Safety
Safe Venting and Recompression of Hydrogen - Final Technical Report
Mar 2026
Publication
This document summarises The Safe Venting and Recompression of Hydrogen project (NIA_NGGT0205) project carried out by Wood PLC for National Gas Transmission from Feb 2023 to Feb 2024. The project explored the possible impacts of transition from natural gas fuel to hydrogen (or to hydrogen/natural gas mixtures) on the requirement to depressurise transmission pipelines and associated equipment for maintenance or other purposes. NGT currently employ gas recompression or venting to atmosphere as a means of achieving safe conditions for intrusive work. The project investigated the impact of the presence of hydrogen on these and other potential technologies for providing safe conditions of work. Details of the work carried out is recorded in Technical Notes TN01 to TN04. High level summaries of each technical note are included at the end of this Executive summary.
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.
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.
Oxygen Ingress - Interim Report 2
Mar 2026
Publication
There was an event at the Redcar EcoHouse where after the isolation of the hydrogen pipework for 27 days over the Christmas break 2023/2024 and shortly after initiation of the flame in a hydrogen gas fire appliance a minor ignition event occurred. Visual investigation led to a discovery that the gas meter was damaged.
Air ingress into the isolated hydrogen pipework (causing a flammable mixture in this pipework) followed by the flame propagation from the ignition system of the hydrogen gas fire were considered to be the most likely reasons for the aforementioned event.
The aim of the work presented in this report was to complement the work undertaken by Steer Energy and to:
• investigate the possibility of the above phenomena to occur in domestic hydrogen installations with main focus on hydrogen appliance design
• to conclude whether there are plausible explanations of the event observed at the Redcar EcoHouse
• to propose possible mitigations which are likely to prevent similar events in the future.
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.
Air ingress into the isolated hydrogen pipework (causing a flammable mixture in this pipework) followed by the flame propagation from the ignition system of the hydrogen gas fire were considered to be the most likely reasons for the aforementioned event.
The aim of the work presented in this report was to complement the work undertaken by Steer Energy and to:
• investigate the possibility of the above phenomena to occur in domestic hydrogen installations with main focus on hydrogen appliance design
• to conclude whether there are plausible explanations of the event observed at the Redcar EcoHouse
• to propose possible mitigations which are likely to prevent similar events in the future.
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.
NIA 344: Ignition Consequences, Final Report
Mar 2026
Publication
The UK government has passed into law its ambition to reach net zero greenhouse gas (GHG) emissions by 2050. The scale of this task is such that policy makers are required to make decisions now in order to enable the necessary reduction in GHG emissions in all aspects of life. One of the largest contributions of GHG in the UK is from heating homes which predominantly uses the natural gas transported via transmission and distribution networks directly to homes where it is burned in boilers to heat water and to a lesser extent to cook with or heat spaces in gas fires. One alternative for reducing GHGs from home heating being examined by the government is to transition the UK gas network to the supply of 100% hydrogen. A key element of any policy decision is to have an informed position on the relative safety of such a change.
Flammable gases such as natural gas and hydrogen have the potential to cause an explosion if an accidental release occurs within a building. Although such explosions are very rare with natural gas it is important to understand how the differing properties of the two gases (natural gas and hydrogen) might influence the explosion behaviour as this will assist in both understanding the relative risks and the benefit provided by additional mitigations that could be introduced at the time of transition.
Gas explosions in buildings are complex as they involve an interaction between the explosion process and structural failure and even subtle changes in phenomena can significantly alter the severity of an explosion. To better understand the effect of changing from natural gas to hydrogen the NIA344: Ignition Consequence Project has been undertaken. The project had the objective of performing experimental research into the various phenomena contributing to the severity of explosions in domestic structures and had three phases. The experiments have mostly been performed in a purpose-built explosion chamber where different configurations inside the chamber and at the venting (front) face were set up.
Measurements of explosion overpressure inside and around the chamber allowed assessments of the comparative severity and consequence of the explosions to be assessed and compared between hydrogen and methane fuel : air explosions (methane representing natural gas of which it is the primary constituent). A general arrangement figure and a collage of an experiment in progress are given in Figure 1 and Figure 2 respectively.
