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HYEX ignition model recommended for hydrogen safety studies in Norway

In July 2021 revised guidelines for quantitative risk analysis (QRA) of facilities handling hazardous substances were issued in a report written by Vysus Group on behalf of the Norwegian Directorate for Civil Protection (DSB). One of the revisions compared to the original 2019 version of the report is that the HYEX model for ignition probability is recommended used for hydrogen studies, see link below (Only Norwegian version revised so far, English version expected soon):

https://www.dsb.no/globalassets/dokumenter/rapporter/andre-rapporter/retningslinjer-for-kvantitative-risikovurderinger-for-anlegg-som-handterer-farlig-stoff-2021.pdf

Observing the lack of well-funded and useful ignition probability models for hydrogen, the HYEX model was first proposed in 2018 in a risk assessment study report for a hydrogen fuelled fast ferry as part of the MoZEES project (www.mozees.no), and later presented in a publication of the same study (Aarskog, Hansen, Strømgren and Ulleberg (2020)).

Due to the stochastic nature of ignition of accidental gas releases an ignition probability model will never be correct or wrong. The model should however reflect the much higher propensity for a hydrogen release to ignite compared to natural gas releases, due to much lower ignition energy and wider flammable range.

Further features of the HYEX model include:

  • Continuous increase of ignition probability with initial release rate
  • Probability for immediate ignition (jet-fire) and delayed ignition (explosion)
  • Delayed ignition model for confined situations where gas can accumulate
  • Increased ignition probabilities may be assumed for areas without ignition proof equipment
  • The model is applicable both for simplified assessments and CFD-studies

From 2018 to 2021 we have used the HYEX model for around 100 hydrogen risk assessment studies for clients in Norway and a handful of other countries and concluded that ignition probabilities and sensitivities to parameter variations seem reasonable. An important feature of the model is its simplicity with relations easy to implement in more comprehensive risk models. Until experience and experiments justify something else, the HYEX model is also recommended used for LH2-releases and tank rupture scenarios (20-50% of H2 is assumed to contribute to delayed explosion).

More details of the models can be found in the DSB-guidance report.