A Dangerous Mix - How to Avoid and Handle Mixed-Pressure Connections

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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Increasing customer demand requires new process applications with needs for reliable lubrication and resistance to galling under stringent conditions of temperature, pressure, vacuum, corrosive environments, process sensitivity to contamination, product life, and maintenance requirements. The various applications within power generation and industrial and chemical processing have advanced lubrication needs. Specifically, this blog is related to the science of molybdenum disulfide coatings in compression fittings. In demanding environments, such as power generation plants, where tubing connections are exposed to stringent requirements and extremes, consistent leak-tight performance is critical and molybdenum disulfide coated fittings provide a solid solution. 

 

Lubrication evolution increases demand

Molybdenum disulfide is a naturally occurring black colored solid compound that feels slippery to the touch. It readily transfers and adheres to other solid surfaces with which it comes into contact. Its mineral form – called molybdenite – was commonly confused with graphite until late in the 1700s. Both were used for lubrication and as writing material for centuries. Wider use of molybdenite as a lubricant was impeded by naturally occurring impurities that significantly reduced its lubricating properties. Methods of purifying molybdenum disulfide and extracting molybdenum were developed late in the 19th century, and the value of molybdenum as an alloying addition to steel was quickly recognized. The demand for a domestic source of molybdenum during World War I resulted in the development of the Climax mine in Colorado, which started production in 1918 and continued into the 1990s.

The availability of high purity molybdenum disulfide spurred extensive investigations into its lubrication properties in various environments during the late ’30s and ’40s. These investigations demonstrated its superior lubrication properties and stability under extreme contact pressures and in vacuum environments. These investigations resulted in extensive applications in spacecraft.

Download our white paper Molybdenum Disulfide - The Ideal Solid Lubricant and Anti-Galling Material for a closer look into the science behind high-performance compression tube fittings required for demanding environments.

 

Exceptional lubricity

Molybdenum disulfide’s exceptional lubricity is a consequence of its unique crystal structure, which is made up of very weakly bonded lamellae. The lamellae tend to align and adhere to contact surfaces, particularly under conditions of sliding and pressure, as shown here. This “burnishing in” of the molybdenum disulfide gives it its exceptional performance life.

 

 

Since molybdenum disulfide is a solid phase, it is not “squeezed out” like liquid lubricants under conditions of extreme pressure. The lamellae are very “hard” to forces perpendicular to them. This combination of properties provides a very effective “boundary layer” to prevent the lubricated surfaces from contacting each other.

The surfaces of objects are generally rough on a microscopic scale. When two objects are in contact with each other, they actually “touch” at very small regions of contact (i.e., asperities).

 

These contact regions have considerably less area than the bulk surface area, typically in the range of 0.5 to 0.001 percent of the bulk area for a machined metal surface, and consequently, the stresses at these contact points are considerably higher than the stresses calculated for the bulk surface area. When these objects slide relative to each other the frictional forces add to the stresses at the contact points, and the resultant stresses may be sufficient to cause deformation of the contact points.

When stainless steel objects slide against each other under high load, they will “gall” or “seize” due to the deformation at the contact points. The objects will actually “cold weld” themselves to each other, which is indicated by the transfer of material from one object to the other on the sliding surfaces. This causes a very rapid increase in friction, quickly to the point that further sliding is impossible without damage to the objects. To prevent this, it is necessary to introduce an “anti-galling” or “anti-seizing” agent between the surfaces. This is a substance that can maintain separation of the surface asperities under high compressive loads – that is, to provide a “boundary layer” between the surfaces. Anti-galling materials are generally very thick grease-like substances or solid materials in powder or plated layer form. Molybdenum disulfide is an ideal anti-galling compound because of its combination of high compressive strength and its adherence (ability to fill or level) to the sliding surfaces.

There are many methods of applying molybdenum disulfide to a surface, from “high tech” techniques such as vacuum sputtering, to simply dropping loose powder between sliding surfaces. The most versatile technique is the application of the powder mixed with a binder and a carrier to form a bonded coating. The binder may be a polymeric material or several other compounds, and the carrier may be water or a volatile organic. The characteristics of the molybdenum disulfide powder, the binder, the carrier, and particularly the application process must be carefully developed and controlled to optimize the performance of a specific product. A properly developed bonded coating of molybdenum disulfide can provide exceptional lubrication performance over a temperature range up to approximately 500°C, under very high pressure and corrosive exposure conditions for extensive lifetimes.

