Protecting LNG processes

The LNG industry is a relatively new development in the global energy market at just roughly 50 years old, but it is one of the fastest growing segments of the global energy industry.

The LNG process chain is complex and diverse and there are a number of different processes from extraction, liquefaction, transport, through to consumption. To ensure the safety of these processes, pressure relief devices (such as safety valves and rupture discs) are required to be used in various aspects of the LNG process chain to ensure that there are no serious accidents caused by overpressure.

A safety valve is a spring-loaded pressure-relief valve 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

Safety valves are widely used within LNG trains for overpressure protection. However, pressure relief valves (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 the API 527² of which the acceptance criteria allow a certain number of bubbles, meaning some level of leakage during normal operation. It has to 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 unites the benefits of both devices. This arrangement is called a combination device as per EN ISO 4126-3.³ When combined, pressure relief valves (PRV) and rupture discs are a reasonable and, above all, economic solution for a wide range of applications.

• 100% isolation of process fluids, no leaks until the disc opens.
• No product loss during normal operation, rupture disc is leak-tight.
• Prevents rogue emission.
• Isolate PRVs from corrosive process media, with the benefits of substantial costs savings on PRV materials and reduction of regular maintenance costs on PRVs to a minimum.
• Prevents blockage of the pressure relief valve.
• It can be used to pop test the PRV in-situ, no need for removal of PRV for testing of it in a maintenance workshop, PRV can be kept in place.

Based on these technical and economic benefits, the use of combination devices are recommended by the current codes and standard and finding more applications in the modern processes.

A typical application within the LNG process where a rupture disc is commonly found in front of safety valves on the vaporisers in the regasification plant.

However, as such processes frequently run on a duty, standby, and redundancy basis (three sets) and as there are many 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 REMBE’s KUB provide longer-term cost benefits. The KUB is the only rupture disc to be manufactured using Euler’s critical load formula to determine the burst pressure. As there is no specific weak-point on the material, as there is on a cross-scored disc, the KUB is an extremely robust solution. The disc is almost immune to damage caused by improper installation or handling which maximises the lifetime of the disc. The unique structure of the KUB also ensures that it can withstand a high level of back pressure without causing any damage to the disc. Boasting an operating ratio of up to 98%, the KUB 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.

References

1. 'API 520 – Part 1 Sizing, Selection and Installation of Pressure relieving Devices', American Petroleum Institute, 9^th edn., (July 2014).
2. 'API 527 – Seat Tightness of Pressure Relief Valves', American Petroleum Institute, 4^th edn., (November 2014).
3. 'EN ISO 4126 -3:2006 Safety devices for protection against excessive pressure – Part 3: Safety valves and bursting disc safety devices in combination', ISO.
4. 'API Standard 521 – Pressure-relieving and Depressuring Systems', American Petroleum Institute, 6^th edn., (January 2014).
5. 'ASME Boiler and Pressure Vessel Code', Sec. VIII Div. 1 (2015).
6. STAHLET, J. S., Dry Gas Seal System Design Standards for Centrifugal Compressor Applications, (2002).

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