MILLION DOLLAR RATHOLE

[Music] in this day of complex automation scenes like this occur all the time at the push of a buggy material flows from the hopper whoops there it is again [Music] [Music] have a problem with raffling [Music] billions of dollars are poured into raffles every year extra man-hours extra equipment lost production time and just plain cussing are necessary to make material flow in bins and hoppers millions of tons of material are used by industry every year and most of the material is routed through storage bins and hoppers several times during handling and processing rattling and arching problems that cause erratic flow and reduce bin capacity are serious bottlenecks to the movement of materials it takes a special effort to design correctly a high-capacity bulk handling installation without flow problems even when been flow is maintained and hopper delivery is at the rated capacity particle segregation of the mixture causes delivery of the finds first then the heavier aggregate the mixture stored in the hopper is not delivered uniformly the flow from bins and hoppers has received a lot of attention many shapes and sizes of pins and hoppers are designed some with rough others with polished walls hopper delivery openings are made round elongated square bins and hoppers are designed square cylindrical cone shaped pyramid shaped rectangular like a box with a hole in it for material delivery and a combination of many shapes [Music] but unfortunately bins and hoppers have usually been designed for a given area or a rated delivery for a maximum required capacity without proper consideration of the material to be stored and delivered the flow characteristics of the material its compaction in storage the moisture changes that occur during storage or the effects of these changes have on the flow characteristics of the material when you attempt to remove it from storage a growing number of flowy devices have been put on the market such as air Lance's vibrators and even this blow problems of bins and hoppers can be solved by calculating the pressures exerted on the materials in the bins and by mathematical analysis to determine why funneling rattling and arching occur [Music] by analytically combining the flow criteria hopper pressures and flow properties the critical hopper dimensions for gravity flow can be calculated bins can be designed to remix the segregated granular materials when the bin is fed from a central delivery point and segregation occurs as a part of the central feeding laboratory scale models show the change in flow conditions with one percent moisture added to dry iron ore concentrate an opening of one sixteenth inch is all that is needed for the dry iron ore concentrate when moisture is added and arch is formed even when flow is started by pounding or prodding it is usually impossible to eliminate rat holing and the loss of bin capacity what has happened why does one percent moisture change the flow patterns and necessitate change in bin design iron ore concentrate and other bulk granular materials have a cohesive strength they will support their own weight they can be compacted to increase their strength Moisture modifies this property preventing greater compactness greater strength most bin and hopper loading results in material compacting when the bulk material has moisture the strength of the material is increased [Music] increased pressure and moisture content make a material stronger change the flow properties however there is a maximum strength the point beyond which normal pressures do not change the flow properties by determining the material strength at various pressures we are able to predict the flow of material from bins and hoppers this knowledge of the strength the bulk granular material permits us to establish the minimum size discharge opening that will permit flow for a given material this minimum opening is proportional to the strength of the bulk material flow properties of the bulk material can be readily measured each of the materials is subjected to a shear test under various pressures to determine the complete strength curve [Music] The Situation's encountered in the field are duplicated material left in the bin for days with changes in temperature chemical composition and moisture material strength depends on pressure and this pressure must be related to the bin or hopper at a place where an arch or rathole occurs everyone knows that the deeper you go in water the more pressure you encounter it would appear that a tall bin would have maximum pressure exerted in the area of delivery opening but it doesn't work that way granular solids cause a pressure distribution that increases for only a short distance from the top surface then decreases toward the bottom of the hopper at the hopper opening there was only a small pressure on the bulk solid [Music] after years of research and calculation this pressure has been computed as a function of the flow properties of the bulk material the hopper shape and the smoothness of the hopper walls now we have the answer to the million dollar rathole when we know the pressure on the bulk solid near the bin opening the strength of the material can be found from the strength versus pressure graph this strength determines whether arching or rattling will occur in the designed bin or hopper we know how to modify the basic design for gravity flow of materials the design capacity of the bin or hopper can be fully utilized an analysis of this charge opening shows that an arch will fail when the width of the slot is larger than the strength to density ratio the diameter of a circular opening or the width of a square opening must be larger than two times the strength to density ratio a rasshole will fail when the diameter of the rathole is greater than four times the strength to density ratio on this lab model the slotted gate is first opened to ask where the material arches over the opening as the slotted gate continues to open to a length to width ratio of about three to one the arch fails we have used iron ore concentrate with one percent moisture content as the bulk material however a stable rathole has now been formed that has a diameter about equal to the diagonal of the hopper opening this scene is typical of many full-scale installations and illustrates clearly that stable rat holes may form even after the arching problems have been eliminated you have probably noticed that rat holes very often form in flat bottom bins and hoppers with rough walls this is the result of the funnel type flow that develops when the material is discharged from these hoppers we can prevent this type of flow with smooth steep hopper walls the wall slope necessary for flow along the walls of a conical hopper can be determined from this ground wall friction has been plotted as the ordinate and wall slope as