(71at) Predicting Far Field Blast Overpressures: An Evaluation of Enhanced Input Parameters for FLACS

Vapor cloud explosions (VCEs) are considered one of the main hazards within process safety, considering the destructive potential of blast waves and the possibility of escalation (i.e.  subsequent jet fire).  In the past decade, the US has experienced severe onshore accidents involving explosion events and BP Texas City (2005) was one of the worst industrial accidents in US history. These types of consequence events have raised the safety concern for the entire onshore safety industry.  Predicting potential explosion overpressure is now a recommended practice for new and existing facilities. Until recently, evaluating potential explosion hazards for onshore facilities has mostly been performed utilizing simplistic models such as TNO and BST.

Empirical and phenomenological models (e.g., TNO Multi-Energy Method and Baker-Strehlow-Tang (BST) Method) can to some extent give useful estimation of VCE consequences for simple explosions and crude geometries.  Empirical models utilize correlations derived from experimental measurements, whereas phenomenological models employ some methods to represent primary physical processes during the explosion (PHAST).

Alternatively, Computational Fluid Dynamics (CFD) models are able to provide more precise predictions of the energy and resulting overpressure of the blast wave, as well as the ability to consider more realistic physical geometries, gas cloud variations and ignition locations.  CFD models can also present detailed dynamic effects in the near and far field, including pressure changes versus time, reflected and redirected blast waves.  FLACS is one of the most widely used CFD-tools for explosion modeling.  Continually improved from its inception in the 1980s, the primary goal was to predict vapor cloud explosion loads in offshore platforms.  As onshore safety restrictions become more stringent, FLACS is becoming an essential tool for onshore safety studies and, in particular, for evaluation of VCE-related overpressure. However, care should be taken when applying FLACS for onshore VCE studies. Investigations into far-field blasts have shown FLACS calculations often underpredict far-field blast pressures when compared to TNO blast curves.  Enhanced parameter settings have been developed for the FLACS code and subsequent studies have suggested that more accurate far-field blast predictions are possible.  These modified parameters will be utilized, compared, and evaluated for discussion in this paper.

In this study, CFD and TNO models are used to predict onshore far-field blast pressure in under- congested areas.  PHAST (Process Hazard Analysis Software Tool) is employed to generate explosion predictions with the simpler models.  Based on the available guidelines, FLACS model parameters are set to resolve the blast waves.  Several explosion scenarios have been simulated with the aforementioned simulation tools PHAST and FLACS and the results discussed.