V. Terminology for Pilot Operated Safety Relief Valves
•
Accumulation: The pressure increase over
the maximum allowable working pressure of
the vessel during discharge through the POSRV,
expressed as a percentage of that pressure or in
actual pressure units.
•
Backpressure: The pressure on the discharge side
of the POSRV:
– Built-up Backpressure: Pressure that develops
at the valve outlet as a result of flow, after the
POSRV has been opened.
– Superimposed Backpressure: Pressure in the
discharge header before the POSRV opens.
– Constant Backpressure: Superimposed
backpressure that is constant with time.
– Variable Backpressure: Superimposed
backpressure that will vary with time.
•
Blowdown: The difference between set pressure
and reseating pressure of the POSRV, expressed
as a percentage of the set pressure or in actual
pressure units.
•
Cold Differential Set Pressure: The pressure at
which the valve is adjusted to open on the test stand.
This pressure corrects for backpressure
when a pop action pilot’s vent is piped to the main
valve outlet.
•
Differential Between Operating and Set
Pressures: Valves in process service will generally
give best results if the operating pressure does not
exceed 90% of the set pressure. However, on pump
and compressor discharge lines, the differential
required between the operating and set pressures
may be greater because of pressure pulsations
coming from a reciprocating piston. The valve
should be set as far above the operating pressure as
possible.
•
Lift: The actual travel of the disc away from the
closed position when a valve is relieving.
•
Maximum Allowable Working Pressure: The
maximum gauge pressure permissible in a vessel
at a designated temperature. A vessel may not be
operated above this pressure or its equivalent at
any metal temperature other than that used in its
design. Consequently, for that metal temperature, it
is the highest pressure at which the primary pressure
POSRV is set to open.
•
Operating Pressure: The gauge pressure to
which the vessel is normally subjected in service.
A suitable margin is provided between operating
pressure and maximum allowable working pressure.
For assured safe operation, the operating pressure
should be at least 10% under the maximum allowable
working pressure or 5 psig (0.34 bar), whichever is
greater.
•
Overpressure: A pressure increase over the
set pressure of the primary relieving device.
Overpressure is similar to accumulation when
the relieving device is set at the maximum
allowable working pressure of the vessel.
Normally, overpressure is expressed as a
percentage of set pressure.
•
Pilot Operated Safety Relief Valve (POSRV):
A pressure relief valve in which the major relieving
device is combined with, and is controlled by, a
self-actuated auxiliary pressure relief valve.
•
Rated Capacity: The percentage of measured flow
at an authorized percent overpressure permitted
by the applicable code. Rated capacity is generally
expressed in pounds per hour (lb/hr) or kg/hr for
vapors, standard cubic feet per minute (SCFM) or
m³ /min for gases, and in gallons per minute (GPM) or
Liter/min (L/min) for liquids.
•
Safety Relief Valve (SRV): An automatic pressure-
relieving device used as either a safety or relief
valve, depending upon application. The SRV is used
to protect personnel and equipment by preventing
excessive overpressure.
•
Set Pressure: The gauge pressure at the valve
inlet, for which the relief valve has been adjusted to
open under service conditions. In liquid service, the
inlet pressure at which the valve starts to discharge
determines set pressure. In gas or vapor service, the
inlet pressure at which the valve pops determines
the set pressure.
12 | BHGE
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