was not able to
impart much of its kinetic energy onto the disc. Consequently, the lift of
the disc was very small within the allowable overpressure. Early efforts
were therefore directed to improve the valve lift.

1
In pop-opening pressure relief valves, blowdown is the difference between the valve
blowdown setting and the inlet pressure loss at the commencement of rapid reclosing. In
modulating pressure relief valves, blow down is due to dynamic friction within the body
of direct acting valves or within the pilot of pilot operated valves with direct acting pilot.