f 351 Hazardous Area Impact Mitigation Phase 1

This programme of work aims to generate empirical evidence of gas concentration with respect to distance from the vent tip for a range of hydrogen releases. The measured data is to be compared to the Zone 2 exclusion distances specified by the IGEM/SR/25 hydrogen supplement.
Test cases have been shared with Steer Energy that calculate the new hazardous areas, as per the hydrogen supplement, for common infrastructure such as pressure regulating installations/stations. The result of these test cases was a significant increase in the calculated hazardous zone distances for hydrogen, compared to those for Natural Gas. The overall programme aims are to measure gas releases replicating these test cases and to compare the measured hazardous zones to the calculated hazardous zones. This report covers Stage 1 of the programme of work which comprised an initial examination using small releases as a fast and economical method to assess the likelihood of differences between measured and calculated zones.
Experimental equipment was setup to release gas at controlled flow rates to match those of the IGEM/SR/25 hydrogen supplement tables. A moveable array of gas detectors was positioned above the vent tip to measure the shape and magnitude of the resulting gas plume from the release.
In all, 22 tests were conducted with gas released from two different vent diameters, 13 mm and 48 mm. Two gas types, hydrogen and methane were used. Ideal and non-ideal vents were tested across a limited range of flows. The measured data enabled colourmaps of the vents to be created, showing the shape and magnitude of the resulting gas plumes.
The results of the study have shown that in all cases, the shape of the plumes from the measured vents are significantly different to the dispersion distances specified in the relevant tables of IGEM/SR/25. In most cases, no gas was detected throughout the majority of the specified hazardous area, instead a thin vertical cylindrical plume of gas was measured often extending above the specified dispersion zones. This was seen in both hydrogen and methane tests.
The test results from this initial phase of the project cast some doubt on the findings from the previous NIA project ATEX Equipment & SR/25 Modification Assessment that used the SR/25 calculator developed from the hydrogen supplement tables. In some instances, the horizontal dispersion distance for hydrogen was calculated to be over 6 times the value for Natural Gas (see Figure 2) with its resulting hazardous area exclusion zone having potentially serious consequences on the viability of the corresponding AGIs without mitigations. However, results from the initial tests undertaken during this phase of work demonstrate significant inconsistencies between the calculated results and empirical tests. This should be further investigated in phase 2 as initial conclusions show that the larger hazardous zones mentioned above are seemingly overstating the risk. The previous work also modelled the hazardous areas using full bore releases whereas relief valves on the network tend to incorporate flow limiting orifices, therefore further exacerbating the perceived increased risk.
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.