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CC-E - Best Practice Design, Maintenance and Troubleshooting of Conveyors and Chutes

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Designed for engineers and technicians from a wide range of abilities and backgrounds, this manual covers basic conveyors, selection, safety, legal obligations, terminology and background. It is an excellent introduction to troubleshooting and maintenance of conveyors and chutes.

It is intended to cover the fundamentals of belt conveying and would be useful for those with little experience in this area. Also featuring numerous tips and case studies throughout, this manual is a collection of important information in one place.

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Table of Contents

Introduction - Best Practice Design, Maintenance and Troubleshooting of Conveyors and Chutes

1       Introduction

1.1          Introduction

Material handling plays an important part in the modern economy. No modern industrial plant: be it a coal mine, power plant, cement plant or a metallurgical plant, would be conceivable without an efficient transport system. Conveying equipment, of one or several types, is usually employed to mechanize material handling, loading and unloading operations. Conveying equipment works in conjunction with process equipment, such as that used for crushing, screening, blending etc. Overall mechanization of the processes becomes effective with appropriate selection of material handling equipment. This equipment not only substitutes manual labor, but also helps in the rational matching with all equipment responsible for the manufacturing processes, thereby enhancing the overall mechanization. Nowadays the operation and control of an entire plant is done from a properly networked centralized control room.  Even the troubleshooting is carried out from there for detecting problem areas and initiating maintenance activities.

The boiler stoking system in a thermal power plant requiring supply of coal round the clock, materials transported in blast furnaces and materials conveyed from underground and open pit mines are some of the important areas where material handling plays a vital role.    

1.2          Classification and characteristics of materials

The type of material handled and its physical as well as mechanical properties are the principal factors determining the type and design of conveying equipment and its accessories. Bulk materials include various heap-loaded, granular and powdered materials such as coal, ore, molding sand, saw dust, food grains and so on.

Bulk materials are characterized by their physical and mechanical properties, such as:

  • Lump size: This refers to the quantitative distribution of the particles of a particular bulk material according to their sizes and is also known as granulometric composition of the material. It is characterized by the particle size denoted by diagonal a(Figure 1.1) in mm. A number of parameters related to conveyors and auxiliary equipment are determined by this characteristic:

 

Figure 1.1

Particle size

Lump size is determined through a consecutive screening of the material through meshes of different sizes. According to the uniformity of lumps in its composition, a bulk material is classified as sized or graded and unsized or non-graded.

A material, in which the ratio between the largest characteristic particle amax and the smallest characteristic particle amin is above 2.5, is considered to be unsized.

In sized materials, i.e. more or less homogeneous ones, amaxamin < 2.5. Sized materials are characterized by their average lump-size, for example:

 

In the unsized material, if the weight of a group of particles of lump size ranging between 0.8 amax and amax is greater than 10% of the total weight of the sample, then the material is characterized by lump-size amax and if it is otherwise, the characterization is done as 0.8 amax.

Tables 1.1 and 1.2 below show sizekWise classification of bulk materials and the recommended maximum lump size for different belt widths.

Table 1.1

Material characteristics

Material Characteristic

Size (mm)

Large-Lumped

Over 160

Medium-Lumped

60 to 160

Small-Lumped

10 to 60

Granular

3 to 10

Fine

0.5 to 3

Very Fine

Below 0.5

Table 1.2

Maximum lump size for different belt widths

Belt width in mm

Uniform lumps (mm)

Mixed with roughly 80% fines (mm)

