Pressures in hoppers filled with fine powders

Abstract

In a previous paper [18] the authors showed that air pressures occurred between the grains of fine powders when they were loaded into hoppers and presented data where the pore air pressure was 60% of the total stress measured at the bunker wall when the hopper was first loaded. The paper included a mathematical model to describe the dissipation with time of these pore air pressures. The model was used to scale up from a bench scale deaeration test to give the rate of air pressure dissipation in a large scale hopper, which was broadly in agreement with that observed. The present paper continues the work, but the emphasis is experimental. The total stresses measured in large scale hoppers of various configuration and dimensions when first loaded with a fine powder are demonstrated to be hydrostatic in nature. The total stresses and pore air pressures near the wall during discharge of a fine powder are then presented. Different types of discharge are considered, namely mass flow, flow from eccentric outlets, and vibration-assisted flow. The total stress is observed to fluctuate with time, but these fluctuations are generally less regular and of lower magnitude than those observed in mass flow with free-flowing materials, such as sand. The total stresses on discharge by mass flow may rise above the initial hydrostatic stresses but it is shown that for the fine particulate material considered, the hydrostatic stress represents a reasonable approximation for one of the loading cases that should be considered during design.