12 |
Baker Hughes
© 2020 Baker Hughes Company. All rights reserved.
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.
