10 Common Questions about Storage Hoppers
Q - Most of the hoppers I come across suffer ‘Hammer Rash’ or have other
problems. Is it true that a hopper should always be of Mass Flow design to work best?
A - No. A Mass Flow hopper has certain benefits, but it also has drawbacks. The choice of flow pattern is the first and most important decision to make in hopper selection therefore it is vital to base it on the correct reasons. The key feature of Mass Flow is that all the contents are in motion towards the outlet during the discharge process. This pattern carries three, separate advantages. One, the absence of static regions of material during out-flow means that there are no pockets of stagnant storage that may deteriorate in condition because they remains undisturbed for an indefinite period. This is a particularly serious hazard if the hopper is re-filled before it completely empties. The second major benefit of Mass Flow is that bulk materials flow better and more consistently in a Mass Flow hopper. Difficult flow materials will reliably pass through smaller outlets than they would in a non-mass flow hopper and three, material cannot ‘rathole’ because wall slip removes the foundation for a stable bed to form and discharge in a more uniform density.
The great drawback of employing a Mass Flow design is that the steep walls required to promote wall slip restricts the vessels holding capacity, so that a taller container is needed to hold a given volume or less storage space is available in a limited headroom. The general criterion is that unless the features offered by Mass Flow are essential or outweigh the headroom penalty, it is not necessary to specify Mass Flow for a duty. See Table of Advantages and Drawbacks of Mass Flow
Q - The above being the case and knowing the nature of the bulk material, I can decide if Mass Flow is needed to avoid indefinite storage periods, but how can I determine when Mass Flow is required to ease flow problems?
A - The most common flow problems are ‘Arching’, (sometimes called ‘bridging’) and ‘Ratholing’, (sometimes called ‘Piping’). These terms describe the shape taken by the unconfined flow boundary condition of a material that will not flow by gravity when the hopper outlet is open. This can be caused by lumps coming together to form a stable structure, (See notes on jamming), but is more usually due to the cohesive nature of the bulk material, in which case, the static material spans completely across the flow channel over the outlet. A ‘Rathole’ or ‘Pipe’ develops when an initial narrow flow channel empties right up to the surface of the stored volume, but no further material collapses into the opening. The ability of the bulk material to sustain a stable condition when there is no restraining surface is due to it ‘unconfined shear strength’. This property is inherent to the nature of the bulk material but is dependent upon the degree to which the bulk is compacted. Obviously, the self-weight of the hopper contents bears on the material in storage, but it is not always obvious how intense this is or how it influences the gain in strength of the bulk. It may be clear from inspection or experience that there is, or is not, likely to be flow difficulties. Unless there is unequivocal evidence that there is absolutely no prospect of potential flow problems it is prudent to examine a representative sample of the bulk material under loading conditions that reflect the magnitude and duration of the forces that will be experienced in practice.
Some specialised help may be needed for this exercise, but this is the time to find out whether there is likely to be a problem, not during commissioning. Mass flow is also needed when the material has a time dependent characteristic, such as a tendency to ‘cake’ or degrade. ‘First-in, first-out’ prevents indeterminate storage times.
Q - What other patterns of flow can prevail, other than Mass Flow, and does Mass
Flow really give ‘First-in, first-out’?
A - The answer to the second part of the question is that no converging flow channel offers totally uniform flow velocity across its cross section, therefore the strict sequence of loading order cannot be preserved during discharge. There is inevitably faster movement in the region directly above the outlet and slower motion against offset walls. One effect of this differential flow rate is that any segregation that takes place during the filling of the hopper is not fully redressed at discharge, even though the effect may be greatly mitigated during the main period of flow. The last portion out will tend to suffer the most segregation because it encompasses the initial peripheral regions of the stored volume.
As for flow patterns generally, a variety of ‘Global flow regimes’ may be formed from compositions of three basic flow components, - ‘Bed flow’, ‘Repose flow’ and ‘’Converging flow’. As the name suggests, ‘Bed flow’ consists of the movement of a bed of material that does not change in cross section. The flow within the bed is not necessarily uniform, although coherent motion does often occur in the parallel body section of many tall, Mass Flow type hoppers.
Differential flow contours are created by wall frictional drag or the imposition of preferential draw down by underlying material resulting from uneven feeder extraction or funnel flow hopper geometry.
