There are currently more than 4500 km of pipelines transporting hydrogen, and almost 1600 km of these are located in Europe. Most of these pipelines are dedicated hydrogen product lines that were designed and built to bring process gas hydrogen from gas producers to industrial users, such as chemical plants and refineries.
The plans for the European Hydrogen Backbone have a different focus; they are established around the vision of hydrogen as a source of energy. In this scenario, the quantities of hydrogen that are needed will be significantly larger than they are today; the network will eventually be able to deliver the gas across the continent. This can be achieved by making use of the existing natural gas grid, adding dedicated new lines only where needed. However, because of the differences between hydrogen and natural gas, these plans for the future pose significantly different challenges to the system, mainly related to volume, pressure and, with this, maintaining pipeline integrity and safety.
For both new and repurposed pipelines, it is necessary to assess the relevant threats and define a strategy for integrity management. This understanding and assessment encompasses a robust knowledge of material properties to form the basis of a ‘Fitness for Hydrogen’ assessment. Currently, the main standard that is used is ASME B31.12.¹ It provides a strategy for both new pipelines and the repurposing of existing lines to transport hydrogen.
Difficult hydrogen issues around steel pipelines
While hydrogen, being a gas, has many similarities with natural gas when it comes to transportation via pipelines, there is one notable effect that differentiates it from other gases. This fundamental feature, which drives much of the integrity concerns and challenges associated with gaseous hydrogen pipelines, is the absorption of atomic hydrogen within the steel microstructure. Under certain conditions, it can diffuse into the steel and interact with the microstructure, leading to a change of the properties that are determined in air. There is a consensus that such interactions lead to a major degradation of ductility and fracture toughness, and an acceleration of fatigue crack growth. However, the material strength remains largely unaffected in the presence of hydrogen. These effects are commonly referred to as hydrogen embrittlement. While the existing codes relate the effects to the steel grade, there are indications suggesting that the effect is rather dominated by the steel microstructure and chemistry.
If this is the case, it could be the source of an increased degree of scatter in the existing data when assessed only against the steel grade. In addition, the factor that describes the relationship between properties in air and in hydrogen could be different for vintage and modern material, as they tend to exhibit a different microstructure. These correlations need to be understood in order to deliver both practical and safe requirements to the industry.
As the need to fully understand steel performance in hydrogen grows, and the design requirements extend, it will become increasingly important to understand the influence of hydrogen on the integrity of new and existing pipelines in the context of mechanical properties as well as chemical composition and microstructure.
Written by Marion Erdelen-Peppler, ROSEN Group, Germany.
Read the article online at: https://www.globalhydrogenreview.com/special-reports/30082023/going-far-beyond/
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