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Answers to 10 Common Questions about Storage Hoppers

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[b]Q[/b] Is it true that a hopper should always be of Mass Flow design to work best?

[b]A[/b] 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 two, 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 can be 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 in a Mass Flow hopper. Difficult flow materials will reliably pass through smaller outlets than they would in a non-mass flow hopper and material cannot rathole because wall slip removes the foundation for a stable bed to form.

The great drawback of employing a Mass Flow design is that the steep wall inclination 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 within limited headroom. The general criteria 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 the duty. Various operational virtues and limitations flow from these features as listed in [b]Table 1[/b]. Typical applications of each type are identified in [b]Table 2[/b].

[b]Q[/b] 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?

[b]A[/b] The most common flow problems are [i]Arches[/i], (sometimes called [i]bridges[/i]) and [i]Ratholes[/i], (sometimes called [i]Pipes[/i]). These terms describe the shape taken by material that will not flow by gravity when the hopper outlet is open. In the first case, the static material spans completely across the flow channel over the outlet. A [i]Rathole[/i] or [i]Pipe[/i] 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 [i]cliff[/i] condition when there is no restraining surface is due to its [i]unconfined shear strength[/i]. This property is inherent to the nature of the bulk material but the effect depends 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. However, 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 service. 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.

[b]Q[/b] What other patterns of flow can prevail, other than Mass Flow, and does Mass Flow really give [i]First-in, first-out[/i]?

[b]A[/b] The answer to the second part of the question is that no converging flow channel offers uniform flow velocity across its cross section, therefore the strict sequence of loading order cannot be preserved during the discharge of a Mass Flow hopper. 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 peripheral regions of the stored volume.

As for flow patterns generally, a variety of [i]Global flow regimes[/i] may be formed from compositions of three basic flow components, - [i]Bed Flow[/i], [i]Repose Flow’[/i]and [i]Converging Flow[/i]. As the name suggests, [i]Bed Flow[/i] 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 hoppers. Differential flow contours may be created by wall frictional drag or the imposition of preferential draw down by underlying material but the essential nature of this flow mode is that internal deformation of the mass is not essential for the material to move.

[i]Repose Flow[/i] is the action of surface layers [i]Pouring[/i] or [i]Draining[/i] 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 [i]Angle of Poured Repose[/i] 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 [i]Angle of Repose[/i] is only meaningful when a consistent value is achieved. Its main purpose is for the calculation of volume of a storage pile or of [i]ullage[/i], i.e., inaccessible top regions in a container that cannot be filled due to the repose slope adopted by the bulk material.

[i]Converging Flow[/i] 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.

[b]Q[/b] What are these Global flow regimes?

[b]A[/b] The simplest overall flow pattern to describe is [i]Mass Flow[/i]. This may either be a simple [i]Converging Flow[/i] channel or comprise a combination of [i]Bed Flow[/i] in the body section of a hopper with a [i]Converging Flow[/i] region underneath that leads to the final outlet. This pattern is characterised by the total movement of the contents during the discharge process.

There is a whole range of [i]Non-Mass Flow[/i] forms of global flow regimes.

A [i]Funnel Flow[/i] pattern has a narrow flow channel leading up from the hopper outlet, which is replenished by a [i]Drained Flow[/i] system collecting material 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 progressively expands to the walls of the container and the conical surface then moves down steadily and uniformly as the hopper empties. A feature of this process is that newly entered material flows through to the outlet before any of the previous contents can empty. Many forms of [i]Eccentric Flow’[/i]occur, but these introduce structural complications.

A [i]Mixed Flow[/i] variation of [i]Funnel Flow[/i] occurs when the converging flow region expands to meet the container walls below the storage level. Contents above this point move down as a local [i]Bed Flow pattern[/i] without the formation of a [i]drained repose cone[/i]. The even 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. A [i]drained repose cone[/i] develops as the surface level of storage falls towards the point where the converging flow channel meets the walls.

A useful technique combination pattern is called [i]Expanded flow[/i]. This exploits the flow benefits of local Mass Flow, without incurring the headroom penalty of total Mass Flow. This design 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. Walls of a less steep inclination are used above this level to secure increased holding capacity, the wall slope allowing the material surrounding the flow channel to eventually self-clear when the central region empties. If the flow channel expands to the wall under the stored surface, the result is a [i]Mixed, Expanded Flow[/i].

[b]Q[/b] How do I design a Mass Flow section and establish the safe width at which a Non-Mass Flow wall angle can be used?

