Data Sheet

Pilot-operated relief valves are used in applications
requiring high-capacity and low-pressure buildup.
Internal Relief
The regulator shown in Figure 5 includes an internal relief
valve. The relief valve has a measuring element (the main
regulator diaphragm), a loading element (a light spring),
and a restricting element (a valve seat and disk).
The relief valve assembly is located in the center of
the regulator diaphragm. Internal relief is often used
in industrial applications where atmospheric exhaust is
acceptable, and low buildup is not required.
Backpressure Regulator Selection
Backpressure regulators control the inlet pressure rather
than the outlet pressure. The selection criteria for the
backpressure regulator is the same as for a pressure
reducing regulator.
Relief Valves and Backpressure Regulators
A pressure relief valve is a control device that opens to relieve
fluid to atmosphere during an overpressure occurrence. A
backpressure regulator is a control device that maintains a
constant upstream pressure throughout a given flow range.
Relief Valve Types
Relief valves are available in four general types: pop type,
direct-operated, pilot-operated, and internal relief valves.
Direct-Operated Relief Valves
System pressure is referenced under a diaphragm and
opposed by a spring. As system pressure increases past the
setpoint, the relief valve opens which allows fluid to escape
and protects the system. The increase in pressure above
the relief setpoint that is required to produce more flow
through the relief valve is referred to as pressure buildup.
Direct-Operated relief valves are commonly used in industry
to protect industrial furnaces and other equipment.
Pilot-Operated Relief Valves
In normal operation, when system pressure is below
setpoint of the relief valve, the pilot remains closed.
This allows loading pressure to register on top of the
main relief valve diaphragm. Loading pressure on top
of the diaphragm is opposed by an equal pressure (inlet
pressure) on the bottom side of the diaphragm. With
little or no pressure differential across the diaphragm, the
spring keeps the valve seated. When the system pressure
increases past the setpoint, the pilot opens and exhausts
the loading pressure from the top of the relief valve main
diaphragm which allows the main valve to open.
Relief Valve / Backpressure Regulator
Relief Valve
Backpressure Regulator
5
Figure 5. Internal Relief
Figure 4. Pilot-Operated Relief Valve
INLET PRESSURE LOADING PRESSURE
EXHAUSTATMOSPHERIC PRESSURE
FLOW
CONTROL
LINE
PILOT
RESTRICTION
PILOT
EXHAUST
MAIN REGULATOR
EXHAUST
MAIN RELIEF
VALVE
INLET PRESSURE
OUTLET PRESSURE
ATMOSPHERIC PRESSURE
LARGE OPENING FOR RELIEF
CLOSED RESTRICTION
OPENED INTERNAL RELIEF
Figure 3. Direct-Operated Relief Valve
SYSTEM PRESSURE LOWER-PRESSURE SYSTEM
(USUALLY ATMOSPHERE)
DIAPHRAGM
SPRING
VALVE
VENT
FLOW