‘Repose flow’ is the action of surface layers ‘Pouring’ or ‘Draining’ over a static bed of product. The top surface of hopper contents that is filled at a single point tends to form a conical pile that reflects the ‘Angle of Poured Repose’ of the material. This is an inherent characteristic of the bulk material, but it does not have a direct relationship with how the material will flow from a hopper outlet. The inclination of this slope may vary with the manner of formation and, as with many fine powders, may not form a pile at all. The ‘angle of repose’ is only meaningful when a consistent value is achieved, and its main purpose is for the calculation of volume of a storage pile or ‘ullage’ lost due to regions in the top portions of storage containers that cannot be totally filled. It may be noted that the depth of moving bed in a drained angle of repose deepens with reduction of radius for two reasons. The converging path focuses the bed material from the outer radii of a collecting area, so this has to travel faster and/or pile deeper to move inwards. Secondly, the surface contour of a ‘Drained Repose’ channel clears an equal layer across the diameter of flow, so extra material is added at every stage of the inwards flow. A further consequence of this pattern is that the surface becomes flatter at increasing diameter, so whilst the angle of convergence is identical, the particle interference is less at greater radii. A planar angle of repose is therefore shallower than a drained repose situation and the latter become steeper with reducing radii. This may only be of academic interest except for level indication reasons.
‘Converging flow’ takes place in flow channels that are bounded either by walls or a bed of static product. As far as the flowing material is concerned, the only difference between these boundary conditions is the degree of resistance offered by the contact surface. This, combined with the amount of work necessary to deform the bulk, comprises the amount of energy required to sustain flow. As the change of potential energy is the only source of power for gravity flow, the bulk density of the material is a crucial factor in relation to overcoming these two ways in which work is dissipated for flow to take place.
Q - What are these Global flow regimes?
A - The simplest to describe is ‘Mass Flow’, which may either be a simple ‘Converging flow’ channel, in which all the material is in motion, or comprise a combination of ‘Bed flow’ in the body section of a hopper that has the ‘Converging flow’ region underneath to the final outlet.
There is a whole range of ‘Non-Mass Flow’ forms of global flow regimes.
‘Funnel Flow’ type has a narrow flow channel leading upward from the hopper outlet that is supplied with material from a ‘Drained flow’ system from the surface layers of the static bed. The internal flow channel tends to expand gradually through the bed, to draw from a cross section that is replenished by material that drains down the surface of a repose cone. The draining cone region typically expands to the walls of the container and the conical surface moves down steadily and uniformly as the hopper emptied. A feature of this process is that newly entered material flows through to the outlet before any of the previous contents can empty.
‘Mixed Flow’ variation of ‘Funnel flow’ occurs when the converging flow region expands to meet the container walls below the surface level of the stored material. Contents above this level then move down as a local ‘Bed flow pattern’ and there is no region of cone repose formed. The way in which the total surface moves down may give the impression that the hopper is working in Mass Flow, but the presence of submerged static regions belie this false assumption. When the surface falls below the level at which the converging flow channel met the walls the surface develops a clear ‘drained repose’ pattern.
A useful technique to deal with poor flow products that do not deteriorate with time is called ‘Expanded flow’. This design exploits the flow benefits of local Mass Flow, without sacrificing the headroom penalty of total Mass Flow. This approach employs a Mass Flow section adjacent to the hopper outlet that continues up to a size of cross section that is too large to sustain an arch or rathole. Above this level it may be permissible to use walls of a less steep inclination to secure increased holding capacity, provided this material will eventually self-clear into the outlet region when the central region empties.
Q - How do I design a Mass flow section and establish the safe width at which a non-Mass flow wall angle can be used?
A - There are four parts to this design process.
Assuming that the basic geometric form has been decided, the first step is to verify whether that a flow situation can be stimulated from a first-fill condition.
This is because the Jenike hopper design method only applies to a condition where flow has been initiated in the bulk material, but not when the hopper or silo has been filled without any discharge having taken place. Only a tiny amount needs to be extracted at an early stage of fill to develop the stress situation in the outlet region that fits in with the Jenike flow analysis. If flow will not take place before the hopper attains a deep bed, see note on ‘The value of the vertical shear cell’ or consider a flow aid device to commence initial flow, See notes on ‘Flow aids’.
