Liquid hydrogen at 20 K was harmlessly released at Turin's Porta Susa station over a period of seven hours on 9 July 1991 through the safety valve of a dewar-type tank on a railway wagon following the loss of the vacuum between its two walls. Commercially available programs were unable to model this type of release in the unusual conditions in which this hydrogen had been stored. A model illustrating the course of the accident was therefore worked out. A start was made by examining the changes in the physical and thermodynamic properties of the hydrogen progress in the dewar to find out how long it had taken to build up the pressure needed to open the safety valve.
Owing to the complex geometry of the insulating layer in the interspace of the dewar on which the liquefaction of the air took place, the heat exchange coefficient could not be determined a priori. It was therefore assumed and subsequently quantified by means of an iterative process. The thermodynamic data were then used to examine the outflow of the hydrogen from the venting line. Flow dynamic calculations showed that the hydrogen was entirely lost through the safety valve and that pressure losses along the approx. 3-m line were negligible. The model also showed that the speed of the outflow was subsonic. The speed evaluated will enable the dispersion of the hydrogen and hence the areas at risk to be evaluated in the subsequent stages of the study. (c) 2005 Elsevier Ltd. All rights reserved.