ultrasonic attenuationThe change in frequency of an ultrasonic signal when caused

measurementsby the interaction with particles. This can be used to measure

particle size.

ultrasonic bathA bath in which pressure waves in the fluid are caused by ultrasonic vibrations.

ultrasonic probeA point source of ultrasonic vibrations.

underflow, of sieving. (See fines). That portion of the feed material that has passed through a screening surface.

undersize controlA screen used for the removal of undesirable fines from a material.

vibrating screenA screen oscillated by mechanical, electrical or ultrasonic means.

warpAll threads or wires running lengthways of the fabric as woven.

weaveThe way in which warp and weft threads cross each other.

weave, plainThe way in which every warp thread crosses alternatively above and below every weft thread, and visa versa.

weave, twilledThe way in which every warp thread crosses alternatively above and below every second weft thread, and visa versa.

wedge wire screen;A screening surface comprising wires of wedge shaped cross section spaced from each other at a fixed dimension. The underflow therefore passes through an aperture of increasing cross section.

weft; shootAll threads running crosswise of the fabric as woven.

wire diameterThe diameter of the wire in the woven fabric.

wire screenA screen produced by a wire weaving process (or by pressure-welding of two layers of parallel wires set at 90 degrees) to form apertures of nominally uniform size.

woven wire clothA sieving medium of wires that cross each other to form the apertures. ■

Section 12 - Bulk Properties and Test Methods

active stressA stress that a bulk material generates against a confining surface as a result of forces acting within the bulk. Note that this has the dynamic capacity to follow a retracting surface. See antonym of passive stress.

adhesion Resistance to separation between two unlike materials under zero applied normal pressure.

aeroflow tester( See avalanche tester).

Ajax tensile testerA tensile test device that applies a tensile failure load at 90 degrees to the direction to the compacting load

Ajax cohesion testerA device that measures the shear strength attained by a uni-axially loaded sample after the removal of the compacting stresses.

angle of effective yield locus The inclination of the effective yield locus, (EYL),

as specified by Jenike.

angle of internal frictionThe angle between the axis of normal stress and the tangent to the

Mohr envelope at a point representing a given failure stress condition for the solid material.

angle of obliquityThe angle between the direction of the resultant stress or force acting on a given plane and the normal to that plane.

angle of reposeThe natural inclination adopted by an unconfined surface formed in a defined manner. Note that the method of formation of the slope and the. rate of deposition can create different curved or flat inclinations. See poured cone, drained cone and planar repose surface. The term is meaningless for cohesive products.

angle of wall frictionSee wall friction

anisotrophyThe feature of not being isotropic. This applies in a various ways to bulk solids. Therefore rigor is needed to define the bulk state. It may be anisotropic by virtue of the particle shape or particulate structure, be subjected to stress or strain in different planes, or any combination of these. Invariably, the composition of a loose solid is not similar in all planes. Strength values must therefore be strictly related to the stress history, the current stresses and the orientation of the applied stresses.

annular attrition cellA rotating trough device that subjects a sample bed of particles to a controlled rate of shear under a selected applied stress. ■

annular shear cellA rotary type of shear cell originally developed by Walker to offer unlimited strain by the use of an annular trough to contain the powder bed that is to be sheared.

ANSI MSDS FormatAmerican National Standards Institute Material Safety Data Sheet. Information to be advised by manufacturers and suppliers to persons who may come in contact with a product.

The American National Standards Institute, 11 West 42nd Street. New York, NY. USA, (ANSI) standard format (Z400.1-1993) was designed to aid the preparation of Material Safety Data Sheets (MSDS)

It has 16 Sections, comprising: -

Section I:Chemical Product and Company Identification

Section II:Composition / Information or Ingredients

Section III:Hazard Identification

Section IV:First Aid Measures

Section V:First Aid Methods

Section VI:Accidental Release Measures

Section VII: Handling and Storage

Section VIII:Exposure Controls / Personal Protection

Section IX:Physical and Chemical Properties

Section X: Stability and Reactivity

Section XI:Toxicological Information

Section XII:Ecological Information

Section XIV:Transportation Information

Section XV:Regulatory Information

Section XVI:Other Information ■

ASTM Standard D – 612DUSA standard test procedure for the Jenike Shear test. See SSTT.

attributesA number of bulk attributes influence the ‘handle-ability’ of a bulk solid. These properties may affect the behaviour of the material and/or the type of equipment that may be used to handle the product.

attributes marked * are related to the flow property of the bulk material

attributes marked ^are related to the type/construction of equipment

attributes with Legends are related to operator protection considerations

LegendsA=Sensitising

C=Corrosive

E=Explosive

Fo=Flammable

Fx=Extremely Flammable

H=Skin Absorption

K=Carcinogenic

Lk=No Classification required

N=Danger to the Environment

M=Geotoxic

O=Oxidising

R=Causes Birth Defects

Tx=Very Toxic

Xi=Irritant

Xn=Harmful

These attributes may be structured according to the nature of the property and its effect on the specification of the equipment, as follows: -

PropertyEffectTypical Material

*Packs under pressuremore difficult to flowHydrated lime, pigments

*Cohesiveforms poor flow massflour, fly ash, titanium dioxide

*Fibrousinterlocks to resist shearhair, wood shavings

gains strength with compaction asbestos

*Fluidisesdifficult to containfly ash, talcum powder

*Fattysticks together to resist flowHigh fat mixes, waxes

*Elasticdeforms at contact pointsground cork, rubber granules

to resist flow

*Plasticdeforms at contact pointsplastics,

to resist flow

*Chemically activeforms solid massground phosphate

Must prevent ■

*Meltscan fuse to solidplastic, ice

*Sintersfuses to solid masswarm plastics,

*Fusesforms solid mass, raw rubber

must prevent

*Cakesforms solids mass salt, sugar, crystals

must prevent

*Wetsticky, may dry to ‘cake’filter & centrifuge cake

*Stickyadhesive to surfacesdamp & fatty products.

*High frictionresists slip on contact surfacetitanium dioxide

^ !Abrasivewears plantsand, aggregate, crystals

^ !Corrosiveattacks surfacessalt, acidic chemicals

^Friabledelicate handling neededtea, coffee granules, flakes

^Explosivemust contain, inert, coal dust, aluminiumsuppress or vent powder, flour.

^Flammablemust prevent or protectwood shavings

Equipment

^ !Dustyhazard and objectionalcement, fly ash.

should contain, collect or suppress

^Hygroscopicbecomes stickysugar, soda ash

Deliquescent

!Noxiousoffensive to operativessewage sludge, waste must contain

!Toxicdangerous to operativesarsenic powder, active

must containDrugs.

!Irritanthazard to operatives

must contain

!Sensitiser hazard to operatives Penicillin intermediates

must contain

^Degradablecleanabilityorganic products ■

^ !Hothazard to operatives, kilned powder

!Sharp, hazard to operativesCullet, metal cuttings.

^Ultra-puresanitarydrugs, meat. Fish.

This list is not exhaustive, as indicated below, but illustrates that interactive factors may determine the overall specification of equipment. Fundamental to the performance of plant however, is that the flow behaviour must be reliable and safe, which means that it must be predictable and the potential hazards recognised and accommodated.

Product Risk Hazards are set out in a detailed format by the International Occupational Safety and Health Centre as below: -

Risk Phrases Used in the Countries of EU

(Phrases in parenthesis) are no longer in use.

Nature of Special Risks Attributed to Dangerous Substances and Preparations

R1Explosive when dry.

R2Risk of explosion by shock, friction, fire or other sources of ignition.

R3Extreme risk of explosion by shock, friction, fire or other sources of ignition.

R4Forms very sensitive explosive metallic compounds.

R5Heating may cause an explosion.

R6Explosive with or without contact with air.

R7May cause fire.

R8Contact with combustible material may cause fire.

R9Explosive when mixed with combustible material.

R10Flammable.

R11Highly flammable.

R12Extremely flammable.

(R13Extremely flammable liquified gas.)

R14Reacts violently with water.

R15Contact with water liberates highly flammable gases.

R16Explosive when mixed with oxidizing substances.

R17Spontaneously flammable in air.

R18In use, may form flammable/explosive vapour-air mixture.

R19May form explosive peroxides. ■

R20Harmful by inhalation.

R21Harmful in contact with skin.

R22Harmful if swallowed.

R23Toxic by inhalation.

R24Toxic in contact with skin.

R25Toxic if swallowed.

R26Very toxic by inhalation.

R27Very toxic in contact with skin.

R28Very toxic if swallowed.

R29Contact with water liberates toxic gases.

R30Can become highly flammable in use.

R31Contact with acids liberates toxic gas.

R32Contact with acids liberates very toxic gas.

R33Danger of cumulative effects.

R34Causes burns.

R35Causes severe burns.

R36Irritating to eyes.

R37Irritating to respiratory system.

R38Irritating to skin.

R39Danger of very serious irreversible effects.

R40Possible risks of irreversible effects.

R41Risk of serious damage to eyes.

R42May cause sensitisation by inhalation.

R43May cause sensitisation by skin contact.

R44Risk of explosion if heated under confinement.

R45May cause cancer.

R46May cause heritable genetic damage.

(R47May cause birth defects.)

R48Danger of serious damage to health by prolonged exposure.

R49May cause cancer by inhalation.

R50Very toxic to aquatic organisms.

R51Toxic to aquatic organisms.

R52Harmful to aquatic organisms.

R53May cause long-term adverse effects in the aquatic environment.

R54Toxic to flora. ■

R55Toxic to fauna.

R56Toxic to soil organisms.

R57Toxic to bees.

R58May cause long-term adverse effects in the environment.

R59Dangerous for the ozone layer.

R60May impair fertility.

R61May cause harm to the unborn child.

R62Possible risk of impaired fertility.

R63Possible risk of harm to the unborn child.

R64May cause harm to breastfed babies.

Combination of R-Phrases

R14/15Reacts violently with water liberating highly flammable gases.

R15/29Contact with water liberates toxic, highly flammable gas.

R20/21Harmful by inhalation and in contact with skin.

R20/22Harmful by inhalation and if swallowed.

R20/21/22Harmful by inhalation, in contact with skin and if swallowed.

R21/22Harmful in contact with skin and if swallowed.

R23/24Toxic by inhalation and in contact with skin.

R23/25Toxic by inhalation and if swallowed.

R23/24/25Toxic by inhalation, in contact with skin and if swallowed.

R24/25Toxic in contact with skin and if swallowed.

R26/27Very toxic by inhalation and in contact with skin.

R26/28Very toxic by inhalation and if swallowed.

R26/27/28Very toxic by inhalation, in contact with skin and if swallowed.

R27/28Very toxic in contact with skin and if swallowed.

R36/37Irritating to eyes and respiratory system.

R36/38Irritating to eyes and skin.

R36/37/38Irritating to eyes, respiratory system and skin.

R37/38Irritating to respiratory system and skin.

R39/23Toxic: danger of very serious irreversible effects through inhalation.

R39/24Toxic: danger of very serious irreversible effects in contact with skin.

R39/25Toxic: danger of very serious irreversible effects if swallowed.

R39/23/24Toxic: danger of very serious irreversible effects through inhalation and in contact with skin.

R39/23/25Toxic: danger of very serious irreversible effects through inhalation and if swallowed. ■

R39/24/25Toxic: danger of very serious irreversible effects in contact with skin and if swallowed.

R39/23/24/25Toxic: danger of very serious irreversible effects through inhalation, in contact with skin and if swallowed.

R39/26Very toxic: danger of very serious irreversible effects through inhalation.

R39/27Very toxic: danger of very serious irreversible effects in contact with skin.

R39/28Very toxic: danger of very serious irreversible effects if swallowed.

R39/26/27Very toxic: danger of very serious irreversible effects through inhalation and in contact with skin.

