Chemical Engineering December 2011 - 29

P
Department Editor: Scott Jenkins
rotecting process systems from overUsing
Rupture
Disks with Pressure
Relief Valves
pressurization is a critical task in the
chemical process industries (CPI), and
rupture disks and safety relief valves are
routinely used for this purpose. In certain
situations, using rupture disks in
combination with safety relief valves
offers advantages that can increase
safety and lower costs. The
combination with safety relief valves
offers advantages that can increase
advantages include a significant
lengthening of the service life of the
relief valve, as well as prevention
of process leakage.
What considerations should be
made when combining devices?
And how can you decide when
the combination is appropriate
versus when it may not be useful?
Reasons to combine the two
When overpressurization occurs in situaWhen
overpressurization occurs in situations
where rupture disks are combined
with safety relief valves, the disk bursts and
a valve release follows. Once the pressure
drops to a safe level, the safety valve
reseats itself and continues to protect the
system (Figures 1 and 2). There are several
situations in which using the two systems
together can lead to significant benefits.
Isolation of relief valve. Rupture disks can
isolate a safety relief valve from process
fluids and materials, so that, under normal
operating conditions, the safety valve does
not encounter the process chemicals. Since
the safety valve is isolated, its internal mechanics
will not come into contact with any
caustic process chemicals or viscous materials
that might interfere with the valve's
operation. Because valve internals are not
routinely exposed to process materials,
they remain in almost new condition, which
allows longer periods between major overhauls.
Also, since the valve is isolated, it is
not necessary to have the valve constructed
in a material designed for continuous
contact. For example, if the process fluid
requires that Hastelloy be the preferred material
of construction for continuous contact,
a carbon-steel valve (with Hastelloy trim)
combined with a Hastelloy rupture disk can
be used. This will save a significant portion
of the valve cost.
Leak prevention. Another major advantage
of combining rupture disks with relief
valves is leak prevention - under normal
operating conditions, the rupture-disk barrier
prevents process fluids from escaping
into the atmosphere. An example described
in Ref. 1 illustrates the savings that can be
realized by the combined arrangement:
For conventional safety valves, American
Petroleum Institute (API) standard 527 (Seat
Tightness of Pressure Relief Valves) allows
for an orifice size of F or smaller to have
a maximum allowable leakage rate of 40
bubbles per minute (approximately 6 ft3
over a 24-h period, or 2,190 ft3
/yr). This
leakage is either lost, eroding profits and
potentially harming the environment, or requires
the installation of a system to recover
the leakage.
Test-in-place. Combining rupture disks with
safety valves allows the safety valve to be
tested in place in the field. With a suitable,
reverse-buckling rupture disk installed at
the valve inlet, the safety relief valve can
be field-tested by a single person with a
portable pressure source. To accomplish this
without opening any process piping, air (or
nitrogen or another fluid) is injected from
the pressure source into the chamber between
the rupture disk and the safety valve
inlet. The test pressure is increased until the
valve releases, and should be within the set
pressure tolerance of the valve.
What to consider
What factors should be considered when
deciding whether to use rupture disks in
combination with pressure safety relief
valves or to use a rupture disk alone? There
are likely many, depending on the particulars
of the application, but here is a set of
basic considerations with which to begin.
Cost. Rupture disks are considerably
less expensive than safety relief valves,
particularly when the valve needs to be
constructed from exotic materials.
Process materials. A rupture disk alone
is a good choice for overpressure protection
in cases where process contents are
inexpensive, nonhazardous and environmentally
safe. A rupture-disk and relief-valve
combination should be the choice when
a leak-tight seal of the pressurized system
is needed, and when the conservation of
product within the pressurized system is
important, because it contains a corrosive,
hazardous or expensive substance.
Speed. The quick-bursting action of a
rupture disk makes it a first consideration
when the potential for runaway reactions
exists. Safety valves alone will not react
quickly enough to protect a process system
lengthening of the service life of the
FIGURE 1. The rupture
disk is used at the inlet
process and the valve
disk is used at the inlet
of the relief valve, acting
as a barrier between the
process and the valve
FIGURE 2. When rupture disks are used
to isolate safety relief valves, the rupup
of
the relief valve, acting
as a barrier between the
ture disk is first to open in the event of
system overpressurization. The vented
process fluid then contacts the safety
relief valve, which releases the fluid if
the pressure is excessive
from the pressure of a deflagration
or a detonation.
Liquid properties. Some
liquids may freeze or
cause icing under rapid
depressurization, leading
to blockage within
a safety valve, and
rendering it ineffective.
Also, highly viscous liquids, such as
polymers, may not relieve pressure
fast enough through a safety relief
valve, and can create a danger of plug
ging the valve.
valve, and can create a danger of plugSizing.
When sizing a relief valve,
engineers need to determine the required
fluid-flow capacity, and simultaneously to
analyze the possible emergency scenarios,
such as fire, loss of process cooling and
equipment failure. The capacity requirements
are then entered into a sizing
equation to determine the relief valve area.
For a rupture-disk-safety-valve combination,
the flow capacity of the combination must
be confirmed to support the selection of
both the valve and the disk. A combination
capacity factor (CCF), which is often determined
from ASME-certified capacity testing,
can be used to support the decision. The
CCF is calculated as the ratio between
the capacity of the disk-valve combination
over the relief valve capacity alone. CCFs
should not exceed 1.
Pressure drop. The proper function of a
relief valve requires that the pressure drop
between the vessel it protects and the valve
inlet is not more than 3% of the valve's set
pressure. Relief valves that are isolated by
a rupture disk contribute to piping pressure
drop, but by selecting rupture disks having
low flow-resistance values, the pressuredrop
target is usually reached.
Differential pressure. In a rupture-disksafety-valve
combination, the differential
pressure across the rupture disk must be
monitored. The assembly shown in Figures
1 and 2 contains an excess flow valve to
maintain atmospheric pressure in the space
between the rupture disk and the safety
valve, as well as a pressure gage on the
relief valve to provide local confirmation of
pressure status.
References
1. Brazier, G., Combining rupture disks with
safety relief valves, Chem. Eng., March 2009,
pp. 42-44.
Editor's note: This edition of " Facts at your
Fingertips " is adapted from the article
referenced above.
liquids may freeze or
cause icing under rapid
depressurization, leading
Chemical Engineering December 2011

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