Publications
Flashback Behavior and Safety Implications of Hydrogen-natural Gas Mixtures
Nov 2025
Publication
Hydrogen blending in natural gas systems is a key transitional strategy for reducing carbon emissions. This study explores the influence of hydrogen on combustion properties including flame flashback risk quenching distance and energy efficiency. Experimental and computational analyses demonstrate that hydrogen addition increases flame speed but reduces calorific value and quenching distance thereby impacting combustion stability and safety. Findings suggest that optimizing burner design and combustion control strategies is essential for safely and efficiently using hydrogen-enriched natural gas. Experimental validation confirmed that a 1.50 mm channel dimension effectively prevented flame flashback for hydrogen concentrations up to 40% in natural gas. As energy systems evolve toward decarbonization this research provides critical insights into the feasibility and challenges of hydrogen integration in residential or industrial applications. The study investigated the combustion behavior of natural gas enriched with various concentrations of hydrogen (up to 25%). Dynamic or fluctuating mixing conditions were excluded as the implementation of such a system in energy sector applications would necessitate a stable and well-defined gas composition.
SIF Alpha Phase - Velocity Design with Hydrogen, Summary Report
Mar 2026
Publication
The current UK natural gas networks operated by the Gas Distribution Networks have the potential to flow blended hydrogen and to be re-purposed to flow 100% hydrogen. The hydrogen networks would therefore have the potential to contribute to Ofgem’s strategic innovation fund (SIF) decarbonisation of heat challenge to help meet national 2030 and 2050 emissions targets.
To demonstrate how the current gas networks can be intelligently and efficiently transitioned to provide low carbon heating the gas velocity constraints for hydrogen applied at the design stage need to be identified. These constraints will directly impact the level of capital investment required in the transition of the system to accommodate blended and 100% hydrogen.
However hydrogen gas does not contain the same level of energy by volume as natural gas so the volume of hydrogen flowing to consumers would have to increase a little over 3 times for an 100% hydrogen network to deliver energy at an equivalent rate compared to natural gas. Without network reinforcement this increase in flow could require a significant increase to the pressure and/or velocity of gas.
Currently IGEM standards specify a nominal maximum velocity of 20 m/s mainly to avoid the risk of debris within the pipes being picked up by the gas stream and causing wear to pipe components possibly then resulting in early failure. A velocity limit of 40 m/s is assumed where the pipe assets are assumed to be clean.
Debris may be present in the system particularly in the lower pressure tiers in the form of dust mainly as a product of the historic manufacture of towns gas. Whilst many metallic mains particularly in the LP pressure tier have been replaced with PE (polyethylene) piping under the ongoing replacement scheme it is anticipated that debris will still be present in the pipes that have not been replaced and may have already been transported into the plastic pipes. Hydrogen has different properties to natural gas so it is not known if debris may be picked up to the same degree or if any other factor will limit velocity. Other factors such as noise and/or vibration may also constrain the design velocity of gas in the system.
Building on this initial work it was envisaged that validation of the pipe network behaviour would require full scale testing to investigate the erosion vibration and noise behaviour associated with transportation of hydrogen and hydrogen blends with natural gas to support the objective of validating and enhancing existing models. To develop the requirements for such testing the “Alpha phase” (this phase) of the SIF project was initiated with the intention of delivering conceptual designs of the full-scale test facilities a detailed test programme and to undertake any associated laboratory testing which would be required to support these activities.
This report summarises the SIF alpha phase conclusions and recommendations from work packages 1 to 5:
Work package 1 Conceptual design of test facilities
Work package 2 Detailed test plan
Work package 3 Laboratory testing
Work package 4 Network engagement
Work package 5 Cost-benefit analysis
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.
To demonstrate how the current gas networks can be intelligently and efficiently transitioned to provide low carbon heating the gas velocity constraints for hydrogen applied at the design stage need to be identified. These constraints will directly impact the level of capital investment required in the transition of the system to accommodate blended and 100% hydrogen.
However hydrogen gas does not contain the same level of energy by volume as natural gas so the volume of hydrogen flowing to consumers would have to increase a little over 3 times for an 100% hydrogen network to deliver energy at an equivalent rate compared to natural gas. Without network reinforcement this increase in flow could require a significant increase to the pressure and/or velocity of gas.
Currently IGEM standards specify a nominal maximum velocity of 20 m/s mainly to avoid the risk of debris within the pipes being picked up by the gas stream and causing wear to pipe components possibly then resulting in early failure. A velocity limit of 40 m/s is assumed where the pipe assets are assumed to be clean.
