Vent sizing in partial-volume deflagrations and its application to the case of spray dryers

Abstract

Abstract The need to provide explosion venting in spray dryers handling combustible dusts has been recognized by the industry. It is generally accepted that the severity of accidental explosions in this type of equipment is mitigated by the fact that the combustible mixture occupies only a fraction of the available volume. The objective of this work was to determine the degree to which vent sizes for spray dryers can be reduced relative to the amounts prescribed by guidelines (VDI 3673, NFPA 68) based on near worst-case full-volume deflagrations (FVD). A model, originally developed to calculate the venting required in FVDs, was modified to account for partial volume effects and was used to evaluate previous work on vented partial volume deflagrations (PVD). The analysis of published data on vented PVDs has revealed inconsistencies which are, at least in part, attributable to inaccuracies in the characterization of the actual filled fraction. Furthermore, the analysis has shown that the use of volume scaling in PVD vent sizing (i.e. vent area calculated on the basis of the volume of the exploding mixture) can seriously underestimate the required area. The PVD model, on the other hand, is found to provide a sound basis for the scaling of the vent area as a function of the filled fraction. As an example, the model predicts that, in the case of low-strength enclosures, there is a relatively weak dependence of the required vent area on the filled fraction. This finding is confirmed by data obtained at Factory Mutual Research Corporation (FMRC) from tests in a 63.7-m3 (2250-ft3) chamber. The paper gives an outline of the model and its implications when applied to the design of vents of spray drying equipment.