Lessons learned from a milling explosion

Abstract

There are many sources of mechanical energy in agricultural processing facilities. This is particularly true when milling is performed. If mechanical energy turns into sufficient thermal energy (heat), it can cause material to smolder. If that smoldering material is fluidized, e.g., through pneumatic conveyance or air/material separation, a deflagration may occur.A case study of an explosion at a wheat milling facility is used to illustrate the hazard. Fugitive grain dust played no role in the event, and no secondary explosion occurred. Although the equipment involved in this incident was vented in accordance with NFPA guidance, a significant portion of the facility was destroyed in the explosion. The severity of the resulting explosion damage resulted, in part, due to the initial conditions assumed in this typical explosion vent design, which did not consider the effects of flame acceleration and pressure piling upstream of the vented filter house.First, we present a basic overview of milling operations, particularly equipment involved in material separation and size reduction. Then we detail the investigation of this incident. Equipment damage, witness observations, and material properties are used to determine the cause. Finally, we discuss lessons learned. Particular attention is paid to milling safeguards and the effects of flame acceleration in the context of the damage observed.