At roughly 50 yr old, the liquified natural gas
(LNG) industry is a relatively new part of the global energy market. However, it
is one of the fastest growing segments of the global energy industry.
The LNG process chain is complex and diverse and comprises
numerous different processes, from extraction, liquefaction and transport to
consumption. To ensure the safety of these processes, pressure relief devices,
such as safety valves and rupture discs, are required in various aspects of the
LNG process chain to ensure that no serious accidents are caused by
overpressure.
A safety valve is a spring-loaded, pressure relief
valve (PRV) actuated by the static pressure upstream of the valve. A rupture
disc is a non-reclosing pressure relief device activated by static differential
pressure between the inlet and outlet of the device.1 Both safety
valves and rupture discs are possible solutions to the problem of overpressure
in LNG processes, but each device has benefits and disadvantages. They are used
separately or in combination depending on the specific requirements of the
process.
Protection of
pressure relief valves by rupture discs: Combination device. Safety valves are
widely used within LNG trains for overpressure protection. However, PRVs can
quickly reach their limits, particularly where there are major requirements in
terms of tightness or if the medium used is viscous, sticky or freezing. In
addition, each PRV has a certain level of leakage. Seat tightness requirements
are defined in API 527: the acceptance criteria allow a certain number of
bubbles, meaning some level of leakage during normal operation.2 It must
be considered that these leakage rates are established on a brand-new valve in
a controlled factory environment.
A solution involving an
upstream rupture disc combines the benefits of both devices. This arrangement
is called a combination device, as per EN ISO 4126-3.3 When
combined, PRVs and rupture discs are a reasonable and, above all, economic
solution for a wide range of applications. The combination device:
Based on these
technical and economic benefits, the use of combination devices is recommended
by the present codes and standards and is finding more applications in modern
processes.
A typical application within
LNG processes is where a rupture disc is commonly found in front of safety
valves on the vaporizers in the regasification plant.
However, as such
processes frequently run on a duty, standby and redundancy basis (three sets)
and because there are many processes on each asset, contractors, engineers and original
equipment manufacturers (OEMs) tend to choose lower technology, cross-scored,
forward-acting rupture discs due to the lower price level. Additionally, the
effect of back pressure from adjacent relief and flare streams on the rupture
discs can be ignored. As end users require more and more output, they put
higher volumes through existing assets to the point where the cycling and
operating pressures increase, pushing the installed rupture discs beyond their
operating limits. Ultimately, the rupture discs can fatigue and fail
prematurely.
High lifecycle. To avoid this
unnecessary, premature failure, rupture discs with a long lifetime are
recommended. Reverse-acting, robust discs such as those produced by the
author’s company provide longer-term cost benefits. The author’s company’s rupture
disca (shown in FIG. 2) is the only one to be manufactured using
Euler’s critical load formula to determine the burst pressure. With no specific
weak-point on the material—as
can be found on a cross-scored disc—the company’s rupture disca is an extremely robust
solution. The disc is almost immune to damage caused by improper installation
or handling, which maximizes the lifetime of the disc. The unique structure of
the disca also ensures that it can withstand a high level of back-pressure
without causing any damage. Boasting an operating ratio of up to 98%, the disca
is not subject to fatigue even at high operating ratios and the non-fragmenting
design is ideal for installation directly in front of a safety valve.
Monitoring can be used
to report when the disc has ruptured. Conventional signalling
devices require cables to be mounted on the rupture disc, which must then be
routed out through the rupture disc holder. This is not the case with the
company’s Non-invasive Monitoring Unitb, shown in FIG. 3. Here, a
signal indicator is attached to the rupture disc during the manufacturing
process. The actual sensor is screwed into a blind tapping in the rupture disc
holder, where it monitors the position of the signal indicator on the rupture
disc. This means that the wiring only starts outside the rupture disc holder.
After an overpressure
event, the outlet part of the rupture disc holder must be removed and the
rupture disc replaced; afterwards, the system can be put back into operation.
The days when the signalling cables also had to be routed again to the
respective switching box are finally over.
The process is
absolutely leak-tight. The blind tapping in the holder replaces the tapping that
is usually required. The absence of cable glands means that they cannot become
porous, thus preventing an escape of the process media.
The end-user now has an
effective engineered solution to provide maximum vessel and personnel
protection. The choice of correct disc design and material leads to increased
service life and eliminates frequent shutdowns for periodic maintenance. Using
the rupture discs rather than other valves, the user can profit from
substantially reduced capital expenditures (CAPEX) and associated installation
costs, and can reduce the overall emissions balance of the process.
Protecting
compressors under challenging circumstances. Another application within the LNG
process where rupture discs are used rather than safety valves is to protect compressors.
Due to their compact, reliable design, centrifugal compressors are often used
for the offshore liquefaction process. Such turbomachinery uses a dry gas seal that
uses clean gas under pressure on the external side of the seal so that any
leakage through the seal is the buffer gas into the compressor. An important
component of a dry gas seal is the vent system. For the primary vent system, a
rupture disc is typically installed in parallel with the vent line to rupture
at a set pressure and prevent excessive pressure to instruments on a primary
seal failure. A signalling system on the primary vent system is recommended to
enable integrity checks while the seal system is still online.
Customer’s using dry
gas seals require rupture discs to be installed to vent excess pressure in the
event of a catastrophic failure of the primary seal. If the pressure increases
beyond its predetermined limit, a higher gas flow or back-pressure in the
piping system is present and must be safely vented. A signalling device is also
required to detect a change in the rupture discs status to initiate a process shutdown
in the event of overpressure.
The author’s company’s rupture
disc with integrated signallingc is a cost-effective, reverse-acting
rupture disc with an integrated polyamide signaling device that is suitable for
the demanding conditions found in dry gas seal applications.
Used extensively in oil
and gas projects, the rupture discc with integrated signalling is
the only reverse-acting rupture disc to be manufactured using the author’s
company’s contour precision lasering technologyd, ensuring high-quality,
accurate burst control even in the harshest of environments. The integrated
signalling device ensures that operators are informed immediately that the disc
has burst and requires replacement. It is physically possible to restart the
compressor with the damaged rupture disc in place, which is why it is important
that signalling is used to alert operators that the disc has burst.
These are only two
examples of how the rupture discs are used across the LNG process industry—other examples range from
the protection of heat exchangers to cryogenic storage tanks. As LNG production
is expected to double over the next two decades with the U.S., Canada, Russia
and Australia leading the development of these global projects, it will become
increasingly important for those involved in LNG contracts to understand the
best and safest way to protect their processes.4–6 HP
NOTES
a
REMBE’s KUB reverse-acting rupture disc
b
REMBE’s NIMU signalling unit
c
REMBE’s IKB rupture disc
d
REMBE’s Contour Precision Lasering (CPL™) technology
LITERATURE CITED
CLAIRE LLOYD has worked for more than 10 yr in the field of process safety, specifically pressure relief, helping to protect processes in a wide range of industries, including chemical, pharmaceutical and oil and gas. Since joining REMBE GmbH Safety & Control in 2012, Lloyd is responsible for REMBE’s European network of partners in process safety.