Special Focus: Environment and Safety

M. SENATORE, HandyTube Corp., Camden, Delaware, U.S.

As commercial and industrial vehicle manufacturers search for more sustainable fuel alternatives to fossil fuels, hydrogen (H₂) fuel cell electric vehicles (FCEVs) are gaining popularity (FIG. 1). H₂-based fuels are beginning to experience their own renaissance, but H₂’s chemical and physical properties challenge its implementation in refueling stations, inhibiting further adoption.

If internal systems are improperly designed, H₂ damage to refueling station components can cause leaks. If there is a leak or ignition source, a fire or explosion can result. To minimize H₂’s harmful effects, it is crucial to design fuel-transfer plumbing systems of H₂ refueling stations with high-strength stainless-steel tubing. Due to tubing’s critical role in these plumbing systems, it is important to understand the key aspects of specifying tubing for H₂ refueling stations. 

This article provides an overview of the challenges of using H₂ fuel and offers best practices to specify tubing that ensures safe, long-lasting and effective refueling station designs. 

An elementary look at H₂. Multiple aspects of H₂ factor into the difficulties of achieving properly designed refueling stations, such as H₂’s high energy, small molecule size, flammability and combustibility.  

The various grades of H₂ and its manufacture are: 

• Green: produced by water electrolysis that is powered by renewable electricity sources, such as wind or solar. This production generates zero carbon dioxide (CO₂) emissions. 
• Blue: produced from fossil fuels. Similar to gray, black or brown H₂ except that CO₂ is captured for storage or repurposed. 
• Gray: natural gas extraction via steam methane reforming (SMR). This is the most common production method. 
• Purple/pink: electrolysis that is powered by nuclear energy. 
• Turquoise: produced by methane pyrolysis, generating solid carbon rather than CO₂ emissions. 
• Yellow: produced by electrolysis that is powered by grid electricity from a variety of sources. 
• White: produced as a byproduct of other industrial processes, such as oil refining or fertilizer production. 
• Brown/black: coal gasification. 

H₂ contains almost three times as much energy per weight as conventional gasoline—making it an attractive fuel—but H₂’s energy density per volume is very low compared to gasoline and other fuels. The comparatively low energy density per volume means that for effective fuel storage, H₂ must be stored as a gas at high pressures or in liquid form under extremely low temperatures. Considering H₂’s high flammability and combustibility, as well as the fact that H₂ flames are nearly invisible, thorough safety considerations are necessary when designing storage and transportation systems. 

High-pressure storage and transfer systems are implemented more often than liquid H₂ alternatives, making tubing that can withstand extremely high pressures necessary. Since H₂ is colorless, odorless and extremely explosive, high-pressure tubing systems must be designed to minimize leak potential.  

H₂ embrittlement. H₂’s small atomic size causes damage to many metals through a process called H₂ embrittlement, also known as H₂-assisted cracking or H₂-induced cracking (HIC). When H₂ comes into contact with austenitic stainless steel, it can diffuse into the material and accumulate within the metal’s crystal lattice. This can cause the metal to become brittle and prone to cracking, even at stresses that would normally not cause failure. 

The mechanism of H₂ embrittlement is complex. It is thought to be caused by the H₂ atoms interfering with the movement of dislocations, which are defects in the crystal lattice structure that allow the metal to deform under stress. When H₂ atoms accumulate at dislocations, movement is prevented. This makes the metal more susceptible to cracking. 

The formation of high-pressure H₂ bubbles within stainless steel can increase the risk of H₂ embrittlement. When H₂ diffuses into austenitic stainless steel, it can combine with other elements, such as carbon, in the steel to form methane (CH₄) or other hydrocarbons. These reactions can produce H₂ gas within the metal, which can accumulate in small bubbles or voids. If the pressure inside these bubbles increases enough, the metal can crack or fail catastrophically. 

