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

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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 a_max and the smallest characteristic particle a_min is above 2.5, is considered to be unsized.

In sized materials, i.e. more or less homogeneous ones, a_max: a_min < 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 a_max and a_max is greater than 10% of the total weight of the sample, then the material is characterized by lump-size a_max and if it is otherwise, the characterization is done as 0.8 a_max.

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

Table 1.2

Maximum lump size for different belt widths

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 g 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)

Table 1.4

Bulk material characteristics

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_v - K_m - K_s

Where:

X is the angle of repose in degrees

K_v is the velocity or slope reduction allowance in degrees

K_m is the material reduction allowance in degrees and

K_s is the special reduction allowance in degrees

Velocity/ slope reduction allowance - K_v

This factor takes into consideration both the belt velocity as well as the angle of the conveyor at the loading point. Values of K_v 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 K_v values in degrees

Material reduction allowance - K_m

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

Table 1.6

Material reduction allowance - K_m

Special reduction allowance - K_s

The value of K_s 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 K_s

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.