The secret is knowing how to right-size
Specialists detail the main requirements for right-sizing the systems and point out the most recurring failures as well as solutions for such problems
The technology used in belt conveyors, widely used in mining and even at construction job sites, has taken such large steps forward in recent years that even the conveyors used moving people and baggage at major international airports are based on the principles of this type of construction equipment. Obviously, these “high tech” belt conveyors that are present in airport boarding and departure areas have special features, such as the appropriate size to meet the needs of airport operation according to the demand for transportation and the speed best suited to the need.
In the case of belts used to transport ore and rock, the same rule applies since the dimensioning or of conveyor belts directly influences the efficiency of the crushing line and even the following stages of processing. Ricardo Deguti, a specialist in crushing and screening at Sandvik, points out that despite being a piece of ancillary equipment, the conveyor belt plays an important role in mining companies that adopt it. “If it is poorly scaled or sized, it can compromise the productivity of the entire operation,” he says.
The same opinion is sustained by Marcelo Gelais, calculation engineer at Tecnometal - a company that evaluates the efficiency of conveyor systems globally. "Design and sizing should be done based on a complete view of the equipment and how it fits into the production process." According to Gelais, in addition to following the standards of safety of operation, the conveyor must meet the specific needs of the process and of every customer, with reliability and good performance without neglecting other equally important aspects, such as the quality of assembly and ease of maintenance.
Deguti, of Sandvik, in turn, underscores that the experience of those in charge of ‘right-sizing’ the equipment is one of the factors counts most. The application for which the conveyor is intended and its position in relation to the crushing line also appear as important data for the definition of the project. “A poorly scaled width will cause problems such as spillage of material from the sides of the conveyor,” he explains.
Considerations for the project
Maurício Zuquim, head of the maintenance department at the Samarco mining company, points out that the bearing applied to rollers and drums is the item responsible for the largest number of failures in a conveyor belt. “Due to the large number of rollers that equip a conveyor belt, we must be especially careful when scaling this component.” His opinion is based on the experience of an intensive user of technology.
According to Zuquim, those who design an operation using a belt conveyor should consider the operating speed and load that roller will have to bear up under. “Roller bearings for belt conveyors which run 24 hours a day are scaled for a theoretical service life of 30,000 hours,” he explains. “Therefore, depending on the speed of the belt, we must choose rollers so that they do not work at over 500 revolutions per minute (rpm).” Zuquim stresses operating at high rotations results in several stops for replacement of rollers, reducing equipment uptime and increasing the cost of maintenance.
In the case of drums, also mentioned by the specialist as an item that deserves special attention, setting of the diameter requires care. In this case, designers must comply with the minimum specifications recommended by belt manufacturers so as to avoid damaging to the seams and shortening the life of the component. “Since the belt is one of the most expensive items of a conveyor and the part that requires a longer amount of time to repair or replace, we must be very careful when specifying the diameter of the drums,” he says.
To the specialist of Samarco, the extent to which the belt will be filled must also be considered at the design stage. “Lines that operate with very high fill rates tend to have a greater spillage of material, which leads to accelerated wear of drum linings, belt and belt scrapers,” he says. “It can also cause misalignment and tearing of the belt due to material that comes back on the return of the conveyor and gets stuck between the belt and the drum.” Zuquim says that it is advisable to work with fill levels of 80% to 90% of the total amounts allowed.
Although the aforementioned items are extremely important, Zuquim has an even more point to stress regarding what drives the belt, which he considers to be the "heart" of the equipment. “Since it is responsible for transmitting motion to the conveyor, the drive mechanism is subjected to the greatest amount of strain during start-up. That's when you have peak voltage and tension, as well as increased demand for engine power.”
In view of that, he assesses that in order to achieve maximum safety and quality in the operation of the complete set up of equipment, the drive of the conveyor must be sized for the most demanding working conditions. Such a situation is established, basically, at the start-up of the conveyor, when bearing a load of 100% of the admissible amount of material in the ascending and straight segments of travel.
