Pipe Conveyors and Environmental Control
The troughed belt conveyor has been the main product for the efficient transport of bulk materials over short, medium and long distances for many years.
The inherent problems of conventional belt conveyors i.e. loss of material by wind scatter or velocity disturbance, spillage due to poor alignment and dust creation and spillage at transfer points has led to the development of totally enclosed methods of transport.
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. Since
this time, the system has become the most proven and reliable enclosed belt type conveyor with the Bridgestone version alone having over 700 installations world-wide.
Design Criteria
Belt Speed Capacity
It is not unusual for Pipe Conveyors to run faster than troughed conveyors on dusty materials. This is possible because the material is enclosed and the fact that the tubular belt is inherently stiffer, resulting in less belt sag between idlers, preventing disturbance of the material at support idlers.
Carrying Capacity
Generally, a Pipe Conveyor will carry an equivalent load to a conventional troughed belt conveyor, when the conventional belt has a width in the order of three times the diameter of the Pipe Conveyor.
However, the capacity of a conventional conveyor will depend on the troughing angle of the idler sets and the surcharge angle of the material being conveyed.
Lump Sizes
Because the material is completely enclosed by the belt and is surrounded by six idlers, there is a limit to the amount of lumpy material which can be handled.
Too many, or too large a lump size will cause crushing of material at the idler stations so for the recommended maximum loading ratio of 75%, it has been found that a maximum lump size of one-third of the pipe diameter can be handled. For larger lumps, the loading ratio should be reduced i.e. for lumps of half pipe diameter, use 58% loading ratio and for lumps two-thirds pipe diameter, use 44% loading ratio.
Curves
Information relating to curves is equally applicable to horizontal and vertical curves.
At the standard panel spacing, curves up to 45° including angle require a curve radius of 300 times pipe diameter with increasing curve radius for up to a 90° turn. Should tighter radii be required, it is possible to reduce the factor to 200 times the pipe diameter by giving additional support to the belt by reducing the panel spacing.
Standard Dimensions
The forming of the belt into a pipe shape results in a comparatively narrow rectangular structure for the Pipe Conveyor with a height/width ratio of 2:1. Table 1 shows the standard panel dimensions together with the spacing between idler panels.
It will be noted that the panel spacing increases with increasing pipe diameter from 1.5m at 150mm diameter to 2.7m at 850mm diameter. For a conventional troughed belt conveyor, carrying idler spacing tends to decrease with increasing belt width and load so that for larger pipe diameters, the number of idlers required can be comparable with a conventional belt conveyor.
Belt
Various belts are available to satisfy the need of industry. As well as normal cover compounds these include:
• High temperature
• Oil resistant
• Fire resistant
• Food quality
• Abrasion resistant
The majority of installations use a ply type belt. Materials used in the manufacture are identical to those used on conventional conveyors, the difference being in the construction of the belt. Accurate control of the weft strength is critical to ensure the belt is flexible enough to form the pipe shape whilst still maintaining sufficient strength to prevent collapsing, support of the load and resistance of fatigue from flexing. In addition, the plies are closed up at the edges of the belt to reduce edge stiffness, which allows the overlap to form an effective seal.
Limitations on the tensile strength are similar to those on conventional belts, at which point steel cord belts are available for the larger capacity systems. These are designed to give the same pipe forming characteristics as the ply belts with breaker plies being used to maintain the pipe shape.
Idler Roller Construction
The idlers usually have a steel shell construction, fitted with greased-for-life ball bearings. The bearings are protected by labyrinth seals and can be fitted with stone guards. Shell diameter, thickness and size of bearing are determined by the duty and life. To ensure good performance without increasing cost, attention is paid to the concentricity, rolling resistance and roller end float. Special idler rollers are available for the food and chemical industries.
Power
The basic power calculation is based on normal practice for troughed belt conveyors. Additional allowance has to be made for forming and maintaining the pipe shape and traversing any curves. Pipe Conveyors use approximately 10% of the power absorbed by pneumatic systems and in theory 10% to 15% more than a troughed belt conveyor. In practice and depending on the route layout it is sometimes the case where Pipe Conveyor absorbed power is less than that of conventional troughed conveyor.
