f NIA 346 H21 Hazardous Area Zoning Summary Report

In order to utilise the existing gas transmission and distribution network to transport 100% hydrogen, the effects of the changes in characteristics of hydrogen from natural gas need to be reviewed and the resultant effect on the network assessed. Hydrogen features a substantially larger range of flammable concentrations than natural gas, which could potentially cause safety concerns if the existing network is not reviewed. Hazardous area zoning of equipment present on the gas transmission and distribution network is modelled in accordance with standard IGEM/SR/25 Ed. 2. A hazardous area is defined in this standard as “an area in which explosive gas/air mixtures are, or may be expected to be, in quantities as such as to require special precautions for the construction, installation and use of electrical apparatus or other sources of ignition.” A supplement to this standard compatible with the use of hydrogen blends up to 20%, in addition to pure hydrogen, was published by IGEM in November 2022. This hydrogen supplement has been utilised to establish the hazardous area zoning of hydrogen gas in 13 sites across multiple networks.
Hydrogen possesses a lower molar mass than natural gas, therefore the mass flow rate of gas escaping relief vent pipework during venting operations is expected to decrease during pressure-driven release. Due to the larger flammable concentration range of hydrogen-air mixtures, the impact on the sizes of hazardous areas was not immediately present. Across all sites the size of hazardous areas was seen to increase upon calculating the hydrogen mass flow rate for a given vent. It was observed on several sites that the hazardous areas of relief vents extended beyond the site boundaries.
Additional consideration was paid to vent pipe geometry in relation to sections 7.8.3 and 7.8.4 of IGEM/TD/13 Ed. 2 Supplement 1 – Pressure Regulating Installations for Hydrogen at Pressures Exceeding 7 bar. These clauses require that the Length/Diameter ratio of a vent pipe be kept below 60:1, to reduce the chance of combustion or detonation due to depressurisation in the vent pipe. This is due to hydrogen experiencing an increase in temperature during depressurisation, as opposed to natural gas which decreases in temperature. This affects all sites due to the prevalence of small-bore pipework (10-15mm) used in impulse and instrumentation pipework. This also has potential to affect smaller relief vent pipework such as that used on district governors (typically 1”/25NB), depending on specific pipe and valve placement.
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.