Maintenance, Safety and Reliability

M. SIMPSON, TÜV SÜD, Fareham, United Kingdom

There is no doubt that hydrogen (H₂) will play a decisive role in future energy systems. However, due to its properties, especially in relation to its small molecular size, H₂ places specific demands on the materials, components and systems that contact with the gas, and they must be qualified for safe use.

For CE marking purposes—a mark that indicates a product meets European Union (EU) safety, health and environmental protection standards—a manufacturer must declare the conformity of their product with the relevant European directives. These framework directives specify the general requirements for product safety but do not make any specific statements as to how product safety should be demonstrated for a specific product or application, such as those using H₂. An exemption is UNECE R1348, which addresses a few H₂ vehicle components [e.g., tank, shut-off valve, check valve, and thermal pressure relief device (TPRD)].

For manufacturers of products that carry H₂, the challenge is that while product safety must be proven before CE marking or release to the market, the specific procedure for this is usually not defined in detail in regulations and standards. While existing regulations require that materials, components and systems must be safe for their intended use, framework regulations typically do not define how that can be proved. To address this dichotomy, there are many influencing factors that must be respected when determining a reasonable test procedure to prove safety.

Qualifications and specifications. For H₂ applications, it is highly unlikely that a qualification purely based on literature will be sufficient, so lab testing will be required in most cases. This means that an appropriate test specification must be identified to fulfil regulatory requirements; however, this specification must also consider the various influencing factors, such as operating pressure, temperature ranges, environmental conditions, etc. The situation is similar for most applications—e.g., , vehicle fuel system applications (UNECE R134) (FIG. 1), stationary pressure equipment (PED 9), or transport and distribution applications (TEPD10 and national guidelines for gas transport like the German DVGW ruleset). To create and implement reasonable test specifications, existing norms and standards specific to H₂ applications can be resourced or adapted.

Product norms do offer detailed testing requirements for specific components; however, if product certification is not referenced in the relevant regulation(s), it is voluntary. Where no product norms exist, a reasonable validation and qualification program should be developed by the manufacturer and supported by an independent third-party test and certification organization. For example, to help manufacturers overcome this issue, the author’s company submits its own test specifications for some products so they can be appropriately evaluated and certified.

Materials testing investigates the effects of H₂-specific damage mechanisms on new and aged material samples, assessing the extent of negative effects on integral material properties such as tensile strength, dimensional stability and flexural fatigue strength. 

Component testing aims to assess the product designed from the previously qualified materials, considering its compressive and cycle strength, functionality over the planned service life, and application-specific requirements for leakage and permeation. A fully comprehensive qualification process will encompass both types of testing to certify a product and confirm its suitability for H₂. 

Influencing factors. Four key damaging mechanisms that can influence the compatibility of materials with H₂ are discussed here. The first metallic materials damaging mechanism is H₂ embrittlement (HE), which can be caused at the interfacial area where H₂ molecules can dissociate into H₂ atoms and atomic H₂ can then diffuse into the alloy. The second metallic materials damaging mechanism is high-temperature H₂ attack (HTHA). Temperatures > 600°C (1,112°F) increase the probability of H₂ molecules dissociating into H₂ atoms and increasing H₂ diffusion into the alloy.

The first plastic materials damaging mechanisms are explosive decompression and blistering, where H2 molecules diffuse into elastomeric and thermoplastic materials quickly—saturation can be achieved within hours. When fast depressurization is experienced, effusion cannot happen quickly enough and the pressure difference results in damage. The second plastic materials damaging mechanisms are shrinking and swelling: when H₂ has diffused into the elastomeric material, it can cause that material to swell or extrude as it increases in volume.

Four key areas that will impact component safety and performance must be considered. These are: 

1. Leakage and permeation  
2. Flow speeds  
3. Heat capacity and thermal conductivity 
4. Pressure and temperature cycles, and thermal shock (cold gas in hot component)—H₂ gas will be pre-cooled [to as low as –40°C (–40°F) for some applications, e.g., vehicle refueling, while components may be warm or hot]. Thermal stresses are applied to the component, sometimes within a short period (thermal shock). 

Gaps in the regulatory framework. While some standards and procedures exist for evaluating H₂ compatibility of metallic materials, these procedures typically do not address problems that may arise with H₂ compatibility when forming the materials. Forming and thermoforming may result in phase change in certain areas of the material (e.g., by generating martensitic structures in originally austenitic materials). While the austenitic material, which was originally tested for H₂ compatibility, may have passed the test without problems, the martensitic structure may be susceptible to H₂ embrittlement in the actual practical application.

For valves for stationary and industrial applications, materials must be compatible for use with H₂ during the entire expected lifetime and under all expected usage conditions. Likewise, functionality and performance must be guaranteed over the entire lifetime. Leakage and permeation must also be considered, as these can represent a serious safety issue.

For non-metallic pipeline designs, materials must be compatible for use with H₂ during the whole expected lifetime and under all expected usage conditions. Failing and/or bursting pipes also represent a safety issue due to overpressure and cycle robustness issues. This is of particular concern towards the end of the component’s lifetime. Again, leakage and permeation can represent a serious safety issue, and permeation behavior in new and aged condition must be considered. Lifetime simulation must be applied during testing to ensure endurance and durability during aging. It is vital to ensure the design is safe and performance can be guaranteed over the whole lifetime.

While the European system for placing components and systems on the market consists of framework directives, in most cases these do not represent the specific requirements of H₂. To fully comply with a manufacturer‘s Product Safety Act (ProdSG) obligations and ensure that safe products are placed on the market, the qualification of H₂-carrying components is required beyond the regulated and standardized range available today. H₂T

About the author

MARCUS SAMPSON is the Business Development Director (Mobility) UK, Ireland, Northern Europe at TÜV SÜD. With 30 yr of engineering experience, Sampson has a broad technical background in fluid power and process control. He earned a BEng (HONS) degree in mechanical engineering and is responsible for the delivery of TÜV SÜD’s testing, inspection and certification services in the mobility sector across the UK, Ireland and Northern Europe.