300

-

-

350

50

100

400

75

125

450

100

150

500

100

175

600

125

200

650

125

250

750

150

300

800

150

300

900

175

325

1000

200

375

1050

200

375

1200

300

450

1350

300

500

1400

300

600

1500

350

600

1600

375

600

1800

450

600

2000

450

600

1.3          Properties of the conveyed material

  • Bulk density:It is the weight of the material per unit of volume in bulk (the volume including the voids or air pockets present in the heap) and is generally denoted by with the units of measurement being tons/cubic meter and pounds/cubic inch. The bulk density of some of the most frequently used materials is mentioned in Table 1.2. It is an important consideration particularly when the capacity of a conveyor and the pressure on the walls and outlet of a hopper is to be calculated. The loose bulk density of a material can be determined by weighing samples of a known volume of uncompacted material. Most ores have varying bulk densities based on the amount of impurities present and the particle size. It is therefore essential to evaluate a reasonable number of samples in order to determine the likely range in the bulk density values.
  • Specific weight:It is the weight of the material particles dried at a temperature of 100 to 105°C, with respect to the volume of water displaced by them. The specific weight of materials must be taken into account in order to calculate the capacity of pneumatic and hydraulic material handling equipment.
  • Particle size and shape: The size of the lumps and the lump to fines ratio can influence the burden surcharge angle, while the particle shape can affect material flow in the chute and also the amount of belt wear.
  • Maximum lump size: This is, in turn, dependent on the material characteristics and the crusher type employed. Large lumps tend to occur on conveyors handling mining products and primary ores. It is important for the maximum lump size to be established, as large slabs of material can pass through crushers.
  • Angle of repose:This defines the mobility or flowability of material and is defined as the angle between the surface of a freely formed pile of the material and the horizontal. When a loose material spills unobstructed on a horizontal plane it assumes a slope. The angle of this slope with respect to the horizontal plane is its angle of repose: j. (Refer Figure 1.2). The Angle of Repose is used as the base value for determining the burden surcharge angle.
  • Angle of surcharge:This is the angle to the horizontal which the surface of the material assumes when the material is at rest on a horizontal supporting surface vibrating vertically. This feature also defines the mobility or flowability of the material. Angle of surcharge is approximately 5 to 15° less than the angle of repose:

 

Figure 1.2

Angle of repose

  • Internal friction angle: Materials with high internal friction angles will normally give higher burden surcharge angles and are less likely to slump when the belt flattens out at the discharge pulley. The internal friction angle can be determined by a material shear test, which in turn gives an indication of the behavior of different materials on a troughed belt conveyor.
  • Coefficient of friction: This factor is taken into account for bulk material  in contact with steel, wood, concrete, rubber and so forth when designing conveying machines and auxiliary equipment. The friction factor determines the angle of inclination of walls and ribs of hoppers, chutes and also the maximum inclination of certain conveyors.
  • Abrasivity: The tendency of the particles of bulk materials to wear away the surface they are in contact with, when in motion, is known as the abrasivity of the material. The extent of abrasion depends on the hardness, surface condition, shape and size of the particles. Some bulk materials such as bauxite, iron ore, sand and coke are highly abrasive.
  • Specific properties: These include moisture content, stickiness, fragility, hygroscopy, toxicity, corrosiveness etc. All these properties need to be considered when designing conveying machines and auxiliary equipment, and effective measures are taken to neutralize their harmful influence. Let us briefly discuss some of these properties:
  • Moisture Content: Tends to have a marked influence on the burden surcharge angle as well as slumping of the conveyed material at the discharge pulley.
  • Cohesion:This property is based on the angle of repose, the method of classifying the cohesive properties of a material is provided by ISO 3435.
  • Temperature:Is a very important consideration. Any material temperature that is significantly higher than the ambient temperature may prove detrimental to the belt cover, necessitating the use of a special heat-resistant rubber (see Table 1.3 and Table 1.4).

