Tubular conveyors and environmental control

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The trough belt conveyor has for many years been the main product for the efficient transport of bulk materials over short, medium and long distances.

The problems inherent in conventional conveyor belts, namely loss of material through wind dispersal or speed disturbance, spill due to misalignment and dust creation and spillage at transfer points have led to the development of methods completely closed transport.

One of them is the pipe or tube conveyor system, developed in the 1970s, it is a
variation of existing belt conveyor technology with the first installation in 1979. Since then
this time around, the system has become the most proven and reliable closed conveyor belt, with the Bridgestone version alone having over 700 installations worldwide.

Design criteria

Belt speed capacity
It is not unusual for pipe conveyors to run faster than trough conveyors on dusty materials. This is possible because the material is enclosed and the tubular belt is inherently more rigid, resulting in less sag of the belt between the rollers, preventing disturbance of the material at the support rollers.

Carrying capacity
Generally, a tubular conveyor will carry an equivalent load to a conventional trough belt conveyor, when the conventional belt has a width of the order of three times the diameter of the tubular conveyor.

However, the capacity of a conventional conveyor will depend on the trough angle of the roller sets and the overload angle of the material being conveyed.

Piece sizes
Since the material is completely enclosed by the belt and surrounded by six rollers, there is a limit to the amount of lumpy material that can be handled.

Too many or too large pieces will cause the material to be crushed at the idle stations. Therefore, for the recommended maximum load ratio of 75%, it has been found that a maximum piece size of one third of the pipe diameter can be handled. For large pieces, the load ratio should be reduced, that is, for pieces of half-pipe diameter, use a load ratio of 58%, and for pieces of two-thirds the diameter of the pipe, use a load ratio of 44%.

Curves
Curve information also applies to horizontal and vertical curves.
At standard panel spacing, bends up to 45 °, including angle, require a bend radius 300 times the pipe diameter with increasing bend radius to a 90 ° bend. If tighter radii are 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
Forming the band into a pipe form results in a relatively narrow rectangular structure for the pipe conveyor with an aspect ratio of 2: 1. Table 1 shows the standard dimensions of the panels as well as the spacing between the intermediate panels.

Note that the panel spacing increases with increasing pipe diameter from 1.5m at 150mm in diameter to 2.7m at 850mm in diameter. For a conventional trough belt conveyor, the carrier roll spacing tends to decrease with increasing belt width and load, so that for larger pipe diameters the number of rollers required can be comparable to that of a conventional belt conveyor.

Belt

Different belts are available to meet the needs of the industry. In addition to the normal covering compounds, these include:

• High temperature
• Oil resistant
• Fire resistant
• Food quality
• Abrasion resistant

The majority of installations use a pleat type belt. The materials used in manufacturing are identical to those used on conventional conveyors, the difference being in the construction of the belt. Accurate control of the weft strength is essential to ensure that the belt is flexible enough to form the shape of the pipe while maintaining sufficient strength to prevent collapse, load bearing and fatigue resistance due to bending. In addition, the pleats are closed at the edges of the waistband to reduce edge stiffness, allowing the overlap to form an effective seal.

Tensile strength limits are similar to conventional belts, in which case wire rope belts are available for larger capacity systems. These are designed to give the same pipe forming characteristics as the pleat bands with break plies used to maintain the shape of the pipe.

Free roll construction
Idler wheels usually have a steel shell, fitted with greased-for-life ball bearings. The bearings are protected by labyrinth seals and can be fitted with stone guards. The shell diameter, thickness and size of the bearing are determined by service and life. To ensure good performance without increasing costs, special attention is paid to the concentricity, rolling resistance and float of the roller. Special idler rollers are available for the food and chemical industries.

