Troughed Belt Conveyors
Jyoti Ranjan Nayak
[email protected]
Troughed Belt Conveyors The troughed belt conveyor is probably the most widely used and well known conveyor design of all types of belt conveyors. The troughed belt conveyor has proven to be a reliable and versatile conveyor in many applications and is probably more forgiving than most other types of conveyors when exposed to adverse operating conditions. Improved technology supporting the design of conveyor systems together with innovations in component design and a better understanding of the dynamics prevalent in conveyor belts during operation, have propelled troughed belt conveyors into broader appli cations and higher conveying capacities. It is imperative that designers, users and l earners are kept informed of the latest late st developments in the field of conveyors, and the associated technical and economic benefits which these technologies hold for the end-user.
Functional Description A troughed belt conveyor comprises an endless, rubberized flat belt (a) suspended between pulleys at either end and supported along its length by a number of rotating idler rollers (b). The belt is driven via one of the pulleys (usually the head pulley (c)) and the tension in the belt is maintained by using a sliding pulley (d) which is tied to a gravity take-up unit (e).
The material (f) is loaded onto the conveyor at the tail-end via a chute (g) and is transported along the carrying-side (h) to the head-end where it discharges into a discharge chute (i) which guides the product onto the downstream equipment. Impact idlers (j) are located at the loading point to support the belt where the load impacts onto the belt as it is dropped down the loading chute. Once the material has been discharged from the carrying belt, the return belt (k) is guided back to the tail pulley on return idlers (l). The impact, carrying and return idlers i dlers are spaced at different intervals. On the carrying-side, the mass of the belt plus the load l oad conveyed is greater than the mass to be supported on the return-side and thus, for the tension in the conveyor belt (by the take-up and induced by the drive unit), the idler spacing is selected accordingly. This 'sag' in the belt between the carrying and return idler sets must therefore be designed on the basis of the heaviest load that the conveyor is to transport. Snub pulleys (m) are sometimes incorporated into the design of a conveyor in order to increase the angle of wrap (n) of the belt on the drive pulley. The greater wrap angle on the pulley allows more power to be introduced into the belt as is passes around the drive pulley without slip occurring. In this way, fewer drives are needed on longer conveyors or conveyors with high conveying loads. For a detailed view of a troughed belt conveyor click here to be routed to the "anatomy" of a trough belt conveyor, or click the appropriate item on the contents bar to the left of your screen
Applications for Troughed Belt Conveyors Troughed belt conveyors are used extensively throughout the bulk materials handling industry and examples of applications where this type of conveyor i s commonly used are listed below (cli ck thumbnail for images) :-
Short plant conveyors - horizontal,
Long overland conveyors - straight or
inclined or declined
curved.
Boom conveyors - stacker-mounted,
Reversible conveyor.
Inclined mine shaft conveyors.
Shiftable conveyors.
Elevated conveyors in gantries.
Two-way conveyors.
slewable.
Tripper conveyors.
Material to be Conveyed Troughed belt conveyors are able to transport a wide variety of materials in bulk. Notwithstanding this, factors which affect the performance of a conveyor belt, its success or failure failu re often depends on whether or not the designer completely understands the properties and characteristics of the material to be transported. Knowing how a particular material will behave when it is dropped down a loading chute onto a belt conveyor below and whether a material is dusty, abrasive, free-flowing or extremely wet, is helpful to the engineer and in the long run, ensures that the client cli ent receives a conveyor that meets his expectations.
Material Class Description
The angle of repose of a material is that angle to the horizontal assumed by the stir-face of a freely formed pile, The angle of surcharge is that angle to the horizontal assumed by the surface of a material at rest on a moving conveyor belt. The surcharge angle may be anything up to 20 less than the angle of repose. The flowability chart on sheet 2 shows the general relationship between the angles of repose and surcharge.
C.E.M.A. Handbook UNIROYAL Conveyor Belt Selection Guide PROK Handbook The classification table are based upon nvrm4e conditions and typical materials. The determination of angles of repose and surcharge and maximum recommended conveyor inclination must be considered with due regard to such properties as size and shape of fine particles and lumps,
roughness of the surface of the particles, proportion of fines and lumps present, moisture content, dustiness, stickiness, abrasiveness, corrosive action, etc. Materials or characteristics omitted from the tables may be roughly appraised by comparison with similar listed materials.
