Abstract
DIERS (Design Institute for Emergency Relief Systems) methodology is the standard framework for sizing pressure relief systems under runaway reaction conditions. Its validated application, however, is largely limited to exothermic systems, where reaction rates are sustained by internally generated heat. This work addresses a key limitation by proposing a vent sizing methodology for endothermic decompositions that produce significant amounts of non-condensable gases but require external heat input. The approach is illustrated through a case study of aqueous formic acid exposed to fire conditions in a storage tank. Experimental characterization was conducted using low thermal inertia calorimetry (PHI-TEC II, Heat-Wait-Search mode) and Differential Scanning Calorimetry (DSC). Results confirm that formic acid decomposition is endothermic, yet generates substantial incondensable gases, as indicated by sustained pressure rise and residual pressure after cooling. The system is therefore classified as either gassy or hybrid under DIERS definitions, but it lies outside the scope of existing DIERS methodologies. Conversely, conventional data treatment based on standards for nonreactive systems may lead to either non-physical or poorly conservative vent sizing. To address this, a revised methodology is proposed based on signal processing of calorimetric data, identification of stagnation point, and evaluation of pressure and temperature derivatives under those conditions. This enables a physically consistent estimation of both gas and vapor generation rates despite the absence of exothermicity; moreover, the application to the formic acid system yielded venting requirements significantly higher than those derived from conventional fire boiling assumptions. The results demonstrated that endothermic gassy/hybrid systems can be treated within a DIERS-consistent framework when external heat input is properly considered. This work can be considered as a first tentative to extend the applicability of DIERS to a broader class of reactive systems and provides a rigorous basis for relief system design beyond current standards.