R39/26/28Very toxic: danger of very serious irreversible effects through inhalation and if swallowed.

R39/27/28Very toxic: danger of very serious irreversible effects in contact with skin and if swallowed.

R39/26/27/28Very toxic: danger of very serious irreversible effects through inhalation, in contact with skin and if swallowed.

R40/20Harmful: possible risk of irreversible effects through inhalation.

R40/21Harmful: possible risk of irreversible effects in contact with skin.

R40/22Harmful: possible risk of irreversible effects if swallowed.

R40/20/21Harmful: possible risk of irreversible effects through inhalation and in contact with skin.

R40/20/22Harmful: possible risk of irreversible effects through inhalation and if swallowed.

R40/21/22Harmful: possible risk of irreversible effects in contact with skin and if swallowed.

R40/20/21/22Harmful: possible risk of irreversible effects through inhalation, in contact with skin and if swallowed.

R42/43May cause sensitisation by inhalation and skin contact.

R48/20Harmful: danger of serious damage to health by prolonged exposure through inhalation.

R48/21Harmful: danger of serious damage to health by prolonged exposure in contact with skin.

R48/22Harmful: danger of serious damage to health by prolonged exposure if swallowed.

R48/20/21Harmful: danger of serious damage to health by prolonged exposure through inhalation and in contact with skin. ■

R48/20/22Harmful: danger of serious damage to health by prolonged exposure through inhalation and if swallowed.

R48/21/22Harmful: danger of serious damage to health by prolonged exposure in contact with skin and if swallowed.

R48/20/21/22Harmful: danger of serious damage to health by prolonged exposure through inhalation, in contact with skin and if swallowed.

R48/23Toxic: danger of serious damage to health by prolonged exposure through inhalation.

R48/24Toxic: danger of serious damage to health by prolonged exposure in contact with skin.

R48/25Toxic: danger of serious damage to health by prolonged exposure if swallowed.

R48/23/24Toxic: danger of serious damage to health by prolonged exposure through inhalation and in contact with skin.

R48/23/25Toxic: danger of serious damage to health by prolonged exposure through inhalation and if swallowed.

R48/24/25Toxic: danger of serious damage to health by prolonged exposure in contact with skin and if swallowed.

R48/23/24/25Toxic: danger of serious damage to health by prolonged exposure through inhalation, in contact with skin and if swallowed.

R50/53Very toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment.

R51/53Toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment.

R52/53Harmful to aquatic organisms, may cause long-term adverse effects in the aquatic environment.

Substances that are identified as posing a specific risk to heath are listed in Directive 76/769/EEC and published as a Consolidated list of C/M/R-Substances. (Classified as category 1 or 2 Carcinogens, Mutagens or Toxic to reproduction). This list runs to 100 A4 pages.

avalanche testerA test method devised by B. Kaye to relate the dynamic repose behaviour of a sample in a cylinder rotating on its horizontal axis to its flow characteristics, by way of chaos theory. The device may be used to characterise the flow behaviour of powders or compare their unconfined shear strength in dynamic conditions

biaxial shear testerA powder testing device that controls shear in a sample in two planes, to reflect steady state flow conditions. ■

BMHBBritish Materials Handling Board.

An organisation set up by U.K. Government to identify useful areas of research and aid the dissemination of technology in handling. It has identified bulk solids handling and processing as major fields in which general industrial practices tends to lag the state of art technology and promoted research projects, co-operative projects and publications.

BMHB Publications include: - This Glossary, and: -

‘Bulk Solids Physical Properties Test Guide’,

‘Guide to the Handling of Dusty Materials in Ports’,

‘Guide to the Effect of Vibration on Bulk Materials and Plant’,

‘Guide to Standards Relating to Materials Handling’,

‘User Guide to Particle Attrition in Mechanical Handling Equipment’,

‘A Survey of Dust Explosions in the UK.’,

‘User Guide to Segregation’.

‘Guide to the Design, Selection and Application of Screw feeders’

‘Code of Practice for the Purchase and Operation of Fabric Filters for Dust Control’.

‘Guide to the design of silos’.

bulk densityThe mass of a quantity of bulk material divided by the total volume that it occupies under defined conditions of preparation. See apparent powder density, loose poured density, tapped density, compacted density.

Fine particulate solids vary in density more than granular materials due to the variable presence of air in the interstitial voids and are more sensitive to applied stress because of the much greater number of points of co-ordination that can be disturbed to allow closer particle packing under load. Coarse granular materials settle readily to a stable density as air can enter or leave the voids easily. Coarse particles also support applied stresses with little deformation because the load path through the points of contact are well established. However, such a loose poured assembly of coarse particles will tend to reduce in volume under the influence of vibration, as the less stable particle-to-particle contact points are disturbed and re-orientation takes place as the particles re-arrange to attain a denser degree of packing.

Bulk Materials Handling Committee A group formed by the I.Mech.E Process Board to further the spread of technical information in solids handling.

caking testerA test procedure that measures the strength of a sample exposed to controlled ambient changes that generate particle-to-particle bonding.

Carr’s compressibility indexA measure of the compressibility of a powder that is determined by dividing the difference between the measured tapped density and aerated bulk densities, as determined by specific methods, by the measured tapped bulk density as a proportion of 100. ■

CEMA Conveyor Equipment Manufacturers Association, USA. publications include: -

‘Book 350‘Bulk Material Properties’.

‘Book 500 ‘Screw Conveyors’,

classification The flow behaviour of a bulk solid may be classified according to the ease with which the material deforms when in a compacted condition. There are no rigid demarcation boundaries, and other factors such as the elasticity of, the bulk material, wall friction, stability, caking and segregation potential will influence the degree of difficulties that impedes flow, but for convenience materials may be grouped in five flow categories.

A secondary classification may be made according to the significant attributes of the bulk material. These are a set of independent features influencing the suitability of the flow route for operations dictated by the handling and process requirements and the sensitivity of the product quality and value to its final condition as delivered from the system.

Group 1These materials are the easiest to store, discharge and convey, having a uniform particle size, with consistent, small length to width ratio.

Generally are hard, granular shapes that do not degrade easily. These materials exhibit no discernable adhesive or cohesive properties and maintain consistent physical properties with time and in variable ambient conditions. The loose solid will not gain strength or significantly change volume under compressive loads and will fall apart as soon as the compacting stress is relaxed.

Examples of Group 1 products are dry sand, plastic pellets, aggregates, dry salt, granulated sugar and coated prills in dry conditions. They have a flow function, ff, greater than 10. Materials that have these characteristics, except that they have a non-uniform size distribution, are strongly prone to segregate during their passage through unconfined flow regimes.

Group 2These materials behave in a sluggish manner and gain shear strength with compaction. Usually the particles are too small to be easily discerned. Their slight cohesive properties are due to irregular particle configuration and/or minor molecular forces. The products will change volume and gain strength under compressive stress, typically will hold together as a weak ‘snowball’ if squashed in the hands. Typical examples are flour, ground limestone, light soda ash, ground coke and dry castor sugar. They have a flow function value, ff, between 4 and 10.

Group 3These materials are usually of a fine composition and are more sluggish than group 2, but still retain sufficient porosity to be fluidised. Gas entrained during dilated handling will not readily settle out due to the fine structure of the voids, so that they can remain in a fluid condition for an extended period. However, once settled by the loss of air they attain a poor flow condition that is exacerbated by compaction. Their flow function value, ff, lies between 2 and 4.Typical product in this class are hydrated lime, cement, silca gel, starch, fly ash, polymers and carbon black. ■

Group 4These materials resist deformation and tend to have high cohesive values due to Van der Waal forces, electrostatic forces, surface texture or surface tension effects, or because of a strong interlocking nature due to particle shape configuration.

Typical examples are organic pigments, centrifuge or filter cake, Soya bean meal, and high-fat bakery products. Their flow function value, ff measures between 1 and 2.

Group 5The toughest type of bulk products to handle due to tenacious matting or interlocking tendencies, strong surface bonds, high inter-particle molecular forces, surface tension or a combination of these with other binding effects. Thread-like materials with elastic structures, flocculent, amorphous solids, and fibrous materials come into this class. Also products that form crystal bridges to set onto firm cakes, products that knit together or sinter.

Typical examples are wood chips, sawdust, plastic regrind, asbestos fibres, fibreglass strands, chopped paper and agricultural residue such as bagasse. Group 5 products have flow function value, ff, less than 1.

cohesionThe shear strength at yield of a compacted powder, after the removal of the forces causing the specific state of compaction,. One value of which is shown by the intersection of the yield loci with the ordinate. Eurocode 1 part 4, Silos and Tanks, defines a particulate solids as having ‘low cohesion’ if the unconfined yield stress is less than 14 kPa after the solid has been pre-compressed by a uniaxial consolidation stress of100 kPa.

Note. 1 – The magnitude and orientation of the formation stresses must be defined relative to the alignment of the plane in which failure occurs.

Cohesive forces normally reduce to a negligible value during sustained unconfined shear, due to dilatation of the particulate structure.

Note 2 - This phenomenon is a combination of the consequence of residual tensile stresses acting on the particulate solid and mechanical interference with shear due to the overlapping of particles in the bed structure, compounded with any temporary effect of void pressure differential with ambient. See tensile strength, void pressure

compaction The process of volume reduction by the application of stress

compaction, biaxialCompaction produced by the application of stresses in two directions at right angles to each other, not necessarily equal.

compaction, isostaticCompaction produced by the application of a stress which is the same in all directions. (A condition rarely achieved in solids handling)

compaction uniaxialCompaction produced by the application of a stress in one direction ■

composition,The constitutive structure of a mass of particles that may be

of bulkcollectively described as a bulk material. Typical expressions include: -

Uniform- whose particles are of similar size and shape.

Non-uniform- consisting of particles that vary in size, shape or other

physical attribute.

Homogenous- A mass if non-uniform particles that are uniformly

diffused in the bulk.

Heterogeneous- A mass of non-similar particles that are unevenly

distributed in the bulk. See segregated

Mixture- Two or more different bulk materials that have been

uniformly inter-dispersed by a shearing process.

Blend- Two or more bulk materials that are uniformly inter-

dispersed by a process of low work input.

Isotropic- Non-uniform particles that are not preferentially

oriented in the mass.

Anisotropic- A mass of non-uniform particles that are directionally oriented in the mass, such that the bulk physical

properties vary in different planes.

Granular- A mass of particles of such size that the individual particles can be visibly discerned.

Powder - A mass of particles of such size that individual

particles cannot be visually discerned in the bulk.

‘Wet cake’- A mass of compacted particles that are held together by

surface tension of contact of the moisture film on the

surface of the particles.

Paste- A mass of particles bound by a liquor that does not

fill all interstitial void space between the particles.

Sludge- A mass of particles mixed with a liquor that

completely saturates the interstitial void space

between the particles

Slurry- A mass of particles within a liquor that fills the

interstitial void space to excess, such that the

particles are unable to develop shear strength.

Suspension- A mass of particles within a liquor base that exceeds the

volume required to fully occupy the voids.

Dispersion- A mass of particles diffused in a volume of liquor

quantity such that the particles are not held in contact

with each other.

compound stress(relating to bulk strength) The sum of external and internal stresses acting on a powder compact to influence its bulk strength. Internal stresses may be due to chemical, physical, electrical or other forces acting to draw the particles together or tending to hold them apart. Typical origins are surface tension, van der Waals molecular forces, and electrostatic forces. Note that for all practical purposes, internal forces that are normally termed tensile forces act as externally applied compressive forces on the particulate solid. ■

compressibility The ability of a powder to be compressed, expressed by the ratio: -

C = 100 (Pv – P) / Pv

WhereP = The loose poured bulk volume of the material.

