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Latest Developments in Belt Conveyor Technology M. A. Alspaugh Overland Conveyor Co., Inc. Presented at MINExpo 2004 Las Vegas, NV, USA September 27, 2004 Abstract Bulk material transportation requirements have continued to press the belt conveyor industry to carry higher tonnages over longer distances and more diverse routes. In order keep up, significant technology advances have been required in the field of system design, analysis and numerical simulation. The application of traditional components in non-traditional applications requiring horizontal curves and intermediate drives have changed and expanded belt conveyor possibilities. Examples of complex conveying applications along with the numerical tools required to insure reliability and availability will be reviewed. Introduction Although the title of this presentation indicates “new” developments in belt conveyor technology will be presented, most of the ideas and methods offered here have been around for some time. We doubt any single piece of equipment or idea presented will be “new” to many of you. What is “new” are the significant and complex systems being built with mostly mature components. What is also “new” are the system design tools and methods used to put these components together into unique conveyance systems designed to solve ever expanding bulk material handling needs. And what is also “new” is the increasing ability to produce accurate computer simulations of system performance prior to the first system test (commissioning). As such, the main focus of this presentation will be the latest developments in complex system design essential to properly engineer and optimize todays long distance conveyance requirements. The four specific topics covered will be: Energy Efficiency Route Optimization Distributed Power Analysis and Simulation Energy Efficiency Minimizing overall power consumption is a critical aspect of any project and belt conveyors are no different. Although belt conveyors have always been an efficient means of transporting large tonnages as compared to other transport methods, there are still various methods to reduce power requirements on overland conveyors. The main resistances of a belt conveyor are made up of: Idler Resistance Rubber indentation due to idler support Material/Belt flexure due to sag being idlers Alignment These resistances plus miscellaneous secondary resistances and forces to over come gravity (lift) make up the required power to move the material. 1 Latest Developments in Belt Conveyor Technology MINExpo 2004, Las Vegas, NV, USA M.A. Alspaugh, Overland Conveyor Co., Inc. September 27, 2004 In a typical in-plant conveyor of 400m length, power might be broken into its components as per Figure 1 with lift making up the largest single component but all friction forces making up the majority. Power- In-Plant Idlers 6% Rubber Indent 11% Alignment 9% Material Flexure 21% Lift 43% Miscellaneous 10% Figure 1 In a high incline conveyor such as an underground mine slope belt, power might be broken down as per Figure 2, with lift contributing a huge majority. Since there is no way to reduce gravity forces, there are no means to significantly reduce power on high incline belts. But in a long overland conveyor, power components will look much more like Figure 3, with frictional components making up almost all the power. In this case, attention to the main resistances is essential. Power- Slope Belt Lift 90% Rubber Indent 4% Idlers 1% Alignment 2% Material Flexure 1% Miscellaneous 2% Figure 2 The specifics of power calculation is beyond the scope of this paper but it is important to note that significant research has been done on all four areas of idlers, rubber indentation, alignment and material/belt flexure over the last few years. And although not everyone is in agreement as to how to handle each specific area, it is generally well accepted that attention to these main resistances is necessary and important to overall project economics. Power- Overland Rubber Indent 48% Idlers 26% Lift 1% Alignment 17% Material Flexure 4% Miscellaneous 4% Figure 3 At the 2004 SME annual meeting, Walter Kung of MAN Takraf presented a paper titled “The Henderson Coarse Ore Conveying System- A Review of Commissioning, Start-up and Operation”2. This project was commissioned in December 1999 and consisted of a 24 km (3 flight) overland conveying system to replace the underground mine to mill rail haulage system. Figure 4- Henderson PC2 to PC3 Transfer House Latest Developments in Belt Conveyor Technology MINExpo 2004, Las Vegas, NV, USA M.A. Alspaugh, Overland Conveyor Co., Inc. September 27, 2004 The longest conveyor in this system (PC2) was 16.28 km in length with 475m of lift. The most important system fact was that 50% of the operating power (4000 kW at 1783 mtph and 4.6 m/s) was required to turn an empty belt therefore power efficiency was critical. Very close attention was focused on the idlers, belt cover rubber and alignment. One way to document relative differences in efficiency is to use the DIN 22101 