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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:
ƒ Built-up Backpressure: Pressure that develops at the
valve outlet as a result of flow, after the POSRV has
been opened.
ƒ Superimposed Backpressure: Pressure in the discharge
header before the POSRV opens.
ƒ Constant Backpressure: Superimposed backpressure
that is constant with time.
ƒ 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
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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.
© 2019 Baker Hughes, a GE company. All Rights Reserved.
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