[b]A[/b] There is three parts to this design process. The first is to determine the orifice size needed for the outlet, second, fix the inclination of the mass flow wall angle and finally calculate the transition width and self clearing angle for the upper region.

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 [i]Standard Shear Testing Technique for Particulate Solids using the Jenike Shear Cell[/i], (SSTT), and also in the ASTM Standard D 6128-00. You should be aware that the testing and interpretation is demanding of technical skills, therefore this full work normally rests in the domain of specialists. Wall friction measurements themselves are relatively easy to conduct and these measurements are invaluable for selecting an optimum contact surface, determining a reliable self-clearing angle for chutes and hopper walls and giving a good guide to the slope of walls needed to generate mass flow. No similar short cuts are 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 outflow, but the situation is much more difficult to correct if the wall angle is wrong.

[b]Q[/b] What are the relative merits of different hopper shapes?

[b]A[/b] Pyramid shaped base sections are easy to fabricate but 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. The reduction in circumference is over three times that of the change in diameter. 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.

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 [i]Diamondback[/i] 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 [i]Diamondback[/i] hopper outlet. Non-proprietary design variations can be employed that [i]play tunes[/i] with different options. A technique evolved by Ajax Equipment Ltd, known as [i]Sigma Two Relief[/i], 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.


[b]Q[/b] What is the best way to avoid Segregation in a hopper?

[b]A[/b] Most segregation mechanisms prevail 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 also 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 introduce 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 non-mass flow hoppers that are refilled during discharge or before the hopper is completely emptied. More detailed descriptions of this behaviour is given in my book [i]User Guide to Segregation[/i], 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.

[b]Q[/b] How can I work out the holding volume of a hopper, taking account of surface repose conditions of the material in various cross sectional shapes?

[b]A[/b] Don’t bother. Visit the web site http://www.ajax.co.uk and submit your data on the forms provided. The answers will pop up in an instance. You can also find the Bulk Density for hundreds of common materials and other forms to calculate for you the surface area and weights of hoppers of different shapes and wall thickness.

[b]Q[/b] How do I characterise a bulk material for flow in a hopper?

[b]A[/b] 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 requirements 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 natural attributes of the bulk material that need to be accommodated within the equipment. Some of these qualities are difficult to quantify, the decision as to suitability resting in some cases entirely with the judgement of a user. 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.

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Physical Properties of Bulk Materials relevant to Storage

The most important are marked (*).

(*) Bulk density of the material in the state relevant to its activity.
(*) Wall friction with respect to a specific contact surface material and finish.
The porosity of the bulk in a given condition of density.
The tensile strength of the mass in a given state of compaction.
The cohesive strength of the bulk in a given condition.
(*) The shear strength of the bulk in varied states of compaction and normal loading.

Attributes of a Bulk Material Relative to - Storage, Personnel, Equipment Wear, Safety,
Environmental issues.

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  [b]Property                          Effect                                          Typical Material[/b]

  Packs under pressure     more difficult to flow               Hydrated lime, pigments

  Abrasive                          wears plant                               sand, aggregate, crystals

  Corrosive                         attacks surfaces                       salt, acidic chemicals

  Friable                             delicate handling needed         tea, coffee granules

  Explosive                         contain, inert,                            coal dust, flour,
                                            suppress or vent                       aluminium powder

  Flammable                       protect                                        wood shavings

  Dusty                               fugitive                                     cement, fly ash

  Wet                                 sticky, may dry to [i]cake[/i]           filter & centrifuge cake

  Sticky                              adhesive to surfaces               damp & fatty products.

  Hygroscopic                  becomes sticky                         sugar, soda ash

  Deliquescent                  dissolves to solution

  Noxious                           objectionable to personnel    sewage sludge, irritants

  Toxic                               dangerous to personnel          arsenic, active drug

  Degradable                     loss of value, contaminant     organic products

  Ultra-pure                        requires sanitary handling      drugs, meat. fish.

  Carcinogenic                  requires total containment      asbestos

  Sharp, irritant,                 Personnel protection needed
  hazardous,
  extreme temperature

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Attributes that may influence the quality or acceptability of a product.


Particle size or shape

Appearance - Colour, [i]sharpness[/i]

Texture - [i]Feel[/i] or taste

Uniformity, consistency

Moisture content

Its flow condition for subsequent handling

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These lists are not exhaustive, but are indicative of features that may dictate certain design requirements.



[b]Q[/b] How can I stop material from [i]flushing[/i] through the outlet of my hopper?