 

Molybdenum disulfide in compression fittings

During fitting make-up, the ferrule(s) is driven forward into the body seat and tubing surface as the nut is turned per the makeup instructions. The ferrules seal at contact points with the fitting body seat and tubing surface. The fitting is carefully engineered such that the ferrule and tubing do not rotate with the nut, moving only in the axial direction. This is critical to forming a high integrity leak-free tubing connection during the first make-up and subsequent remakes. Therefore, the sliding takes place between the back of the ferrule and the flange of the nut under very high pressure. This region of contact between the nut and the ferrule must have excellent lubrication for the proper action to occur during make-up to ensure ease of assembly, low make up torques, and optimum fitting function. The nut is “pulled” against the ferrule during make-up by the threads, which are also sliding under very high pressure and require high-performance lubrication.

Stainless steel compression fittings have the additional problem of preventing galling in these areas of sliding under very high contact pressures. This requires a “boundary layer material” – a substance that maintains separation of the surfaces during sliding.

Our instrumentation engineers have developed and used a bonded molybdenum disulfide coating on the nuts of our premium CPI™ compression fitting products for 30+ years. These fittings are readily recognized in the field by the “black nut” – the molybdenum disulfide coated nut, and have been providing exceptional service in many demanding applications.

The molybdenum disulfide coating has been carefully formulated and processed to optimize the performance of the CPI™ compression fitting. This fitting, with its single ferrule and the molybdenum disulfide, coated nut, has easy initial make-up with very low torque, consistent remake, and exceptional, leak-tight performance under demanding power generation applications including pressure, temperature, corrosion exposure, and vibration.

 

One product, multiple applications

We also offer a “Moly Inside” nut for use with our CPI™ and A-LOK® compression fittings. This is an optional version of our premium compression fitting products with the molybdenum disulfide coating only where it is needed, on the inside surfaces of the nuts. This offers the same low make-up torque and consistent remake-ability of the standard CPI™ compression fitting in both the CPI™ and A-LOK® versions due to the use of molybdenum disulfide on the critical mating surfaces of the interior threads and flange of the nut, but without the molybdenum disulfide on the external surfaces to rub off onto hands, gloves, or other equipment for industries requiring a “clean” appearance.

 

Download our white paper Molybdenum Disulfide - The Ideal Solid Lubricant and Anti-Galling Material for a closer look into essential attributes of high-performance compression tube fittings required for extreme environments.

 

 

 

 

 

 

Article contributed by Kevin Burke, marketing manager, Instrumentation Products Division, Parker Hannifin.

 

 

 

 

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In the quest for a decarbonised society there is no doubt that Hydrogen as an environmentally friendly fuel source is gaining a lot of popularity to become the fuel of the future. And the hydrogen revolution is happening now. Driving the ecological transition, hydrogen-based technologies are rapidly evolving, mass entering the market and becoming part of our daily lives. From clean power generation to environmentally friendly cars, the possibilities are endless.

The transportation sector is a prime example of how hydrogen technologies are taking off and making truly sustainable mobility more tangible than ever. Heavy trucks with hydrogen-powered cells are already hitting the roads, and although developing a global hydrogen refuelling infrastructure might take several years or even decades, the commitment to hydrogen economies from governments all around the world will certainly accelerate its pace.
 

Download our white paper: Addressing the Challenge of Hydrogen Embrittlement in Metallurgy.

 

 

 

 

 

The hydrogen challenge

Hydrogen is the most abundant element in nature and its versatility can offer compelling advantages as an accessible, sustainable and efficient alternative source of energy.

However, hydrogen can be very damaging to most metallic materials, causing what is known as hydrogen damage or hydrogen attack. Hydrogen is an extremely small particle and hydrogen degradation is directly connected to its capability to be easily absorbed by metals coupled with the high mobility those particles have at microstructural level.

Nearly every metallic material can be susceptible to hydrogen damage, and there are several forms of hydrogen degradation. Hydrogen embrittlement cracking is the most common form and affects the three main areas of industries that use hydrogen:

• Production
• Transportation
• Storage

  What causes hydrogen embrittlement?

