D5.2 Report on safe isolation, depressuring, and evacuating of high-pressure hydrogen pipelines and installations for maintenance purposes

Maintenance on high-pressure hydrogen pipelines, such as the Hydrogen Network Netherlands needs strategic consideration and planning. In this report, we provide an overview of the guidelines and methodology for isolating and evacuating a high-pressure pipeline or installation for a maintenance operation. The dialogues with various experts from the hydrogen gas industry provided a diverse range of perspectives and knowledge, helping to validate the nuances of different methodologies better. The general methodology of the maintenance procedure consist out of a few steps:

1. Identify the sections that need maintenance
2. Isolate the section that needs maintenance, utilizing valve schemes or stopples
3. Depressurize the system
4. Evacuate hydrogen with nitrogen, utilizing pigging, purging, or dilution techniques
5. Ensure secondary isolation and a bleed mechanism is in place
6. Execute maintenance operations
7. Flush the maintenance area with nitrogen before reintroducing hydrogen

The different evacuation and isolation methods impose slightly different methodologies, which will be explored in this report.

When isolating between valve schemes, evacuation with a separation PIG minimizes the mixing of hydrogen and nitrogen. The report discusses leakage rates along the pig for different types of pigs. These rates will be approximately 20-30% higher for hydrogen operations compared to using natural gas. Although the leakage is higher, the amount of back mixing is still very limited compared to the other techniques described in this report. Separation pigs will have an increased stick-slip behaviour since the reduction of the acoustic impedance increases 3 to 4 times. However, valve schemes with pigging facilities can be typically distanced 50 km to 100 km apart, this method requires closing down a large section of the pipeline. The process of pigging becomes less feasible for very long pipelines due to the large loss of gas volume and possible disruption of flow from suppliers and to industrial consumers.

If it is not feasible to start a pigging operation over a large section of pipeline due to loss of large volumes of hydrogen, a smaller section, between two valve schemes without the necessary facilities to perform a pigging operation, can be isolated and evacuated by performing a purge or dilution-based purge with nitrogen gas. The section cannot always be evacuated by purging due to physical constraints, such as stratification or the presence of dead volumes, dilution can be used to lower the concentration of hydrogen gas till acceptable levels. Evacuation by displacement or purging has its challenges, especially for long pipeline sections where stratification can occur. The minimum velocity requirements to prevent stratification for hydrogen and natural gas are computed and show that the velocity requirements are higher for hydrogen. The diffusion fronts of hydrogen and natural gas are computed in this report, demonstrating that the diffusion front length is velocity-independent and results in approximately equal volumes of remaining gas-air mixtures at different velocities. The distance of these valve schemes varies within the network, but can be up to up to 50 km apart. When these distance between valve schemes is high, the amount of hydrogen volume lost can still be quite large.

When it is more feasible to isolate and evacuate a smaller section, stopples can be installed to provide temporary isolation. Sections isolated with stopples can be sufficiently small to allow for the installation of a bypass, thus ensuring the continuity of gas flow to preserve gas supply within the hydrogen network.. The current procedures used for the natural gas network will not suffice for the hydrogen network, since they do not provide a double block and bleed. Alternatives techniques, such as hydraulic stopples or a stopple train, due provide a double block and bleed and are discussed. These alternative techniques will need different equipment than the equipment currently in use and will need further research before applied in the field.

In complex systems or installations, such as the hydrogen storage facility HyStock, it is impossible to avoid dead volumes or spaces where the hydrogen flow is limited in the installation. Here, a dilution-based purge should be used. Alternating the pressure in the spaces with restricted gas exchange can also contribute in achieving a suitable dilution-based purge. Although this method will use an increased amount of nitrogen since it requires multiple purge-cycles for a successful purge, theoretical analyses and experimental data indicated that pockets of hydrogen would mix more efficiently with nitrogen compared to natural gas pockets, approximately 3.8 times faster.