Upgrading the gas infrastructure
For hydrogen to be able to be used as an important part of the energy transition comprehensively in industry, mobility and energy supply in Germany, new lines must be built and existing natural gas pipelines must also be upgraded for hydrogen transport. This can be challenging, as hydrogen is explosive and attacks the materials of the pipes. Professional material testing creates the necessary security regarding this.
With the national hydrogen strategy, the federal government is relying on hydrogen as an alternative energy source for industry, mobility and energy supply. H2 is a promising solution to support the clean energy transition: Hydrogen has a wide range of possible applications, be it in power generation, in the operation of fuel cells for mobility applications, in industry or in heating technology. This means that hydrogen has great potential for reducing emissions; sustainability arises from the use of green hydrogen. H2 can additionally serve as long-term storage, because it or its derivatives have a better storage capacity than electricity.
For all these areas of application, hydrogen must be transported as a gas or in liquid form: in pipelines as the backbone of an H2 infrastructure or in tanks on the road, rail or at sea. However, ensuring the necessary security represents a technical challenge, because hydrogen is highly flammable and has a wide flammability range in which it could explode. Leaks must therefore be avoided at all costs, and materials and lines must be tight and H2-resistant.
Hydrogen can affect pipe materials
This is demanding, as hydrogen reacts with other materials and influences their properties: Hydrogen embrittlement (HE) can occur when hydrogen atoms penetrate metals. H atoms diffuse into the metal structure and accumulate on lattice defects such as grain boundaries, dislocations or cavities. This reduces the strength and ductility of the metal, so its properties, leading it to plasticly deform under the load, before it gives out. This makes it more susceptible to cracks and breaks under stress.
High-strength steels and alloys (tensile strength: Rm > 1,000 MPa) are particularly affected as well as weld seams. With repeated mechanical stress, such as pressure surges like those that occur during pipeline operation, cracks can spread more quickly. Additionally, thermal-mechanical effects can be observed: At higher temperatures, hydrogen atoms can penetrate faster and deeper into the metal, and, depending on the material, another damage mechanism called HTHA (high temperature hydrogen attack) can come into play.
At higher pressure, the amount of hydrogen that can penetrate the metal increases. In humid environments, hydrogen and water can work together and accelerate corrosive attacks. Shifting temperatures and pressures then pose further challenges.
The result of these effects is a reduced service life of the transport lines: The material fatigues more quickly, cracks can occur and premature material failure can occur. That makes more frequent maintenance, inspections and the replacement of parts of the systems necessary, which leads to downtime. On top of that are safety risks such as leaks and the risk of explosion.
Natural gas pipelines for H2 transport?
Planned is the repurposing of existing natural gas pipelines for the transport of hydrogen. In parts of Germany, ten percent hydrogen is currently mixed with natural gas. The switch to 100 percent hydrogen is currently being tested in pilot projects. Many of the materials used in the natural gas pipelines are fundamentally also suitable for transporting hydrogen. However, attention must be paid to compatibility, which is why material testing is essential. This means that the material must be tested for hydrogen embrittlement and its suitability.
On the other hand, with hydrogen, whose molecules are smaller than those of methane, there is increased diffusion through seals and therefore a higher risk of leaks, which sometimes makes it necessary to replace seals and valves. In addition, better monitoring and control systems are necessary for (early) leak and situation detection.
Measuring the influence of H2 pressure on the infrastructure
How hydrogen affects the materials of the infrastructure is being examined in material testing using a combination of laboratory tests, microstructure analyses, simulations and long-term field tests: In tensile tests, for example, material samples are loaded under various H2 pressure conditions in order to measure strength, ductility and fracture behavior. Charpy impact tests evaluate the toughness of the material and its ability to absorb energy before it breaks. Hydrogen can significantly reduce notch toughness.
During pressure and fatigue tests, materials are exposed to cyclic and different pressure conditions to investigate their fatigue strength and behavior under repeated loading. Further insights into material behavior and reliability can also be gained from field reports and data analyzes of existing hydrogen infrastructures.
Additional factors critical for assessing the suitability of materials for hydrogen pipelines include fracture toughness and crack growth behavior. They allow conclusions to be drawn about the safety, reliability and service life of the pipeline: Fracture toughness indicates how well a material can resist the propagation of a crack or defines the lowest value that a material must have in order to be considered safe for use. The tests allow precise service life predictions and, through the selection of suitable materials, longer operating times.
The quality of the pipeline welds is determined through visual inspections and non-destructive and destructive tests such as fracture mechanical analysis. International norms and standards such as ASME B31.12, ISO 11114 and others provide guidelines and minimum values that materials must meet. For example, the minimum fracture toughness is typically in the range of 50 to 100 MPa·m1/2.
Because there are still regulatory gaps, especially in national and European standards, the DIN has initiated the standardization roadmap Normungsroadmap Wasserstoff. For example, with DIN EN 13445-15 and DIN EN 13480-11 additional requirements for pressure vessels and pipelines for hydrogen applications are currently being developed.
Testing by accredited bodies
Material tests should be carried out by an accredited laboratory in order to meet the high quality and safety requirements. TÜV Hessen as an approved company for materials testing for example offers comprehensive testing and certification services, including non-destructive and destructive tests as well as special tests such as H2 qualification. The accreditation in accordance with ISO/IEC 17025 certifies that the company meets the requirements of an internationally recognized standard for expertise in testing and calibration laboratories. DIN EN ISO/IEC 17025 is the globally valid standard for laboratory accreditation in the area of testing and calibration: It defines general requirements for competence, neutrality and working methods. An approved testing laboratory brings the necessary expertise through technical knowhow and experience, and guarantees independence and objectivity and compliance with international standards and thus conformity. For companies, this means increased security, risk reduction and cost savings in the long term.
Summary
To upgrade natural gas pipelines for the transport of hydrogen, a testing of the materials must occur. Because hydrogen embrittlement, which can arise from operation with H2, leads to premature material fatigue and can impair safety. The necessary tests and trials should be reliably carried out by accredited testing laboratories.
Author: Dr. Stephan-Lederer, TÜV Hessen, Darmstadt
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