Debris may be present in the system particularly in the lower pressure tiers in the form of dust mainly as a product of the historic manufacture of towns gas. Whilst many metallic mains particularly in the LP pressure tier have been replaced with PE (polyethylene) piping under the ongoing replacement scheme it is anticipated that debris will still be present in the pipes that have not been replaced and may have already been transported into the plastic pipes. Hydrogen has different properties to natural gas so it is not known if debris may be picked up to the same degree or if any other factor will limit velocity. Other factors such as noise and/or vibration may also constrain the design velocity of gas in the system.
Building on this initial work it was envisaged that validation of the pipe network behaviour would require full scale testing to investigate the erosion vibration and noise behaviour associated with transportation of hydrogen and hydrogen blends with natural gas to support the objective of validating and enhancing existing models. To develop the requirements for such testing the “Alpha phase” (this phase) of the SIF project was initiated with the intention of delivering conceptual designs of the full-scale test facilities a detailed test programme and to undertake any associated laboratory testing which would be required to support these activities.
This report summarises the SIF alpha phase conclusions and recommendations from work packages 1 to 5:
Work package 1 Conceptual design of test facilities
Work package 2 Detailed test plan
Work package 3 Laboratory testing
Work package 4 Network engagement
Work package 5 Cost-benefit analysis
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.
LTS Futures Technical Report No. 2: Charpy Impact Testing & Transverse Strip Tensile Testin
Mar 2026
Publication
This report covers the Charpy impact testing and the transverse strip (flat) tensile testing of SGN pipes. The testing has been conducted on specimens extracted from three types of X52 steel grade linepipe: (a) Pipe A seemless; (b) Pipe B spiral seam welded; (c) Pipe C longitudinal seam welded.
MOBs Phase 3: Task 3 - Network Pipeline Capacity
Mar 2026
Publication
There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from supplying Natural Gas to Hydrogen in multi-occupancy buildings (MOBs). Previous work undertaken during project ‘MOBs Work Pack 2 Asset Information Review’ identified the following gap in technical evidence relating to network pipeline capacity:
♦ The adequacy of the diameter of existing risers and laterals to supply the energy required with Hydrogen need to be investigated.
♦ The effects of an increased flow rate velocity or increased pressure (pipe integrity) should it be required to meet the demand without increasing the diameter of risers and laterals. This would need to consider the effect of altitude on Hydrogen riser systems the pressure drops from existing fittings and additional safety devices installed (e.g. excess flow valves) and the minimum pressure required to ensure safe operation of Hydrogen appliances.
SGN is leading a feasibility project with some applied testing to understand the steps needed to convert MOBs to Hydrogen. The program is formed through 4 main work packs broken down into 2 main stage gated programmes (Part A = WP1 2 & Part B = WP3 4). This report is part of Work Pack 3 and summarises Task 3. The objective of Task 3 is to address evidence by examining the effects of increased volumetric flowrate velocity and/or increased pressure (pipe integrity) using the OLGA (V2021.2) pipeline simulator.
An earlier report described the survey of eighteen multi-occupancy buildings of various heights ages and construction methods. Of the eighteen multi-occupancy buildings eight were selected for analysis of capacity. A further two buildings representative of standard riser and lateral design were modelled with the data taken from the SGN management procedure SGN/PM/RL/1.
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.
♦ The adequacy of the diameter of existing risers and laterals to supply the energy required with Hydrogen need to be investigated.
♦ The effects of an increased flow rate velocity or increased pressure (pipe integrity) should it be required to meet the demand without increasing the diameter of risers and laterals. This would need to consider the effect of altitude on Hydrogen riser systems the pressure drops from existing fittings and additional safety devices installed (e.g. excess flow valves) and the minimum pressure required to ensure safe operation of Hydrogen appliances.
SGN is leading a feasibility project with some applied testing to understand the steps needed to convert MOBs to Hydrogen. The program is formed through 4 main work packs broken down into 2 main stage gated programmes (Part A = WP1 2 & Part B = WP3 4). This report is part of Work Pack 3 and summarises Task 3. The objective of Task 3 is to address evidence by examining the effects of increased volumetric flowrate velocity and/or increased pressure (pipe integrity) using the OLGA (V2021.2) pipeline simulator.
An earlier report described the survey of eighteen multi-occupancy buildings of various heights ages and construction methods. Of the eighteen multi-occupancy buildings eight were selected for analysis of capacity. A further two buildings representative of standard riser and lateral design were modelled with the data taken from the SGN management procedure SGN/PM/RL/1.
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.
MOBs Phase 3: Task 7 - Combined Effect of Hydrogen and Thermal Loading on Material Integrity
Mar 2026
Publication
There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from Natural Gas to Hydrogen in multi-occupancy buildings (MOBs). SGN is leading a feasibility project with some applied testing to understand the steps needed to convert to Hydrogen. This report is part of Work Pack 3 and summarises Task 7: combined effect of hydrogen and thermal loading on material integrity.
