A simplified approach to the assessment of domino events caused by external fires

Domino effect is responsible of severe accidents that took place in the chemical and process industry. Several studies pointed out that the more critical step in the quantitative assessment of domino hazards is the availability of reliable models to estimate the possibility of escalation due to the effects of the primary accident. In the case of fires caused by the accidental releases of flammable substances, it is well known that secondary events with catastrophic consequences may result as a consequence of flame impingement on equipment and pipes. Several technical standards suggest to evaluate the possible damage to process equipment caused by fire using threshold values for radiation intensity that do not take into account safety and site-specific factors, as the presence of improved thermal protection systems or the possible mitigation due to effective emergency response. An alternative to this oversimplified approach are very complex and time consuming models available for the detailed calculation of the time to failure of storage vessels, requiring a detailed description of vessel geometry and other design data. An important benefit to the safety management of possible domino hazards would come form the availability of an approach to the calculation of the possibility and probability of vessel damage following external fires based on simplified correlations able to take into account specific protection factors. The present study was focused on the development of a simplified methodology for the calculation of the damage probability of process vessels aimed to the quantitative assessment of domino effect triggered by fire scenarios. The methodology is based on simple analytical functions relating the time to failure of vessels to the radiation intensity. These were validated by an integrated approach, based on the use of available experimental data, of the ANSYS finite elements code for complete thermal and mechanical simulations of the behaviour of vessels exposed to fires and of a simplified model for vessel failure based on thermal nodes. The correlations were obtained for atmospheric as well as for pressurized storage vessels. Specific correction factors were introduced in order to take into account the effect of protection materials. Damage probability was estimated by a probabilistic function derived from layer of protection analysis (LOPA). LOPA was used to estimate the probability of effective mitigation on the basis of the calculated time to failure and of site-specific factors The presence and the delay time for the activation of protection systems were also considered. A fundamental issue in the development of the correlations resulted the presence of thermal protection layers on storage or process vessels. In particular, the possible use of innovative materials for passive protection systems as basalt rock fibres resulted in a high impact on the time to failure. Due to the lack of literature data on the properties of these materials, experimental data were obtained from a specific facility. The data were used to correctly analyse the effect of these protection panels on the time/temperature profile of vessel wall and to determine the physical properties of the materials, such thermal conductivity and emissivity, necessary input for finite elements simulations and for simplified threshold correlations. The approach evidenced that important differences in the possibility and probability of domino effect triggered by external fires should be expected if differences among vessel characteristics and protection systems are taken into account. This was confirmed by the quantitative assessment of the risk caused by domino effects triggered by fires, performed using a specific software and the damage probability models discussed above.