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Baker Hughes
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VIII. Terminology for Pressure Relief Valves
A. Accumulation:
The pressure increase over the
maximum allowable working pressure of the vessel
during discharge through the valve, expressed as
a percentage of that pressure or in actual pressure
units.
B. Backpressure:
The static pressure existing on the
discharge side of a pressure relief valve:
1. Built-up Backpressure:
Pressure that
develops at the valve outlet as a result of flow,
after the valve has opened.
2. Superimposed Backpressure:
Pressure in
the discharge header before the valve opens.
3. Constant Backpressure:
Superimposed
backpressure that is constant with time.
4. Variable Backpressure:
Superimposed
backpressure that will vary with time.
C. Blowdown:
The difference between set pressure
and reseat pressure of a valve, expressed
as a percentage of the set pressure or in actual
pressure units.
D. Cold Differential Set Pressure:
The pressure at
which the valve is adjusted to open on a test stand.
This pressure corrects for backpressure when a
pop action pilot’s vent is piped to the main valve
outlet.
E. 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.
F. Lift:
The actual travel of the disc away from the
closed position when a valve is relieving.
G. 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 relief valve is set to open.
H. 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. The suitable margin depends on the type
of pressure relief valve in service.
I. 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.
J. 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.
K.
Set Pressure:
The inlet static pressure at which
a pressure relief valve begins to operate under
service conditions. In liquid service, the inlet
pressure at which a valve starts to discharge
determines the set pressure. In gas or vapor
service, the inlet pressure at which the valve pops
or starts to leak, determines the set pressure.
L. Simmer:
The audible passage of a gas or vapor
across the seating surfaces just before a “pop”.
The difference between this start-to-open pressure
and the set pressure is called “simmer”. Simmer
is generally expressed as a percentage of set
pressure.
