Health, Safety and Handling

THE EVOLVEMENT OF SOLIDS HANDLING TECHNOLOGY

Lyn Bates
Ajax Equipment Ltd
Milton Works Mule
Street Bolton BL2 2AR

1 INTRODUCTION

Solids Handling Technology is a multi-discipline science, with roots in civil and mechanical engineering, with stimulus from structural and chemical engineering. Its emergence as a single, comprehensive field of study followed a long gestation period in soil mechanics and a belated recognition in the bulk handling industry that particulate solids represented a fourth state of matter. The technology combines features from liquids, solids and gases, but embraces infinitely greater variation due to the interaction of a host of factors. The effect of mechanical, chemical, thermal, electrostatic and molecular features in a two or three phase media brings in further complications and the situation is made even more complex by the influence of scale, ambient conditions, industrial equipment factors and operating sequences. No wonder that the accumulation of technical knowledge took a long time to mature in order to bring a coherent structure to the subject. Any attack on a subject of this complexity must involve simplifying assumptions. Treating the media as a continuum is appropriate under certain conditions and this step allowed advances to be made.

2 HISTORY

Bulk solids have been handled and stored for thousands of years. One famous engineer from ancient times, Archimedes, calculated the number of grains of sand that would fill the then known universe as 1051, displaying a basic understanding of the packing characteristics of bulk solids. His perception of density being mass divided by displacement, regardless of container shape, is also applicable to particulate solids and their variable voidage condition.

His invention of using helical screws to pump water evolved to ubiquitous applications for handling of bulk solids. Evidence of grain stores in Babylon, ancient Egypt and through the Roman Empire show that relative large volumes of bulk materials have been shipped, stored and handled from antiquity. However, it is only in recent times, with the growth of cities and the industrial revolution, that concentrations of a wide range of loose solids have been held in large gravity flow structures other than by manual handling. Whereas grain is comparatively free flowing, many other mineral and processed products display difficult flow and handling characteristics. Solids handling remains a mature industry with an immature technology.

Modern solids handling technology has its roots in soil mechanics. The stability of bridges, buildings, earthworks, dams and military fortifications has attracted the attention of engineers for centuries. The work of Coulomb and Rankine on friction and Reynolds, who observed the dilatancy effects on sand during deformation are especially relevant to bulk solids flow. The one and only known paper of an obscure German engineer in Hamburg in 1895 was a landmark in solids handling technology, developing a theory to explain the findings of an English engineer, Isaacs Roberts of the effect of wall friction on silo wall pressures. By way of wood and glass models and simple calculus Janssen developed a theory of pressure distribution in grain silos that remains today the most widely used method of assessing forces on silo walls. One might question how many papers published in this year of high technology, will be widely quoted a hundred years from now?

The maturity of centuries of bulk technology progressed at a snail pace over the first half of the next century. Verification and refinements to Janssen followed, mainly being concerned with structural aspects of silo construction. Airy, Prante Toltz, Ketchum, Jamieson, Lufft, Pliessner, Bovey and many others, constructing experiments and calculations. Interestingly, the findings of Janssen were re-discovered by Shaxby in a joint paper with Evans, during experiments with powders. Hvorslev, examining the stability of cohesive soils, introduced the important concept of 'critical state' to the study of the failure characteristics of bulk material. He showed the peak stress at failure to be a function of the effective normal stress and the void ratio, and independent of the stress history of the material. The void ratio has a direct correlation with bulk density and hence provides a basis for specifying the fundamental strength potential of a compact. This concept is particularly important to an understanding of the mechanism of consolidation and flow of solids. Meanwhile, the concentration of industry, increase in the scale of production, and growing automation, highlighted the shortfall in progress in solids handling technology. The growth of mess and pollution, a thriving 'flow aids' industry and the increasing number and scale of silo failures reported by Theimer all pointed to remaining deficiencies in the technology.

3 THE BREAK THROUGH

By a quirk of history a young, Polish army officer stood on a hill in 1939, with Germans advancing up one side and Russian forces up the other. It was time to pack up fighting and seek refuge in England. Andrew Jenike studied for his engineering degree in London after the war and later settled in America, working for US steel. He decided to examine a range of industrial problems and collected all the information available on about thirty subjects, collecting all the information in separate boxes that he rooted through systematically. One day he suddenly made the crucial decision that the flow of solids was one of the most important problems of the day and, being a man of very positive action, immediately scrapped all the boxes of papers collected on other topics. He approached various universities before agreeing with Utah University to research on bulk solids flow for one dollar per year.

This must have been the best scientific bargain of the century. With the aid of a young student named Jerry Johanson they developed a theory, powder property measuring instrument and design methodology that offered a solution to the age old problem of designing a silo that would guarantee the reliable discharge of non-free flowing materials. His knowledge of Russian was fortunate in that he...