Historically, it was common for the output from longwall face equipment to greatly exceed belt conveyor system capabilities. With recent advancements in drive and control technologies and increased idler capacities, belt conveyor capabilities are now a much closer match to the output generated by high-capacity longwall equipment. Not that many years ago, underground coal mines could achieve 5000 short tph only with ideal mining and material transport conditions. Today, as the result of these advancements, it is possible for an underground coal mine running a dual longwall system to produce 10 000 short tph.
The implementation of medium-voltage controllers, improvements in controlling belt tensions and the introduction of idlers with increased carrying capacities have all contributed to the most productive underground conveyors and slope belt conveyors ever designed and installed. These advancements have dramatically improved existing conveyor power, control and reliability, as well as the carrying capacity levels required to keep up with advanced longwall equipment production. Joy Global is at the forefront of these belt conveyor advancements and has recently applied that knowledge in several key underground coal mining operations in the US.
Joy Global provides custom-designed controls as part of the overall conveyor system to meet the demands of each individual installation. With controllers for head drives and tripper drives, it is now possible for conveyor systems to function effectively even in the most complex mining situations. With proven start packages, conveyor systems are now capable of extended start times in excess of 120 sec., with optimal load sharing between head drive and multiple tripper drives. All of the new conveyor installations listed in Table 1 use medium-voltage variable frequency drives (VFDs). Medium-voltage controllers are not new, but have only recently been used in high-tonnage, high-horsepower slope belt conveyor systems. The advantages includes:
- Controlled, full-load torque at various speeds, when designed properly.
- Advanced load-sharing in multiple motor drive arrangements (VFDs offer better control and reaction times when compared to other drive options).
- Extended conveyor start times that minimise shock and stress to all conveyor components.
- Consistent, repeatable conveyor starting regardless of material load on the belt.
- Reduced take-up tension, resulting in lower overall belt tension – in some cases allowing use of a belt with a smaller PIW (pounds per inch of belt width).
- Electrical cable size is much smaller, meaning the cable is less costly and distance from the drive to the motor can be much greater.
All-electric winches have recently been applied successfully in high-capacity slope belt installations, resulting in efficient conveyor operation and increased production.
Constant tension winches are an alternative to the traditional sheaved cylinder take-up designs. Greater line pull and longer travel are the main advantages of the constant tension winch design. Joy Global acquired Continental Conveyor in 2008 and has continued to develop take-up technologies that are advancing the capabilities of some of the world’s most productive conveyor systems. These take-up technologies directly affect operational belt tensions and ultimately influence stresses throughout the entire conveyor system. With belt capacity ranging from 40 ft to 1500 ft and line pull ranging from 25 000 lb to 150 000 lb, constant tension winches are capable of successfully meeting belt tensioning requirements for lengthy, high-tension applications.
All-electric winches have been successfully used in the conveyor industry for close to 20 yr, but have only recently been accepted to control slope belts. Historically, slope belts used gravity take-ups located outside the mine. Gravity take-ups typically require more maintenance and are limited in their ability to achieve the desired belt tension in complicated systems that have dynamic stopping conditions.
Much of the time, winch drums remain stationary, with 100% torque and zero speed, as long as belt tensions are correct. When tensions fluctuate, the all-electric winch responds accordingly. This reaction time is almost immediate.
When properly equipped, all-electric winches allow a user to monitor belt tensions and apply variable rope pull to the take-up carriage or belt storage unit. The all-electric winch is ‘smart’ in that it can adjust to the dynamic load conditions found in complicated systems – large and small. The all-electric winch allows the motor to reach full torque at zero speed. These features allow the winch to maintain proper belt tension regardless of load conditions and belt speed.
High-capacity conveyor components
The demand for increased conveyor capacities has also led to the development of conveyor components capable of carrying up to 10 000 short tph. For example, the HXF-40 idler from Joy Global exceeds CEMA F idler load standards and is designed for today’s super-capacity operations, including highly productive underground longwall coal mining operations attempting to more closely match tonnage rates to the output of highly productive longwall mining machines.
The HXF-40 idler uses a spherical roller bearing and super-duty frame. Roll features include a triple labyrinth seal, counter-bored shell and a lip seal designed to provide added protection for the bearing.
In addition, for applications exceeding CEMA D and CEMA E standards, Joy Global recently introduced its patented engineered shaft for HD-1500 (CEMA D+) and the SDX-2200 (CEMA E+) conveyor idler rolls. These new idlers exceed load ratings for their respective CEMA class by 20% to 40%. The HD-1500 idler is rated up to 1500 lb and the SDX-2200 idler is rated up to 2200 lb. Using Statix conveyor simulation software, idler spacing is calculated for each application. Idler spacing can be extended based on load for a particular installation. In these installations, fewer idlers are required, leading to potential cost savings for the mine operator.
The combination of a CEMA F centre roll and CEMA E wing rolls is used to transport material from underground to the surface at a rate of 10 000 short tph.
Selecting the best conveyor drive system
Many conveyor drive system options are available and not all systems require medium-voltage VFDs. Proper selection for a particular mine location varies depending on many factors, including system environment, maintenance expertise (hydraulic vs. electric), total horsepower requirements and system complexity.
The goal is to select a power system that is actively controllable and friendly to the conveyor system from a starting, stopping and load-sharing standpoint, thereby protecting the conveyor belt and the many components that make up the conveyor system. Very high belt tensions can lead to extreme stress, which can result in component and/or system damage, extended downtime and lost production. Insufficient belt tension in the conveyor system typically leads to belt sag and possible pulley slippage. The result is an underperforming conveyor system.
A conveyor drive system that allows a consistent, repeatable, linear time ramp up during acceleration and can balance load distribution during operation (regardless of material load) offers the most protection for a conveyor system. Over the years, conveyor systems have used several different control systems. As the need for larger and more productive conveyors is now more common, technologies and control methods have advanced to meet those needs.Several types of control technologies are available, including controlled clutch slip systems and controlled fill fluid couplings, as well as low-voltage and medium-voltage VFD controls. Each of these technologies has advantages and limitations related to performance and cost, which is why each application should be reviewed by a knowledgeable representative on a project-by-project basis to determine what system best fits a particular requirement.
Dual 1000 hp drives are used as part of a slope belt conveyor drive system to transport coal from underground to the surface for processing.
While many things in the conveyor business have changed, one thing remains constant: successful conveyor system installations start with proper design and engineering. Joy Global uses conveyor simulation software to model system behaviour under a wide variety of loading conditions before the conveyor is installed. Mines should select a supplier with design and application experience to provide highly efficient and reliable conveyor systems.Mines in every part of the world are focusing on new methods and technologies that can effectively increase their levels of productivity and profitability. The newest and most productive underground coal mines in the US have selected conveyors with medium-voltage controllers and super-capacity belt idlers for increased conveying capacity and the resulting increase in profitability.
Read the article online at: https://www.worldcoal.com/special-reports/21092015/maximising-production-2881/