The blockage of a hopper outlet or other flow channel by lumps

By Lyn Bates

Ajax Equipment Limited, U.K.

This danger not as common a problem as cohesive arching but design guidance on how to avoid this hazard tends to suggest simple multiples of particle size for cones and slots, without regard to the many other factors that can influence this danger. It is not practical to lay down a specific formula or hard and fast rules; theoretically because the process involved is stochastic and there is no definitive safe dimension, but in practice this is because the phenomenon is affected by many interacting factors whose effects can be awkward to quantify. However, extended tests and experience allows a slightly more refined assessment of a reasonably ‘safe’ orifice size that takes account of the main influential variables that bear on the prospect of a stable arch structure forming within a flow channel. These figures should be taken as a guide only, with further investigation needed for applications that are particularly sensitive to flow stoppages and those where the size distribution is highly irregular.

Flow blockages become increasingly probable for cohesionless bulk solids when the ratio of particle diameter to discharge orifice diameter exceeds a critical value, the size of which depends on various physical and operational factors. Obviously, the probability is certain if a particle is larger than the orifice but even orifices that are several multiples of a particle size can be vulnerable to blockage. When the particles in the flow stream converge towards the discharge outlet, jamming occurs if the size of the particles is large enough to come together to sustain a stable, interlocking arch. Jamming phenomenon as structural blockages of this kind cannot be predicted from current theories, (Jenike and others), that apply for cohesive materials and can only be assessed in an approximate manner. However, a review of the factors and experience provide a useful basis for practical application.

Jamming is statistical in nature. The probability of jamming to occur at the outlet depends on the nature of packing as the particles move towards the discharge outlet. The jamming probability increases with particle size. Since the phenomenon is stochastic, there is no absolutely ‘safe’ value of orifice, although the probability falls off rapidly above a certain value of orifice size to particle size ratio. The problem is further complicated if the bulk material has a wide particle size distribution or multi-modal size distribution and the particles are non-spherical.

Designers of bulk solids equipment generally use an experience-based approach with crude rule of thumb guidance since experimental data tends to be scattered, and there are no usable theoretical models. Published figures do not discriminate between differing operating circumstances, which do influence the blockage prospects, so the following figures provide an improved starting point for design purposes. These suggested values do not apply to soft, sticky, elastic or plastically deformable particles, for which special investigations should be performed. Also, note that rounded or cube-shaped granules are less likely to adopt a stable contact relationship than those with significant flat surfaces or have an extended, elongated form.

The prospects of particles coming together in a converging flow channel to form a stable structure and block the flow depends on many factors. The three major factors are the span and shape of the flow channel and the ‘effective size’ of the particles in the media. . Flow channel span is key a design choice. Planar flow through a slot opening offers less prospect of a blockage than radial flow where the particles converge in two dimensions. ‘Effective particle size’, in this context, must take account of size distribution, shape, surface roughness and other features of the particle. Many bulk materials have a particle size distribution so an ‘effective’ dimension must be allocated to form the basis of an assessment of what may be considered a practical size of opening that has a very low probability of blocking. Account must be taken of any accumulative effect of segregation concentrating the proportion of larger fractions at some stage of the flow route.

Two other main features are the shape of the particles, as those with flat, rough or interlocking surfaces are more prone to resists slip against each other, and whether the flow boundary is smooth or rough enough to provide a stable foundation for an arch. A hopper designed for mass flow offers a wall surface on which the particles will slide so the boundary of the flow channel does not offer robust support to oppose flow and jamming is less probable than an equivalent size of opening which is surrounded by static product. A third feature is the velocity of flow as fast moving particles tend to have a more dilate condition due to the flow dynamics and greater inertia to overcome weak structures that may occur from time to time as the particles are jostled into ever changing conditions of packing in converging flow.