Provided that flow has been, or can be, developed, the Jenike method should be followed. The next step is to determine the orifice size needed for the outlet to guarantee that reliable flow can be sustained. The Following step is to fix the inclination of the mass flow wall angle and finally, to calculate the overall size of the hopper, select the transition width and height of body to suit the capacity required. (If an ‘expanded flow’ type is to be employed, there are further steps of establishing the self clearing angle for the upper region, making sure the transition from mass flow to self-clearing is at a larger size than the ‘critical rathole dimension’).
It is necessary to conduct a series of shear and wall friction tests on the bulk material and follow a design procedure, as originally set out by Andrew Jenike, to establish the required orifice size. The calculations take account of the shape of the opening and the geometry of the hopper. Details of this method is given in a publication by the Institution of Chemical Engineers entitled ‘Standard Shear Testing Technique for Particulate Solids using the Jenike Shear Cell”, (SSTT), and also in the ASTM Standard D 6128-00.
The procedure for designing for ‘first-fill’ conditions should follow the Johanson technique with his ‘Indisizer’ or the Ajax ‘Vertical Shear Cell’.
You should be aware that the testing of powders and interpretation of the results is demanding of technical skills, therefore this full work normally rests in the domain of specialists. By contrast, wall friction measurements are relatively easy to conduct and these measurements are invaluable for selecting an optimum contact surface, establishing a reliable self-clearing angle for chutes and hopper walls and determining the slope of walls needed to generate mass flow. No similar short cut or easy method is available to predict a proven orifice size that will guarantee reliable flow. Some consolation is that a range of retrofit actions may be taken to stimulate discharge, but the situation is much more difficult to correct if the wall slope is inadequate.
Q - What are the relative merits of different hopper shapes?
A- Pyramid shaped base sections are easy to fabricate, nest well and provide maximum holding capacity is a rectangular cross section. The flat sides do not provide good resistance to containment pressures, so usually require stiffening. However, their main drawback is that they offer poor flow characteristics because of the retention characteristics and reduced slope of the gullies compared with smooth walls. These features virtually inhibit their potential for Mass Flow because it is almost impossible to generate total slip in the corners and the steep wall inclination required for the gullies tend to impose uneconomic use of headroom. For these reasons, pyramid shaped hoppers tends to be used for rough duties, such as the storage of minerals, and applications where solids flow and extended residence periods do not present any difficulty.
Cones offer simplicity of construction and for interfacing with a circular body section. They also provide excellent internal pressure resisting characteristics compared with flat surfaces. This is a useful asset for bearing storage pressures, and particularly so should ambient design or such requirements as explosion containment impose additional pressures, even if the vessel is vented or the explosion suppressed. For these reasons, a conical form of hopper construction is popular with manufacturers of standard hoppers and large silos. Unfortunately, a cone is not a particularly good shape for flow because the material is required to converge in both the radial and circumferential direction and the reduction in circumference is Pi times that of the change in diameter during flow. This leads to the generation of a hoop stress in the bulk and accounts for why a stable rathole can form around a hole in the mass above a hopper outlet.
Vee, or Wedge shaped base sections offer a better flow shape than a cone because the moving material is only required to converge in one plane. Wall angles are therefore typically 10 degree lower that those required for flow in a cone and the material will discharge through a slot width approximately half the diameter needed by a circular outlet. The holding capacity of the hopper section is also greater, ever than that available from a cone of equivalent height, but further enhanced by the use of reduced wall angles. The big snag with a slot outlet is that a feeder is usually needed to extract material from the whole length of the outlet slot and this feeder must provide ‘live’ flow over the whole length to enable Mass Flow to take place from the hopper. As previously pointed out, Mass Flow is not always essential and in those circumstances the extraction pattern generated by the discharge device is not so critical, as long as the whole contents of the hopper can be emptied. This point emphasises that the design of a feeder is an integral feature of a hopper and their specification must be determined together, rather than planning a hopper and selecting a feeder to fit.
Special hopper shapes have been developed to enhance flow potential. Using radius corners in place of sharp corners invariably results in improved flow behaviour but far greater benefit is given by altering the flow channel shape A proprietary form of construction, termed ‘Diamondback’ hopper, essentially comprises of two wedge-shaped hoppers with radius corners in series. These cause the material to converge via two plane flow channels in sequence, to a circular outlet that offers similar flow reliability to a slot outlet that has a width equal to the diameter of the ‘Diamondback’ hopper outlet. Non-proprietary design variations can be employed that ‘play tunes’ with different options. A technique evolved by Ajax Equipment Ltd, known as ‘Sigma Two Relief’, relaxes the confining walls at 90 degrees to the main converging flow. Deformation of the bulk is eased and flow enabled though walls that are less steep and from narrower openings than is possible with conventional construction.