Table 1.3

Properties of most commonly used bulk material (approximate values)

         Material

Bulk Density, g

Tons per cu.m

Angle of Repose, j

degree

Anthracite, fine, dry

0.8 to 0.95

45

Gypsum, small-lumped

1.2 to 1.4

40

Clay, dry, small-lumped

1.0 to 1.5

50

Gravel

1.5 to 1.9

45

Foundry sand, shake-out

1.25 to 1.30

45

Ash, dry

0.4 to 0.6

50

Limestone, small lumped

1.1 to 1.5

38

Coke

0.36 to 0.53

50

Wheat

0.65 to 0.83

35

Saw dust

0.16 to 0.32

39

Sand, dry

1.4 to 1.65

45

Iron ore

2.1 to 2.4

50

Coal, run of mine

0.8 to 1.0

38

Cement, dry

1.0 to 1.3

40

Crushed stone, dry

1.8

45

Slag, blast furnace, crushed

1.3 to 1.4

25

 

Table 1.4

Bulk material characteristics

Material

 

Material bulk/density (kg/m3)

Surcharge angle degrees

 

Recommended maximum inclination degrees

Code

 

Alum, fine

721-802

 

 

B35

Alum, lumpy

802-962

 

 

D35

Alumina

802-1042

10

10-12

B27M

Ammonium nitrate

721

 

 

•C36NUS

Asbestos shred

320-401

 

 

E46XY

Ash, black, ground

1683

15

17

•B35

Ashes, coal, dry, 13mm and under

561-641

20

20-25

C46TY

Ashes, coal, dry, 76mm and under

561-641

 

 

 

 

 

D46T

 

Ashes, coal, wet 13mm and under

721-802

 

25

 

23-27

 

C46T

 

Ashes, coal, wet 76mm and under

721-802

 

 

 

 

 

C46T

 

Ashes, fly

641-721

20

20-25

A47

Ashes, gas-producer, wet

1250

 

 

D47T

Asphalt, binder for paving

1283-1363

 

 

 

 

 

C45

 

Asphalt, crushed, 13mm and under

721

 

 

 

 

 

C35

 

Bagasse

112-160

 

 

E45Y

Bark, wood, refuse

160-320

20

27

E46Y

Barley

609

10

10-15

B15N

Barytes, powdered

1924-2245

 

 

B26

Bauxite, ground, dry

1090

15

20

B26

Bauxite, mine run

1283-1443

15

17

037

Bauxite, crushed 76mm and under

1202-1363

 

 

 

20

 

D37

 

Bentonite, crude

561-641

 

 

D46X

Bentonite, 100 mesh and under

802-962

 

 

 

20

 

A26XY

 

Bones

545-641

 

 

*

Bonemeal

882-962

 

 

B36

Borax, 50mm to 100mm lumps

962-1042

 

 

 

 

 

D36

 

Borax, 40 to 50 mm lumps

882-962

 

 

 

 

 

D36

 

Brewer's grain, spent, dry

401-480

 

 

C45

Brewer's grain, spent, wet

882-962

 

 

C45T

Brick, hard

2004

 

 

D47Z

Brick, soft

1603

 

 

D47

BuckWheat

641-673

10

11-13

B25N

Carbon, black, pefletised

320-401

 

 

B15Q

Cardon, black, powder

64-112

 

 

•A35Y

Carborundum, 61mm and under

1603

 

 

 

 

 

D27

 

Cement, Portland

1507

20

20-23

A26M

Cement, Portland, aerated

962-1202

 

 

A16M

Cement rock (see limestone)

1603-1764

 

 

 

 

 

D36

 

Cement clinker

1202-1523

15-20

18-20

D37

Cement mortar

2132

 

 

37Q

Chalk, lumpy

1202-1363

 

 

D26

Chalk, 100 mesh and under

1042-1202

 

 

 

 

 

A46MXY

 

Charcoal

289-401

15

20-25

D36Q

Chips, paper mill

320-401

 

 

E45

Chips, paper mill, softwood

192-480

 

 

 

 

 

E45

 

Clay (see also bentorite, diatomaceous earth, fullers earth, kaolin and Marl)

Clay, calcined

1283-1603

 

 