The following design features are suggested as ways to avoid the prospects of blockages occurring as a result of the formation of structural arches in hoppers

1Select a design for mass flow in the outlet region. If the hopper is not of mass flow design there is a static bed of product around the discharge orifice. Invariably, some particles will overlap the opening to reduce its effective size and flow has to pass through the rough boundary surface of a static bed of particles. By contrast, mass flow takes place with slip on a smooth boundary that offers very poor support for a stable, structural arch to form. The following link to a video demonstrates the reduced jamming tendency in mass flow configuration versus non-mass flow. http://www.ajax.co.uk/model1.htmmodel1.htm

2It is well established that a slot outlet with plane flow is less prone to jamming than circular outlet with three-dimensional convergence. If the outlet is square or circular, the preceding shape can still converge in one direction only to secure the benefits of a plane flow channel.

3Avoid use of a butterfly valve if at all possible as the central blade reduces the flow cross section to two semicircles that are approximately equivalent to openings half the diameter of the valve.

4Minimize segregation, as the accumulation of larger fractions increases the prospect of jamming. Segregation most commonly occurs during the filling of a container at a single point. Dispersing the feed stream by way of spreading the feed over an area or diverting to a multipoint feed is effective in reducing segregation. Other tips are given in my book, 'User Guide to Segregation', published by The British Materials Handling Board

5Do not restrict the hopper discharge as a slow moving bed of material has less inertia to disturb an instantaneous blockage. The bulk is also in a denser state when flow is restricted because a flow stream is more dilated when moving at higher flow velocities, therefore, there is less room for slow moving particles to escape from a structural load path forming and arch as they touch in a dynamic state.

6In marginal cases, the provision of a vibrator will reduce the potential for particle jamming an opening by structural blockage and aid its failure, should one tend to occur. . However, sustained vibration on a blockage that does not break down quickly will tend to increase the local packing density and create a more stable array that is more difficult to destroy. Vibration should not be applied when the material is static The optimal location for application of vibration, specific energy input, amplitude and frequency will depend on the bulk material and system configuration but should essential be directed as close to the outlet as practical. The effect of vibrations on jamming probability is an open area for further research.

7Whilst research is handicapped by the numerous factors that influence particle jamming, it would be useful to fit individual research contributions into a framework that distinguishes Plane Flow from Radial Flow, Mass Flow from Non-Mass Flow, Dynamic Flow from Incipient Flow, rounded particles from angular particles & so on, and include polydispersity.

8Fitting one or more bars across the flow channel, at a distance and spacing above the outlet that offers a larger flow channel than the final outlet, will dilate the flow underneath and allow more latitude for particle re-arrangement in passage through the outlet.

9If there is a prospect of an occasional lump of uncertain size forming, such as an agglomerate, than the fitting of an inverted cone in a conical hopper, that has a slot opening similar to the outlet size, can also serve as a flow dilator and capture significant oversize particles without blocking the whole flow channel.

10For ultimate protection, a grid may be fitted with openings fractionally smaller than the outlet and access or facilities provided for removing or breaking down the lumps.

For the reasons outlined, the following multiple of maximum particle size are suggested as the minimum allowable orifice size for round or irregular shaped particles, based on the form of flow channel, whether mass flow or not and if dynamic or starting from a static condition. A separate assessment must be made in circumstances where the extent or distribution of particle sizes within the bulk is irregular, as with agglomerates or segregation effects.

Suggested minimum orifice dimension as a multiple of maximum Particle Size, form of Flow Channel and Particle shape



Radial non-mass flow pattern, (from static condition) Round 8 Irregular 10

Plane non-mass flow pattern, (from static condition) Round 6 Irregular 8

Radial mass flow pattern, (from static condition) Round 6 Irregular 8

Plane mass flow pattern, (from static condition) Round 5 Irregular 7

Radial non-mass flow pattern, (from dynamic condition)Round 7 Irregular 8

Plane non-mass flow pattern, (from dynamic condition) Round 5 Irregular 6

Radial mass flow pattern, (from dynamic condition) Round 6 Irregular 7

Plane mass flow pattern, (from dynamic condition) Round 4 Irregular 5

If it is important that flow stoppages do not occur, then further allowances should be made on these guidelines, or the flow behaviour verified in representative trials.

For more information, please visit:

https://edir.bulk-online.com/profile...-equipment.htm1083 ajax equipment.htm

http://www.ajax.co.uk/ ■