; A balanced safety relief valve
incorporates a means of minimizing the
effect of back pressure on the operational characteristics (opening pressure, closing pressure and relieving
capacity) of the valve (Figure 5).
; A pilot operated pressure relief
valve is a pressure relief valve in
which the major relieving device is
combined and controlled by a self-actuated auxiliary pressure relief
valve (Figure 6).
; A vacuum relief valve is a pressure
relief device designed to admit fluid
to prevent an excessive internal vacuum; it is designed to reclose and
prevent further flow of fluid after
normal conditions have been
restored (Figure 7).
Common Causes of
Now we are going to look at three
common types of overpressure:
a boiler, or may be unplanned, such
as a fire in a tank farm.
3. Thermal expansion is as simple as
the sun’s heat on a trapped liquid.
1. Many times the over-pressure
condition is caused by a blocked
discharge or the overpressure
could create an explosion if it was
allowed to continue….fortunately
the pressure relief valve is calibrated to open at Maximum
Allowable Working Pressure.
2. External fire can be planned, such as
In each case, pressure is exerted on a
piping system or pressure vessel with
potential danger to people, property and
Education & Training Committee member WILLIAM
TRAVIS—training manager, Dresser Consolidated
( www.dresser.com) in Alexandria, LA—is the lead
author for the PRV section of Valves 101. Reach
him at email@example.com.
The Importance of PRVs
Let’s face it: Pressure relief valves don’t
have any fancy lights or indicators. Usually they are automatic, and plant personnel normally don’t pay any attention
to them (unless they are leaking). However, the National Board [of Boiler and
Pressure Vessel Inspectors] thinks
Part II of the National Board of
Boiler and Pressure Vessel Inspectors’
Inspection Code NBIC states: “The
most important appurtenances on any
pressurized system are the Pressure
Relief Devices.” For the uninitiated,
appurtenances can be defined as necessary equipment.
Part II of the NBIC goes on to state:
“These devices are not designed or
intended to control the pressure (that’s
a function of the control room). Instead,
they are intended to function when normal operating controls fail.”
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