B37

Clay, dry, fines

1603-1924

15

20-22

C37

Clay, dry, lumpy

962-1202

15

18-20

D36

Coal, anthracite, river or culm, 32mm and under

962

15

18

 

B35TY

 

Coal, anthracite, sized

882-962

15

16

C26

 

Coal, bituminous, mined 50 mesh and under

802-866

20

24

B45T

Coal, bituminous, mined and sized

721-882

 

15

 

16

 

 

 

Coal, bituminous, mined, run of mine

721-882

20

18

D35T

Coal, bituminous, mined slack, 13 mm and under

689-802

20

22

C45T

Coal, bituminous, stripping, not cleaned

802-962

 

 

D36T

Coal, Lignite

641-72

20

22

D35T

Coke, loose

369-561

 

18

D47QVT

Coke, petroleum calcined

561-721

 

20

D36Y

Coke, breeze. 64mm and under

401-561

15-20

 

20-22

 

C37Y

 

Concrete, 51mm slump

1764-2405

 

24-26

D26

Concrete, 102mm slump

1764-2405

 

20-22

D26

Concrete, 152mm slump

1764-2405

 

12

D26

Copper ore

1924-2405

 

20

•D27

Copper ore, crushed

1603-2405

 

 

D27

Copra, lumpy

353

10

9

D25

Com grits

641-721

 

 

B25W

Cryolite, dust

1202-1443

 

 

A36

Cryolite, lumpy

1443-1603

 

 

D35

Diatomaceous earth

176-224

 

 

A36MY

Dolomite, lumpy

1443-1603

 

22

D26

Earth, as excavated - dry

1122-1283

15

20

B36

Earth, wet, containing clay

1603-1734

 

20

 

23

 

B46

Feldspar, 13mm screenings

1122-1363

 

20

 

18

 

B36

 

 

Feldspar, 38mm to 76mm lumps

1443-1734

 

15

 

17

 

D36

 

Feldspar, 200 Mesh

1603

 

 

 

Fish, meal

561-641

 

 

B45W

Flour, wheat

561-641

 

21

A45PN

Flue dust, boiler house, dry

561-641

 

 

 

 

 

A17MTY

 

Fluorspar, 13mm screenings

1363-1683

 

 

 

 

 

C46

 

Fluorspar, 38mm to 76mm lumps

1734-1924

 

 

 

 

 

046

 

Flay ash, dry (see flue dust)

 

 

 

 

 

 

 

 

Foundry sand, loose (see sand)

1283-1443

 

 

 

 

 

B47

 

Foundry refuse, old sand cores, etc

1122-1603

 

 

 

 

 

D37Z

 

Fullers earth, dry

481-561

10

 

B26

Fullers earth, oily

962-1042

 

 

B26

Fullers earth, oil filter, burned

641

 

 

 

 

 

B26

 

Fullers earth, oil filter, raw

561-641

 

15

 

20

 

•B26

 

Garbage, household

802

 

 

•E45VW

Glass batch

1283-1603

 

20-22

D27Z

Granite, 13mm screenings

1283-1443

 

 

 

 

 

C27

 

Granite, 38mm to 76mm lumps

1363-1443

 

 

 

 

 

D27

 

Granite, broken

1523-1603

 

 

027

Gravel, bank run

1443-1603

20

20

 

Gravel, dry, sharp

1443-1603

 

15-17

D27

Gravel, pebbles

1443-1603

15

12

Q36

Gypsum dust, non-aerated

1491

 

 

 

 

 

 

 

Gypsum dust, aerated

962-1122

20

23

A36Y

Gypsum, 13mm screenings

1122-1283

 

20

 

21

 

C36

 

Gypsum, 38mm to 76mm lumps

1122-1283

 

15

 

15

 

D26

 

Ice, crushed

561-721

 

 

D16

llmenite ore

2245-2565

 

 

B27

Iron ore

1603-3206

15

18-20

•D36

Iron ore, crushed

2164-2405

 