Material Angle of Repose and Surcharge Angle
If details about the material to be conveyed are known and fall within the experience of the designer, the engineer is able to cater for the specific features necessary on the c onveyor which will ensure that the material can be loaded, conveyed and discharged in accordance with the users specification. This will in turn lead to a more reliable and efficient troughed belt conveyor operation. From the above it is obvious that the conveyor designer must have a firm grasp on the properties of the material which is to be transported along the conveyor in addition to a fundamental understanding and appreciation of belt conveyor design.
Conveyor Selection Criteria For the novice, a brief outline of the basic considerations and limitations of the troughed belt conveyor are given below. Reference is also made to alternative types of belt conveyors which may be more suitable than troughed belt conveyors in some instances. As can be seen below, it is incumbent on the engineer to establish the most cost-effective and practical conveyor system in each application. W here necessary visitors should consult the appropriate sections dealing with these alternative types of conveyors by clicking on the title bar at the top of your screen. Note that detailed design by a competent engineer is required to establish the specific layout parameters for each conveyor. The information provided below serves merely as a guide.
Layout
For 'short' troughed belt conveyors (whose length is less than approximately 500 m) and troughed belt conveyors installed in factories or plants should be straight in plan view. These conveyors may be inclined, horizontal or declined in elevation, or parts of a conveyor may be inclined, horizontal and declined.
For 'long' troughed belt conveyors (where the conveyors' length is in excess of 500 m) and where the conveyor is an overland-type conveyor following ground line, horizontal curves of radius greater than 1000 m can be considered. As above, these conveyors may include inclined sections, horizontal sections and declined portions between the loading and discharge points.
Profiles of Troughed Belt Conveyors
The possible profiles which a troughed belt c onveyor can follow are almost unlimited. In the process of determining whether a troughed belt conveyor is the correct type of conveyor for any given application, the layout of the conveyor plays a significant role. One aspect of the layout of a conveying system is the profile whic h the conveyor must follow. To a large degree the vertical profile is limited by the character of the material being conveyed e.g. whether the material will roll back along the belt while being transported up an inclined section of a conveyor. To ensure that this does not occur, the designer must consider the ma terial properties and must tailor the profile of the t he conveyor accordingly. Assuming that the material properties, physical obstacles on the Plant and the pr ocess requirements are taken into consideration, the profile which a troughed belt conveyor can follow is almost unlimited. Some examples of troughed belt conveyor profiles commonly seen are provided below :-
At loading points, the maximum inclination of the belt should not be greater than approximately 5 degrees to the horizontal. This maximum inclination is dependent on the belt speed, the material characteristics and to a lesser degree the layout of the feed point. Inclining the loading point to a greater angle may result in spillage at the loading chute. A further consideration at loading points is the height required above the conveyor belt for the loading chute work. It is important that material is loaded onto the center of the conveyor belt to ensure that the belt runs centrally along the conveyor structure. To achieve this the loading/feed chute must be wide enough at the top of the chute to collect all material being fed into it, while the bottom of the chute must be no wider than 2/3 of the width of the conveyor belt. The side walls of the chute must be steep enough to prevent material accumulating on the side walls and corners of the chute, which could lead to blockages. Typically side walls of chutes range between 45 degrees and 70 degrees depending on the material to be conveyed. It can be seen in the sketch therefore that careful consideration must be given to the layout of the tail station. The height required above the new conveyor belt for the feeding system and chute work must not be overlooked. The discharge of material from a troughed belt conveyor is usually via a chute onto another conveyor, into a silo, etc. As with the feed chute, the discharge chute must be carefully laid out in order to establish the required elevation of the head / discharge pulley to ensure that the discharge chute does not block under full load conditions. It has been explained above that trou ghed belt conveyors are generally designed as straight conveyors (plan view), certainly in the case of short belt conveyors. When a designer is considering the layout of a conveyor in a factory for example, the desired route which the conveyor is to follow may not in fact be possible due to physical obstacles located between the feed po int and the desired discharge point.
In such a case the designer is presented with a number of options which should be investigated prior to finalising the design of the belt conveyor system to be used. A first option may be to use two a troughed belt conveyors with a transfer point between the two conveyors, as can be seen in this sketch. A second option may be to use a different type of belt conveyor for example the pipe conveyor or Sicon conveyor which offer the ability to negotiate corners without the need of transfer points. Each of these alternative types of belt conveyors has their own layout constrains, advantages and disadvantages, which the designer must take into consideration as part of the investigation. The inclination of a belt conveyor is also a factor which must be considered by the engineer, when selecting the type of belt conveyor to be used. The maximum inclination or decli ne of a conveyor generally depends on the characteristics of the material to be transported. To assist the learner in determining which type of conveyor should be used for a particular application, a graph may be viewed by clicking here.
