f Hazardous Area Impact Mitigations (HAIM) Phase 2a: Full Scale Testing, Interim Report

This project has built on the Hazardous Area Impact Mitigation (HAIM) Phase 1 project (NIA2_SGN0041) results that identified a disparity between hazardous zones measured during initial testing and those specified in the IGEM/SR/25 Hydrogen supplement. The aim of the HAIM Phase 2 project is to scale up the measurements to confirm the behaviour of larger vents, equivalent to the test cases presented in the ATEX Equipment & IGEM/SR/25 Modification Assessment (NGNG_NIA_346) project.
The formation of a technical review group has informed the project team of the parameters and some of the assumptions used for the modelling leading to the development of the SR/25 Hydrogen Supplement. The difference between the modelled and measured data seen in the HAIM Phase 1 project has been attributed to the modelled data being carried out under a minimum of 0.5 m/s cross winds. Completely still conditions are not expected to occur, hence this 0.5 m/s minimum. The result of this wind on the model leads to a significant reduction of the height of the resulting plume, and a corresponding increase in the radial displacement of the plume from the vent tip. This has shifted the focus of this project towards examining wind influenced vents.
Two sets of experiments are provided in this interim report: measurements of plumes from wind influenced vents, and plumes from fixtures and fitting leaks. The report also includes early results from plume ignition studies which have shown that ignition is not instantaneous for high velocity plumes.
The wind influenced plume tests have measured 0.0005 kg/s hydrogen releases from 50 mm and 15 mm vent pipes. The largest hazardous zone for these releases, stipulated in IGEM/SR/25 hydrogen supplement, is Xr = 2.5 m and Xh = 1.5 m, so these were used as the extent of measurement. With no wind, the plume rises vertically from the vent tip with no radial deflection. Measured concentration peaks have exceeded the lower flammable limit (LFL) at the 1.5 m measurement height. The influence of wind radially displaces the plume, the higher the wind, the larger the displacement. Concentration peaks are reduced, but a wind of 0.5 m/s still permitted levels above the 4 % LFL value. Wind levels of 1.0 m/s displaced the plume to the end of the 2.5 m measurement array. Wind levels of 1.5 m/s broke up the plumes, potentially driving pockets of gas beyond the 2.5 m measurement array.
Partial ignition of both vent types was possible at 1.5 m above the vent tip, but complete sustained ignition was only possible when closer than 1 m to the vent tip.
Plumes from higher pressure (above 0.1 barg) fixture and fitting leaks have shown a good correlation between the shapes of modelled and measured vents. Except for the lowest pressure leaks, which are momentum-dominated jets, the resulting plumes are long and thin, unaffected by buoyancy. The concentration decay in measured plumes is observed to be faster with distance compared to modelled values. Typically, the measured distance to reach 2 % volume from the leak position is about half of the specified zone distances.
Limited ignition tests have been conducted, but ignition from a 2 barg adverse downward pointing leak was challenging beyond 30 cm from the leak. The hydrogen jet also repeatably extinguished the methane flame used as pilot light during tests.
The next steps for the project are to carry out more measurements and to scale up the magnitude of the gas releases. This will provide more evidence supporting specified magnitudes of hazardous zones. In addition, it is proposed that mitigation measures are explored that could reduce the specified hazardous zones for given vents. This could include design guidelines for hydrogen vents.
Further ignition tests will also be conducted to assess required conditions such as flow, direction and gas concentration required to achieve both partial and stable ignition of hydrogen vents.
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.