(47ab) Using Explicit Finite Element Analysis to Simulate the Dynamic Response and Predict the Structural Damage Associated with a Real-Life Process Equipment Failure Due to an Internal Detonation

Simulating the realistic blast loading associated with an internal detonation occurring within a pressure vessel or heat exchanger is extremely challenging.  Unlike evaluation of external blast loading on structures due to far-field explosions, where typical overpressure-time histories can be reasonably defined based on empirical data, investigating confined detonations presents additional complications such as accounting for internal blast wave reflection that can yield multiple blast waves and considering potential pressure build up due to the accumulation and temperature rise of gaseous explosion by-products.  The subsequent impulsive peak reflected overpressure from confined detonations acting on a structure can be extremely high due to the often times, close proximity of the blast source to the vessel wall or pressure boundary.  This establishes the possibility of significant structural damage for process equipment subjected to an internal detonation, even for relatively modest amounts of concentrated explosive products.   

Performing dynamic computational simulations of internal detonations is valuable in assessing the structural response and possible failure modes of critical process equipment such as process piping, pressure vessels, or heat exchangers.  Furthermore, assessing the potential damage from an internal detonation within such a structure and predicting ensuing damage to surrounding equipment if the containment vessel catastrophically fails can provide valuable information about protecting structures from explosions and implementing designs that promote blast damage mitigation and process safety.  Additionally, determining a damage threshold for a given amount of concentrated chemical explosives can guide process engineers in mitigating the potential for accidental detonations that pose a significant risk to the structural integrity of pressure containing equipment.

This paper discusses the underlying theory of blast analysis and examines the practical application of non-linear, finite element based, explicit computational techniques for simulating the load acting on a structure due to internal and external blasts.  The investigation of a recent, real-life industry failure of a heat exchanger due to a suspected internal detonation is discussed.  This particular failure resulted in gross rupture of the pressure boundary and significant damage to nearby equipment.  Explicit, three-dimensional blast analysis is performed on the heat exchanger in question, and an internal detonation is simulated to reasonably replicate the considerable damage actually observed in the field.  This analysis permits the determination of an approximate amount of concentrated product that caused the accidental explosion; that is, the plausible equivalent amount of explosives is back-calculated based on the predicted damage to the finite element model of the equipment in question.  Computational iterations of varying charge amounts are performed and the predicted amount of permanent damage is documented so sensitivity to the hypothesized charge amount can be quantified from a structural response standpoint.  Furthermore, explicit blast analysis of nearby equipment is performed to predict the magnitude of plastic deformation actually observed due to external blast loading. 

The primary computational tool used to generate realistic blast wave profiles in this study is based on the Conventional Weapons Effects Blast Loading Model or CONWEP.  The CONWEP blast loading model is based on a collection of conventional weapons effects calculations established by the United States Military.  The main advantage of using CONWEP is that realistic overpressure amplitudes and other blast wave parameters are calculated based on a defined charge amount, and the appropriate impulsive loading is automatically employed in the explicit finite element analysis.  Furthermore, spatial decay and blast wave reflection effects are also explicitly calculated and accounted for.  Conveniently, the CONWEP blast loading model does not require modeling the fluid medium to account for overpressure reflection, eliminating the need for coupled structural-acoustic analysis and offering a substantial improvement in computational efficiency, especially for large, three-dimensional simulations involving complex geometry.

In this study, computational results for both the heat exchanger (subjected to internal blast loading) and surrounding equipment (subjected to external blast loading) are in good agreement with the measured plastic deformations and failure modes that are actually observed in the field.  Commentary on the likely detonation event that lead to the damage observed in the field, supplemented by explicit computational finite element results, is provided.  Additionally, an advanced finite element failure criterion is employed where portions of the computational model are removed from the simulation once a certain strain threshold is reached.  This approach facilitates simulation of the gross heat exchanger pressure boundary failure actually observed in this case.  The explicit, three dimensional finite element based analyses discussed in this study reasonably predict the structural response and damage characteristics corresponding to a recent, real-life industry failure.