In this programme a total of 95 experiments have been conducted to study the effect on explosion severity of five different phenomena individually and in combination:
♦ The effects of gas concentration and accumulation patterns (established in other programmes) on pressure development in an explosion (Build-up effects).
♦ The vent face was covered with a polythene sheet only.
♦ The effects of weak structures on explosion severity (Confinement).
♦ The effects of furniture / obstacles on pressure development (Congestion).
♦ The effects of interconnecting rooms (Multi-room).
Detonation in hydrogen-air mixtures either through deflagration to detonation transition (DDT) or direct initiation (Detonation Initiation);
One important factor affecting explosion severity is the laminar burning velocity which is a fundamental property of flammable gases in well-defined conditions. The laminar burning velocity of hydrogen : air mixture at 15 %vol is similar to the maximum for methane : air mixtures (observed at around the stochiometric concentration of 9.5 %vol) and falls well below this as the hydrogen concentration reduces further. Based on laminar velocity alone hydrogen explosions at a concentration of 15 %vol should be no worse than the worst case with methane (or natural gas) and considerably less at lower hydrogen concentrations. However real explosions in domestic settings are likely to feature multiple explosion phenomena together. Given that proposed mitigations for end users have the objective of limiting the potential for higher hydrogen concentrations it is important to understand if explosion severity is characterised by laminar burning velocity alone or whether other factors play a part.
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.
Flammable gases such as natural gas and hydrogen have the potential to cause an explosion if an accidental release occurs within a building. Although such explosions are very rare with natural gas it is important to understand how the differing properties of the two gases (natural gas and hydrogen) might influence the explosion behaviour as this will assist in both understanding the relative risks and the benefit provided by additional mitigations that could be introduced at the time of transition.
Gas explosions in buildings are complex as they involve an interaction between the explosion process and structural failure and even subtle changes in phenomena can significantly alter the severity of an explosion. To better understand the effect of changing from natural gas to hydrogen the NIA344: Ignition Consequence Project has been undertaken. The project had the objective of performing experimental research into the various phenomena contributing to the severity of explosions in domestic structures and had three phases. The experiments have mostly been performed in a purpose-built explosion chamber where different configurations inside the chamber and at the venting (front) face were set up.
Measurements of explosion overpressure inside and around the chamber allowed assessments of the comparative severity and consequence of the explosions to be assessed and compared between hydrogen and methane fuel : air explosions (methane representing natural gas of which it is the primary constituent). A general arrangement figure and a collage of an experiment in progress are given in Figure 1 and Figure 2 respectively.
In this programme a total of 95 experiments have been conducted to study the effect on explosion severity of five different phenomena individually and in combination:
♦ The effects of gas concentration and accumulation patterns (established in other programmes) on pressure development in an explosion (Build-up effects).
♦ The vent face was covered with a polythene sheet only.
♦ The effects of weak structures on explosion severity (Confinement).
♦ The effects of furniture / obstacles on pressure development (Congestion).
♦ The effects of interconnecting rooms (Multi-room).
Detonation in hydrogen-air mixtures either through deflagration to detonation transition (DDT) or direct initiation (Detonation Initiation);
One important factor affecting explosion severity is the laminar burning velocity which is a fundamental property of flammable gases in well-defined conditions. The laminar burning velocity of hydrogen : air mixture at 15 %vol is similar to the maximum for methane : air mixtures (observed at around the stochiometric concentration of 9.5 %vol) and falls well below this as the hydrogen concentration reduces further. Based on laminar velocity alone hydrogen explosions at a concentration of 15 %vol should be no worse than the worst case with methane (or natural gas) and considerably less at lower hydrogen concentrations. However real explosions in domestic settings are likely to feature multiple explosion phenomena together. Given that proposed mitigations for end users have the objective of limiting the potential for higher hydrogen concentrations it is important to understand if explosion severity is characterised by laminar burning velocity alone or whether other factors play a part.
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.
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