Experimental and numerical investigation of passive fire protection for pressurized tanks engulfed by fires

Accidental fires may impinge equipment and pipes, leading to their thermal collapse and to accident escalation triggered by fire. The resulting secondary events may be far more severe than the primary fire, especially when pressurized storage or transport units are involved, resulting in severe domino accidents. Passive fire protection (PFP), which consists in systems able to limit the temperature rise of equipment involved in fires, may be a robust and effective solution to reduce the probability of fire escalation. The assessment of the behaviour of the material exposed to fire is a critical issue for determining the effectiveness of the PFP system. As a matter of facts, thermal coatings may undergo degradation that may cause the variation of key physical properties during prolonged fire exposure. Though the degradation, in terms of weight losses and devolatilization, may be an inherent part of the protective action of the coating (for example in the case of intumescing coatings, vermiculite sprays, etc.), the progressive deterioration of the material may lead to a decrease of the effectiveness of the protection. In the present study, an approach to the assessment of the effectiveness of passive fire protection is presented. The approach is based on the combination of experimental results and finite element modelling. A preliminary experimental analysis was carried out on several reference materials, selected among those more commonly used for PFP in Oil&Gas processing and storage facilities. The aim was to analyze the critical phases of the devolatilization and degradation processes, in order to assess the dynamic behaviour of the thermal coating exposed to fire. Thermogravimetric analysis (TGA) was used to determine the degradation profile of the coating and to identify the devolatilization rate. Thus, the temperature scale of the reference phases of the coating behaviour were identified. A reference value for the thermal conductivity was selected for each phase, thus building a dynamic function for the coating performance during fire exposure. The results obtained were implemented in a finite elements model (FEM), in order to simulate the behaviour of insulated tanks engulfed by fire. The FEM is based on two modules, a thermal and a mechanical one. The first was employed to simulate the vessel heat-up due to fire, and thus, the possible limitation in temperature increase due to the coating. The implemented dynamic behaviour of the coating also allowed investigating on the resultant degradation effect. The second FEM module was used to obtain the stress intensity distributions combining thermal and mechanical loads on the structure. A simplified failure criterion allowed combining the temperature and stress distribution for the evaluation of time to failure of the structure. Thus the effectiveness and critical limits of the considered passive fire protection coatings were identified.