Pv = The bulk volume in defined conditions of compaction.

consolidated, overSee over-consolidated

consolidation, criticalSee critical consolidation

consolidation locusLocus of the shear and normal stress causing an under-consolidated particulate solid of a given initial bulk density to deform plastically.

consolidated, underSee under-consolidated

critical consolidation(of shear test sample) The condition where the consolidating load acting on a sample at shear failure is just adequate to contain the sample at constant volume during the shearing process. See critical state.

critical sample sizeThe minimum sample required to obtain the same value from repeated measurements by a given test apparatus. See representative sample.

critical stateThe condition of a bulk material during steady state flow at an equilibrium density condition of the bulk material, where there is a unique relationship between the stresses causing shear of the bulk and the stresses acting at 90 degrees to the plane of shear. A sample of bulk solid is in a critical state when it shears under the stresses applied without change of density. For a given density condition the critical state is the at the end point of a yield locus. See critical state line, yield locus. Hvoslev surface.

critical state lineThe locus of end points of a family of yield locus of differing density conditions in axis of normal stress, shear stress and void ratio. A change of stress would cause the mass to adopt a different bulk density.

critical void ratioThe void ratio of a particulate solid at critical state.

deformation, elasticA deformation that is totally recoverable when the stress causing the deformation is removed.

deformation, plastic1The permanent deformation that remains after elastic recovery is

complete, following the removal of the stress that caused the deformation. Plastic deformation and yield are the more general expressions whereas the term ‘incipient failure’ is associated with plastic deformation of an over-consolidated particulate solids and the term ‘flow’ is used for steady state flow.

Cont.

2.Continuous deformation under the influence of an applied stress in excess of the yield stress. ■

Note: -Yield, failure and flow are widely used synonymous expressions for plastic deformation. The term ‘yield’ is normally associated with incipient failure and the commencement of deformation, being the lowest limit of stress that will incur permanent deformation, whereas flow relates to continuous deformation, which is by nature a dynamic, irreversible condition.

density, apparent powderThe mass of a powder divided by its apparent volume.

density, aerated The mass of a quantity of bulk material that is in a defined state of fluidity divided by the volume that it occupies. (This condition reflects that achieved during rapid flow in an unconfined channel and as discharged from a lean phase pneumatic conveying system. This value is used for calculating a conservative, mass-holding capacity for a storage container.

density, bulk( See bulk density).

density, effectiveThe density of powder particles as determined by a given fluid

soliddisplacement method.

density, greenThe apparent density of a green compact.

density, immersedThe mass of powder per unit volume of suspended media displaced.

density, poured( See aerated bulk density).

density, pressedThe mass per unit volume of a powder bed compacted by way of a defined uniaxial stress.

density, settledThe lowest stable density conditions attained by a bulk material when settling from a dilated condition.

density tappedThe apparent density obtained under prescribed conditions of tapping within a container of given dimensions.

deviator stressThe difference in values between the major and minor principal stresses in a triaxial test.

discontinuity surfaceAny surface across which the properties of the solid are not continuous, such as a fracture or plane separating a flow channel from a static bed.

effective angle of frictionSynonymous with the angle of effective yield locus

effective solid density(See density, effective solid)

effective solid volume The mass of the particles divided by the effective solid density. ■

effective yield locusStraight line passing through the origin of the shear stress/principal stress plane and tangential to the Mohr circle corresponding to steady state flow conditions of a particulate solids of given bulk density.

elevator A synonym for silo, commonly used in the grain industry.

end point (of yield locus) The peak shear stress value of a steady state re-shear.

factor, recovery( See recovery factor).

factor spring( See spring factor).

fill weight;Mass of powder in a container of stated dimensions, when this had

been filled under stated conditions.

fine sand( See sand, fine)

flow factorA factor, ff, originally defined by Andrew Jenike, relating to the geometry of a hopper’s ability to apply stress to the flow channel, expressed as the ratio of the major principal stress in material flowing in a converging channel to the major principal stress that would cause it to cease flowing.

The value of this factor depends on the angle of wall friction, the form and slope of the hopper walls and the angle of internal friction of the bulk solid, as described by the angle of the effective yield locus.

flow functionThe ratio, FF, defined by Jenike, of the major principal stress at steady state flow to the unconfined yield strength of a specific particulate solid.

( FF = 1 / Fc . A ‘flow’ classification proposed by J.Thomas is : -

FF <1‘hardened’

FF <2very cohesive

FF <4cohesive

FF <10easy flowing

FF >10free flowing

(These descriptions of flow condition relate to a compacted bed).

formation stressesThe stresses applied that conditioned the particulate material to its specific, current state, as normally characterised by its bulk density.

Freeman rheometerA dynamic powder testing device that measures the torque to rotate a blade or impellor that is submerged in a confined mass of particulate solids. This device may be used for qualitative comparisons of material behaviour but the complex stress conditions generated within the bulk cannot be quantified.

‘Guide to the specification of bulk solids for storage and handling applications’ Book prepared by Bulk Materials Handling Committee of I.Mech.E. ■

hang-up indicizer( See Johanson indicizer).

hazardsSee attributes

Hosokawa TestsA series of basic measurements and empirical tests devised by Carr, an engineer at BIF, to correlate powder behavioural factors to flowability.

incipient failureThe onset of permanent deformation. See yield.

Indicizer( See Johanson indicizer).

instantaneous compactionThe condition of a sample of bulk material that has been subjected

to a uniaxial compacting load for a short period in preparation for a shear test.

instantaneous yield locusThe yield locus measured immediately following consolidation.

internal frictionThe portion of shear strength indicated by the term .tan in Coulomb’s equation s = C + .tan due to the combination of the effects of particles interlocking and their resistance to slip over each other.

isostatic compactionSee compaction, isostatic.

Jenike cellA shear cell developed by Andrew Jenike to provide data for hopper design. See Bul. 123. Univ. of Utah. 1964

Jenike & JohansonAn empirical test based upon determining the flow behaviour of a

quality control test sample under pressurised stimulation.

Jenike testA procedure developed to determine the stress conditions at failure of an unconfined surface in order to establish the critical orifice size of a given form of flow channel that will sustain reliable flow, once a flow regime has been fully mobilised. See SSTT, ASTM Standard D – 6128.

Johanson indicizerA set of test devices for indicating the flow related properties of a bulk solid. Comprising:-

1Shear Test Device. This measures the initial failure strength of a compacted sample.

(Johanson)Note that this test reflects initial failure conditions, not steady state

(critical state), behaviour that is achieved during flow. From these measurements, certain factors are derived that are used to assess the size of orifice required to secure reliable flow. No supporting theory is provided to justify these criteria.

.

2Wall friction tester This operates on a tilting plate mechanism that reflects static, rather

(Johanson)than dynamic friction. This measurement is utilised to determine the required steepness of inclination of a hopper wall to stimulate mass flow of the container contents. ■

3Porosity tester The rate of air flow through a compacted bed is used to assess the

(Johanson)expansion characteristics of a settled bed during the initiation of flow. is used for assessing the effect of flow rate of a fine bulk material from the outlet of a storage container.

Johanson porosity testerSee Johason Indiciser, item 3

Johanson wall friction tester See Johason Indiciser. Item 2.

limestone CRM-116See CRM 116

Limitations DirectiveSubstances classified as dangerous and can only circulate freely when packaged and labelled in accordance with Directive 67/548/EEC, (for dangerous substances), and Directive 99/45/EC, (for dangerous preparations). In a relatively small number of cases the rules for classification, packaging and labelling are insufficient to reduce risks and must be supplemented by Directive 76/769/EEC

load/compaction testA test in which an increase in uniaxial stress applied to a confined sample of bulk material is related to the change of volume of the sample. The rate of increase in resistance of the material to compaction is used as a measure of how the product will gain in strength under load

major consolidating stressThe major principal stress, as denoted by the largest value intercept of the Mohr stress circle of steady state flow that is tangential to the effective yield locus with the axis of principal stress on a shear stress/normal stress diagram.

Material Safety Data SheetsSee ANSI-MSDS

mean stressThe expression if often used to denote the mean of the normal stresses on two mutually perpendicular planes. This stress is independent of the orientation of the planes and the mean stress, as normally quoted, is the mean of the corresponding principal stresses. The mean stress corresponds to the centre of a Mohr stress circle on the axis of normal stress. This value should not be confused with the intermediate principal stress, which can have any value between the major and minor principal stresses according to the pertaining conditions.

mechanical arching( See structural arching)

Mohr circleA circle drawn in the co-ordinates of normal stress and shear stress for a specific bulk density condition, that is the locus of equivalent normal stress and shear stress combinations for a specific state of applied stress. ■

moisture contentThe proportion of moisture, or other liquor, that is part of the composition of a bulk material may be expressed as a percentage of the total weight or a percentage of the weight of the solid content, (weight to weight basis). The difference in these values is small at low levels of moisture content but increases significantly at higher liquor levels, therefore care must be taken to identify the correct format. For example, equal quantities of solids and liquor may be expressed as containing ‘50% moisture’, whereas the latter is defined as having ‘100%’ moisture content on a weight to weight basis.

Moisture content values normally relate to free moisture, (constitutional water), excluding bound water of crystallization. This occurs as a surface film that, in quantities below about 1%, is in the form of water held by specific hydrophilic sites on molecules of the particles, (ie other than on water itself), as a monolayer on the surface of particles. From about 1% to 5% moisture, the water takes a multi-layer form, with additional layers forming over the monolayer. This surface film forms liquid meniscous bridges cusps at points of coordination, to create a pendular state of induced tensile strength due to the surface tension effect of the contact film curvature. For a given state of particle packing, as characterised by the dry content density of the bulk material, increases of fluid content raises the bulk material strength, but above a specific, critical value of moisture content the bulk strength declines.

The transitional condition at which the bulk strength declines is because higher proportions of liquor tend to fill local regions of interstitial voids, forming pockets of saturation that are incompressible amid regions of partially filled voids as a fenicular condition. A change of total volume as a result of compaction reduces the available voidage space, leading to a higher degree of the available space being filled with the fluid. The degree of void occupancy by the liquor increases with higher fluid content, until fluid occupies the total voidage space to form a capillary fluid network. At this stage external stresses on the bulk are hydrostatically contained and there is no gain in strength of the material. The product essentially changes in nature from a wetted bulk solid to an incompressible paste. The water is however entrapped from flowing freely from the bulk, being held by capillary attraction by a matrix of gel structures or tissues. Damp material in a deep bed will suffer different compacting loads and the fluid may drain to leave excess fluid in a lower saturated bed, and entrapped moisture in the upper layers.

The weakness of a fully saturated particulate bed does not ensure that the

material is free flowing. In fact, the virtual impenetrability of the bulk offers a high resistance to dilatation due to lack of relief for void demand of expansion imparting a high tensile strength to the bulk. Should the overall volume change through particle re-orientation to allow the liquor to fill the total voidage with surplus free liquor, the excess fluid leads to separation of ■

the constituent particles and a progressively change of nature from a particulate solid to a paste, then to a slurry with no incipient shear strength.

These conditions arise with some ores transported by sea, which settle and are ‘worked’ by movement of the vessel. The stability of the mass is compromised and movement of the cargo may erode the integrity of buoyancy by shifting of the vessels centre of gravity. (See ‘safe transportable moisture content’.)

Any further increase in the fluid content beyond the critical saturation

value serves to separate the particles and reduces the shear strength of the bulk material, eventually changing the condition to paste and then to a slurry, that is unable to sustain a shear stress and behaves as a viscous, non-Newtonian fluid. A slurry is further weakened by excess fluid to becomes a suspension, and eventually a dispersion of disconnected particles in a sea of liquid.

