Page 174 - Valve Selection Handbook

Pressure Relief Valves 161

Balanced safety relief valve: A pressure relief valve that incorporates a
means for minimizing the effect of back pressure on the performance
characteristic.

Supplementary loaded pressure relief valve: A direct-loaded pressure
relief valve with an externally powered auxiliary loading device that is
designed to inhibit valve simmer by introducing a supplementary seating
load during normal operation of the pressure system. Depending on the
Code of Practice, the supplementary-loading force augmenting the spring
force may be restricted or unrestricted. The load is automatically
removed at a pressure not greater than the set pressure of the valve. 1

Assisted pressure relief valve: A direct-loaded pressure relief valve with
an externally powered auxiliary assist device that is designed to inhibit
valve chatter by introducing a supplementary lifting force when the valve
is called upon to open. When the safe operating pressure is restored, the
assist device is deactivated and the valve closes normally. If the power to
the assist device should fail, there is no interference with the normal
2
operation of the valve.

Pilot-operated pressure relief valve: A pressure relief valve consisting of
a main valve that is the actual pressure relieving device, and a self-actuat-
ed pilot that controls the opening and closing of the main valve by either
pressurizing or depressurizing the dome of the main valve. 3

Pilot-operated pressure relief valve with direct-acting pilot: A pilot-oper-
ated pressure relief valve in which the pilot represents a pressure-actuated
4
three-ported valve that controls the operation of the main valve.
Pilot-operated pressure relief valve with indirect-acting pilot: A pilot-
operated pressure relief valve in which the pilot represents a spring-
loaded pressure relief valve that controls the operation of the main valve
by the fluid being discharged. 5

1
U.S. Code of Practice: ASME, Sect. HI, NC 7511.3.
Draft European Standards: EN 1268-1 and 5.
2
U.S. Code of Practice: ASME, Sect. HI, NC-7512.1.
Draft European Standard: EN 1268-1.
3
Draft European Standard: EN 12688-4.
4
Draft European Standard: EN 12688-4.
5
Draft European Standard: EN 1268-1.