Q - How should I make more than one outlet on a hopper
A - Well, what you should not do without detailed consideration, is make extra necks on the sloping sides, above the lower outlet. Apart from leaving dead pockets of material this gives rise to eccentric flow patterns that can have serious structural implications and would certainly negate the operation of a mass flow design. The best way is to split the final outlet or arrange the discharge mechanism to reverse, bearing in mind that this can normally only serve a short outlet. A technique developed at Ajax Equipment employs two, cantilever screws that allows material to be extracted to either of two outlets, or both outlets at the same time.
A special form of flight construction by Ajax extends the live-flow reversing capacity of either a single or axial twin screw of this type by up to four times the outlet width.
Q - How long can I make the outlet of a slot shaped hopper outlet.
A - This depends largely on the extraction circumstances, particularly for a mass flow hopper that demands ‘live’ flow from the total cross sectional area of the hopper outlet. With an unrestricted outlet there is no theoretical limit, but practical considerations apply to the outlet control and where the material goes. A screw uniform in pitch and diameter will only take product from the initial flight region, but may extract from a longer slot if the product is free flowing and ‘dead; regions of storage is acceptable. This situation is limited by a realistic span of the screw and powder needs. A well designed extraction screw can extend the ‘live’ draw length to about six to eight times the screw diameter, but the rate will be uneven at different regions and long slots should be served by specialists.
Belts are commonly used and can ultimately extract from extensive lengths of openings, if ‘dead’ regions of storage and power demands are acceptable. Tapering the outlet width allows a degree of progressive extraction, but large changes in width can result in large loads being imposed on the belt.
Q - What is the best way to avoid Segregation in a hopper?
A - Most segregation mechanisms operate during the filling process due to the dilate condition in which bulk material is handled and deposited and the various forces that act on the in-feed stream that cause differential routes to be taken by the various fractions of composition. It follows that any means that inhibits the freedom of the constituent particles to move relative to its neighbours will reduce the potential for segregation to occur. The use of distributed fill or multi-point loading will limit the degree of repose flow that takes place, and thereby eliminate some opportunities for segregation to occur. Flow inserts and mechanical devices can be used to diffuse the material during filling, but some care is needed to ensure that the results reduce segregation and do not exacerbate it or other undesirable consequences.
A Mass Flow form of discharge will mitigate much of the segregation that has taken place during filling of the hopper, but generally tends to concentrate some coarser fractions in the terminal portion of material to discharge. Very peculiar effects can be found in funnel flow hoppers that are refilled whilst discharging or before the hopper is completely emptied. More detailed descriptions of this special hazard is given in ‘User Guide to Segregation’, published by the British Materials Handling Board.
Various forms of flow inserts can be used to reduce segregation, amongst their many other uses. Whilst these fittings can be very effective, this is not a recommended approach for amateurs in the field, except perhaps for controlled experimentation in small installations where flow pressures and the consequence of failures are trivial. Specialist can advise on their use and this type of facility is especially valuable in retrofit situations, where existing plant suffers serious operating problems.
Q - How can I find the bulk density of a material to calculate the capacity of a hopper
and work out its overall weight?
A - Don’t bother. Visit the web site and submit your data on the forms provided. The answers will pop up in an instance.
Q - How do I characterise a bulk material for flow in a hopper?
A - There is no single measurement or value that will give an unequivocal reference because in any given situation there are many different requirements to satisfy.
These can be conveniently separated into two lists. The first is a register of the physical properties of the bulk material, each of which can be independently measured and related to specific functional aspects of hopper performance. The second list is an assembly of the functional or value attributes of the bulk material that need to be accommodated within the equipment. Some of these qualities are difficult to quantify and the decision as to suitability resting in some cases entirely with the judgement of a user. Examples of limiting grades for acceptance/rejection may be required, with a qualifying description. Nevertheless, it is important to ensure that both operating conditions and final product condition meet the aspirations as well as the essential needs of a user so all relevant circumstances have to be taken into account. A graphical means of showing the multiple characteristics in a single diagram is the ‘Spider diagram’, developed by Eddie McGee, Technical Director of Ajax Equipment.