The condition of a nearly saturated cake or paste, such as some filter cakes and centrifuged products, is sensitive to compaction. If there is inadequate liquor to fill the voids, the bulk will gain strength rapidly with compaction. If the voids are completely saturated, external forces are then supported by hydrostatic continuity of the liquor and the volume will not decrease nor the strength increase further with increased applied stress values

Bulk materials that contain salts or other soluble components of composition are prone to significant changes of nature. Deposition from solution that occurs when the product dries out tends to form crystal bridges at points of co-ordinatation that bind the particle structure to a rigid ‘caked’ mass of considerable bulk strength.

mono-axial cellA cell in which a uniaxially compacted sample is stressed to failure in line with the compacting load. E.g. unconfined failure test

MSDSSee ANSI MSDS

negative void pressure An effect produced by the expansion of a particulate solid. As the total volume increase is occupied by the original solids content, the fluid occupying the interstitial voids must expand to fill the balance of space. In the case of a fully liquor saturated mass, which will not expand, the void expansion demand is the full ambient pressure.

Cont.

For coarse, dry powders the reduced air pressure is alleviated by the permeation of air through the bed. The resistance to expansion therefore depends upon the degree of change in void volume and the permeability of the particulate bed. During such time as there is a negative differential between pressure in the voids and ambient pressure, the effect is similar to a ■

tensile stress acting on the mass or an external compressive stress. See positive void pressure.

normalised stressThe total of internal and external stresses acting on a particulate mass. i.e. adding tensile stress and void pressure differential to compacting stresses, to sum all loads on a particulate bed as a unified whole. See compound stress ,tensile stress, negative void pressure, positive void pressure.

over-consolidatedA state of consolidation and applied stress where the confining loads acting at shear failure of a (of shear test sample) sample are inadequate to prevent the particulate structure expanding during shear.

passive stressThe stress generated by a bulk solid in resisting an external force. Note that this decays on removal of the external force. See antonym active stress.

Peschl shear cellA proprietary form on rotating shear cell, similar to an annular shear cell but with zero internal diameter.

plastic deformationDeformation that does not recover on the removal of stress.

planar repose surfaceThe flat surface repose formed by pouring or draining to a straight edge. This value is useful for non-cohesive particulates, being independent of the convergence or divergence inherent in flow on conical surfaces.

plane stressThe term commonly used to describe the condition of deformation taking place in one plane only, as with a Vee shaped hopper. The effect of the end walls of the rectangular flow channel distorts the plane stress, but the influence diminishes to a negligible value if the length exceeds three times the width of the section.

porosity, (of bulk)The volume of voids within a quantity of particulate solids divided by the total volume of the mass. I.e. The volume of voids divided by the volume of voids plus the volume of the solid content. Synonym for voidage.

positive void pressurePressure of the ambient fluid in the interstitial voids of a particulate solid opposes the external stresses acting on the bed, thereby reducing the particle-to-particle compacting forces. Excess gas under sufficient pressure to sustain the weight of the bed will negate particle contact loads and allow the bulk to deform with minimal resistance, in a fluid-like manner. The rate of escape of such pressurised gas depends upon the pressure differential between the void gas and ambient, the porosity of the bed and the geometry of the system. Fine powders are prone to such fluidised behaviour when poured or strongly agitated.

pressure-void ratio curveA curve representing the effective pressure and void ratio as obtained from a consolidation test. The curve has a characteristic shape when plotted on semi-log paper with pressure on the log scale. The various parts of the ■

curve and extensions to the parts have been designated as recompression, compression, virgin compression, expansion, rebound and other descriptive names by various authorities.

principal planeEach of three mutually perpendicular planes through a point within a particulate mass on which the shearing stress is zero. These planes are unique for a given state of stress. The planes are described as follows: -

Major principal plane - The plane normal to the direction of the major principal stress

Intermediate principal plane – The plane normal to the direction of the intermediate

principal stress.

Minor principal plane - The plane normal to the direction of the minimum

principal stress.

principal stress ratioThe ratio of the minor principal stress to the major principal stress acting within a particulate solid.

progressive failureFailure in which the ultimate shearing resistance is progressively mobilised along the failure surface.

properties A bulk material has many type of properties. These may be classified

(of bulk material)under headings of - physical, (which includes mechanical, chemical, thermal and electro-static properties that affect the rheological nature of the bulk, and attributes, which introduce other characteristics of interest).

pseudo-stereo photogrammatic analysis

A technique of investigating planar flow regimes by way of time lapse photographs that, when viewed as a stereo-pair, translates the strain deformation to the image of a virtual contour map of displacement. Static regions appear as plains at the reference depth, coherent motion show as raised flat surface or plateau, shear discontinuities are seen as cliffs and the inclination of the apparent slope reflects the local rate of strain of the sample.

relative densityThe ratio between the void ratio of a cohesionless particulate mass in its loosest stable condition to that in its most compact condition.

ring shear tester( See annular shear cell).

safe transportable A degree of moisture held by a bulk solid which is inadequate to fill the

moisture contentinterstitial voids, to create an unstable, plastic mass when the bulk is compacted and sheared to a maximum particle packing structure.

(See moisture content).

Schulze ring shear cellA proprietary annular shear cell ■

shearThe permanent dislocation of a particulate structure in a plane subjected to a shear stress

shear forceA force directed parallel to the surface element across which it acts.

shear planeA plane along which failure occurs by shearing.

shear strainThe change in shape, under the influence of stress, expressed by the relative change of the right angle at the corner of what was, in the un-deformed state, an infinitesimally small rectangle or cube.

Strain may be elastic or plastic, the latter being a form of irrecoverable deformation, or flow, when the applied stress exceeds the elastic limit of the bulk. Strain is essentially anisotropic in nature.

shear strengthThe strength of a compacted powder to resist shear in defined conditions. Note - When shear takes place without change in volume of the bulk material, as occurs in a solids flow situation, there is a unique relationship between the shear stresses and the stresses acting normal to the failure surface and the material is said to be in a ‘critical state’. In circumstances where the volume is changing to a denser or more dilate condition during shear, whether commencing from a static condition or not, the failure conditions are transient and the relationship between the shear stress and normal stress varies. In all cases of powder testing, the stress history of the prepared sample must be clearly defined.

shear stressA force acting parallel to the surface of a plane of a particulate bed, divided by the area over which it acts.

spring-back factor1.The proportional change in dimensions of a metallic

compact on ejection from its die or mould.

2.The degree of strain recovery on the removal of a

compacting stress from a powder bed.

SSTTStandard Shear Cell Testing Technique. A standardised procedure for the conduction of Jenike shear cell test developed by the Working Party for the Mechanics of Particulate Solids of the European Federation of Chemical Engineers and published by the I.Chem.E. See ASTM D – 6128.

strainDeformation or displacement, causing a change in length per unit of length in a given direction, due to the application of a stress

strain energyThe total work input to deform a mass. The work done by forces affecting a change in volume comprises the work content absorbed by friction, that retained by elasticity in the bulk, and that absorbed by plastic deformation, and sometimes, to a limited extent particle fracture. In the case of gravity flow, the energy balance is that the loss of potential energy equals the work input to the bulk, less the frictional loss on the flow boundary contact surface. ■

Note that the volume of the mass may increase or decrease during flow, depending on the stresses acting on the bulk within the flow channel. The energy change involved in expansion or contraction must be taken into account. A work balance approach offers a fundamental understanding of the mechanistic nature of particulate solids behaviour.

tapped density( See density, tapped).

tensile strengthThe strength of a compacted powder to resist separation of the particle bed under the influence of a tensile stress. It is the external manifestation of attractive forces between the constituent particles. The magnitude of these forces depends on the closeness of particles packing, hence tensile strength is related to the state and stress history of the particulate solid. Tensile stress is equivalent to a compressive stress acting on the mass.

See compound stress, normalised stress, negative void pressure

Note: – The magnitude and orientation of the formation stresses must be defined in relation to the alignment of the plane in which failure takes place.

E.g. - ‘Co-axial’ tensile strength relates to failure stress in the opposite direction

to the compacting stress of formation. See Univ. of Bradford tensile tester

-‘Transverse’ tensile strength relates to failure stress at 90 degrees to the orientation of the formation stress. See Ajax tensile tester

-‘Ultimate’ tensile strength is a measure of the failure stress of a tri-axially compacted sample. (A theoretical, rather than a practical test)

tensile test The stressing to failure of a compacted sample by the application of a tensile stress. The relationship between direction of compacting stress and that of failure stress must be defined. (see tensile strength).

time consolidationThe compacting of a bulk material due to stresses applied over a period of time, such as may happen in long-time bulk storage or in preparation of a sample for shear testing purposes to represent bulk conditions experienced after extended periods of storage.

time yield locusPlot of shear stress versus normal stress at failure of a bulk solid that has been subjected to time consolidation.

tri-axial testerA test device common to soil mechanics where the sample is stressed along a cylindrical axis whilst the cylindrical body section is subjected to hydrostatic compaction by way of a tubular membrane.

unconfined compressive strengthThe load per unit area at which an unconfined cylindrical specimen of compacted powder will fail under a simple axial load. See unconfined failure test ■

unconfined failure test A test on an axially compacted sample, usually in a cylinder, that is stressed to failure by an axial load after the confining cylinder is removed.

Note that this test reflects initial failure conditions, as under the surface of a static arch, not those of steady flow, critical state conditions,

Uniaxial test( See unconfined failure test).

University of Bradford A device that measures the tensile strength attained by a

tensile tester powder bed compacted at 90 degrees to the plane of failure.

vertical shear testThe shearing of a compacted powder bed by way of a vertical load applied on an unsupported, defined cross section

voidageSee porosity.

Walker cellThe original annular shear cell for powder strength investigations named after its inventor.

wall cohesionThe resistance to slip offered by a material, separate and additional to, any frictional resistance due to a normal load acting against the contact surface.

wall frictionThe relationship between the force necessary to cause slip on a contact surface and the force acting on the bulk normal to the surface. The effect should be measured under a range of applied normal forces and the results graphed to produce show the relationship and the effect of surface cohesion in circumstances of zero normal load. The value of wall friction is a major factor in the determination of wall inclination for a mass flow hopper. (See friction, wall friction- dynamic and wall friction- static).

wall friction-dynamic The relationship between the force necessary to sustain slip on a contact surface and the force acting on the bulk normal to the surface.

wall friction-staticThe relationship between the force necessary to initiate slip on a contact surface and the force acting on the bulk normal to the surface. The angle between the abscissa and the tangent of the curve representing the relationship of the shearing resistance to the normal stress acting between the bulk solid and the surface of another material.

wall normal stressThe stress acting at 90 degrees to the bulk at a wall boundary surface.

wall shear stressThe shear stress mobilised by frictional resistance to slip at a confining wall

wall yield locusA plot of the wall shear stress against the wall normal stress. see wall friction. ■

yieldThe boundary of fully elastic behaviour. A condition on which the applied stresses exceeds the bulk strength of a particulate mass causing the material to fail and plastically or catastrophically deform

yield locusThe plot of a series of failure shear stresses of commonly consolidated samples to a specific bulk density, measured by a shear cell under the influence of a range of applied normal stresses. Note that the magnitude of the normal stresses are all less the stress of consolidation of the sample as adding an extra shear stress to the consolidating stress would change the density of the sample. The shape of the curve as the shear stress increase is noted, to establish the normal load condition at which the stress will increase to a sustained value without change of volume as the sample shears.

This represents the critical state of the powder. as flowing under these stress conditions any increase of normal load would cause the sample to increase in density and hence be in a different bulk condition. If the shear stress increases to a maximum value and then reduces, it implies that the sample has inadequate normal force to hold the sample in its prepared state. See under-consolidated. The yield locus, (YL), is sometimes called the instantaneous yield locus to differentiate it from the time yield locus.

yield strengthThe stress that a particulate bed, in a defined condition of density and loading, will sustain before deforming to failure. (See yield stress).

yield stressThe value of stress that will cause failure of a bulk compact in a defined state of prepared compaction and of applied loading normal to the plane of stress application.

yield surfaceThe envelope of the yield loci on a three dimensional plot of density, shear strength and principal stress. It extends from the tensile strength value to the critical state line in the shear/principal stress plane, and from a point at the origin in a condition of dilatation that has zero shear strength, to a degree of consolidation at maximum solidification in the density plane.

