Zone Negligible Extent: Example of Specific Detailed Risk Assessment for Low Pressure Equipment in a Hydrogen Refuelling Station

The MultHyFuel project aims to develop evidence-based guidelines for the safe implementation of Hydrogen Refueling Stations (HRS) in a multi-fuel context. As a part of the generation of good practice guidelines for HRS, Hazardous Area Classification (HAC) methodologies were analyzed and applied to case studies representing example configurations of HRS. It has been anticipated that Negligible Extent (NE) classifications might be applicable for sections of the HRS, for instance, a hydrogen generator. A NE zone requires that an ignition of a flammable cloud would result in negligible consequences. In addition, depending on the pressure of the system, IEC 60079-10-1:2020 establishes specific requirements in order to classify the hazardous area as being of NE. One such requirement is that a zone of NE shall not be applied for releases from flammable gas systems at pressures above 2000 kPag (20 barg) unless a specific detailed risk assessment is documented. However, there is no definition within the standard as to the requirements of the specific detailed risk assessment. In this work, an example for a specific detailed risk assessment for the NE classification is presented:
• Firstly, the requirements of cloud volume, dilution and background concentration for a zone of NE classification from IEC 60079-10-1:2020 are analyzed for hydrogen releases from equipment placed in a mechanically ventilated enclosure.
• Secondly, the consequences arising from the ignition of the localized cloud are estimated and compared to acceptable harm criteria, in order to assess if negligible consequences are obtained from the scenario.
• In addition, a specific qualitative risk assessment for the ignition of the cloud in the enclosure was considered, incorporating the estimated consequences and analyzing the available safeguards in the example system.
Recommendations for the specific detailed risk assessment are proposed for this scenario with the intention to support improved definition of the requirement in future revisions of IEC 60079-10-1.