the abscissa the wall conditions identified by the blue area provide flow along the walls the wall conditions identified by the green area do not provide blow along the walls for a relatively smooth wall the flow pattern for both 70 and 50 degree cones can be compared a 70 degree cone according to the graph should have flow along the walls the 50 degree cone is outside this region and the flow therefore is confined to the central portion of the hopper [Music] as you see our laboratory models confirmed these flow patterns the material in the 70 degree cone moves as a mass with flow along the walls in the 50 degree cone there is no flow along the walls [Music] not all 70-degree cones provide necessary flow along the walls in case of a rough wall conditioned flow does not occur along the hopper walls the effect of rough walls on a material is clearly illustrated by these laboratory models both of these 70-degree cones have the same material in them but the rough wall forces a funneling flow pattern movement of material along the wall is restricted these same flow patterns were observed in this full-scale bin installation materials flowed through these bins without problems however holes began to appear in the hopper walls because of the sliding motion of the material along the smooth hopper walls for example let's consider this hopper section to reduce wear rings of angle iron were welded inside the hopper to trap material which would protect the walls from where the walls stopped wearing but unfortunately the material also stopped flowing especially along the walls some materials arched others formed stable rat holes and the capacity of the bin was greatly reduced a lot of material just stayed in the bin as would be expected under these wall conditions now the bin has replaceable smooth wear liners in the hopper sections flow problems have disappeared the design capacity of the bin is being fully utilized [Music] [Applause] the 70-degree conical hopper displays an effective flow pattern however conical hoppers require additional fabrication to form the curved surface to reduce cost many pyramidal hoppers are used because of their simple construction the pyramidal hopper usually has a funnel type flow pattern that may form stable rattles with sticky material the sharp corners of the pyramid trap material creating a rough walled cone mass flow can be achieved with the pyramid by eliminating sharp corners and designing steep enough walls mixtures containing two or more particle sizes present a special problem in bin and hopper design when the mixtures are Center loaded segregation occurs as part of the loading the large aggregate moves to the bin walls leaving a concentration of fines in the center as the material is drawn from the bin flow occurs only in the central portion where the fines are concentrated because the hopper walls are not steep enough the coarse particles come out last this segregation can result in severe quality control problems bins can be designed to remix the material a mass flow hopper with a tall vertical section permits the material in the vertical section to move as a mass the material is removed in the same order that it was deposited remix is accomplished in the hopper section the live anti-segregation capacity of the bin is that portion of the material above the hopper section when the material level drops below the vertical section the solids move faster in the central portion of the bin and segregation results [Music] the design criteria developed can be used to modify flow problems that may be present in existing installations [Music] segregation problems in non mass flow bins such as these 60-degree comb and pyramid shaped hoppers can be helped by using inserts the conical inserts change the flow pattern but are not large enough to cause arching [Music] the inserts changed the flow pattern to typical mass flow in the vertical portion of the bin a mixed delivery will be maintained until the material drops to the critical level then segregation will occur the position and size of the inserts can be calculated based on the flow properties of the bulk solid [Music] relatively free-flowing materials can be anti segregated in flat bottom bins provided a slotted opening or series of openings extends across the diameter of the bin the material level must be maintained high enough to ensure mass flow in the upper portion of the bin proper been designed based on the flow properties of the material readily solves the flow problems of even multiple hoppers such as used on these coal bunkers at the coke plant this Larry car above a coke battery is used to charge the coke ovens the hoppers at the coal bunkers are specially arranged to feed the four pockets of the Larry car the 60-degree pyramidal hoppers are typical of the hoppers used to control feed from these bins a model of this hopper illustrates the problem with the fine coals used today the stable rat holes are typical of those that develop in large bunkers [Music] consider as an alternate design these mass flow hoppers located so that their flow patterns join each other when the same coal with the same moisture content as before is drawn from these hoppers no rat holes occur and the full live capacity of the bunker is realized [Music] Ben and hopper material flow problems have been solved but now there are additional design factors to be considered in the past bins were designed on the basis of process requirements only the required process speed rate determines size and type of feeder and been opening necessary surge capacity determined the bin volume and the bin was usually fit into whatever space was available in the overall plant layout often the design that evolved had a discharge opening that caused arching or a hopper shape that caused stable rat holes the live capacity of the bin was only a fraction of bin volume segregation of mixtures occurred excessive wear with abrasive materials resulted when wear liners were eliminated to keep capital costs to a minimum and hoppers had to be replaced in a short period of time and bins and hoppers that were constructed cost industry millions of dollars every year just to keep material flowing from hoppers and to stop production shut down economics and capital investment cost will always be primary factors in bin and hopper design just as they are with other capital equipment [Music] but it is never economically sound to build a machine a building a crane or a bin and hopper that will not do the job it was intended to do [Music] by considering the material to be stored and delivered from the hopper its strength density the hopper delivery opening necessary friction versus wall slope the flow pattern desired as well as the process requirements of the installation bins and hoppers can be economically designed without the million dollar rathole [Music]