See Hvoslev surface. ■

Section 13 - Bulk Flow

air-retarded flowThe resistance to surface failure and expansion of the bulk material due to low permeability of the bulk material that inhibits the rate at which flow can take place from an unconfined surface.

archingThe transfer of stress from a yielding region of solids to an adjoining restrained section of the mass. This is characterised by the formation of a stable bridge across a flow channel due to the effect of either a structural assembly or a cohesive structure. See structural arching and cohesive arching.

asymmetric flowA flow channel that is biased to one side of the stored contents in a silo or hopper. Note that differences in walls pressure that arise from a asymmetric flow regime can result in extra-ordinary wall stresses that threaten the integrity of a silo structure designed for symmetric stresses.

avalanchingIntermittent surges of loose material on poured or drained repose surfaces due to transient instabilities of the flow restraining surface. See repose flow

bed flowA flow pattern characterised by the movement of a mass of bulk solids in a parallel flow channel. Wall slip must take place on all contact surfaces but the flow velocity is not necessarily uniform across the whole cross section of flow. See coherent flow.

bin activatorA device for stimulating discharge from a bulk storage container by vibrating a base conical bowl section that has a conical convergence to the outlet neck. This section is supported from the bin walls by a number of links having flexible end fittings. The rim of the bowl is connected to the outlet of the main bin by a flexible skirt. Within the bowl is a concentric inverted cone to shield the outlet and create an annular flow gap.

Binsert®A proprietary form of Cone-in-Cone. Used to secure mass flow in hoppers with shallow walls and regulate flow velocities to achieve flow blending or mitigate the effects of segregation.

bridgingThe formation of arches of particles keyed, jammed or cohered together across the pathway of flow to form a stable obstruction.

( See arching, structural arching and cohesive arching). ■

bullet insert A rigid, circular flow insert. It is shaped with a top section as a steep inverted cone connected to a lower reverse cone section that may or may not be truncated. It is normally supported from the bin walls by vertical, radial ribs to form a flow annulus above the outlet. These generate mass flow at reduced cone wall angles of the bin and/or minimise segregation.

cakingThe bonding of particles, normally by way of crystal bridges between points of contact in the bed, to form a hard, brittle compact.

coherent flowMass movement of the bulk material without re-ordering of the particulate structure. See bed flow. This is not strictly a flow process but is typically the manner of movement of product in the body section of a mass flow hopper until it nears entry to the converging section of the hopper, at which point the velocity tend to increase in the centre of the flow channel.

cohesive archingThe formation of a stable blockage in a flow channel resulting from the bulk strength of the material being sufficiently high to form a stable arch with an unconfined under-surface. (See also structural arching).

cohesive strengthThe resistance to incipient shear of a compacted mass in the absence of a normal load on the shear surface. The stress conditions and history of the sample must be known for the value to have any meaning. See cohesion.

‘Cone-in-cone’ insertA technique of incorporating a secondary hopper inside a bulk storage container to modify the flow pattern by changing the velocity gradient across the flow channel. One use is to convert a hopper that would otherwise not operate in a mass flow mode to one that will mass flow. Further uses are to induce blending of the container contents and mitigate segregation that would otherwise take place during discharge

confined flow Flow taking place within a channel that is constrained by firm boundaries.

conical flowA flow channel that uniformly converges in two planes at 90 degrees to each other.

convective mixingThe separation and displacement of regions from a zone of material in a mixer, to new locations within the bed of powder.

converging mass flow A flow channel that uniformly converges in one or two horizontal planes with slip taking place on all the confining boundary surfaces.

core flowA flow channel that develops within a static mass of bulk material during discharge, that is replenished from the surface layers of the stored material by way of drained repose. The expression was coined by Arnold Redler in a patent application of 1921. ■

critical arching diameterThe size of largest size of opening over which a stable arch can form in a stored bulk solid that has developed a fully mobilised flow channel. This measurement is derived from Jenike shear tests and the application of his formula and charts. Its utility specifically relates only to a mass flow hopper as, if the hopper is not of the mass flow type, a secondary form of flow secession that occurs at a larger cross section takes the form of a rathole within a body of static material, See critical rathole diameter. SSTT.

critical ratehole diameter The maximum size of stable hole that can form through a bed of stored bulk material. In the case of a non-mass flow hopper the size of outlet opening must exceed this size in order to sustain gravity flow and a rathole cannot form. The powder shear cell test procedure developed by Jenike to establish this dimension is given in SSTT.

dead region, of flowA static zone with a stored mass that has developed a flow channel within the bulk. Note that this is incompatible with mass flow. The prospect of mass flow is negated if any region of the container outlet is not mobilised to flow, as with the case of a feeder that does not develop a progressive extraction profile along the length of a hopper outlet slot.

de-aerationThe process of excess air escaping from the voids, to eventually bring the void pressure to ambient in a settled state of particle structure.

de-aeration constantA measure of the decay rate of de-aeration of a fluidised bed proposed by Mainwaring and Reed for the evaluation of the materials potential for dense phase conveying.

diffusive mixingA description of small scale interchange of fractions of a mix brought about by local migration and interactions between adjacent zones in an agitated powder bed.

dilated bedA bed of particles in an expanded state due to agitation, flow conditions, or the presence of excess air in the voids that develops a state of quiescent fluidisation.

drained coneSee drained repose

drained reposeThe surface inclination of a conical depression formed by material emptying from the surrounding area into a core flow channel or orifice, or the slope formed when material is taken from the bottom of a pile. This feature is associated with the surface profile of a non-mass flow hopper as it empties. It may also develop during the emptying of the portion of material in the converging hopper section of a mass flow hopper, due to the velocity gradient of flow across the cross section. This latter characteristic can negate the remixing of segregated product on the final stages of discharge, as the peripheral, segregated region empty is last to empty.

dynamic archingThe formation of unstable flow obstructions caused either by : - ■

a.Transient cohesive arches that form within the flow channel, or

b.Air retarded flow, causing waves of dilation to progress through the mass causing temporary flow fluctuations, or

c.Erratic flow obstructions resulting from intermittent and unstable particulate structures that form in the flow channel.

cont.

The phenomenon is characterised by regions of dilated bulk forming under a moving, unstable arch region. An irregular outflow rate may develop from the storage container as flow is erratically restrained by the temporary obstructions to smooth deformation of the mass.

dynamic reposeThe surface profile of a powder bed in a defined state of motion.

eccentric flowA flow pattern that is not concentric with the boundary walls of the container. The flow channel may, or may not, intersect with the wall surface but if it does the intersection can be at differing heights and possibly fluctuating around the container walls. It should be noted that this pattern of flow gives rise to unbalanced wall pressures that cause complex, and possibly dangerous, wall stress situations.

effective transitionThe location of change in the flow channel from bed flow, with active boundary stresses, to converging flow, where passive confining stresses act on the flow boundary. In the case of a mass flow hopper this occurs at the change from a body section to a hopper section of a bulk storage container. For a mixed flow type of bulk storage unit, the effective transition is unpredictably varies on the wall of the body section. The sudden change of confining stress raises high ‘kick’ pressures at the transition, which may place high structural loads on the container. See appendixII note on flow

expanded flowA flow pattern comprising of a lower mass flow region that converts to a non-mass flow construction of container in the upper region of storage by transition of the wall geometry to a more shallow wall inclination on which the material will not slip in a confined state. This construction is normally selected to secure the reduced size of outlet, anti-arching and anti-ratholing benefits that are given by mass flow and securing the additional storage capacity given by lower wall angles at a cross section greater than that at which arching can take place, without the penalty of extensive headroom requirements for a total mass flow design.

fines expressionThe process of large particles in a flow stream pushing away fines on

(of segregation) impact with a dilated bed that has a preponderance of fine particles.

flooding( See flushing).

flowPlastic deformation of a bulk material, due to the influence of external forces. ( See gravity flow, confined flow and unconfined flow). ■

flow channelA cross section of moving bulk material bounded by confining surfaces or a static bed of similar product

flow insertsFittings added to the internal construction of a bulk storage container to change the stress distribution within the material stored or the flow regime generated in order to secure some favourable feature. See cone-in-cone, Binsert, Lynflow inserts, homogenising inserts.

flow pattern The shape of a local flow channel that develops through a solids flow route. See poured and drained repose, core flow, mass flow, flow regime.

flow regimeThe fully mobilised form of flow channel developed by a particulate solid during discharge from a container. A primary classification may be made between mass flow and non-mass flow. Mass flow may be completely converging mass flow or incorporate a region of bed flow. Non mass flow variants includes expanded flow, mixed flow, core flow with drained repose and funnel flow, all of which are combinations of sequential flow patterns.

flow, steady stateContinuous plastic deformation of a particulate solid in a critical state condition.

flowability A expression is cited as a measure of how easily a material will flow. This property may be expressed in comparative terms, or as an index with defined examples to form a scale but, in the case of a compacted particulate solid, is better quantified by the flow function.

Note that this expression may be concerned with both the instantaneous and time consolidated values of the flow function, but the instantaneous value is normally the figure used to express the potential flow behaviour of a bulk material in a fully de-aerated and compacted conditions.

Also, the flow potential of a particulate solids may be stress dependent, be influenced by time effects, particle-to-particle caking, bonding, fusion, elastic or plastic deformation at the points of particle contact and the degree of confinement. In essence, flow potential cannot be expressed by a single value, but by relating its prospects to specifically defined conditions, taking account of the relevant properties of the bulk material.

fluidisationA condition of zero internal strength of a powder bed brought about by the presence of excess fluid, usually air, in the interstitial voids of a fine bulk solid that dilates the bulk sufficiently to offer the constituent particles unrestrained freedom for rearrangement and hence behave like a fluid of low viscosity. (See flushing).

fluidisation, aptitudeThe ability of a bulk material to be fluidised. See Geldarts classification

of a product ■

fluidisation, The minimum speed of fluidisation , theoretical method, is given by the

minimum speed of,formula : (Valid only in laminar rating i.e. if inequality below applies)

v0 = 1.03 . g (∂s - ∂g) 2 . d2 ifv0 . d . ∂g > 10

200 . ( 1 - 0 )

Where : -

d=Average diameter of particles, equivalent to the diameter

of a sphere having the same volume.

g=acceleration of gravity

0=vacuum rate = volume of interstices

Total volume filled by bulk product

0 can vary from 0.4 to 0.6 for homogeneous particles of same dimensions).Cont -

=shape factor = diameter of sphere of same surface

diameter of sphere of same volume

=1 for a sphere

∂s=density of solid particles

∂∂=density of gas

=factor of dynamic viscosity of gas

The theoretical minimum speed of fluidisation, v0 is compared to the aptitude of a product real speed of fluidisation, vfm deducted from the gas flow qvfm

vfm = qvfm/S Where S in the area of the porous cloth.

If v0/ vfm = 1 The product is easily fluidisable

1< v0/ vfm < 1/2 The product is fluidisable

1/2 < v0/ vfm < 1/4 The product is difficult to fluidise

If v0/ vfm > 1 The product can be considered unfluidisable



flushingThe fluid-like behaviour of a dilated fine particulate material flowing with negligible restraint to deformation. The internal friction of the bulk is negated by the presence of air or other gas holding the particles sufficiently apart as to allow them total freedom of relative movement.

