Performance of passive fire protection for liquefied petroleum gas vessels: An experimental and numerical study

Fire is among the most dangerous accident scenarios that may affect the process industry. Vessels contain-ing pressurized liquefied gases are particularly vul-nerable to external fires, since an increase in the ves-sel temperature by a fire scenario may result in both the rise of the internal pressure and the weakening of the vessel structure. Vessel failures due to accidental fires may yield a significant escalation of accident severity, either by the release of the vessel content or by overpressure generation in the case of catastro-phic failure (BLEVE). Passive fire protection (PFP) is a robust and effective solution to reduce the prob-ability of accident escalation. In particular, fireproof-ing delays the temperature rise of the protected sur-faces retarding vessel heat up and failure (CCPS 2003). The assessment of the behaviour of the materials exposed to fire is a critical issue for determining the effectiveness of the PFP system. In particular, ther-mal coatings may undergo degradation (e.g. devola-tilization) that causes the variation of key physical properties during prolonged fire exposure. Though the thermal degradation may be an inherent part of the protective action of the coating (e.g in the case of intumescing coatings), the progressive deterioration of the material may lead to a decrease in the per-formance of the protection. Although some large scale tests on vessels have been carried out (Landucci et al. 2009, VanderSteen & Birk, 2003), further studies are required in order evaluate the effectiveness of PFP systems. It has to be considered that this type of tests is expensive and hazardous. Coping with smaller scale tests and mod-eling is thus advisable to obtain a more effective route to explore this issue. Finite element modelling (FEM) combined with experimental tests at small scale can be an option which allows to study in depth these systems in a safe and reliable way. In the present study, an approach to the assess-ment of the effectiveness of PFP systems is pre-sented. The approach integrates experimental results and finite element modelling. This allowed predict-ing the expected behaviour of real scale pressurized tanks engulfed by fires. The behaviour of a commer-cial epoxy intumescent PFP material exposed to temperatures up to 800ºC was analyzed using Ther-mogravimetric Analysis. The results allowed identi-fying the main decomposition regions of the PFP. Small samples of PFP were exposed to different temperature histories in a fixed-bed tubular reactor, allowing further characterization of the material deg-radation behaviour (e.g. morphological, swelling). The data collected in these lab-scale experiments can be used to define an apparent kinetic model for prop-erty variation of the PFP material, as well as correla-tions which allow to predict the expansion, the bulk density and the thermal conductivity of the material with temperature. At the same time, a finite elements model (FEM) for LPG vessels was developed for simulation of real scale vessels. The model considers a simplified fail-ure criterion, combining temperature and stress dis-tribution, for the evaluation of the time to failure. A preliminary study, resorting to the obtained ex-perimental data, was carried out comparing the be-haviour of coated and uncoated tanks. The results evidenced that the time to failure is clearly increased by PFP coating, but the correct modelling of PFP modification during fire exposure is identified as critical for the definition of PFP effectiveness.