SELECTION STRATEGY OF
RUPTURE DISC
t has been a practice in the
process industry to fit in
rupture discs in pressure
equipment to provide
means of relieving excess
pressure. The rupture disc ensures safety
of life and plant equipments in case of
overpressure. A rupture disc is a non-
reclosing pressure relief device designed
to provide overpressure relief in chemical,
petrochemical, and sanitary applications.
It can also be defined as a thin metal
membrane designed to burst at a certain
pressure and temperature in order to
prevent over pressurization of the attached
vessel. The rupture disc can be scored,
machined, or injection molded. Rupture
discs are also many a times known as
burst discs due to its characteristic way of
bursting when relieving pressure build-up.
I
Rupture discs is similar to a pressure relief
valve due to its function of reducing over
pressurization. But differs in the sense that,
while a pressure relief valve will reseat
after discharging, a rupture disc remains
open after discharging.
A rupture disc consists of a thin metal
disc held at both ends by specially
machined flanges. The disc can be made
of aluminum, silver, copper, nickel etc. And
its of the disc material, and thickness that
it will burst at a predetermined pressure.
Since the rupture disc metal disc is thin,
the stresses are high, and non-corrosion
allowance is allowed. However, the rupture
disc material must be more resistant to
corrosion than the metal from which
the pressure vessel is constructed. As a
preventive alternative, lining and coatings
or other materials can be employed to
protect the disc against corrosion.
The rupture disc with accessories consists
of selected instruments that work as a unit
with the single objective of controlling
pressure build-up. A rupture discs
accessories consists of: burst checks, burst
indicators, clamps, ferrules. A rupture disc
can be thermally or mechanically actuated.
Regardless of the rupture disc design,
it should be made with a “safety factor”,
which means that a rupture disc designed
94
PECM Issue 32
to burst at 80psi must burst at 80psi (It
must burst within the agreed tolerance).
R upture D isc T ypes
There are different types of rupture
discs used in the oil and gas, sanitary
etc industries. They are categorized as
forward-acting rupture discs and reverse-
acting rupture discs. These different types
of rupture discs are employed, depending
on the suitability of the process.
F orward -A cting R upture D isc
The forward-acting rupture disc is dome
shaped, and is oriented into a system with
the process media (phase application)
pressure against the concave side of
the rupture disc (fig 2). When there is an
increase in the process pressure beyond
the designed or allowable operating
pressure, the tensile strength of the
material is reached and rupture occurs.
This type of rupture disc is employed in
systems that typically have operating
ratios at about 80% or less. They are
usually designed to act in tension, more
like a balloon. There are different types
of forward-acting rupture discs, they are:
forward-acting composite disc, forward-
acting solid metal disc, forward-acting
scored metal disc, and graphite disc. They
have an advantage of being cheaper than
the reverse-acting rupture disc.
be non-fragmenting. Reverse-acting
rupture disc can also be used in vacuum or
larger backpressure applications without
special supports. They have an advantage
of controlling burst pressure at close
tolerances.
O perating a R upture D isc
While using a rupture disc, back-pressure
build-up in the space between the disc
and the pressure relief valve should be
controlled. The contrary will lead to the
disc not bursting at its designed pressure.
This back-pressure build-up could be
caused by leakage, as a result of physical
damage or corrosion. Proper inspection
for corrosion and physical damage of the
rupture disc will give satisfactory service
and enhance process safety.
Rupture discs are used on vessels, piping
and pressure relief valves, where the
pressure build-up is formed as a result of
unavoidable mechanical malfunctions,
runaway chemical reactions. They are also
used to protect heat exchangers, and large
equipment like compressors and pumps.
Rupture discs can be termed “non-return”
when the relief is discharged to the
atmosphere, while they can be termed
“return” when they discharge to some
downstream collection and treatment
systems, like the knockout drums (fig 1).
R everse -A cting R upture D isc R upture D isc S election C riteria
The reverse-acting rupture disc is also
dome shaped, but is installed with process
pressure against the convex side of the
disc, thereby placing the rupture disc in
compression (fig 3). It is designed, such
that as the designed burst pressure of the
rupture disc is attained, the compression
loading on the disc causes it to reverse
back into a forward-acting disc and then
burst. This causes the disc to open by a
predetermined knife blade or scoring
pattern penetration. Reverse-acting
rupture discs are employed in systems
where operating ratios is as high as 95%
or less. They can be used in combination
with relief valves, which means they can It is believed in some industrial sectors
that over 60% of rupture disc installed
don’t perform satisfactorily and improper
selection strategy accounts for the
majority of these problems. Proper
selection of a rupture disc is more
than performing sizing calculations to
make sure it is adequately sized for the
emergency event. Failure to select the
correct rupture disc for an application can
result in significant plant down time due
to ‘nuisance failures’. In a worst case, an
improperly specified disc can fail to open
during an over pressure event, resulting to
a catastrophic failure.