The condition can arise from high agitation, as with free fall, the collapse of a cohesive arch or as delivered from a pneumatic conveying system. Note, that such a flow condition is subjected to hydrostatic pressures. It is highly ■

searching and cannot be restrained other than by gas tight seal mechanisms, such a rotary valves or cut-off valves.

free flowingA particulate solids that offers no cohesive or rigid structural resistance to

of bulk material deformation under gravity flow.

funnel flowAn expression originally coined by Jenike in Bull 123 to describe the characteristic funnel shape flow pattern having a composite form. This comprises of a core flow channel, formed by a narrow cross section of flow emanating from the outlet, that is replenished by a drained repose surface. The term is often utilised to generically describe all flow patterns that are not mass flow. In this context it is held to be a misnomer and the use of ‘non-mass flow’ is a preferred expression to describe flow regimes in a hopper of any form that are not of a mass flow character.

Geldart’s classification A chart devised by D.Geldart to group the fluidising potential of powders into four classes according to their relationship on a graph of particle density against particle size.

Geldart Group AThe powders most easy to aerate are generally fine, but of very limited cohesion. They are slow to settle from an expanded condition and uniformly permeable in the expanded state. In their eventually settled state they may form a firm bed but this can be expanded and broken up by an upward gas flow.

Group BThe sand-like particle size of materials in this group are

considerably larger than in group A for any given particle density. Bubble of gas grow with bed depth and gas velocity and any expanded bed formed collapses quickly when gas flow ceases.

Group CPowders in group C are difficult to fluidise because inter-

particle forces exceed the hydrodynamic force created by the rising gas stream. Gas escape channels develop through cracks or fissures through planes of local weakness, or by bubbles, to inhibit the penetration necessary to separate the particles to a fluid condition.

Group DAre generally coarse and tend to be of high particle density

due to the easy with which air can pass through the interstitials and gas velocity needed to carry the weight of the particles, these powders are difficult to fluidise without the expenditure of great volumes of air that tends to spout through the bed. ■

gravity flowFlow of a mass of bulk stimulated by the force of gravity. (This is such an important aspect of bulk technology that a detailed review of the process is given in appendix 2). See flow

homogenising hopperA hopper used to blend the stored contents by means of air injection or the inclusion of flow modifying devices that change the zone order of filling and/or emptying the container.

homogenising siloSee homogenising hopper.

impact penetrationThe process of bed penetration and capture of large or dense particles

(of segregation) from the flow stream within the impact zone of a forming repose pile thereby giving rise to a concentration of these particles in the centre of the pile and fine being displaced to the periphery of the pile or container. This behaviour pattern arises when the forming bed is of weak composition and the kinetic energy of the larger, denser particles penetrate to a depth than prevents their unconfined rolling down the repose slope.

intensity ofThe degree of differentiation, due to segregation processes, of

segregation particulate fractions that have different physical properties. Note that the discretion of intensity tends to be related to the scale of scrutiny, therefore the acceptable degree of segregation intensity may have different values at different scrutiny scales.

internal flowThe formation of a flow channel within a mass of static product. ( See core flow, funnel flow).

live flowAn active flow channel

mass flowA flow pattern in which the entire contents of storage are mobilised to flow when discharge takes place from a bulk storage container. It is characterised by the fact that no stagnant zones are present during discharge when flow is fully mobilised. Slip essentially occurs on all wall contact surfaces. The flow pattern may relate to a container that has a converging region only, or to a two-stage pattern where the lower region converges and the upper portion moves in a bed flow pattern.

The flow velocity is not necessarily uniform across the flow channel and is invariably not so in a converging channel It should be noted that this mode of flow is dependent upon a combination of a specific bulk material in a container of given geometry and construction media and is not a feature of a specific form, type or geometry of storage vessel in isolation.

mass flow binSee mass flow hopper.

mass flow bunkerSee mass flow hopper.

mass flow hopperA storage container where the contents move in a mass flow pattern. ■

mass flow siloSee mass flow hopper.

mixed flow A flow pattern comprising of two zones in a bulk storage container. The lower region of flow channel is a core flow regime that expands within an outer zone of static material to intersect with the container walls some distance below the upper surface of the stored mass, to give an upper zone that extends as a bed flow pattern. cont.

Note, vertical movement of the total surface cross section should not allow this mode of flow to be confused with mass flow, where the total contents of the container are in motion. The term may also be applied to an expanded flow form that expands to intersect the container wall to create an upper bed flow section.

non-mass flowA preferred generic term for any flow pattern that does not embrace the movement of the total contents of a bulk storage container.

percolation1 - (of segregation)

The penetration of smaller fractions through a static bed of larger particles.

2 - (of gas permeation)

Leakage of gas into, through or from a bed of particles.

pipe flowA region of initial discharge down a narrow flow channel that is not sustained because the cross section is smaller than the critical rathole diameter. ( See rathole, piping and core flow).

pipingA narrow flow channel that develops within a static mass of bulk material that exhausts itself to leave an open void channel from the container outlet to the upper surface of the stored bulk material. See core flow, funnel flow.

plane flow Flow in a confined channel that converges in one plane only, as with a Vee or wedge shaped hopper.

poured coneThe profile of heap a bulk solid created by a single point fill. Note that, even with a steady feed stream, the slope is formed by material cascading in a series of surges, or small avalanches, in successive radial dispositions around the point of fill. Materials that tend to segregate deposit fines in strata of decreasing thickness from the peak as these surges peter out down the widening slope. Cont -

The cross sectional disposition of the segregated fractions within the formed pile is characterised by a ‘Christmas tree’ type of dispersion. Any bias of segregation in the feed stream leads to severe diversion of the fractions in the circumference of the pile. This effect can be mitigated by regular, minor variations of the fill position ■

poured reposeThe surface inclination of a pile formed by material building up from a feed point of steady supply. Normally of conical shape surface, but may be a plane surface if the boundary of the slope is a linear overflow plane or a moving dispensing device. In view of the variable flow condition of many bulk materials it is essential to closely define the conditions of the feed stock and the circumstances of the supply stream. See angle of repose, drained repose.

cont.

The poured repose and angle of repose measurement have no direct relation to the flow behaviour of a bulk material in a confined flow channel. Unless the inclination of the surface formed is independent of the method of preparation, or the test procedure reflects the specific application conditions of interest, the measured value has little meaning.

powder bedA mass of powder in a settled state, unless otherwise defined.

quakingA cyclic pattern of movement and stoppage of flow of a significant quantity of a stored bulk product that induces substantial inertial forces upon a storage container and its foundation. The phenomenon arises from various causes, such as frictional stick-slip, slip-stop, cyclic oscillation of a flow pattern that is marginal between mass and non-mass flow, ‘slurping’ and surface repose instability, as in a discharge chute or bin activator.

radial flowA flow pattern whereby the cross section reduces uniformly in radius as the channel progresses. See conical flow.

ratholeThe void left when a core flow pattern or pipe evacuates all the material in the flow channel, to leave a stable unconfined surface. Note: this behaviour is not applicable to a plane flow channel, unless the wall inclination is not self-clearing, as the absence of a central continuity across the flow channel leaves an un-restrained mass resting on each side wall. (See piping).

scale of segregationThe magnitude of the overall mass that is affected by segregation.

segregationThe migration of disparate fractions along separate paths of a particulate solids flow route as a result of the influence of common applied forces on the differing physical properties of the constituent particles that leads to a diversion of the particle paths such that the various fractions accumulate in different locations. The consequence is such that a bulk material of initially homogeneous composition becomes heterogeneous in nature.

Mechanisms prevailing in various regimes of flow lead to many types of segregating processes. The intensity and scale of segregation are dependent on the nature and scale of the operation. For further information on this behaviour, means to reduce, counter and rectify the effect, see ‘User Guide to Segregation’, published by the British Materials Handling Board.

self clearingA bulk solids container that empties without leaving residue. ■

shear mixingThe process of random migration of particles across a plane of shear generated within a mixer. ( See also diffusion and convective mixing).

sifting,(of segregation) The process of the dynamic penetration of smaller fractions through larger ones, in a dynamic bed of particles. See percolation.

sigma-two reliefA flow pattern that converges in one plane and diverges in the plane at right angles, thereby relaxing the minor principal stress of confinement. This feature allows the major dimension of deformation to occur at lower stresses, such that a particulate material will slip on converging hopper walls at reduced inclinations, have a smaller critical arch span in a Vee shaped hopper, reduce the prospect of structural arching and discharge material at higher flow rates through a given slot width. Flow must takes place over the whole cross section of the outlet of such a flow channel.

slip-stickA ‘Chattering’ motion of a solid on a contact surface due to the relaxation of flow generating stresses when movement takes place. The difference between static and dynamic friction brings the contact layer to rest, until the elasticity of the flow system accumulates adequate slip generating forces to again overcome static friction. See slip-stop.

slip-stopA behaviour pattern whereby a wave of dilatation moves through the bed from a discharge opening, until the forces supporting the static bed on the growing span of the flow boundary are relaxed sufficiently for the weight of the superimposed mass to exceed the mobilised wall friction. A sudden, gross movement of the contents compresses the dilated region and the mass is brought to a sudden rest because the gross rate of flow far exceeds the capacity of the outlet. Note, this differs from Slip-stick because the restraint is a compacted mass, and not static friction. The scale of inertial impact is generally greater than developed by slip-stick. See quaking.

slurpingA phenomenon of erratic discharge arising from the unconfined failure surface above the outlet orifice of a container suffering dilated or air-retarded flow. The rate of disengagement of particles from the body of bulk material is limited by the low permeability of the bed, which offers resistance to penetration of the ambient gas to allow void expansion for the particles to separate. The effect of this cyclic increase in span of the draining or dynamic arch at the approach to the container outlet, followed by collapse, is to develop a fluctuating rate of flow as the surface area of failure increases with surface release until it reduces rapidly to a smaller value. When the unconfined span attained is larger than the critical orifice size, the internal stresses overcome the strength of the bulk material, such that the arch collapses. The cycle of progressive surface failure is re-commenced from the material deposited around the container outlet. See Slip-stop, quaking.

solifluctionCreep of repose slopes. Characteristic of, but not restricted to, regions ■

solifluxionsubjected to periods of alternate freezing and thawing.

steady state flow Flow in a condition of steady density as each section of the flow channel is characterised by a unique relationship between the shear stress and the normal stress acting on the yielding surface.

structural archingThe formation of a flow obstruction caused by the coming together in the flow path of discrete large particles, agglomerates or lumps that interact by physical contact to form a stable structure over the dimensions of the flow channel. This is a stochastic process. The probability of a stable arch forming is dependent upon many variables, of different importance, as -

-The size and shape of the orifice in relation to the particle size(s). (Major factor)

-The inclination and surface conditions of the approach walls of the flow channel.

-The rate of flow, whether commencing or sustained.

-The size, shape, size distribution of the constituent particles. (Major factor)

-The surface condition of the particles, (internal angle of friction of the bulk).

-whether the material is slipping on the approach walls to the opening .

-The viscosity of the interstitial fluid in the voids (A minor influence).

A general rule of thumb is that structural blockages, held by mechanical forces only, will not occur if a round orifice is greater than eight times the largest particles size, or five times in the case of a slot shaped opening. (cohesion introduces extra considerations)

Some prudence and interpretation is required to assess suitable opening sizes when there is a wide particle size distribution, the lumps are infrequent or erratic, or are of indeterminate lump size. Precautions are best made by the introduction of lump traps or flow channel shapes, such as slots or annuluses, that will not block by a single lump. See, structurally retarded flow and dynamic arching.

structurally impeded flowThe. See structural retarded flow, structural arching.

structurally retarded flowRestraint to smooth flow and reduction of flow rate due to the transient formation and collapse of dynamic or transient structural arches in the flow stream induced by structural re-ordering of the particulate array that intermittently form a temporary obstacle to reduce the flow velocity within a confined channel by way of an unstable mechanical blockage. A stochastic process that results in an erratic rate of discharge.

