f H21 Phase 2A Testing - Part A: Minimum Purge Velocity Report

In line with the UK governments decarbonisation strategy, gas networks within the UK are continuing to investigate the potential to convert the UK gas networks to hydrogen. Northern Gas Network's (NGN) H21 project aims to enable the conversion to 100% hydrogen.
The process of converting the existing UK gas infrastructure involves, primarily, purging operations. Therefore, there is a requirement to understand in more detail how hydrogen affects the requirements to conduct a safe and efficient purge.
The increased buoyancy forces associated with hydrogen is expected to increase the minimum purge velocity required to avoid stratification. This can easily be calculated theoretically for 100% hydrogen and can be used as a starting point for experimental investigations.
NGN's H21 project has focused on converting natural gas pipelines to 100% hydrogen via indirect purging, which involves purging the air out with inert nitrogen first before commissioning with hydrogen. This avoids the potential to form a flammable mixture in the pipe, as inert nitrogen is used as a buffer. However, a minimum purge velocity is still required for the commissioning procedure to ensure efficiency and no pockets of nitrogen are left within the pipeline that could reach customers appliances.
This report provides the results of 32 set of tests, which investigate stratification and the extent of mixing zones formed during the purge of the gases in pipe in relation to the purge speed. The programme is concerned with determining the minimum speed for successful purge in light of difficulties in satisfying Froude and Reynolds number criteria traditionally used as a basis for ensuring that stratified mixtures do not result and failed purges do not occur.
This report was submitted to HSE for their assessment of the safety evidence for 100% hydrogen heating, which can be found at Hydrogen heating: HSE assessment of the safety evidence - GOV.UK.
Queries should be directed to DESNZ: https://www.gov.uk/guidance/contact-desnz.