Power
The basic power calculation is based on normal practice for trough belt conveyors. Additional allowance must be made to form and maintain the shape of the pipe and pass through any bends. Tubular conveyors use about 10% of the power absorbed by pneumatic systems and in theory 10 to 15% more than a trough belt conveyor. In practice and depending on the route of the route, it sometimes happens that the power absorbed by Pipe Conveyor is less than that of a conventional trough conveyor.
We attribute this to the following factors:

1) The cleaner environment of the closed conveyor, leading to less spillage and therefore roller friction.
2) Not all rollers on the pipe conveyor hexagon are rotating constantly.
3) Less sag between rollers, reducing lifting / lowering and material disturbance.
4) Less indentation of the rubber cover.

Conveyor length
The limitations on the maximum length of tubular conveyors are the same as those of conventional conveyors (i.e. belt resistances available with some installations exceeding 5 km. The minimum length is in the order of 20 to 25 m .

Maximum tilt angle
Due to the increased contact area between the strip and the material, much steeper angles can be negotiated. Typically, the tubular conveyor will handle materials at an incline 50% greater than that achievable with conventional trough belt conveyors with a maximum incline of 30 °.

Advantages.

The main advantages of the system are:

• Ability to negotiate relatively tight radius curves (eliminating transfer points).
• Steeper tilt angles can be accommodated.
• Enclosure for transported equipment.
• Elimination of spills and wind dispersal.
• Containment and control of dust.
• Product safety.
• No spillage or dust from the return belt as it is included.
• Reduced noise levels.

Dust control

In today’s climate of environmental control, the tube conveyor is particularly suited to dust control.

As the web is formed around the product, dust from the material cannot escape, reducing contamination of surrounding areas and facilities, reducing maintenance and cleaning costs.

It is only at the ends of delivery and reception that it is necessary to provide covers to eliminate the creation of dust when necessary as these areas are the same as for conventional conveyors.
In addition, if necessary, dust filters can be installed at these points to
reduce dust.

Noise
Typically, noise levels along the path of a tubular conveyor are lower than those of conventional conveyors.

Typical applications
There are a number of facilities around the world that transport a variety of materials, but they all have common requirements to transport the material safely or to cope with difficult routing.

High angle / curved conveying
A large glass manufacturer needed to increase its capacity by building a new line of float glass. The flexibility of the pipeline system allowed them to overcome the geographic routing problems of the raw material handling system. (Fig. 1)

The system used three 250mm diameter tubular conveyors, the first carrying sand and dolomite at a rate of 100 tph. The conveyor has two feed points which are sealed and dust free. Once shaped into a pipe, the conveyor negotiates a tight, 20 ° angled “S” bend and enters the concrete batching plant in line with the mixing equipment. This will keep the installation compact and eliminate dumping and transfer points.

The other two conveyors have capacities of up to 200 tph and adopt the curved characteristics of pipe conveyors to overcome fixed obstacles along their route. Both have additional charge points, with cullet (broken glass) being added to one of these points.

Case of the cement industry.
In 1995, a system was ordered to undertake a cement and clinker pipe conveyor by Blue Circle Industries for their import facility at Port Ventanas, Chile (Fig 2 and Fig 3).

The system measuring 1100 meters long and treating 1000 tonnes per hour of clinker or 800 t / h of cement was chosen for:

a) Reduce spills and contamination of the surrounding area

b) Negotiate the required route with a carrier.

c) Reduce the manpower required for the operation.

Case of the energy industry.
One of the latest projects involved the supply of 2 300mm diameter biomass handling pipe conveyors to the Ferrybridge power station in the UK. (Fig. 4 and 5)

The system includes 1 tubular conveyor 471 m long and 1 tubular conveyor 223 m to process 350 cubic meters per hour of biomass (wood pellets) from a building receiving trucks to feed the power station.

The tubular conveyor was selected for its ability to negotiate the route without
transfer points and maintain a local dust-free environment using the closed system.

The system will be put into service during the summer of 2006 and will then contribute to the production of electricity from alternative fuels.

Operating in a wide range of industries, it offers alternative solutions to material handling problems. The demand for this mode of material transportation is expected to continue to grow in the future, with environmental planning becoming more of an issue.

Author – Paul Holt


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