Note that the boundary between structurally retarded flow and the prospect of formation of a stable structural arch is indeterminate, being dependent on fine interactions of many factors. See structural arching.

surchargeMaterial piled above the level of the peripheral storage edges of the confined contents of a container.

surface reposeThe maximum sustainable inclination of surface profile adopted by a bulk material under given conditions of formation. See angle of repose ■

transitionThe junction between the body section and the converging hopper section of a bulk storage container.

transitional stateThe interim condition of a deforming bulk material during flow initiation or change whereby the bulk density is changing in response to variable stress condition acting on the material.

unconfined flow The flow of a bulk material where part of the flow channel is not constrained by confining boundaries.

unconfined surface A product bed or flow stream boundary that is not restrained by a contact surface.

waves of dilationThe progression through a bed of bulk material of a gravitational wave of dilation and local collapse following the opening of an outlet valve or the commencement of a feeder or other discharge device. ■

Section 14Pneumatic Conveying

actual gas velocityThe conveying gas volume flow, at the existing pressure and temperature of location, divided by the area of the empty pipeline. It is normally expressed in unit distance in unit time. Whereas the mass of conveying gas is usually constant over the entire length of the conveying pipe, the actual gas velocity increases as the pressure reduces with distance from the origin. The figure normally cited is an average value, because the local gas velocity differs in different parts of the cross section.

air retentionThe ability of the material conveyed to retain air, (or another gas), in the voids after discharge from the pneumatic conveying system in a dilated condition. This ability depends upon the permeability of the bulk material for the specific viscosity of the gas concerned. See fluidity, de-aeration

‘angle hairs’Long thin ‘streamers’ that are formed in the lean phase handling of low melting point granules, such as plastics, due to the high velocity frictional contact of the particles on the pipe walls melting the particle surface and forming extended length of narrow thin films or fibres that become entangled in the subsequent bulk.

average gas velocity The mean of the actual gas velocity at the commencement of the pipeline and the terminal gas velocity.

blinded bendA change of direction affected by a closed end of pipe in the initial direction of flow. The gas stream is re-directed by a static bed of captured particles in the closed end of the pipe to a side outlet.

choking The phenomenon of the formation of a slugging, fluidised bed forming in a vertical section of a pneumatic conveying line as the gas velocity falls below the level at which it can entrain the solids.

choking velocityThe superficial gas velocity at which choking occurs.

Note: 1.However, for mixed sized particles the velocity at which choking occurs is usually lower than the saltation velocity

Note: 2.An alternative definition for choking velocity takes the superficial velocity at which the entire suspension collapses into slug flow as the choking velocity. However not all powders can be made to collapse into slug flow and the former definition is preferred.

conveying line exit velocitySee terminal gas velocity ■

combined vacuum/pressure systemPneumatic operation under suction to a single gas and material receiving vessel with the same unit providing a pressure system to one or several gas and material receiving vessels.

dense phaseA pneumatic conveying system characterised by low gas velocities, ( 1 – 5 m/sec), high solids concentrations, ( . 30% by volume), and high pressure drops per unit length of pipe, ( typically > 20 mbar/m).

Note: The products carried in this mode uses gas velocities lower than those required for lean phase conveying. The nature of dense phase flow is very varied, for it depends upon the properties of the bulk solid and the conveying gas velocity.

Typical dense phase flow patterns include flow over a deposited layer, which may itself be moving slowly, and flow in discrete slugs of material that may form naturally, sometimes with material falling away from the ‘tail’ of one slug to be picked up and carried forward by the following slug.

Whilst this mode is more energy efficient than lean phase and treats the product more gently, the system design is more demanding and the range of bulk solids that can be transported in this mode is limited

dilute phase See lean phase.

dune flowMovement of a saltated layer on product along the conveying pipe in the form of waves of the material.

ending gas velocitySee terminal gas velocity

fluidised flowThe pouring or otherwise causing to move of fluidised solids under the influence of gravity or pressure gradient.

free air conditionsThose conditions at which p = 101.3N/m2 absolute (standard atmospheric pressure) and t = 150 C (standard atmospheric temperature)

Note: Free air conditions are generally used as the reference for the specification of positive pressure air movers.

free air delivered. (FAD) See volumetric gas flow

free air velocityThe velocity of the air at free air conditions.

interstitial gas velocitySee actual gas velocity. ■

lean phaseA pneumatic conveying system characterised by high gas velocities, (< 20m/sec), low solids concentrations, (< 1% by volume), and low pressure drops per unit length of conveying line. (typically < 5mbar/m).

Note: It is necessary to exceed a minimum value of conveying line inlet gas velocity to produce sufficient drag on the solid particles to convey by lean phase. The vast majority of particulate materials can be conveyed in this mode.

material massflow rateThe mass of material conveyed over a specific time period. Also known as conveying rate or system capacity.

material velocityThe velocity of the solid conveyed. This is always less than the local gas velocity and is usually specified as either an average (or mean) velocity, or the terminal velocity of the product from the conveying pipe. This is almost invariably an estimated figure because reliable means to measure the progress of individual particles are very limited.

mean gas velocitySee average gas velocity

minimum conveying velocityThe lowest gas velocity that can be used to ensure stable pneumatic conveying conditions for the material. This velocity must pertain at the feed point of the system, because the gas accelerates with the reduction in pressure along the flow route. Hence it is also known as the pick-up velocity.

multi-discharge systemPneumatic conveying operation by blowing or suction, with discharge into several gas and receiving gas and material separators and material receiving vessels.

multi-pick up systemPneumatic conveying operation under suction with multiple feeder units.

null pointThe position in a system where the pressure is equal to the ambient pressure.

Note: This is often used in relation to closed loop systems and can identify a natural point of access to the system for monitoring or conditioning.

pick up velocitySee minimum conveying velocity

plug flowThe movement of plugs of solid that occupy the full cross section of the conveying pipe

saltationDeposition of product in pipe as it is no longer carried forward within the gas stream. ■

saltating flowProgression of the product along the lower boundary of the conveying pipe. Particles are conveyed in suspension above a layer of settled solids. Particles may be deposited and re- entrained from the static or slow moving layer.

saltation velocityThe gas velocity in a horizontal pipeline below which particles mixed homogeneously with the conveying gas will fall out of the gas stream.

simple pressure systemPneumatic conveying operation under pressure by blowing with discharge into a single gas and material separator and material receiving vessel.

simple vacuum systemPneumatic conveying operation under suction, with discharge into a single gas and material separator and material receiving vessel.

slip velocityThe difference between the gas and particle velocity.

superficial gas velocityThe velocity of the gas disregarding the volume of solid particles or porous media at the temperature and pressure conditions under consideration within the pipeline.

Note: In a pipeline it is the velocity based upon the cross sectional area and neglects the space occupied by the conveyed product. For flow through a membrane or across a filter, it is the open duct velocity normal to the surface. Gas velocity is dependent upon both pressure and temperature, and so when conveying gas velocities are evaluated at any point is a system the local values of pressure and temperature at that point must be used.

suspension flowSee lean phase

terminal gas velocityThe velocity of the gas as it exists the system.

volumetric gas flowIt should be noted that several different terms can be used to describe volumetric gas flow. The volumetric gas flow during conveying is expressed as free air delivered, (FAD). The output of most air movers are specified in terms of FAD, being the volumetric gas flow at the suction port of a positive-pressure blower or compressor, or at the discharge port of a vacuum blower or vacuum pump. This reflects the volumetric gas flow in the actual conditions where the equipment is located.

wall heatingThe gain of local temperature on contact surfaces by virtue of high velocity frictional contact during lean phase conveying. This energy input may cause unwanted heating of the product, e.g. when handling frozen or heat sensitive products See ‘angle hairs’. ■

Appendix I -Rand Report Summary

Rand reportEd Merrows 1985

A review of performance in 37 new Solids Handling plants showed remarkable shortfall compared with the industrial norm in plants handling liquids and gasses.

Main findings: -

Two thirds operated at less than 80% capacity at end of first year and a quarter at less than 40%. The average working capacity at end of year, one was 64%, compared with 90 – 95% for industry as whole. Only 6% of the facilities had no ‘major problem’ in first year of operation, (defined as a shutdown for one week or more).

Most difficulties lay in flow and material behaviour problems, rather than due to the quality of engineering or basic process chemistry.

No general improvement showed over earlier study in 1960.

Most R & D is invested in process chemistry. Physical problems often not considered worthy of study. Handling problems that may arise are expected to be resolved during start-up.

Conclusions -More research needed on the behaviour of solids.

-More education required in solids behaviour.

-Better feedback needed from plant to design.

-More routine solids testing needed for plant design.

Editors Views-The current position shows only limited improvement.

-Performance requires total flow route reliability.

-Capital cost tends to dominate purchasing decisions, probably

because handling is perceived as a non-value adding item, rather than a fundamental necessity to be optimised in cost and effect

-Industry requires a simple and inexpensive ‘screening’ test to determine the nature and magnitude of potential flow problems, to evaluate the extent of further investigation that is warranted.

-Attention to detail most often lacking on low cost items. ■

Appendix II-Gravity Flow Stresses in bulk materials

A grasp of the difference between active and passive stresses is necessary to understand how bulk materials behave in storage hoppers and silos. An active stress is one that presses onto a contact surface due to the forces generated within the body of the material. If the surface were to be slightly moved away from the material, such stresses will follow and continue to act with virtually the same pressure. A passive stress is caused by the resistance offered to a bulk material against any surface that is trying to compact the mass of product. If the surface is withdrawn slightly, this pressure ceases. A simple case to illustrate the difference is that of retaining walls for a stockpile. Product that is piled against the walls exerts a force caused by the horizontal stresses generated within the bulk material. If the wall is withdrawn slightly, the material will normally collapse and form a new shape pressing with a similar force. An exception may occur if the material were cohesive, in which case the pile may stand as a vertical cliff because the internal strength of the material is sufficient to contain these internal stresses.

A contrasting situation is if an attempt were made to push the walls into the pile. This would give rise to a passive resistance with a magnitude larger than the original active stress. It is easy to see that the effort required must be enough to overcome the original force pushing out and have the extra work of compressing the bulk and/or pushing up the level of the pile. Contrary to what the words ‘active’ and ‘passive’ may suggest, this example graphically indicates that passive stresses can be substantially greater that active stresses.

What generates an active stress in the first place? Well, a void gas under pressure is a classic form of an active stress. When a bulk material is delivered into a container it is usually in a dilate condition. The particles move closer together as the bulk settles into a stationary bed. Air is expressed from the voids through the remaining interstitial gaps, the escape path becoming longer as the bed depth increases. With fine particles it can be a long process for the void air pressure to come to equilibrium with the surrounding ambient atmosphere. Until this happens the void pressure supports part of the superimposed weight of the mass, thereby delaying the particle-to-particle contact pressure from achieving its ultimate value. Initially, the particle contact forces maybe so small that they can move against each other with ease, and the bulk can behave as a fluid. In these circumstances the active pressure on the wall is hydrostatic, dependent on the bed depth and the effective density of the product in this state.

With coarse materials this effect is less dramatic and short lived because the air can readily percolate through the larger void gaps. Nevertheless, there is almost invariably a temporary, declining void pressure in a recently filled bed of loose solids. The other, growing and longer lasting source of active pressure is that arising from the particles being ‘squashed’ by the overburden. Any solid will tend to deform under stress, reducing in dimension in line with a compressive stress and, unless restrained, expanding to a less degree at right angles to the axis of the compressive stress. The ratio of this dimensional change is termed the ‘Poission ratio’, and is generally of the order of 0.4. A simple compressive load of this type causes the ‘major principal stress’ in the mass and the transverse stress generated by this is called the ‘minimum principal stress. The force needed to contain the ‘sides’ of the material to its initial position is equal to this minimum principal stress. ■

Now consider what occurs when a stored product slides from a parallel, body section of a hopper into the converging section. In the parallel section the material moves in ‘bed flow’ without change in cross section, so the walls need only contain the active bulk stresses, which remain virtually unchanged during this phase. Moving into the converging section, forces are exerted by the walls to compress material to a smaller cross section. There is a resistance to this deformation requiring work input, in addition to that needed to overcome the original active stresses. It is considerably more difficult to overcome the initial structural resistance to deformation than it is to continue the failure process. There is also a reduction in the containment pressure as the material approaches the final outlet. For these reasons, the severe change in stress that occurs at the transition point is followed by a reduced value as the material continues downwards.