20-22

•C26

Iron oxide, pigment

401

20

25

A45

Kaolin clay 76mm and under

1010

 

15

 

19

 

036

 

Kaolin talc, 100 mesh

673-898

20

23

A46Y

Lead ores

3206-4329

15

15

'B36RT

Limestone, agricultural 3mm and under

1090

 

20

B26

Limestone, crushed

1363-1443

20

18

C26X

Limestone, dust

1283-1363

 

20

A46MY

Malt, meal

577-641

 

E25

 

Manganese ore

2004-2245

20

20

'D37

Nickel-cobalt, sulphate ore

1282-2405

 

 

 

 

 

•D27T

 

Oats

417

10

10

C25M

Paper pulp stock

641-962

 

 

•E15MV

Phosphate, acid fertilizer

962

15

13

B25T

Phosphate, triple super ground fertilizer

801-882

20

30

B45T

Phosphate rock, broken, dry

1202-1363

 

15

 

12-15

 

026

 

Phosphate rock, pulverized

962

 

20

 

25

 

B36

 

Pyrites, iron 52mm to 76mm lumps

2164-2325

 

 

 

 

 

D26T

 

Pyrites, pellets

1924-208

 

 

C26T

Quartz, dust

1122-1283

 

 

A27Y

 

 

Quartz, 13mm screenings

1283-1443

 

 

 

 

 

C27Z

 

Quartz 38mm to 76mm lumps

1363-1523

 

 

 

 

 

027Z

 

1.3.1          Estimation of surcharge angle

In the absence of reliable information on the surcharge angle, the following method may be employed to serve as a guide in the selection of a suitable value. The process is based on reducing the angle of repose and allows for:

  • Belt velocity and angle of inclination at the loading point
  • Material properties
  • Special allowance for trippers

The nominal surcharge angle in degrees is given by the equation,

Surcharge angle = X - K- K- Ks

Where:

X is the angle of repose in degrees

Kv is the velocity or slope reduction allowance in degrees

Km is the material reduction allowance in degrees and

Ks is the special reduction allowance in degrees

Velocity/ slope reduction allowance - Kv

This factor takes into consideration both the belt velocity as well as the angle of the conveyor at the loading point. Values of Kv for a wide range of belt velocities and loading angles are given in the table below. These values are, in turn, proportional to the time taken for accelerating the material at the loading point, assuming the coefficient of friction between the belt and the material as 0.5. These values are for typical transfers in which some amount of material is redirected at the loading point. These values could reduce further in the event of there being effective material redirection. On the other hand, they could increase in the event of the feed chute not providing any material redirection (see Table 1.5).

Table 1.5

Typical Kv values in degrees

Conveyor angle at loading point

Belt velocity in m/sec

1

2

3

4

5

6

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

2

2

2

2

2

2

3

3

3

3

3

3

4

4

4

4

5

4

4

4

4

5

5

5

5

6

6

6

7

7

8

8

9

10

6

6

6

7

7

7

8

8

8

9

9

10

11

11

12

13

15

8

8

9

9

9

1

1

1

1

1

1

1

1

1

1

1

2

10

10

11

11

12

12

13

13

14

15

16

17

18

19

21

22

24

1

1

1

1

1

1

1

1

1

1

1

2

2

2

2

2

2

Material reduction allowance - Km

Table 1.6 gives the values of Km for different materials as described.

Table 1.6

Material reduction allowance - Km

 

Material

K in degrees

Fine material having 5% moisture or interlocking material

Dry material with low fines content such as crushed rock

10°

 

Dry, free flowing fine material

15°

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Special reduction allowance - Ks

The value of Ks for belt sag, trippers and horizontal curves will depend partly on the design features of the conveyor and also the nominal surcharge angle obtained from other factors. In case the nominal surcharge angle is high, say more than 15°; there must be some additional reduction in the surcharge angle to account for these special features.

