Page 16 - Valve Selection Handbook

Introduction 3

trols the opening and closing of the main valve in response to the system
pressure.
Direct-acting pressure may be provided with an auxiliary actuator that
assists valve lift on valve opening and/or introduces a supplementary
closing force on valve reseating. Lift assistance is intended to prevent
valve chatter while supplementary valve loading is intended to reduce
valve simmer. The auxiliary actuator is actuated by a foreign power
source. Should the foreign power source fail, the valve will operate as a
direct-acting pressure relief valve.
Pilot-operated pressure relief valves may be provided with a pilot that
controls the opening and closing of the main valve directly by means of
an internal mechanism. In an alternative type of pilot-operated pressure
relief valve, the pilot controls the opening or closing of the main valve
indirectly by means of the fluid being discharged from the pilot.
A third type of pressure relief valve is the powered pressure relief
valve in which the pilot is operated by a foreign power source. This type
of pressure relief valve is restricted to applications only that are required
by code.

Rupture Discs

Rupture discs are non-reclosing pressure relief devices that may be
used alone or in conjunction with pressure relief valves. The principal
types of rupture discs are forward domed types, which fail in tension, and
reverse buckling types, which fail in compression. Of these types,
reverse buckling discs can be manufactured to close burst tolerances. On
the debit side, not all reverse buckling discs are suitable for relieving
incompressible fluids.
While the application of pressure relief valves is restricted to relieving
nonviolent pressure excursions, rupture discs may be used also for reliev-
ing violent pressure excursions resulting from the deflagration of flam-
mable gases and dust. Rupture discs for deflagration venting of atmos-
pheric pressure containers or buildings are referred to as vent panels.

Units of Measurement

Measurements are given in SI and imperial units. Equations for solv-
ing in customary but incoherent units are presented separately for solu-
tion in SI and imperial units as presented customarily by U.S. manufac-