Ignited Releases of Liquid Hydrogen - Safety Considerations of Thermal and Over Pressure Effects

If the 'Hydrogen Economy' is to progress, more hydrogen fuelling stations are required. In the short term and in the absence of a hydrogen distribution network, these fuelling stations will have to be supplied by liquid hydrogen (LH2) road tankers. Such a development will increase the number of tanker offloading operations significantly and these may need to be performed in close proximity to the general public. LH2 was first investigated experimentally [1] as large-scale spills of LH2 at a rate of 60 litres per minute. Measurements were made on un-ignited releases which included the concentration of hydrogen in air, thermal gradients in the concrete substrate, liquid pool formation and temperatures within the pool. Computational modelling on the un-ignited spills was also performed [2]. The experimental work on ignited releases of LH2 detailed in this paper is a continuation of the work performed by Royle and Willoughby [1]. The experimental findings presented are split into three phenomena; jet-fires in high and low wind conditions, 'burn-back' of ignited clouds and secondary explosions post 'burn-back'. The aim of this work was to determine the hazards and severity of a realistic ignited spill of LH2 focussing on; flammability limits of an LH2 vapour cloud, flame speeds through an LH2 vapour cloud and subsequent radiative heat levels after ignition. An attempt was made to estimate the magnitude of an explosion that occurred during one of the releases. The results of these experiments will inform the wider hydrogen community and contribute to the development of more robust modelling tools. The resulting data were used to propose safety distances for LH2 offloading facilities which will help to update and develop guidance for codes and standards.