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Like other pressure relief
valves (PRV), pilot operated relief valves (PORV) are used for emergency relief
during overpressure events (e.g., a tank gets too hot and the expanding fluid
increases the pressure to dangerous levels). The difference between PORV and
conventional PRV is that pilot valves use system pressure to seal the valve. A
PRV typically uses a spring to hold the disc or piston on seat. The essential
parts of a PORV are a pilot valve (or control pilot), a main valve, a pitot
tube, the dome, a disc or piston, and a seat. The volume above the piston is
called the dome.
PORV are also called
pilot-operated safety valve (POSV), pilot-operated pressure relief valve
(POPRV), or pilot-operated safety relief valve (POSRV), depending on the
manufacturer and the application. Technically POPRV is the most generic term,
but PORV is often used generically (as in this article) even though it should
refer to valves in liquid service.
Mode
of functioning
The pressure is supplied from
the upstream side (the system being protected) to the dome often by a small
pilot tube. The downstream side is the pipe or open air where the PORV directs
its exhaust. The outlet pipe is typically larger than the inlet. 2 in × 3 in
(51 mm × 76 mm), 3 in × 4 in (76 mm × 102 mm), 4 in × 6 in (100 mm × 150 mm), 6
in × 8 in (150 mm × 200 mm), 8 in × 10 in (200 mm × 250 mm) are some common
sizes.
The upstream pressure tries to
push the piston open but it is opposed by that same pressure because the
pressure is routed around to the dome above the piston. The area of the piston
on which fluid force is acting is larger in the dome than it is on the upstream
side; the result is a larger force on the dome side than the upstream side. This
produces a net sealing force.
The pressure from the pilot
tube to the dome is routed through the actual control pilot valve. There are
many designs but the control pilot is essentially a conventional PRV with the
special job of controlling pressure to the main valve dome. The pressure at
which the control pilot relieves at is the functional set pressure of the PORV.
When the pilot valve reaches set pressure it opens and releases the pressure
from the dome. The piston is then free to open and the main valve exhausts the
system fluid. The control pilot opens either to the main valve exhaust pipe or
to atmosphere.
Snap
acting
At set pressure the valve
snaps to full lift, it can be quite violent on large pipes with significant
pressure. The pressure has to drop below the set pressure in order for the
piston to reseat (see blowdown in relief valve article).
Modulating
The pilot is designed to open
gradually, so that less of the system fluid is lost during each relief event.
The piston lifts in proportion to the overpressure. Blowdown is typically
short.
Advantages
- Smaller package on the larger pipe sizes.
- More options for control.
- Seals more tightly as the system pressure approaches but does not reach set pressure.
- Control pilot can be mounted remotely.
- Some designs allow for changes in orifice size within the main valve.
- can be used in engines
Disadvantages
- More complex, resulting in various fail-open failure modes.
- More expensive at smaller sizes (starts to even out as pipe size increases).
- Small parts in pilot valve are sensitive to contaminant particles.