standard definition of “equivalent friction factor- f” as a way to compare the total of the main resistances. In the past, a typical DIN f used for design of a conveyor like this might be around 0.016. MAN Takraf was estimating their attention to power would allow them to realize an f of 0.011, a reduction of over 30%. This reduction contributed a significant saving in capital cost of the equipment. The actual measured results over 6 operating shifts after commissioning showed the value to be 0.0075, or even 30% lower than expected. Mr. Kung stated this reduction from expected to result in an additional US$100, 000 savings per year in electricity costs alone. Route Optimization Figure 5- Tiangin China Horizontal Adaptability Of course the most efficient way to transport material from one point to the next is as directly as possible. But as we continue to transport longer distances by conveyor, the possibility of conveying in a straight line is less and less likely as many natural and man-made obstacles exist. The first horizontally curved conveyors were installed many years ago, but today it seems just about every overland conveyor being installed has at least one horizontal change in direction. And todays technology allows designers to accommodate these curves relatively easily. Figures 5 and 6 shows an overland conveyor transporting coal from the stockpile to the shiploader at the Tianjin China Port Authority installed this year. Designed by E.J. ODonovan however the system engineer can seldom test the finished system until it is completed on site. Therefore computational methods and tools are absolutely critical to simulate the interactions of various diverse disciplines and components. Dynamic Starting and Stopping When performing starting and stopping calculations per CEMA or DIN 22101 (static analysis), it is assumed all masses are accelerated at the same time and rate; in other words the belt is a rigid body (non-elastic). In reality, drive torque transmitted to the belt via the drive pulley creates a stress wave which starts the belt moving gradually as the wave propagates along the belt. Stress variations along the belt (and therefore elastic stretch of the belt) are caused by these longitudinal waves dampened by resistances to motion as described above. 7 Many publications since 1959 have documented that neglecting belt elasticity in high capacity and/or long length conveyors during stopping and starting can lead to incorrect selection of the belting, drives, take-up, etc. Failure to include transient response to elasticity can result in inaccurate prediction of: Maximum belt stresses Maximum forces on pulleys Minimum belt stresses and material spillage Take-up force requirements Take-up travel and speed requirements Drive slip Breakaway torque Holdback torque Load sharing between multiple drives Material stability on an incline It is, therefore, important a mathematical model of the belt conveyor that takes belt elasticity into account during stopping and starting be considered in these critical, long applications. A model of the complete conveyor system can be achieved by dividing the conveyor into a series of finite elements. Each element has a mass and rheological spring as illustrated in Figure 26. Figure 26 Many methods of analyzing a belts physical behavior as a rheological spring have been studied and various techniques have been used. An appropriate model needs to address: 1. Elastic modulus of the belt longitudinal tensile member 2. Resistances to motion which are velocity dependent (i.e. idlers) 3. Viscoelastic losses due to rubber-idler indentation 4. Apparent belt modulus changes due to belt sag between idlers Since the mathematics necessary to solve these dynamic problems are very complex, it is not the goal of this presentation to detail the theoretical basis of dynamic analysis. Rather, the purpose is to stress that as belt lengths increase and as horizontal curves and distributed power becomes more common, the importance of dynamic analysis taking belt elasticity into account is vital to properly develop control algorithms during both stopping and starting. Using the 8.5 km conveyor in Figure 23 as an example, two simulations of starting were performed to compare control algorithms. With a 2x1000 kW drive installed at the head end, a 2x1000 kW drive at a midpoint carry side location and a 1x1000kW drive at the tail, extreme care must be taken to insure proper coordination of all drives is maintained. Figure 27 illustrates a 90 second start with very poor coordination and severe oscillations in torque with corresponding oscillations in velocity and belt tensions. The T1/T2 slip ratio indicates drive slip could occur. Figure 28 shows the corresponding charts from a relatively good 180 second start coordinated to safely and smoothly accelerate the conveyor. Latest Developments in Belt Conveyor Technology MINExpo 2004, Las Vegas, NV, USA M.A. Alspaugh, Overland Conveyor Co., Inc. September 27, 2004 Figure 27- 120 Sec Poor Start Figure 28- 180 Sec Good Start Latest Developments in Belt Conveyor Technology MINExpo 