Conveyor Duty & Material Specification
It is important that the duty which the conveyor is expected to fulfill is clearly established at the commencement of the design phase, as this will i mpact on numerous items in the detailed det ailed design of the conveyor and component selection. The capacity of the conveyor (in tons per hour or cubic meters per hour) as well as the average operating hours per day or or per year are important design criteria. Details pertaining to the material material to be conveyed must also be specified. Having established the duty that the conveyor is expected to fulfill and obtained the material specifications the designer is in a better position to determine the most cost-effective conveying method to be employed.
Environmental Considerations
The need to improve the local environment at factories and plants is gaining support throughout the world and is often driven by legislation. As such the type, layout, design and correct operation of belt conveyors and their associated equipment is becoming more important. Environmental considerations cover an extremely wide scope of issues ranging from the degree of pollutants generated by the working conveyor to the visual impact of a conveyor on the local community. The designer cannot therefore neglect to include the environmental considerations into the assessment of the most appropriate type of conveyor.
Environmental legislation differs from country to country and from factory to factory and it is therefore important that the specific environmental legislation and r ules be established and introduced into the overall conveyor feasibility and design procedure. For the purposes of list Beginners Guide, some of the most obvious environmental-related issues are itemised below to provide the learner with an holistic approach to conveyor design.
Products which are dusty or which tend to generate dust at transfer points for example powder cement and dry fly ash, may be conveyed in enclosed conveyors. Examples of enclosed conveyors include pipe conveyors, Sicon conveyors, bucket elevators, screw conveyors and pneumatic transportation systems.
In considering the layout of the conveying system, the number of transfer points between conveyors should be minimised to reduce the number of dust and spillage generating points in a conveying system, thereby minimising the impact of the system on the local environment.
Using a pipe conveyor which incorporates a gradual 90 degree curve can eliminate one transfer point in an equivalent troughed belt conveyor system, where two troughed conveyors would otherwise be necessary. Alternatively, a Sicon conveyor may be a better solution due to its superior cornering ability and increased flexibility. Should a troughed belt conveyor be used however, the conveyor can be installed into an enclosed gantry or tunnel to reduce the possibility of material being blown off of the c arrying side. Covers can also be installed over the carrying belt to reduce the risk of material blowing off of the conveyor and / or contain the dust.
The environment in which the belt conveyor is to operate may be subject to extreme e xtreme variations in weather conditions. A conveyor located on a jetty for example, may be subject to high and low temperatures, strong wind conditions and an extremely corrosive environment. In such a case issues such as whether or not the conveyor is to be enclosed becomes extremely important. In addition, the materials of construction of the structure may be different to an equivalent conveyor located in a dry, noncorrosive environment. The corrosion protection specification applicable to the co nveyor structure and components would typically demand a far greater degree of control over the application a nd maintenance of the coating.
The material to be conveyed may inherently be a commodity of high value where the client wishes to limit access to the product and the opportunity for spillage. The transportation of diamond-bearing Kimberlite to and from a process plant is an example of such an environment. In such a case Sicon conveyors are employed for their ability to negotiate difficult conveying routes without the need for transfer points as well as the fact that they enclose the material.
The 'environment' in which a conveyor is to be installed may dictate that the space available for a conveyor is extremely limited. Transporting material from ground level to the top of a bin or silo may require the conveyor to be installed vertically up the side of the silo. In this case the equipment to be used would typically be a bucket elevator or a pocket belt or a sandwich type conveyor, all of which are able to transport material vertically.
The location of the site where the conveyor is to be installed may be in a cold, mountainous area. In this case (assuming ambient temperatures drop to well below freezing), special consideration must be given to the selection of the conveyor belt as well as to items such as the mechanical components (bearings) and the lubricant proposed. Starti ng belt conveyors in extremely cold conditions requires among other things additional power to overcome the friction imposed by cold lubricants and a stiffer belt.
In addition to the temperature aspect, it may be necessary to route the conveyor down the side of a mountain and the conveyor route must negotiate the natural contours of the terrain thereby forcing the designer to incorporate vertical and horizontal curves into the conveyor layout. Where the environment imposes extremely arduous parameters/limitations on the design of the conveying system, it may be necessary to reconsider whether in fact a belt conveyor is appropriate for the application.