Typical values of Ks

For conveyors with tripper – 5 to 10°

For high belt sags exceeding 1% of the idler centers - 5°

Refer to Appendix C for surcharge angles

1.4          Classification of conveying machines

Owing to the wide range of conveying machines available, such as those differing in the principle of operation, design features, direction and means of conveyance, a general classification of material handling equipment is almost impossible.

According to their principle of operation, conveying machines can be categorized as those based on intermittent action and continuous action, the salient features of which are mentioned below.

1.4.1          Intermittent action machines

  • Cyclic operation is a characteristic feature of these machines
  • They operate on an alternately reciprocal principle; they run loaded in one direction and idle in the other
  • Examples of intermittent action machines include cars, trucks, rail mounted cars, cable cars and tractors
  • Loading and unloading are generally accompanied by stoppages
  • They possess great flexibility in the path of transport, with the path being provided with a number of branches at times
  • They are suitable for small and medium capacity work
  • They are difficult to put into automatic operation

1.4.2          Continuous action machines

  • A feature specific to continuous action machines is that their load carrying member conveys the load in a practically uninterrupted stream or in small successions (buckets, tubs etc)
  • They move along a precisely determined path
  • Examples of this type include various types of conveyors and pneumatic and hydraulic transport installations
  • They are suitable for all capacity ranges, from small to very high
  • They are most suited for automatic operation

1.4.3          Auxiliary equipment

Auxiliary equipment forms a special group and is designed for operation in conjunction with conveying machines. They are not an independent means of conveyance. Auxiliary equipment comprise chutes, troughs, hoppers, gates, feeders and so on.

1.5              Selection of conveying machines

Following are the technical factors to be considered when selecting a conveying machine:

Nature and properties of the material to be conveyed:

  • Required capacity of the equipment – If the capacity is high, economic considerations will dictate selection of the equipment that is compact and low in cost.
  • Direction and length of conveying run –This is of prime importance in selecting the equipment type. Certain types of machines easily permit change of direction in one or both planes; others operate in a straight path and in one direction. While some are adopted to convey materials a considerable distance, others are limited by their length.
  • Storage of material at the head and tail end –The method of loading and unloading of material also has an important bearing in the selection of a conveying machine. While some of them are self loaders, others may require certain additional loading devices. Loose material can be stored in heaps, from which they are loaded on to the conveying machines with the help of buckets, scrapers or by other means. The material stored in a bin is discharged on to the conveying machine by gravity.
  • Processing steps and the movement of loads –In most cases, conveying machines are related to the overall manufacturing cycle, depend on it and serve to carry a load processed en route.
  • Specific local conditions – These include the area of the site at disposal, its topography, type and design of the building, mutual layout of handling machines and processing equipment, humidity, ambient temperature, environment protection etc. It is also important to know whether the machine will be installed outdoors or indoors.

After selecting the machine on the basis of the technical factors discussed above, a detailed review also has to be carried out from the economic point of view. An optimum solution would be the type of conveying machine that meets all the processing requirements while ensuring a high degree of mechanization and favorable working condition. Such equipment will, in the long run, ensure minimum per unit handling cost and will recoup the initial outlay in the shortest possible time.

1.6          Trends

Following are the most visible modern trends in bulk material handling:

  • Reduction in the amount of movement of bulk load to a minimum. This means that load is to be handled from the initial to the final point of conveyance, with minimum number of transfers, for example, by using a single or minimum number of machines. It must be ensured that the shortest path is always taken. On the other hand, there is a trend towards bringing the process plant closer to the source of material.
  • Increase in handling capacity.
  • More reliable operation, improvement in working condition and minimum maintenance requirement. Which is an essential prerequisite for automation of the manufacturing process.
  • Automatic control of individual conveying machines and group of installations, automatic loading/ unloading operations and distribution of loads.
  • Light weight machines of small size.

 

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