Page 215 - Valve Selection Handbook

202 Valve Selection Handbook

Part of the equipment is a lap top computer with special software and
a printer. This equipment delivers the measured data and a pressure/lift
diagram. The output allows a complete performance evaluation of the
tested valve.
The set pressure may be verified at any system pressure and at the test
bench. The other data may be evaluated successfully only at an operating
pressure of not less than 80% of the set pressure.

PILOT-OPERATED PRESSURE RELIEF VALVES

Pilot-operated pressure relief valves differ from direct mechanically
loaded pressure relief valves in that the system fluid is the medium for
both opening and closing the valve. Such valves consist of a main valve,
which is the actual pressure relief valve, and a self-actuated pilot that
controls the opening and closing of the main valve in response to devel-
oping and receding overpressure.
Over the years, two types of pilot-operated pressure relief valves have
evolved.
One of these is of U.S. origin in which the pilot controls directly three
flow passages of which one interconnects with the pressure sense line, a
second one with the dome of the main valve, while a third one repre-
sents the vent. This type of pilot is described in the following as direct-
acting pilot.
The other type is of European origin in which the pilot represents a
spring-loaded pressure relief valve that controls the operation of the main
1
valve by the fluid being discharged. This type of pilot is described in the
following as indirect-acting pilot.

Pilot-Operated Pressure Relief Valves with Direct-Acting Pilot

Figure 5-35 shows the basic layout of a pilot-operated pressure relief
valve with direct-acting pilot. The actual pressure relief valve is the
main valve in which the piston is the closing member that is controlled
by the pilot.
At a system pressure lower than the set pressure, the system pressure
is allowed to act on the top of the main valve piston while the vent to the
atmosphere is closed. Because the piston area facing the dome is larger
than the nozzle area, the system pressure introduces a closing force on

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Draft European Standard EN 1268-1.