Fig. 1. Embrittlement process.

Embrittlement occurs when a material loses ductility and becomes brittle as a result of the diffusion of hydrogen into the material. The hydrogen atoms find preferential places in the structure of the material, modifying its physical properties and its mechanical behaviour. The result is a loss of ductility which makes the material more brittle and more susceptible to cracking. Hydrogen can be a silent assassin, weakening the material slowly and without any clear signs of damage, often leading to critical failure.

 

The effect of hydrogen embrittlement is determined by three main challenges:

 

• The Environment
• The Mechanics of the material
• The Microstructure characteristics of the material.

Environment

Hydrogen and stress need to be present on a susceptible material for hydrogen-assisted fracture to happen. Firstly, hydrogen absorption can happen at both production and service stages. Processes such as uncontrolled melting, electroplating or welding can promote the pre-charge of hydrogen into a given metal.

Microstructure

In terms of microstructure, and as a rule of thumb, materials that bestow high mechanical strength or show a great number of defects and inclusions are likely to be more susceptible to this type of failure. The severity of hydrogen embrittlement is also a function of the operating temperature, with low temperatures being the worst case scenario in terms of material ductility and higher temperatures in terms of hydrogen absorption rate.

The factors that can affect the quality of the microstructure are numerous and have been widely documented by the materials society. Due to the complexity of the subject, the effect of microstructure as a major contributing factor to hydrogen behaviour, cannot be evaluated in simplistic terms. Taking one variable in isolation is not enough to guarantee the quality or performance of a given component and can be misleading. For example, a material grade with a ‘perfect chemistry’ or with high levels of a particular ingredient can still result in a very low quality product.

The common consequences of improper and non-controlled material processing, heat treatment and/or manufacturing operations are high densities of undesirable phases and inclusions in the raw material. These will inevitably lead to fatal and premature hydrogen assisted cracking during service in demanding H2 environments.

Application mechanics

The mechanics of the application also play a major role. Stress states in components can be caused by the presence of residual stresses associated with certain fabrication techniques as well as stresses applied during service. Improper product design and improper installation can cause overloading of stress onto the material. All of these factors can cause premature failure of components in Hydrogen service.

Can hydrogen embrittlement be prevented?

When it comes to handling hydrogen, material and equipment selection becomes, more than ever, an essential ingredient for success.

According to the International Industry Standard ISO 15916-2015, due to the fact that most metals are susceptible to different levels of H2 embrittlement, materials of construction and suitable equipment must be carefully selected to avoid failure when hydrogen exposure is anticipated. The positive news is that hydrogen embritllement can be prevented. End users need to pay special attention to the materials of construction and the quality of the equipment that goes into their assets.

How Parker is at the front end of innovation for the hydrogen market

As a manufacturer of pressure containing equipment, Parker has decades of experience in serving hydrogen applications. Parker products are designed to minimise the risk associated with corrosion and hydrogen attack and deliver successful performance in the field. The raw materials that we use are fully traceable and closely controlled from melting stage to the finished product.

In addition, our manufacturing processes are selected to ensure minimum operating risk in hydrogen environments. As well as Stainless Steels (the prime material of choice for the H2 transportation sector), we can offer a variety of nickel alloys for a wide range of other applications.  

Parker's portfolio also includes EC-79 approved products. The EC-79 approval (the Regulation of the European parliament and the Council of 14th January 2009 on type-approval of hydrogen powered motors) is an EU normative for components and systems which are installed on hydrogen-driven vehicles. Product ranges certified to this regulation are extensively tested to guarantee the safety and performance of H2 equipment under different pressures, electric, mechanical, thermal or chemical conditions.

As a leading manufacturer in motion and control technologies Parker offers a wide range of products orientated to the hydrogen market. 

 

 

Fig. 4. Parker A-LOK® two ferrule tube fittings

EC-79 approved for use on-board hydrogen vehicles up to 350 barg pressures.

 

Download our white paper and learn the main causes of hydrogen embrittlement and best practices on how to prevent it. 

 

 

Article contributed by Clara Moyano, innovation engineer - Materials Science, Parker Hannifin, Instrumentation Products Division Europe.

 

 

 

 

 

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