The IGEM/SR/25 hydrogen supplement reveals significant differences in the dispersion distances between Natural Gas and hydrogen, potentially extending hazardous areas beyond current site perimeters following a conversion to hydrogen. The Hazardous Area Impact Mitigation (HAIM) Phase 1 project (NIA2_SGN0041) revealed discrepancies between the calculated values in the hydrogen supplement and small scale empirical test results, necessitating a follow-on phase. Phase 2 will look to engage wider stakeholders, refine experiments to incorporate the impact of wind on releases, map additional vent types like angled vents, and conduct larger scale tests to validate Phase 1 findings across all pressure tiers. Additional work will also be carried out by DNV using industry standard packages to provide further comparison from modelling with the data tables from the IGEM/SR/25 hydrogen supplement.
Benefits
This project will build on the results from Phase 1 by conducting larger scale tests, incorporating alternative vent types, and considering external factors such as wind to further refine results. These tests will inform our understanding of how hydrogen behaves when a release occurs, which in turn will enable the development of a roadmap to update industry standards if required. This has potential to benefit the current gas network industry along with the network conversion to hydrogen.
Learnings
Outcomes
The HAIM Phase 2 project aim was to measure a range of gas releases from specific vent cases, providing empirical evidence of hazardous areas for hydrogen. A technical review group was established early in the project and this included members of the team who contributed to the IGEM/SR/25 hydrogen supplement.
The first group of experiments covered leaks from infrastructure such as flanges and fittings. Tests were conducted using 0.25 mm2 and 2.5 mm2 leaks driven by a variety of pressures up to 4 barg. Findings were that gas releases from these leaks travel in the direction of the leak unless impacting on a surface. Buoyancy has little effect on all but the lowest pressure releases. Concentration decay is rapid, the rate of decay increasing as pressure increases, and 2% v/v concentration gas reaching less than half of the specified zone distance from the hydrogen supplement. The limit of partial ignition was as little as 20% of the specified zone distance and the limit of sustained ignition was less than 10% of the specified zone distance.
The second group was from small vent releases, carried out in the workshop under the influence of artificial wind. Gas release displacement was compared to the horizontal zone (Xr) and the vertical zone (Xh). The gas concentration tests demonstrated that is possible to drive 2% v/v gas concentrations to the horizontal limit of the specified zones with the correct release and wind conditions. In low wind conditions, gas could rise above the specified Xh. The limit of partial ignition, however, was much lower than the specified Xh and the limit of sustained ignition 20% of Xh.
The final group of tests was the large-scale offsite tests. These explored three of the test cases from the ATEX Equipment & IGEM/SR/25 Modification Assessment project. Tests were conducted outside under the influence of variable wind conditions. The dominant effect of wind was to break up the plumes from gas releases. The greatest horizontal displacement was achieved with the low-pressure gas releases, with the non-ideal (capped) vent seeing the greatest effect. It was possible to drive pockets of gas large distances; however, the measurement of the pockets became challenging due to the transient and broken up nature of the wind influenced plume. The best indication of gas concentration and hazard came from ignition tests. An array of 48 igniters was used with a high definition FLIR video camera to map out the limits of partial and sustained ignition for all the tests. All the tests were compiled into collated ignition maps for each test case. The results indicated that the low-pressure gas releases were much more prone to wind influence, especially for driving the plume horizontally.
The ignition maps from the large-scale tests resulted in defining three separate areas, the limit of sustained ignition which resulted in light-back to the vent tip, the limit of partial ignition which created blooms of flame that did not burn back and a heat map which was the total extent of heat. For the high-pressure case, the limit of sustained ignition was less than 20% Xr and just over 25% Xh, the limit of partial ignition which was about 35% of Xr and Xh, and the heat map extended to about 50% of Xr and Xh. Low pressure tests saw horizontal limits of ignition up to 60% of Xr but only 25% of Xh under wind influence.
Closing out the large offsite tests involved heat mapping ignited vents from the plumes and comparing these to the previous limits of ignition. This was found to be representative for the high-pressure release tests and should be investigated for potential further, higher pressure, larger volume release tests.
The main project conclusions are that it is possible to drive low concentration gas pockets large horizontal distances, but they are not representative of the actual hazard posed by the gas releases. The limits of partial and sustained ignition are a much better method for investigating and defining the hazards from gas releases.
Lessons Learnt
Steer Energy performed heat map analyses of the outdoor releases to explore the accuracy of this method when assessing hazardous zones. More research is required but initial findings show that this may be a good measurement for limits of ignition and thereby the main hazard from gas releases. Sensors are limited in the fact they only measure gas concentration at specific spatial and temporal points. The infrared camera on the other hand records the full image of the release in real time. This could become a useful tool for future assessment of hazardous areas.
