API-RP-576

Type
Characteristics
Mediums
Limitations
Pilot-operated
safety-relief valves
This type of safety valve
uses the flowing medium
itself, through a pilot
valve, to apply the
closing force on the
safety valve disc. The
pilot valve is itself a
small safety valve.
Clean,
non-viscous
mediums.
should not be used in service;
(1) Dirty, fouling mediums causing blockage to the
small bore piping..
(2) Viscous mediums increase the operation time.
(3) Corrosive mediums that cause blockage, impair
actuation.
(4) Chemical compatibility and temperature limits
that affect the pilot valve seating.
1. large relief area and/or high set pressures are required.
2. Back pressure is high and balance design is required.
3. low differential exists between the normal vessel operating pressure and the set pressure of the
valves.
4. Pilot operated safety valves offer good overpressure and blowdown performance (a blowdown
of 2% is attainable).
5. Pilot operated valves are also available in much larger sizes, making them the preferred type of
safety valve for larger capacities.
6. Remote sensing & control at different locations.
7. inlet or outlet piping frictional pressure losses are high.
8. In-situ, in-service, set pressure verification is required.
http://www.spiraxsarco.com/resources/steam-engineeringtutorials/safety-valves/types-of-safety-valve.asp
Fion Zhnag/ Charlie Chong