f MOBs 357 Purging Hydrogen Risers (MOBS), Final Report

Multiple Occupancy Buildings (MOBs) account for 21% of the UK’s domestic heating demand and tackling the challenge to decarbonise these properties will be key to meeting Government net zero targets.1 There is therefore a requirement for gas distribution network operators (GDNOs) to understand the cost, safety, and practicality of converting gas supplies to hydrogen. This project aimed to address evidence gaps centred around commissioning and decommissioning of risers associated with MOBs in particular purging operations.
The project has carried out a review of processes, procedures and tooling used for purging MOBs, examined site surveys and discussed purging with operators. Riser systems in MOBs are branched systems often comprising many vertical and horizontal elements taking a single supply source and distributing it to multiple individual dwellings in the building. Purging this network of elements is caried out in a routine manner as dictated by standards and procedures. Routine purging of MOBs is not challenging and this will continue to be the same when using hydrogen. The greatest challenge identified to purging MOBs is when each individual dwelling needs to be accessed to complete the purge. If an individual dwelling is inaccessible and individual lateral isolation valves are not installed, then unpurged branches can remain. A consequence of leaving branches unpurged is a mixing of the air and fuel into a flammable mixture in the riser.
An experimental programme of work has been developed to investigate dispersion in unpurged branches of risers using methane and hydrogen. The experiments started with single pipes and developed in complexity to a branched system with six laterals. The main conclusions are: • If an unpurged branch is left over time, the flammable volume at the interface between purged and unpurged sections will increase. Pipe diameter is the dominant parameter that dictates the speed of mixing of the two gases. • Gas dispersion occurs through a combination of buoyancy and diffusion, buoyancy effects are diameter dependent becoming more dominant in pipe diameters greater than 50 mm. Below 50 mm gas dispersion is slow, being dominated by diffusion alone. • Diffusion driven dispersion acts in the direction of concentration gradient from high to low. This acts to reduce the driving concentration gradient and slows down subsequent diffusion. In vertical pipes, concentration gradients have been seen to act upwards or downwards. • Buoyancy effects act preferentially upwards, but also promote mixing of different density gases in horizontal pipes. • In tests, hydrogen dispersion was up to twice as fast as methane dispersion.
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.