The list of critical points to consider also includes the factor of friction (friction coefficient) between the driving drum and the belt. Depending on the type of surfacing material utilized, the efficiency of power transmission varies, as does the available power (from the motor). “An alternative is to vary the angle of belt wrap on the drive drum up to 120 ° using the end drum to increase transmission efficiency,” indicates Zuquim.
Defining the speed
André Silva of Metso assesses that right-sizing, in this case, involves knowledge of the following: characteristics of the material to be transported (type, size/mesh, specific weight, angle of repose, temperature, moisture, abrasion), profile of the conveyor (length between centers of end drums, lifting height or slope of descent), desired capacity, operating conditions (environment, regime of operation) and special conditions (reversible belt, with tripper, etc.).
Zuquim of Samarco, literally gives us the formula for sizing the capacity of a conveyor: Q = S x V x Y x E x k. In other words, the load capacity (Q) is the product of cross-sectional area of the belt (S) multiplied by the operating velocity (V), the specific weight of the material (Y), degree of filling (E) and the capacity correction factor due to inclination (k).
Simple? Not quite. Zuquim explains that the area of the conveyor depends on the width of the belt used, the configuration of the bridge which refers us back to the number of rollers and their inclination and the angle of accommodation of the material. “To define the minimum width of the belt that we will use, we must know the ‘top size’ of the material to be moved, i.e., the dimensions of the largest particle within the particle size distribution range projected for the operation,” he says.
Width vs. Velocity
According to him, the high speeds have a greater impact on shorter conveyors, since in these cases belt wear increases since each of its points is subject to greater contact with the drums and scrapers. In the movement of finer and more fragile/friable material, it is advisable to use low speeds and to avoid dust and material degradation during transfers. Heavier materials in turn, of larger dimensions or with sharp edges, should be moved at a medium speed in order to avoid wear to the belt covering and to the wall of transfer chutes.
Lowering the speed of the conveyor, in turn, leads to the need to adopt a larger belt width, increasing the cost of the system. “When defining the speed of operation, one should take into account the costs related to maintenance and belt availability,” adds Zuquim.
He recommends that conveyor belts that are narrower and shorter should operate at speeds below 3.5 m/s. Lines that are wider and longer, in turn, can reach a speed of 4.5 m/s. The executive also cites information contained in the manual issued by CEMA (Conveyor Equipment Manufacturers Association), with the American standard for calculation of conveyors, as a source for the development of a higher quality project. Another resource are the Brazilian standards on the topic; all geared to achieve maximum performance of the conveyor belts based on efficient dimensioning of the equipment.
Criteria for maintenance of the system
Deguti also warns of the possibility of seized rollers. If for some external reason or due to wear, a bearing jams or seizes, the roller will also seize with the belt over it. In that case, the friction generated may even cause the belt to catch fire.
Marcelo Gelais, calculation engineer at Tecnometal, includes among primary care of equipment attention to lubrication and monitoring of noise and temperature. The condition of lining/surfacing materials and of wear items in general should also be carefully monitored, as well as the operating condition of instruments. According to him, besides acting in providing preventive maintenance, some service providers already perform predictive maintenance; monitoring vibration, heating and conveyor lubrication, among other factors.
How to avoid tear propagation
One of the most recent among such ‘solutions’ is the installation of antennas along the conveyor belt, attached to the conveyor structure, for detecting cuts. It is based on the principle of “signal interruption” because in the event of a continuous cut, the antenna signal is no longer received and that triggers the alarm. “These technologies are normally used together so as to capitalize on the advantages of each one and also to provide redundancy for greater safety of the conveyor,” argues Gelais.
Besides establishing the methods of tear detection, their distribution along the conveyor belt must also be taken into account. In view of this, it is advisable to install such detection devices at points where there is a greater risk of tearing. The list, in this case, establishes the exits at the transfer points as the priority, but the reliability of detection increases with a wider use of detectors, although the cost involved will also grow proportionally.
QUARTA-FEIRA - 19 de JUNHO de 2013
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|Desenvolvido e atualizado por Diagrama Marketing Editoral - Apoio editorial: Revista M&T. Reprodução apenas com permissão dos editores e com o devido crédito.|