A DESIGN PROCEDURE TO PREVENT PIPING IN THE AERATED DISCHARGE OF COHESIVE POWDERS FROM A SILO

The formation of a stable pipe or rathole can occur during the silo discharge of cohesive powders. when the inclination of the hopper is not steep enough to guarantee mass-flow and when the outlet size is too small. To disrupt the pipe and to induce powder flow in a wider region of the silo (either in funnel flow or in mass flow regime), flow aid devices, such as aeration pads, are commonly used in the industry. However, design criteria are not available yet for such a technology Piping occurs when the strength of the consolidated material in the dead zones exceeds the load stress and a stable pipe is formed. Jenike Errore. L'origine riferimento non è stata trovata. and Jenike and Yen 0 proposed a relationship between the diameter of a stable pipe and the properties of the compressed powder, namely the bulk density and the angle of internal friction. This theory is substantially unquestioned and based on the evaluation of the critically stable conditions, that are the conditions in which the pipe is on the verge of its failure. More recently, Johanson [3] proposed a modification of the Jenike theory for the case of aerated silos. The stability of the pipe was analyzed accounting also for the effective components of the gas pressure gradient through the powder bed. However, the limits of the latter analysis was its application to non realistic values of the pressure gradient and the lack of a closure of the model to estimate the air flow rate necessary to cause the pipe collapse. In this work the pipe stability analysis for aerated powders was completed. The stability criterion was derived comparing the strength of the powder and the overall hoop stress acting in the powder due to the gravity and the gas pressure gradient. Aeration was accounted for also in the estimate of the effective consolidation of the powder. The effective components of the gas pressure gradient were calculated as a function of the air flow rate injected from the silo bottom by means of a simple permeation model based on the Darcy law. The solution of the gas pressure field through the porous powder bed was obtained numerically and used to derive a general correlation between the injected air flow rate and the radial component of the gas pressure gradient. Alternatively, an analytical solution of an approximate model was also proposed. The results of the complete model in terms of air flow rate values necessary for the pipe collapse were finally compared with the experimental values published in a previous paper [4]. References [1] Jenike, A.W. Bulletin of the University of Utah N.108 - Vol 53, 1961 [2] Jenike, A.W. and Yen B.C. Proc. 5th Rock Mechanics Symposium, 1963 p689-711 [3] Johanson, K. Powder Technol. 2004; 141: 161-170 (2004)