2004, Las Vegas, NV, USA M.A. Alspaugh, Overland Conveyor Co., Inc. September 27, 2004 Mass Flow at Transfer Points One of the reasons for using intermediate drives and running single flight conveyors longer and longer is to eliminate transfer points. Many of the most difficult problems associated with belt conveyors center around loading and unloading. The transfer chute is often sited as the highest maintenance area of the conveyor and many significant production risks are centered here. Plugging Belt and Chute Damage and Abrasion Material Degradation Dust Off Center Loading/Spillage In the past, no analytical tools have been available to the design engineer so trial-and-error and experience were the only design methods available. Today, numerical simulation methods exist which allow designers to “test” their design prior to fabrication. Numerical simulation is the discipline of designing a model of an actual physical system, executing the model on a computer, and analyzing the results. Simulation embodies the principle of “l(fā)earning by doing. To understand reality and all of its complexity, we build artificial objects in the computer and dynamically watch the interactions. The Discrete Element Method (DEM) is a family of numerical modeling techniques and equations specifically designed to solve problems in engineering and applied science that exhibit gross discontinuous mechanical behavior such as bulk material flow. It should be noted that problems dominated by discontinuum behavior cannot be simulated with conventional continuum based computer modeling methods such as finite element analysis, finite difference procedures and/or even computational fluid dynamics (CFD). The DEM explicitly models the dynamic motion and mechanical interactions of each body or particle in the physical problem throughout a simulation and provides a detailed description of the positions, velocities, and forces acting on each body and/or particle at discrete points in time during the analysis. 8 In the analysis, particles are modeled as shaped bodies. The bodies can interact with each other, with transfer boundary surfaces and with moving rubber conveyor belt surfaces. The contact/impact phenomena between the interacting bodies are modeled with a contact force law which has components defined in the normal and shear directions as well as rotation. The normal contact force component is generated with a linear elastic restoring component and a viscous damping term to simulate the energy loss in a normal collision. The linear elastic component is modeled with a spring whose coefficient is based upon the normal stiffness of the contact bodies and the normal viscous damper coefficient is defined in terms of an equivalent coefficient of restitution (Figure 29). Figure 29 Figure 30 Figure 30 shows particles falling through a transfer chute. The colors of the particles in the visualization represent their velocity. The RED color is zero velocity while BLUE is the highest velocity. Perhaps the greatest benefit that can be derived form the use of these tools is the feeling an experienced engineer can develop by visualizing performance prior to building. From this feel, the designer can arrange the components in order to eliminate unwanted behavior. Other quantitative data can also be captured including impact and shear forces (wear) on the belt or chute walls. Latest Developments in Belt Conveyor Technology MINExpo 2004, Las Vegas, NV, USA M.A. Alspaugh, Overland Conveyor Co., Inc. September 27, 2004 Future Bigger Belt Conveyors This paper referenced Henderson PC2 which is one of the longest single flight conventional conveyors in the world at 16.26 km. But a 19.1 km conveyor is under construction in the USA now, and a 23.5 km flight is being designed in Australia. Other conveyors 30-40 km long are being discussed in other parts of the world. Belt manufacturers have developed low rolling resistance rubber with claims of 10-15% power savings as methods to quantify indentation have become known. Together with improved installation methods and alignment, significant power efficiencies are possible. Underground coal mines and tunneling contractors will continue to use the proven concept of distributed power to their best advantage, but now at least two of the longer surface conveyors in development will be installing intermediate drives in 2005. In Germany, RWE Rheinbraun operates coal conveyors with 30,000 tph capacities and other surface coal mines have plans to soon be approaching these loads. With capacity increases, comes increases in belt speed; again demanding better installation, manufacturing tolerances and understanding of resistances and power. Each time we go longer, higher, wider or faster, we stretch the limits of our analytical tools to predict system performance. And because each conveyor is unique, the only way we have to predict performance is our numerical analysis and simulation tools. Therefore it is imperative we continue to improve our design tools as our goals get bigger. Belt Conveyors for Bulk Materials, 6th Ed, CEMA The Conveyor Equipment Manufac