Simplified Assessment of Protective Measures to Prevent Fire Escalation Involving Pressurized Vessels

The ignition of accidental releases of flammable substances followed by escalation due to flame impingement on equipment and pipes may cause severe accidental events in the process industry. Past accident data analysis evidences that BLEVE hazard is a critical issue in the risk management within the chemical and petrochemical industry and, more generally, in the risk management of storage vessels containing pressurized flammable gases. The prevention of this hazard requires the preliminary assessment of the possible escalation events triggered by primary jet fires and pool fires, and the consequent application of inherent, passive and active strategies for the control and the reduction of the risk associated to each piece of equipment. The present contribution focuses on the development of specific strategies for the protection of process equipment from catastrophic failure caused by external fires. A simplified methodology was developed for the calculation of the time to failure of vessels impinged or engulfed in fires, based on the correlation of the time to failure to the radiation intensity. These were obtained from an integrated approach, based on the use of available experimental data, and modelling activity. A model based on "thermal nodes" was developed. The model was used to generate and extend the data for the times to failure to the more common equipment pieces and the more likely fire exposure conditions. The model was validated using experimental results and the results of simulations carried out with a detailed finite element model. Correlations for the time to failure as a function of radiation mode and of radiation intensities were derived from the data set developed. Damage probability was estimated by a probabilistic function derived from layer of protection analysis. This approach 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 these correlations resulted the presence of thermal protection layers on the vessels. In particular, the possible use of innovative materials for passive protection resulted in a high impact on the time to failure. 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.