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 barg), 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 kilograms per hour (kg/hr) for vapors, standard
cubic feet per minute (SCFM) or cubic meters
per minute (m. /min) for gases, and in gallons
per minute (GPM) or Liter per minute (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.
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