CON-2
Page 4
II.
Terminology for Safety
Relief Valves
•
Accumulation
Accumulation is the pressure increase over the
maximum allowable working pressure of the
vessel during discharge through the pressure
relief valve, expressed as a percentage of that
pressure, or actual pressure units.
•
Back Pressure
Back pressure is the pressure on the discharge
side of a safety relief valve:
1.
Superimposed Back Pressure
Superimposed back pressure is the pressure
in the discharge header before the safety
relief valve opens.
a)
Constant - Specify single constant back
pressure (e.g., 20 psig/1.38 bar).
b)
Variable - Specify variable back
pressure range using min. and max.
limits (e.g., 0 to 20 psig/1.38 bar).
2.
Built-up Back Pressure
Built-up back pressure is pressure which
develops at the valve outlet as a result of
flow, after the safety relief valve has been
opened.
•
Blowdown
Blowdown is the difference between set pressure
and reseating pressure of a pressure relief valve,
expressed as a percentage of the set pressure, or
actual pressure units.
•
Cold Differential Set Pressure
Cold differential set pressure is the pressure at
which the valve is adjusted to open on the test
stand. This pressure includes the corrections for
back pressure and/or temperature service
conditions.
•
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. It is
recommended that the valve be set as high
above the operating pressure as possible.
•
Lift
Lift is the actual travel of the disc away from the
closed position when a valve is relieving.
•
Maximum Allowable Working Pressure
Maximum allowable working pressure is 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 safety relief valve is set to
open.
•
Operating Pressure
The operating pressure is 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 psi
(.34 bar), whichever is greater.
•
Overpressure
Overpressure is 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.
•
Rated Capacity
Rated capacity is 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)
for vapors; standard cubic feet per minute (SCFM)
or m
3
/min for gases; and in gallons per minute
(GPM) for liquids.
•
Relief Valve
A relief valve is an automatic pressure-relieving
device, actuated by static pressure upstream
from the valve, a relief valve is used primarily for
liquid service.
•
Safety Relief Valve
A safety relief valve is an automatic pressure-
relieving device which may be used as either a
safety or relief valve, depending upon application.
A safety relief valve is used to protect personnel
and equipment by preventing excessive
overpressure.