The aim of this task is to fill the following evidence gaps identified in ROSEN report entitled ‘16357-1 Document Landscape Review Report Issue 1-0’:
♦ Validity of current diameter height lateral length and material limitations and permitted jointing methods.
♦ Susceptibility of low strength steel to hydrogen cracking when subjected to stresses resulting from expansion and contraction and effect of hydrogen on likelihood of failure of risers which do not have the required allowance for expansion and contraction.
♦ Applicability of existing thresholds including minimum permitted wall thickness before isolation and corrosion damage categories for pipe designed to operate at stress levels not greater than 40% SMYS with hydrogen.
Finite Element Analysis (FEA) has been performed to assess the performance of existing carbon steel gas riser configurations when subjected to thermal loading to understand the suitability of converting the existing pipework to hydrogen.
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.
The aim of this task is to fill the following evidence gaps identified in ROSEN report entitled ‘16357-1 Document Landscape Review Report Issue 1-0’:
♦ Validity of current diameter height lateral length and material limitations and permitted jointing methods.
♦ Susceptibility of low strength steel to hydrogen cracking when subjected to stresses resulting from expansion and contraction and effect of hydrogen on likelihood of failure of risers which do not have the required allowance for expansion and contraction.
♦ Applicability of existing thresholds including minimum permitted wall thickness before isolation and corrosion damage categories for pipe designed to operate at stress levels not greater than 40% SMYS with hydrogen.
Finite Element Analysis (FEA) has been performed to assess the performance of existing carbon steel gas riser configurations when subjected to thermal loading to understand the suitability of converting the existing pipework to hydrogen.
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.
MOBs Phase 3: Task 8 - Electrical Safety Report
Mar 2026
Publication
There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from natural gas to hydrogen in multi-occupancy buildings (MOBs). Previous work undertaken during project “MOBs Work Pack 2 Asset Information Review” [1] considered the requirements for domestic electrical safety in MOBs following a conversion to hydrogen and identified gaps in technical evidence.
SGN is leading a feasibility project with some applied testing to understand the steps needed to convert MOBS to Hydrogen. Task 8 relates to potential electrical safety requirements associated with the conversion of MOBs from natural gas to hydrogen. It was proposed that ROSEN review the projects “EUSE – Hazardous Areas Within Buildings” [2] and “ATEX Equipment & SR/25 Modification Assessment” [3] to confirm their applicability to MOBs. The objectives of this review are to:
1. Confirm whether the standards to which electrical equipment is currently specified relates to natural gas only or to any flammable gas
2. Determine whether requirements for electrical equipment are impacted by hydrogen being gas group IIC versus gas group IIA for natural gas
3. Investigate whether the existing separation distance between natural gas pipes/meter and electrical equipment remains the same with hydrogen.
ROSEN has reviewed the outputs from these research projects as well as relevant standards to determine electrical safety requirements for the conversion of MOBs from natural gas to hydrogen.
The review of standards and research projects undertaken has drawn the following conclusions in regards to electrical safety.
1. To comply with the requirements of DSEAR meter banks energy centres and common areas within MOBs must be risk assessed to determine the location and extent of explosive atmospheres and their classification once repurposed for use with hydrogen. The risk assessment includes hazardous area classification of the pipework and components.
2. For pure hydrogen the necessary air change rate per hour to allow classification as Zone 2 NE is increased from 0.5 (for natural gas) to 1.5 air changes per hour (ACH). Where this cannot be achieved uncertified electric lighting and other electrical equipment will need to be relocated or replaced with certified gas group IIC equipment.
3. There will be no requirement for a hazardous area classification for hydrogen in a domestic environment (individual dwellings) as DSEAR does not apply.
4. The minimum separation distances between electrical equipment and gas equipment are independent of the gas being transported and research projects have concluded that these do not need to change.
5. Lightning protection requirements are independent of the gas being transported and are in place to protect the pipe structure from damage and existing IGEM/G/5 guidance can remain unchanged.
6. Research indicates that most incidents relating to electrical safety are due to non-compliance with current standards.
7. Any electrical equipment located in the vicinity of gas installations should be assessed for compliance by a competent person prior to the conversion of MOBs to hydrogen.
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.
SGN is leading a feasibility project with some applied testing to understand the steps needed to convert MOBS to Hydrogen. Task 8 relates to potential electrical safety requirements associated with the conversion of MOBs from natural gas to hydrogen. It was proposed that ROSEN review the projects “EUSE – Hazardous Areas Within Buildings” [2] and “ATEX Equipment & SR/25 Modification Assessment” [3] to confirm their applicability to MOBs. The objectives of this review are to:
1. Confirm whether the standards to which electrical equipment is currently specified relates to natural gas only or to any flammable gas
2. Determine whether requirements for electrical equipment are impacted by hydrogen being gas group IIC versus gas group IIA for natural gas
3. Investigate whether the existing separation distance between natural gas pipes/meter and electrical equipment remains the same with hydrogen.