High-pressure H₂ bubbles are more likely to form in areas of the metal where there are high stresses or defects, such as near welds or in areas of the metal where there are sharp changes in geometry or surface roughness. The presence of high-pressure bubbles can increase the metal’s susceptibility to H₂ embrittlement by acting as stress concentration points, increasing the likelihood of the metal cracking. 

H₂ embrittlement compromises a metal’s structural integrity over time. Special material considerations are required to mitigate this effect when designing H₂ refueling stations. 

TUBING DESIGN BEST PRACTICES 

When designing the plumbing systems for H₂ refueling stations and other transport infrastructure, it is crucial to specify tubing that withstands H₂ embrittlement and minimizes the potential for leaks and other structural failures. Some considerations are discussed in the following sections. 

Select the correct material. Embrittlement-resistant metals provide the protection necessary to minimize the harmful effects of H₂, which often diffuses into steel, aluminum and titanium. Stainless steels generally provide better protection due to their high chromium content. 316L austenitic stainless steel is an alloy that provides the greatest balance between embrittlement protection and cost, making it ideal for H₂ fuel applications. 

Additionally, this type of stainless steel benefits from excellent corrosion resistance. Many H₂ fuel lines are placed underground or in similar environments that simplify design, allowing transport between storage and dispensers with fewer tube fittings and supports. While this configuration reduces the number of joints, it increases the likelihood of corrosion. Tubing with high corrosion resistance can withstand such harsh conditions longer. 

Eliminate welds. H₂ is typically stored at pressures up to 10,000 psi, which is much higher than storage pressures for natural gas or gasoline. These extreme pressures make leak prevention an additional design priority for H₂ refueling stations. Typically, the plumbing systems of these refueling stations consist of tubes connected by welds, fittings and other connections, all of which are potential leak sites. 

A design strategy that reduces leak potential minimizes the number of tubing sections, thereby reducing the total number of necessary welds. Following this strategy, H₂ refueling stations should incorporate long-length tubing, minimizing leak potential and ensuring greater safety and more successful operation. Additionally, long lengths of seamless tubing simplify designs, reducing installation time and maintenance efforts since there are far fewer welds to perform or inspect.  

Find a high-strength tubing supplier. Since H₂ is a relatively new fuel source, many engineers lack extensive experience and familiarity with designing and specifying the proper infrastructure. Regulations are being developed to standardize H₂ fuel applications. In the meantime, it is important to understand an application’s requirements and work with a supplier to properly specify the best tubing solution.  

Seamless coiled tubing, like that shown in FIG. 2, provides the advantages of longer length and safe interconnections to remote H₂ storage and dispensers with only two connections in each tube-run: one at the source and one at the dispenser. Many facilities that transport high-pressure or flammable gases can benefit from long-length, seamless coiled stainless-steel tubing.

The author’s company’s advanced tube manufacturing capability allows it to produce seamless austenitic stainless-steel coils as long as 2,900 ft and as smooth as 20 Ra. This ensures the delivery of precision-engineered, high-quality tubing that provides H₂ delivery systems with improved safety. 

Because the company’s austenitic 316L stainless steel can withstand H₂ embrittlement without sacrificing flexibility, it is workable enough to bend. Depending on the application’s pressures, the tubing can be cold-worked or cold-hardened to withstand up to 1,000 bar (~14,500 psi). The 316L stainless steel also features high nickel and molybdenum content, which further boosts the tubing’s corrosion resistance. 

To provide excellent material traceability and ensure its products meet the highest standards, only Defense Federal Acquisition Regulation Supplement (DFARS)-compliant materials are used. All coiled tubing is 100% eddy current and hydrostatic tested to ensure structural integrity, as well as analyzed for material purity.H₂T

About the author

MARCELO SENATORE works as the Business Development Director for alternative Energy at HandyTube Corp., and is a recognized leader in the alternative energy sector with a deep focus on low-carbon and renewable technologies. Previously at Sandvik Materials Technology, Senatore was instrumental in introducing low-carbon energy-aligned products and driving market development initiatives. He holds a degree in metallurgical engineering from FEI, Brazil, and an International Executive MBA from IBE/FGV and the University of Miami.