Protecting LNG processes
C. LLOYD, REMBE, Brilon, Germany
While the liquified natural gas (LNG) industry is a relatively new part of the global energy market (roughly 50-yr-old), it is one of the fastest growing segments. The LNG process chain includes several complex and diverse 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 throughout various aspects of the LNG process chain to prevent serious accidents 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 PRV activated by the static differential pressure between the device’s inlet and outlet.1 Safety valves and rupture discs are possible solutions to overpressure in LNG processes, and each device has benefits and disadvantages. They are used separately or in combination, depending on the specific requirements of the process.
Protecting PRVs with 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 regarding the medium’s tightness or if it is viscous, sticky or freezing. In addition, each PRV allows a certain level of leakage. Seat tightness requirements are defined in American Petroleum Institute (API) 527—the acceptance criteria allow for a certain number of bubbles, meaning some leakage is inevitable 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 unites the benefits of both devices. This arrangement is called a combination device per the International Organization of Standardization (ISO) 4126-3.3 When combined, PRVs and rupture discs are a reasonable and economical solution for various applications. The benefits of the combination include:
- Isolating 100% of process fluids
- Avoiding product loss during normal operation
- Preventing rogue emissions
- Isolating PRVs from corrosive process media, resulting in cost savings on PRV materials and maintenance
- Preventing PRV blockage
- Pop testing the PRV onsite, with no need for removal (FIG. 1).
FIG. 1. A combination device with a rupture disc for onsite testing of PRVs.
Current codes and standards recommend the technical and economic benefits of combination devices, which are finding more applications in modern processes. For example, an application where a rupture disc is commonly found within the LNG process is in front of safety valves on the vaporizers in regasification plants.
However, as such processes frequently run on a duty, standby and redundancy basis (three sets)—and 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 backpressure on the rupture discs from adjacent relief and flare streams can be ignored.4 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. Rupture discs with a long lifetime are recommended to avoid premature failure. Proprietary reverse-acting rupture discsa provide longer-term cost benefits. The author’s company’s rupture disca was manufactured using Euler’s critical load formula to determine the burst pressure. The disc is nearly immune to damage caused by improper installation or handling, which maximizes the disc’s lifetime. The rupture disca also ensures that it can withstand a high level of backpressure without causing any damage to the disc (FIG. 2).
FIG. 2. A reverse acting rupture disc for absolute leak tightness.
The system is leak-proof and back in operation quicker. Monitoring can also be used to report when the disc has ruptured. Conventional signaling devices require cables to be mounted on the rupture disc, which must then be routed through the holder. The author’s company’s sensorb operates differently. In this case, a signal indicator is attached to the rupture disc during manufacturing. The sensor is screwed into a blind tapping in the rupture disc holder, where it monitors the position of the signal indicator. 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; the system can then be put back into operation. The signaling cables no longer need to be rerouted to the respective switching box.5
The process is leak-tight, and the blind tapping in the holder replaces the tapping, which is usually required. The absence of cable glands means they cannot become porous, preventing an escape from the process media (FIG. 3).
FIG. 3. When the rupture disca opens, the sensorb will release information to the system’s process control unit.
The end user received an engineered solution for maximum vessel and personnel protection. Choosing the correct disc design and material leads to an increased service life and eliminates the need for frequent shutdowns for periodic maintenance. The user can profit from substantially reduced capital expenditure (CAPEX) and associated installation costs using the rupture discs compared to using any other valve, reducing the process’ overall emissions balance.
Protecting compressors under challenging circumstances. Protecting compressors is another application within the LNG process where rupture discs are used rather than safety valves. Due to their compact, reliable design, centrifugal compressors are often used for offshore liquefaction.6 Such turbomachinery uses a dry gas seal, which uses clean gas under pressure on the external side of the seal, so the only leak 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 on instruments during a primary seal failure. A signaling system on the primary vent system is recommended to enable integrity checks while the seal system is still online.
Customers using dry gas seals require rupture discs to be installed to vent excess pressure in case the primary seal experiences a catastrophic failure. If the pressure increases beyond its predetermined limit, a higher gas flow or backpressure in the piping system is present and must be safely vented. A signaling device is also required to detect a change in the rupture disc status to initiate a shutdown of the process in the event of overpressure.
A rupture discc with integrated signaling is cost-effective and reverse-acting with an integrated polyamide signaling device, suitable for the demanding conditions in dry gas seal applications. The rupture discc uses laser technologyd, ensuring high-quality, accurate burst control even in the harshest environments. The integrated signaling device informs operators immediately that the disc has burst and must be replaced. It is possible to restart the compressor with the damaged rupture disc in place, which is why it is critical that signaling is used to alert operators to avoid incident.
Takeaway. These are only two examples of how rupture discs are used across the LNG process, but there are many other uses, such as protecting heat exchangers and 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.
Notes
a REMBE’s KUB
b REMBE’s NIMU
c REMBE’s IKB
d REMBE’s unique Contour Precision Lasering™
LITERATURE CITED
1 API, “API standard 520: Part 1─Sizing, selection and installation of pressure-relieving devices,” July 2014, online: https://www.api.org/~/media/files/publications/whats%20new/520pt1_e9%20pa.pdf
2 API, “API standard 527: Seat tightness of pressure relief valves,” November 2014, online: https://www.api.org/~/media/files/publications/whats%20new/527_e4%20pa.pdf
3 ISO, “ISO 4126-3:2006: Safety devices for protection against excessive pressure: Part 3: Safety valves and bursting disc safety devices in combination,” March 2006, online: https://www.iso.org/standard/34203.html
4 API, “API standard 521: Pressure-relieving and depressuring systems,” January 2014, online: https://www.api.org/~/media/files/publications/whats%20new/521%20e6%20pa.pdf
5 The American Society of Mechanical Engineers (ASME), “Boiler and pressure vessel code, Sec. VIII Div. 1,” 2017, online: https://www.asme.org/getmedia/c041390f-6d23-4bf9-a953-646127cfbd51/asme-bpvc-brochure-webview.pdf
6 Stahley, J., et al., “Dry gas seal system design standards for centrifugal compressor applications,” Texas A&M University, 2001, online: https://oaktrust.library.tamu.edu/bitstream/handle/1969.1/163307/t31pg145.pdf;jsessionid=F0C003C24086CEC4A4C55D8251A34E6E?sequence=1
Comments