Cost Like any other equipment, there are costs associated with belt conveyors and these costs must be determined to a reasonable degree of accuracy in order for a commercial evaluation to be developed which will accompany the technical evaluation. It is generally accepted practice for the 'costs' of a conveyor to be presented in two categories namely; the initial capital outlay or capital expenditure for the system i.e. CAPEX and an ongoing operating and maintenance expense i.e. OPEX which the client will incur for the remaining life of the conveyor system. CAPEX should take into consideration costs such as :
the cost of the mechanical, electrical, civil, structural and instrumentation equipment purchased and installed on site as part of the conveyor.
the project management fee which a contractor / supplier will charge the user for executing the project.
the cost of any modifications to existing equipment which may be required on site to incorporate the new conveyor into the factory.
the cost to install, commission and test the conveyor system on site.
OPEX should take into consideration the following costs :
the cost of spare parts which the user will have to procure over the operational life of the conveyor. This cost is based on a projection of the life of components within the conveyor and the cost of each of those components, based on the duty of t he conveyor.
the cost of personnel required to operate the conveyor.
the costs of personnel and equipment required to maintain the conveyor.
the cost of electrical power to drive the conveyor.
Depending on the layout of the conveyor system and possibly the type of conveyor system to be used, the CAPEX and OPEX costs will differ. It is important therefore that the calculation of the costs for each option is performed according to the same basic parameters for example, similar costs for man hours and for electric power. It is possible for the total life cycle costs (CAPEX and OPEX) for any given option to be represented in a number of ways. Examples include; all capital and operational costs may be shown as a present value cost, escalation and/or discount factors may be included or excluded from the financial analysis, etc.
Standardisation
If a belt conveyor is to be installed on either a new plant or existing facility, the choice of type of conveyor to be used may depend to a large extent on the need to standardise on the type of equipment used. In the case of a new factory for example where a number of troughed belt conveyors are to be used throughout the facility then, in the interests of standardising and minimizing on spares to be held by the client and the training of personnel to operate and maintain the conveyors, the type of conveyor to be used should as far as reasonably possible be kept the same. Notwithstanding the need to investigate the optimum layout and cost for a particular conveyor, practicality dictates that standardisation of equipment is very often in the long-term interest of the end-user of the equipment and the engineer should bear this in mind in his a ssessment.
Examples of Troughed Belt Conveyors Having understood some of the basic c onsiderations relating to the choice a nd feasibility of troughed belt conveyors, it is important that the learner is left with an indication of the capabilities of this type of conveyor. The longest single-flight troughed belt conveyor in t he world is installed in Zimbabwe, Southern Africa, by Bateman Engineered Technologies. The conveyor length measured from tail pulley to head pulley is 15.6 km (belt length = 31,372 km) and transports iron ore at a rate of 500 tons per hour. The belt is 750 mm wide, travels at a speed of 4,7 m/s and the installed drive power is 1000 kW. The troughing and return idlers are spaced at intervals of 4.5 m and 9.0 m respectively and the 80 ton counter weight has a travel of 50 m. The longest multiple-flight troughed belt conveying system is installed in the Sahara desert and transports phosphate rock at a rate of 1000 tph over a distance of 110 km. The conveying system comprises 11 individual conveyors. This system was designed and installed by Krupp. Among the highest capacity troughed belt conveyors in the world are mounted on mobile bucket wheel reclai mers in Europe. While these conveyors are relatively short (approximately 100 m pulley centre distance) the belts measure up to 3.0 m wide and travel at 8 m/s. The peak design capacity of these conveyors is up to 25 000 m3/hour and material lump size ranges up to 1.0 m particle diameter.
Conveyor Selection Guide
Choosing the correct type of the conveyor for any given application depends on a number of factors including material characteristics, conveyor system layout, duty, environment, etc.
The following graph has been developed to assist learners with the selection of the correct type of belt conveyor system. This graph is intended for use as a guide only and Users would be well advised to approach suppliers of these conveyor systems in order to confirm whether the preferred type of conveyor is appropriate.
Anatomy of a Troughed Belt Conveyor
There are many possible variations in the design of a troughed belt conveyor depending on the purpose and duty for which the conveyor is being designed. Similarly the choice of individual components, features and accessories found on a conveyor should be selected on the basis of the functions which have to be performed by the conveyor. At this point in the Beginners Guide the learner is introduced to the different components and basic arrangement and features which are found on the majority of troughed belt conveyors throughout the world. In this section a graphic image of a typical troughed belt conveyor is provided together with detailed images and photographs of specific components. The learner should also study the nomenclature and terms used by engineers and designers when describing troughed belt conveyors.
