Chemical Engineering July 2017 - 46

Figure 4A
Holder
outlet
Ring
Rupture
disc
Holder
inlet
Process media
FIGURE 3. Leaks can occur outside of the planned
flow path in rupture disc systems if all components
are not correctly installed
Figure 4B
seal, this designed force creates a
leak-tight seal between the inlet side
of the holder and the process side
of the rupture disc. This arrangement
makes the rupture disc and
holder much less prone to damage
when torque forces exceeding the
holder's individual torque specifications
are applied to the flanges. It
also minimizes the damaging effects
of uneven torque application. Today,
nearly all high-performance scored
rupture discs are mounted in flatseat
holders.
Pre-torque holders are flat-seat
holders that allow the user to install the
rupture disc into the holder and apply
torque to cap screws in a controlled
environment.
Applying controlled
amounts of torque to the preload cap
screws better ensures proper disc
seating before the holder is installed
between the actual piping flanges.
Many facilities use this type of holder
so that a trained individual can clean
and inspect the holder for damage
that may cause leakage. The installer
can then verify that the correct disc is
being installed and that the disc is in
its proper orientation. Proper torque is
then evenly applied to the preload cap
screws to ensure the disc-to-holder
seal
is achieved. The pre-torqued
disc and holder assembly can then
be passed to an individual with less
training and expertise to install it into
the piping system. This type of assembly
may help reduce installation and
leakage issues when combined with
proper training and maintenance.
Usually, the technologies described
previously provide satisfactory
performance under the right
44
Leak path
FIGURE 4. Shown in Figure 4A is a rupture disc with a ring on the outlet. The concentration of stress on the
ring (Figure 4B) can make the system more susceptible to leaks outside the planned flow path
circumstances.
These
assemblies
must be carefully cleaned and inspected
by well-trained individuals.
They must then be installed in properly
aligned piping precisely according
to all installation procedures.
A new technology
One-piece units are the most recent
evolution in rupture-disc assembly
technology. They are a variation of
flat-seat holders, and are beginning
to emerge in chemical processing
plants. These units consist of a rupture
disc and holder combination that
is typically assembled at the factory
utilizing high-integrity welds. Companies
that have experienced problems
involving overpressure-relief systems
understand the value derived from
a sealed rupture disc and holder assembly.
One-piece units eliminate
installation errors due to improper
torquing, mismatched discs and holders,
improper disc orientation, inadequate
cleaning, corrosion damage,
holder wear and lack of inspection. A
hermetically sealed assembly keeps
the process in the piping, effectively
reducing a potential leak point in the
system. This technology eliminates
the need for installers to determine
if holders are too worn, corroded or
damaged to be reused. The use of
one-piece units ensures there will
be no unplanned downtime resulting
from leakage related to a damaged
holder. Finally, the simplified installation
process leads to faster installation
times and less required training
for maintenance teams.
Leaks through the relief system
As discussed, rupture discs are typically
mounted in holders designed for
a specific brand and type of rupture
disc. These holders are designed in
such a way that, barring any specification
or installation errors, the only
way for a rupture disc to leak through
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