We attribute this to the following factors:
1) The cleaner environment of the enclosed conveyor, leading to less spillage and hence friction from the idlers.
2) All the idlers on the hexagon of the Pipe Conveyor are not constantly turning.
3) Less sag between idlers, reducing the lifting/lowering and disturbance of the material.
4) Less rubber cover indentation.
Conveyor Length
The limitations on the maximum pipe conveyor length is the same as that of conventional conveyors (i.e.) the available belt strengths with some installations being over 5 km. The minimum length is in the order of 20 - 25m.
Maximum Incline Angle
Because of the increased contact area between belt and material, much steeper angles can be negotiated. As a general rule, the pipe conveyor will handle materials on an incline 50% greater than can be achieved with conventional troughed belt conveyors with a maximum incline of 30°.
Advantages.
The main advantages of the system are:
• Ability to negotiate curves of relatively sharp radii (eliminating transfer points).
• Steeper incline angles can be accommodated.
• Enclosure of the transported material.
• Elimination spillage and wind scatter.
• Containment and control of dust.
• Product security.
• No spillage or dust from return belt as this is enclosed.
• Reduction in noise levels.
Dust Control
In the current climate of environmental controls, the pipe conveyor is particularly suitable for the control of dust.
As the belt is formed around the product, the material dust cannot escape therefore reducing contamination of surrounding areas and plant resulting in lower maintenance and clear up costs.
Only at the delivery and receiving ends is it necessary to provide covers to eliminate dust creation when required as these areas are the same as for conventional conveyors.
Additionally, when required, dust control filters can be installed at these points to further
reduce dust.
Noise
Typically noise levels along the route of a pipe conveyor are lower than for conventional conveyors.
Typical Applications
A number of installations around the world carrying a variety of materials but they all have common requirements to carry the material securely or to accommodate difficult routing.
High Angle/Curved Conveying
A large glass manufacturer had the need to increase its capacity by building a new float glass line. The flexibility of the pipe system allowed them to overcome geographical problems in routing the raw materials handling system. (Fig 1)
The system employed three 250mm diameter Pipe Conveyors, the first carrying sand and dolomite at a rate of 100 tph. The conveyor has two feed points that are sealed and dust-free. Once in the pipe shape, the conveyor negotiates a tight “S” bend, inclined at 20° and enters the batch plant in line with the mixing equipment. This keeps the installation compact and eliminates spillage and transfer points.
The two other conveyors have capacities up to 200 tph and adopt the curved features of Pipe Conveyors to clear fixed obstacles along their route. Both have additional charge points, with cullet (broken glass) being added at one of these points.
Cement Industry Case.
In 1995 a system was commissioned to undertake a cement and clinker pipe conveyor by Blue Circle industries for their Port Ventanas import facility in Chile (Fig 2 and Fig 3).
The system being 1100 meters long and handling 1000 tonnes per hour of clinker or 800tph of cement was chosen to:
a) Reduce spillage and contamination of the surrounding area
b) To negotiate with one conveyor the required route.
c) To reduce the required manpower for the operation.
Power Industry Case.
One of the latest projects was for the supply of 2 off 300mm diameter biomass handling pipe conveyors at Ferrybridge Power Station, UK. (Fig 4 and 5)
The system comprises of 1 pipe conveyor 471 m long and 1 pipe conveyor 223 m to handle 350 cubic meters per hour of biomass (wood pellets) from a lorry reception building to the power station in-feed.
The pipe conveyor was selected for its ability to negotiate the route without additional
transfer points and to maintain a dust free local environment by using the enclosed system.
The system will be commissioned during the summer of 2006 then contributing to the power production using alternative fuels.
Operating in a diverse range of industries it offers alternative solutions to material handling problems. It is expected that the requirement for this mode of materials transport will continue to grow in the future with environmental planning becoming more of an issue.
Author - Paul Holt