ROSEN has reviewed the outputs from these research projects as well as relevant standards to determine electrical safety requirements for the conversion of MOBs from natural gas to hydrogen.
The review of standards and research projects undertaken has drawn the following conclusions in regards to electrical safety.
1. To comply with the requirements of DSEAR meter banks energy centres and common areas within MOBs must be risk assessed to determine the location and extent of explosive atmospheres and their classification once repurposed for use with hydrogen. The risk assessment includes hazardous area classification of the pipework and components.
2. For pure hydrogen the necessary air change rate per hour to allow classification as Zone 2 NE is increased from 0.5 (for natural gas) to 1.5 air changes per hour (ACH). Where this cannot be achieved uncertified electric lighting and other electrical equipment will need to be relocated or replaced with certified gas group IIC equipment.
3. There will be no requirement for a hazardous area classification for hydrogen in a domestic environment (individual dwellings) as DSEAR does not apply.
4. The minimum separation distances between electrical equipment and gas equipment are independent of the gas being transported and research projects have concluded that these do not need to change.
5. Lightning protection requirements are independent of the gas being transported and are in place to protect the pipe structure from damage and existing IGEM/G/5 guidance can remain unchanged.
6. Research indicates that most incidents relating to electrical safety are due to non-compliance with current standards.
7. Any electrical equipment located in the vicinity of gas installations should be assessed for compliance by a competent person prior to the conversion of MOBs to hydrogen.
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.
Hydrogen in Multi-occupancy Buildings Feasibility Study - Final Report
Mar 2026
Publication
There is a requirement for gas distribution network (GDN) operators to understand the cost safety and practicality of converting network pipelines from Natural Gas to Hydrogen in multi-occupancy buildings (MOBs). Previous work undertaken during project ‘MOBs Work Pack 2 Asset Information Review’ considered the requirements for converting MOBs to Hydrogen and identified gaps in technical evidence. SGN is leading a feasibility project with some applied testing to address the evidence gaps identified.
This report summarises the work which has been undertaken as part of Work Pack 3 – QRA and Testing. This includes the development of Quantitative Risk Assessments (QRAs) for Hydrogen MOBs conversion and an overall impact review of the conversion of MOBs.
Work Pack 3 included the development of a Quantitative Risk Assessment (QRA) for Hydrogen MOBs conversion which should:
• Easily integrate with wider network QRAs (e.g. GB QRA) to help complete understanding of safety across the entire system
• Take account of the network and end user parts of the system in the building
• Provide a record of underlying evidential basis or where this is lacking the justified assumptions or simplifications made in reviewing existing evidential basis (e.g. can it be assumed that occupants will react to a Hydrogen leak in the same way as Natural Gas?)
• Provide a quantified assessment of risk either in the form of:
o Absolute risk
o Comparative risk
o ALARP (“as low as reasonably practicable”)
• Obtain agreement from HSE on conclusions of QRA.
Work Pack 3 also included a review of the overall impact of conversion of MOBs considering:
• The cost and practicality of converting the MOB stock
• The safety of Hydrogen in MOBs vs alternatives
• This study could draw on feasibility type studies – i.e. feasibility review of the conversion of a limited number of real MOBs
• Overall recommendation for the suitability of Hydrogen versus alternatives with potential split between different categories of building
• Further recommendations for transition NIA to SIF project:
o Trials
o Further confirmatory evidential work
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 summarises the work which has been undertaken as part of Work Pack 3 – QRA and Testing. This includes the development of Quantitative Risk Assessments (QRAs) for Hydrogen MOBs conversion and an overall impact review of the conversion of MOBs.
Work Pack 3 included the development of a Quantitative Risk Assessment (QRA) for Hydrogen MOBs conversion which should:
• Easily integrate with wider network QRAs (e.g. GB QRA) to help complete understanding of safety across the entire system
• Take account of the network and end user parts of the system in the building
• Provide a record of underlying evidential basis or where this is lacking the justified assumptions or simplifications made in reviewing existing evidential basis (e.g. can it be assumed that occupants will react to a Hydrogen leak in the same way as Natural Gas?)
• Provide a quantified assessment of risk either in the form of:
o Absolute risk
o Comparative risk
o ALARP (“as low as reasonably practicable”)
• Obtain agreement from HSE on conclusions of QRA.
Work Pack 3 also included a review of the overall impact of conversion of MOBs considering:
• The cost and practicality of converting the MOB stock
• The safety of Hydrogen in MOBs vs alternatives
• This study could draw on feasibility type studies – i.e. feasibility review of the conversion of a limited number of real MOBs
• Overall recommendation for the suitability of Hydrogen versus alternatives with potential split between different categories of building
• Further recommendations for transition NIA to SIF project:
o Trials
o Further confirmatory evidential work
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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