In this converging section the stress pattern has radically changed from that pertaining before flow commenced. Originally, the greatest stress was due to the vertical pressure of material weight. Now, the maximum stress direction is across the bed, causing the cross section to reduce in width during flow. A shear force also acts on the wall boundary due to the frictional drag on the contact surface. The combination of compressive and shear stress forms a maximum principal stress direction inclined upwards from the wall surface. This stress line bends over into an arc within the bulk to meet the opposing wall at a similar angle. The magnitude of this stress diminishes as the material nears the outlet, because the material is eventually not confined at the outlet location as the material falls or is taken away. The most likely location for a stable arch t0 form is near the outlet, where the span is small. This will happen if the unconfined failure strength of the material in this region exceeds the stresses available to cause failure.

Such a situation is more probable from first-fill condition because, as previously described, the stress required to ‘switch’ the state of wall stress from active to passive is much greater that the stress needed to sustain the passive stress. This situation will apply to material in the whole converging section, including the critical outlet region of smallest span, unless flow has taken place. For this reason, the Jenike method requires that a small amount of material is withdrawn from the outlet to initiate flow stresses in the region local to the outlet, before the vertical stresses raised by the weight of the material being loaded develops a stronger bulk material. It is not usually important to initiate a flow field through the total converging hopper section because the span of the flow channel at the upper levels is normally greater than that of a potential arch. Flow opposing stresses within the material in this upper region are readily are overcome by the large work content available from the prevailing forces when flow eventually takes place.

The stress pattern developed in the bulk during flow remains in place as flow stops. The Jenike method of hopper design is founded upon measuring this state of stress on the basis that once this flow state has been achieved, it will reliably restart again when similar circumstances re-occur, i.e. when the outlet once again allows material to escape. This is provided the material is not allowed to stand for some time and allow the bulk to gain strength as the dilated flow condition settles to a firmer bed. Measurement of any increase in bulk strength with time under a steady compacting load is measured by ‘time compaction’ tests, where test samples are loaded in a static condition for an equivalent period to the time of standing without discharge taking place.

An understanding of these flow mechanisms is a useful introduction to the terminology relating to mass flow and the basics of modern practice in hopper design. ■

Appendix III-Relevant Standards

B.S. 410:1976, Specification for test sieves

B.S. 598-104:1989, Sampling and examination of bituminous mixes for roads and other paved areas.

Method of test for the determination of density and compaction.

B.S. 812:Methods for sampling and testing of mineral aggregates, sands and fillers

Part 1: 1975, Sampling, size, shape and classification

Part 2: 1975, Physical properties

Part 3:1975, Mechanical properties

B.S. 1016, Methods for analysing and testing colas and coke.

Part 1:1973, Total moisture of coal

Part 2 Total moisture of coke

Part 3 Proximate analysis of coal

Part 4 Moisture, volatile matter and ash in the analysis sample of coke

Part 13:1980, Tests special to coke

Part 17:1979, Size analysis of coal

Part 18:1981, Size analysis of coke

Part 19:1980, Determination of the index of abrasion of coal

Part 20:1981, Determination of Hardgrove grindability of hard coal

B.S. 1017, Methods for sampling coal and coke.

Part 1:1977, Sampling of coal

B.S. 1377:1975, Method of test for soil for civil engineering purposes.

B.S. 1460:1967, Method for determining apparent density after compaction of precipitated calcium carbonate

B.S. 1703:1977, Specification for refuse chutes and hoppers.

B.S. 1743:1992, Method of analysis of dried milk and dried milk products.

Determination of bulk density.

B.S. 1796:1976, Methods for test sieving

B.S. 1902. Part. 3 Method of testing refractory material. General and textural properties.

Section 3.4:1981, Determination of true density, (photometric method).

Section 3.5:1981, Determination of true density, (powder method 1902-304).

Section 3.6:1984, Determination of grain density, (method 1902-305).

Section 3.7:1989, Determination of bulk density and true porosity of shaped insulating

products. (method 1902-308).

Section 3.8:1989, Determination of bulk density and apparent porosity of shaped insulating

products. (method 1902-317).

Section 3.7:1989, Determination of bulk density and true porosity of dense shaped products.

Section 3.17:1990, Determination of bulk density and volume of dense shaped products.

B.S. 2701:1956, Specification of Rees-Hugill powder density flask.

B.S. 2782-Method 824A:1966, Method of testing plastics. Other properties. Sheet and film. Determination of the coefficient of friction.

B.S. 2975, Method of analysis and sampling of glass making sands.

B.S. 2955:1958, Glossary of terms relating to powders

B.S. 3029:1958, Method for determining the compressibility of metal powders.

B.S. 3272:1960, Aluminium food storage bins.

B.S. 3400:1967, Method of test for dust in filling materials

B.S. 3406: Methods for the determination of particle size of powders

Part 1:1961, Sub-division of gross sample down to 0.2ml

Part 2:1963, Liquid sedimentation methods

Part 3:1963, Air elutriation methods ■

Part 4:1963, Optical microscope method

Part 6:1985, Recommendations for centrifugal sedimentation method for liquids and powders.

Part 7:1988, Single particle light interaction method.

Part 8:1997, Photon corrolation spectroscopy.

Part 9:1989, Recommendations for filter blocking method. (mesh obscuration).

B.S. 3482:1991, Method of test for desiccants. Determination of bulk density. (dry basis).

B.S. 3483, Methods for testing pigments for paints

Part B8, Determination of density relative to water at 40C

B.S. 3625:1963, Eyepiece and screen graticules for the determination of particle size of powders

B.S. 3810-2:1965, Glossary of terms in materials handling. Terms connected with conveyors and elevators. (excluding pneumatic and hydraulic handling).

B.S 3762-4:1986, Analysis of formulated detergents. Physical test methods.

Methods for determination of apparent bulk density.

B.S. 3900:1993, Method of test for paints. Testing of coating powders.

J.2, Determination of particle size distribution of coating powders by sieving.

J.4, Determination of storage stability of coating powders.

J.6, Determination of density of coating powders by gas comparison pyknometer. (Referee method)

J.7, Determination of density of coating powders by liquid displacement pyknometer.

J.8, Calculation of lower explosion limit for coating powders

J.9, Determination of flow properties of a coating powder/air mixture.

J.11, Determination of flow properties of a coating powder. (Incline plane method).

J.13, Particle size analysis by laser diffraction.

B.S. 4140, , Method of test for Aluminium Oxide.

Section 8:1986, Determination of absolute density using a liquid displacement pyknometer.

Section 8:1986, Determination of un-tamped density.

Section 21:1980, Method of test for aluminium oxide particle size analysis.

Section 22:1987, Determination of fine particle distribution (method using electroformed

sieves).

B.S. 4317, Method of test for cereals and pulses.

Section 23:1990, Determination of bulk density of cereals called ’mass per hectolitre’. (reference method).

Section 32:1990, Determination of bulk density of cereals called ’mass per hectolitre’. (routine method).

B.S. 4359: Methods for determination of specific surface of powders

Part 1:1969, Nitrogen adsorption (BET)

Part 2:1971, Air permeability method

Part 3:1970, Calculations from the particle size distribution

B.S. 4409-1:1991, Screw conveyors. Specification for feed trough type.

-2:1991, Screw conveyors. Specification for portable and mobile type. (augers).

-3:1982, Screw conveyors. Method for calculating drive powers.

B.S. 4550, Methods of testing cement

Part 3:1970, Section 3,2:1978, Density test, Section 3.3.1978, Fineness test

B.S. 5551:Part 3, Section 3.5:1986, Fertilizers. Physical properties. Method of determining particle size by test sieving.

B.S. 5600, Powder metallurgical materials and products. Method if testing and analysis of hard metals

2:1981, General information. Glossary of terms.

Part 4, Section 4.17:1981, Compression test

B.S. 5663:1979, Method of testing ores - Determination of moisture content

B.S. 5667:1979, Specification for continuous mechanical handling equipment – Safety requirements.

- 1, Loose bulk materials.

- 2, Pneumatic handling equipment.

- 3, Storage equipment fed by pneumatic handling equipment. ■

- 4, Mobile suction pipes suspended by derrick jibs using pneumatic hoses.

- 5, Couplings and loose components used in pneumatic handling.

- 6, Rotary feeders.

- 7, Rotary drum feeders and rotary valves.

- 8, Hand operated power shovels.

- 9, Bulk throwers.

-10, Vertical screw conveyors

B.S. 5752:10:1986, method of test for coffee and coffee products. Instant coffee size analysis.

B.S. 5958-1:1991 Code of practice for the control of undesirable static electricity. General considerations

B.S. 6043: Method of sampling and test for carbonaceous materials used in aluminium manufacture.

2.9:2000, Determination of particle size distribution

2.12:1994, Determination of particle size distribution. Fine coke.

2.31:1996, Determination of tapped bulk density of green and calcined coke.

3.3:2000, Determination of bulk density (apparent density) of cathode blocks and pre-baked anodes.

3.4:2000, Determination of the open porosity and bulk density (apparent density) of cathode blocks and pre-baked anodes.

B.S. 6049-8:1998, method of testing for tea. Classification of grade by particle size analysis.

B.S. 6147:1979, Method of test for bulk density of iron ores

Part 1:1982, Determination of the bulk density of iron ore samples having a maximum

particle size of 40mm or smaller.

Part 2:1981 Determination of the bulk density of iron ore samples having a maximum

particle size greater than 40mm

B.S. 6318:1982, Classification of bucket elevators.

B.S. 6379:1984, Sampling of coffee and coffee products. Method of sampling of instant coffee in cases

with liners.

B.S. 6989:1989, Guide to safety of storage equipment for loose bulk materials.

B.S. ISO 130.9276., Representation of results from a particle size analysis.

Part 1, Graphical presentation.

Part 2, Calculation of average particle size/diameter and moments from particle size distribution.

Part 4, Characterisation of a classification process.

ISO 3435-1077 (E) Continuous mechanical handling equipment – Classification and symbolization of

bulk materials

B.S. ISO 11323:1996, Iron ores. Vocabulary.

B.S. ISO 13317, Determination of particle size distribution by gravitational liquid sedimentation methods.

Part 1, General principals and guidelines.

Part 2, Fixed pipette method.

Part 3, X ray gravitational technique.

B.S. ISO 13320:19999, Particle size analysis, Laser diffraction method. General principals.

B.S. ISO 14887-2000 Sample preparation. Dispersing procedures for powders in liquids.

B.S. EN, 481:1993, Workplace atmospheres. Size fraction definitions for measurement of airborne

particles.

BS EN 617:2002, Continuous handling equipment and systems- Safety and EMC requirements

for the equipment for the storage of bulk materials in silos, bunkers, bins and hoppers

BS EN 618:2002, Continuous handling equipment and systems- Safety and EMC requirements

for mechanical handling of bulk materials except fixed belt conveyors

BS EN 620:2002, Continuous handling equipment and systems- Safety and EMC requirements for fixed

belt conveyors for bulk material

B.S. EN 725:1986, Advanced technical ceramics. Method of test for ceramic powders.

Determination of particle size distribution. ■