Why Making the Right Tube Connections for LNG Plants Is Vital

Small and mid-scale LNG plants are dramatically changing the application sphere for natural gas as an alternative fuel for transportation, both onland and for marine shipping use. They are helping drive adoption in various high horsepower applications and making gas capture and distribution from remote areas and wells without pipelines a reality.

LNG plant requirements drive need for smaller line and fittings

As the requirement for small and mid-scale LNG plants grows, so does the need for tube connection technology for smaller line sizes. In the past, connection technology was limited to either orbital welding or double ferrule compression fittings. Today, driven by cryogenic applications, there are four connection options in the less than two-inch space: instrumentation compression double ferrule fittings; instrumentation compression single ferrule fittings; O-ring face seal fittings; and permanent fittings.

Connection in a cryogenic application should focus on five critical areas:

• Safety
• Leakage and fugitive emissions
• Ease of assembly
• Longevity
• Code compliance

Safety first

Safety is critical in small and mid-scale LNG plants. In these complex plants, electricity, gas and water will often coexist in close proximity. Add cryogenic temperatures to the mix and the environment becomes extreme.

The most important rule of safety in an LNG plant is to keep the media contained. Good tube connections prevent catastrophic failure at connection points. A failure mode and effect analysis reveal the threat that a bad connection – or a poorly made one – can pose to a plant. In addition to explosion leaks, the danger of frostbite exists with cryogenic temperatures. A good connection is a first defense in preventing exposure and harm to personnel.

Leakage and fugitive emissions

Leakage of methane, a greenhouse gas -- while still not regulated -- is a major concern in LNG plants. Overall, fugitive emissions will become increasingly important; keeping hundreds and thousands of connections in an LNG plant from leaking will be key.

As a basic requirement, ensure bubble-tight connections that follow appropriate leak specifications to future-proof against fugitive emissions while not limiting solely to safe exposure levels. Apart from methane leaks, cryogenic fluid leaks can cause frostbite and major freezing hazards. A small, seemingly safe leak is also a loss of revenue and efficiency. Leak-tight connections can add up to savings and notable percentage points of efficiency.

Ease of assembly

A big challenge for small-scale, modular LNG plants is the flexibility to connect on-site to a dispensing system or a point-of-use system. This often means a lot of field connections and the capability to carry out repairs. Other times it means making certain connections at the time of installation. While welding sometimes seems to be the most straightforward strategy for a permanent joint, finding certified and trained welders may not be simple or easy.

Longevity

With the operational life of an LNG plant in the range of 15 to 25 years or more, it is critical to select corrosion-resistant, robust and reliable connection technologies that outlast the moving parts. Longevity can be achieved by choosing materials that resist corrosion from environmental sources and that are compatible with the media or fluid being transported.

Code compliance

Finally, it is essential to consider applicable codes of construction that, in some environments, may include marine standards such as those for ABS or DNV GL. Code compliance helps ensure that all the above topics are covered. Codes to consider include:

• In the US: NFPA 59A
• In Europe: EN 1473:2007 and EN 13645
• Globally: ISO (draft) 116901, ISO TC 67, ISO 18638, ISO TC 252

The end-use must take into account all possible interface points such as vehicles, tankers, vessels or rail cars to define application standards.

Connection technology challenges and advantages

• The double ferrule compression fitting is a well-known component to most fitters in the industry. A single ferrule fitting allows the use of the same skill set as a double ferrule fitting but reduces the risk of making a wrong connection since one ferrule is eliminated. Both types provide good field make-and-break capability.
• The O-ring face seal fitting and flanged tubes may be tightened and remade many times, and to set torque, provide zero-clearance tube entry. Additionally, the metal seal ensures LNG compatibility.
• Finally, the permanent fitting can replace a normally welded joint or a fitting joint that typically would never need to be reopened in the field. A permanent fitting can also be made much faster and without field orbital welding.

Better protection against fugitive emissions

In both single and double ferrule fittings, stainless steel parts that rub together under high pressure have a strong tendency to cold weld and seize, leaking to leaks and the resulting fugitive emissions.

Using fittings coated with molybdenum disulfide can avoid this problem. Solid molybdenum disulfide readily adheres to surfaces, is noted for its lubrication and anti-seizing properties, and does not squeeze out under extreme pressure. The result is low assembly torque and consistent, leak-free performance, even with repeated remakes.

Many fittings manufacturers also combat fitting leaks – and the resulting possibility of fugitive emissions – through proprietary ferrule-hardening processes designed to heighten corrosion resistance. For example, Parker treats its ASTM 316 stainless steel, single and double ferrule fittings with Suparcase®. The chemical process provides greater resistance to pitting, as well as excellent stress corrosion performance to provide longer, leak-free service life.

 

Article contributed by the Process Control Team. Original article published at LNG Industry in September 2015.  

 

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Categories

• Fluid and Gas Handling
• Process Control
• Safety
• Transportation Technology