[b]A[/b] The material [i]flushes[/i] because it is in a very dilate condition, probably because it has been dropped into the hopper into a loose condition and is composed of fine particles such that the bulk tends to retain excess air in the voids for a considerable period. As a result, the void atmosphere sustains much of the storage pressure and prevents the bulk from developing shear strength by strong particle-to-particle interference. Loose material in this condition can behave like a liquid and is very searching. It will not be restrained by discharge screws, belt feeders or by any device or container that has a gap through to an external region.

One method of containing material in this condition is to use a positive seal mechanism, such as a rotary valve or an air lock system. As it is virtually impossible to prevent bulk material from dilating during unconfined flow when filling a hopper, a method commonly adopted is to contain the product for a sufficient residence time that will allow the excess air to escape. The material can then settle to a stable flow condition before commencing to discharge. Unless sufficient time is allowed to elapse between filling and starting to empty the hopper, a Mass Flow type of hopper should be used. This must retain a [i]heel[/i] of material to seal the outlet and provide the required residence time for fresh material to de-aerate before reaching the outlet.

Do not depend upon the level of material in a mass flow hopper reducing at a uniform rate when assessing the period needed for the material to achieve a stable flow condition. The hydrostatic pressure of a material in a fluid condition will tend to reinforce any preferential draw down potential by overcoming the transverse flow stresses of more settle product. Once product in a fluid state emerges through a bed to an unrestrained outlet, it will rage away to empty all the loose contents in the hopper.


A technique is available to accelerate the de-aeration process by means of a vibrating frame. This incorporates tuned rods that are activated to resonate and whirl at a critical speed to create multiple, short-cut air escape paths from the depths of a fluidised mass. See www.ajax.co.uk for further details. Without these, the air has to percolate through fine tortuous paths between the narrow, interstitial voids. During this slow process air from lower regions is re-placing air escaping from the upper layers so, despite the relatively large pressure differential between the lower regions and ambient, de-aeration of a deep bed can take a long time. The situation is made worse when ambient conditions or the material are warm or hot because air is more viscous at elevated temperatures.

[b]Q[/b] What about the use of flow inserts?

[b]A[/b] You’ve had your ten questions and a review of the many uses of flow inserts is a major subject that must await a further series. I can assure you that the wait is well worthwhile because these devices are most interesting and offer immense opportunities for exploitation.

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   [B]Advantages[/B]                                                                                [B]Drawbacks[/B]

No [i]dead[/i] regions of flow                                              Tall headroom / reduced storage capacity

More predictable storage times                                  Potential wear on walls with abrasive products

Secures flow through smaller outlets                        The outlet must be fully [i]live[/i]

Generally reduces segregation                                   Powder tests are essential to measure
                                                                                        wall friction and shear strength. Their
                                                                                        conduction and interpretation demand
                                                                                        considerable expertise.

Resists [i]Through-flow flushing[/i]                                 The design relates only to the specific condition
                                                                                       of the product and the particular wall contact
                                                                                       material as tested

The flow pattern is predictable                                 High wall pressure are generated at the hip joint


[i]First in - First out[/i] - almost                                       Flow can be exploited to blend contents. Any
                                                                                       property change may negate mass flow

Proven design guarantees reliable flow                   The flow rate is less than that of non-mass flow
                                                                                       with the same size of outlet

[b]Notes[/b]  -  The flow velocity is not uniform across the converging section of the hopper.

              -  Mass flow at the outlet region only, ([i]Mixed flow[/i]), secures the orifice
                 size benefits of Mass flow, without the full headroom penalty
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[b]Table 1 Advantages and Drawbacks of Mass Flow[/b]


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             [b]Type[/b]                                         [b] Typical Applications[/b]


    [b]Non-Mass Flow[/b]                                 Inert, easy-flow materials –
                                                                 Abrasive products
                                                                 Sand, gravel, minerals, coarse coal
                                                                 Plastic granules
                                                                 Batch loads of easy flow materials

    [b]Mixed Flow[/b]                                       Poor flow, inert products -
                                                                Cement, titanium dioxide, carbon black
                                                                Fine coal, damp products
                                                                Batch loads of poor flow materials

    [b]Mass Flow[/b]                                        Poor or easy flow degradable products
                                                                Easy or poor flow products demanding hygienic storage
                                                                Hygroscopic and deliquescent materials
                                                                Products that [i]cake[/i], (sugar, salt),
                                                                Materials that tend to [i]flush[/i], (flour, talcum powder)
                                                                Products and mixtures that tend to segregate

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[b]Table 2 Hopper Flow Regime Selection[/b]