Process safety: Protecting LNG processes

C. Lloyd, REMBE GmbH Safety & Control, Brilon, Germany

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:

1. Provides 100% isolation of process fluids, no leaks until the disc opens
2. Allows no product loss during normal operation, rupture disc is leak-tight
3. Prevents rogue emissions
4. Isolates PRVs from corrosive process media, with the benefits of substantial costs savings on PRV materials and the reduction of regular maintenance costs on PRVs to a minimum
5. Prevents blockage of the PRV
6. Used to pop test the PRV in-situ (FIG. 1), with no need to remove the PRV for testing in a maintenances workshop (i.e., the PRV can be kept in place).

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 disc^a (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 disc^a 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 disc^a 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 disc^a 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 Unit^b, 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 signalling^c 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 disc^c with integrated signalling is the only reverse-acting rupture disc to be manufactured using the author’s company’s contour precision lasering technology^d, 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

1. American Petroleum Institute (API) Standard 520, “Sizing, selection and installation of pressure-relieving devices—Part I: Sizing and selection,” 9th Ed., July 2014.
2. American Petroleum Institute (API) Standard 527, “Seat tightness of pressure relief valves,” 4th Ed., November 2014.
3. International Organization for Standardization (ISIO) 4126-3, “Safety devices for protection against excessive pressure—Part 3: Safety valves and bursting disc safety devices in combination,” 2006.
4. American Petroleum Institute (API) Standard 521, “Pressure-relieving and depressuring systems,” 6th Ed., January 2014.
5. American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC), Sec. VIII, Div. 1, “Rules for construction of pressure vessels,” 2015.
6. Stahley, J. S., “Dry gas seal system design standards for centrifugal compressor applications,” Dresser-Rand Co., Olean, New York.

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.