For more than 100 years the troughed belt conveyor has been used to transport bulk materials over short, medium and long distances in the mining, power generation, cement, ship port and industrial plants. The main benefit of belt conveyors compared to other material handling systems like trucks, trains and barges is the higher efficiency in the mechanical system, energy consumption and total cost over the long term, especially when conveyor system is design optimized.
Despite of all these advantages there are also some disadvantages and problems associated with the conventional troughed belt conveyor for transporting the bulk material. Exposure of material to atmosphere results in material property deterioration, air pollution, loss of material by wind scatter or velocity disturbance and spillage of product at transfer points and at the return side of the conveyor due to inadequate cleaning of the belt at the head section.
The increased demands for the reduction of spillage & dust and a need to provide a totally enclosed system to protect the product from the environment or contain dribble from the return belt have been a stimulus for the development of several kinds of enclosed/folded belt conveyor systems. One of these is the pipe or tube conveyor system, being developed in the 1970's, it is a
variation of the existing belt conveyor technology with the first installation in 1979. The pipe conveyor solves problems in training, spillage of materials, pollution, and limited angles of inclination, associated with conventional conveyor systems.
Pipe conveyors have become the preferred method of transportation for all bulk solids since it was first introduced. The system has become the most proven and reliable enclosed belt type conveyor with thousands of installation worldwide. Pipe conveyor saw not only increased number of installations and growth in the size and complexity of the system, but also better recognition by the general material handling sector as a feasible material handling system. The popularity of the pipe conveyor lies in its ability to ensure protection of the environment it passes through, besides proving to be cost-effective, calling for low labour and operating costs.
Mechanism and Working of Pipe Conveyor
In a Pipe Conveyor system the belt runs flat over the head, tail and snub pulleys. The belt is troughed by idlers at the feeding point. Once the belt is loaded, it is formed into a pipe by overlapping its edges. At the discharge end the belt opens out to a troughed belt running flat over the pulleys. Material can be transported on the return belt. Intermediate loading can be provided at any point by opening and closing the belt at the required feed point.
In contrast to the common troughed belt conveyor, which just arches the belt on its sides up to a certain trough angle, in the pipe conveyor as the name implies - the belt totally encompasses the bulk material. Thus, the cross-section can be designed either round or oval. To achieve those cross-sectional shapes, the idler assembly is apart from some exceptions arranged hexagonally.
The top rollers help the belt to transform into pipe shape and the bottom rollers is used to support the load and material. At the horizontal and vertical curved sections, rollers surrounding the belt support the load depending on the direction of the resultant force.
The idler in pipe conveyors lies in hexagonal position. The belt used in pipe conveyor consists of special cross rigid ply which plays a vital role in constant surface contact with the idlers. If a conventional rubber belt would be used as a pipe conveyor, after a short while the belt would experience severe sagging at the upper part of the pipe-shape, causing loss of surface contact with the idlers.
The belt has to have sufficient stiffness to form and maintain the pipe shape as it passes through the circle forming idler rolls. At the same time it must remain flexible for the transition from the flat shape at the pulleys to the pipe shape at both the loading and discharge ends of the conveyor.
The reason that the bottom or return strand of the belt is kept in the pipe shape rather than the flat shape, as on a conventional conveyor, is to permit the same size structure to be used and to guide the belt through the vertical or horizontal curves.
For pipe conveyors, the capacity, bulk density, maximum material lump size and the belt speed determine the pipe diameter and therefore the belt width, which determine application limits, in principal.
Advantages and Disadvantages of Pipe Conveyors
While the pipe conveyor technology is relatively new as compared to the more conventional belt conveyors, it offers many advantages over other material handling systems by making use of all the standard equipment developed for the conventional belt conveyor and by incorporating some additional features.
The material being transported by a pipe conveyor is fully enclosed by the conveyor belt for the major portion of its travel. This has several benefits including minimizing environmental pollution as dust generation and spillage are eliminated; protection from outside contamination and possible theft; and product can be transported in the return pipe. It also addresses the challenges of noise pollution and pollution of environment through smell and particulate emission enroute as well. This ultimately helps the industry in going forward with such projects in eco-sensitive zones.
Pipe conveyors can negotiate tighter horizontal curves and can take steeper inclines and declines than conventional trough conveyors. Pipe conveyors with their ability to keep the bulk material enveloped and at the same time negotiate horizontal and vertical curves with equal ease, have provided the much needed solution for in-plant conveying and conveying over long distances crossing forests, water bodies, human settlements, roads, highways and railway tracks addressing the environmental & statutory requirements. Pipe Conveyors are ideally suited to situations where transport by conventional conveyor systems would prove too hazardous or costly due to environmental or population concerns.
The pipe conveyor also eliminates the need for transfer points to change direction. The pipe conveyor has the ability to form horizontal curve over a much smaller radii than conventional trough belt conveyors, since the belt is constrained on all sides by the idler rolls. This eliminates all the environmental problems and expense of belt cleaners, pulleys, drives, chutes, dust collectors, power distribution and the cost of maintenance associated with transfer points.
They also allow the flexibility to convey bulk materials in both directions either simultaneously or one at a time depending upon the requirement. This has found significant use in port based facilities and pit head based power plants, for e.g. conveying raw materials (coal and limestone) from the port to the cement plant and conveys back products (cement and clinker) to the port for further transportation through ships.
Pipe conveyors reduce operational costs. They can be easily integrated into existing facilities, and as a rule they are more cost effective than trough conveyors for facilities with two or more transfers. Plus, their closed design greatly reduces belt-edge damage and buildup on idlers, which reduces maintenance costs.
Inspite its obvious benefits, pipe conveyors have additional costs and engineering problems compared to other conveyors in many applications. Total cost is generally double comparable to other conveyors due to increased number of idler rollers, bigger drive size, and higher belt cost. Similarly, for operating cost it is high due to increased power consumption. One of the main engineering and maintenance concerns is how to control the belt rotation, twist and collapse, especially when the conveyor routing has small radius curves. The manufacturing of long length and large ton capacity pipe conveyors requires specialized engineering that is not readymade available to the general professionals in the material handling sector.
Use and Application of Pipe Conveyors
The use of the pipe conveyor is only limited by the imagination of the individual looking for the possible application. Pipe conveyors can be used for transportation of almost all bulk materials; any material that can be placed on the pipe conveyor can be conveyed. The typical materials handled are ores, coal, coke, limestone, crushed stone, shale and overburden. Some of the more difficult materials that are being handled on the pipe conveyor are copper concentrate, petroleum coke, clay, flue dust, ready mixed concrete, sludge, humidified fly ash, coal tailings, alumina and filter dust.
Pipe Conveyors have been used in various industries including power plants, steel, cement plants, refineries, port terminals etc. for:
Transporting of raw materials e.g. limestone and ore from mine to process plant.
In port terminals, loading and unloading materials in environmentally sensitive areas.
In power plants, transporting coal
In refineries, transporting sulphur.
