D5d.1 Safety impact of permeation in hydrogen distribution grids

Safety in the management and operation of the gas distribution network is a top priority for Dutch gas distribution network operators to minimize risks to people and the environment, not just for natural gas grids, but also for hydrogen grids. This research includes examining the effects of hydrogen and oxygen permeation on the gas composition in gas distribution pipelines, gas pressure reduction and regulation stations, and casing pipes. Permeation is a natural and unavoidable process. Due to the higher concentration of oxygen outside the pipeline, oxygen from the surrounding environment will permeate through the pipe wall into the gas network. Conversely, hydrogen from the gas network can permeate through the pipe wall to the environment. As a result, a hydrogen-oxygen mixture may form at various locations in the distribution network.

Permeation reduces gas quality, which can be detrimental to users and combustion appliances. Additionally, permeation may lead to the formation of a flammable mixture, potentially impacting safety during normal operations or maintenance work on the network. However, how the hydrogen-oxygen ratio evolves over time and in which situations a flammable mixture may form is not yet well understood.

Using a combination of research data and literature-based permeability coefficients and rates, Kiwa has modeled the evolution of the hydrogen-to-oxygen ratio due to permeation for selected scenarios. An essential condition for these calculations is that no gas exchange (no flow or diffusion) occurs within the pipeline. The models indicate that in a number of situations, a flammable mixture may form inside a pipeline, technical installation of a gas pressure reduction and regulation station, or casing pipe. However, a flammable mixture in a pipeline does not pose a danger if an ignition source is missing. One potential ignition source that has been considered is static electricity caused by dust transport within the piping system. The presence of an ignition source due to static electricity can be ruled out in all cases where a flammable hydrogen-oxygen mixture could be present according to this research. This is because in the absence of gas flow static charge from dust transport cannot build up. When the gas flow resumes, the pipeline length, the amount of dust present, and the duration of dust transport are insufficient to generate a static charge capable of producing a spark with enough ignition energy.

In situations where a flammable mixture may be present in the pipeline due to permeation, and maintenance work is carried out on a hydrogen network in pilot projects, it is concluded that the implementation of the appropriate precautionary measures is sufficient to mitigate risks. Potential safety risks can be mitigated by including them in work instructions, flushing pipelines before performing maintenance, and using appropriate indoor combustion appliances. Additionally, practical tests can further improve insights into the possible formation of a flammable mixture in various scenarios.