A coal mine is no place to fool around. These vast, subterranean complexes are home to some of the heaviest and most dangerous industrial equipment, which is even more difficult to manage because much of it must be moved continually to new areas as coal is removed. Also, even the biggest mines are still enclosed environments, where coal dust and methane gas can result in fires and explosions. In such environments, intrinsically safe (IS) solutions—not only for at-the-face mining equipment, but also for supporting communications—can provide significant economic advantages compared to conventional systems using other explosion-protection methods, according to MTL Instruments, which is a division of Cooper Crouse-Hinds.
For instance, Daw Mill colliery is the flagship, deep mine of UK Coal. However, due to earlier closings of mines and privatization, UK Coal recently decided to review its control and monitoring, and find more supportable technologies. As a result, Daw Mill chose to deploy IS PLCs and IS Ethernet systems on all of its underground production and conveying equipment.
Located above the Warwickshire coal field, Daw Mill is favored by its geology, including the width and depth of its seam, and by the quality of the coal in it. The mine was opened in 1961 after this rich seam was found during the sinking of a ventilation shaft for another nearby mine. Production is nearly 3 million metric tons per year, and 96% of the extracted material is saleable. At a rate of 1200 metric tons per hour, almost all of Daw Mill's coal is used for power generation. This environment has to be carefully managed—and full control maintained—to make sure production is consistent and economically competitive with imported coal (Figure 1).
Securing Safer Systems
Daw Mill reports that, above all other considerations, safety is paramount in its machine operations, transport equipment, ventilation and environment systems, and gas monitoring devices. Automation has been fully implemented at the mine with machinery being controlled by IS programmable logic controllers (IS PLCs). This underground intelligence allows the use of commercially available real-time SCADA software to supervise, monitor and handle alarms. In addition, data is extracted from the mine's equipment that's used to provide real-time information for many management functions.
The present upgrade program began in 1998 when the overall coal industry had shrunk, and questions were raised about the ongoing viability of the mine's legacy, highly customized data-gathering solution. Also, Daw Mill reports its existing system's architecture was inherently slow and prohibited effective control. Investigating and selecting a new solution for the mine involved visits to other process industry sites, and discussions with several suppliers. Eventually, Daw Mill's engineers decided to use IS PLCs with an IS Ethernet network to enable communications with the surface.
Initially, the mine used MTL's 9260 range of IS Ethernet infrastructure components, but more recent extensions employed MTL's 9466-ET five-port IS Ethernet managed switch together with the option of Power-over-Ethernet (PoE) to distribute low-level DC power. IS Ethernet cards in PLCs throughout the mine are connected to a local 9466-ET switch together with 9461-ET IS Ethernet serial gateways used to communicate with a range of IS serial devices. The 9466-ET switches are all interlinked with lengths of fiberoptic cable via 9465-ET IS Ethernet copper-to-fiber media converters, and ultimately reach the surface to link into the control system servers. The use of multimode fiberoptic Ethernet connections is invaluable for the long distances involved in coal mining, supporting 2-kilometer (km) distances at 100 Mbps or 5 km at 10 Mbps, while IS Ethernet over copper supports the standard 100Tx Ethernet distance of 100 m.
Consequently, PLCs in the mine are monitored and controlled in the control room at the surface using Wonderware InTouch SCADA system from Invensys Operations Management. Daw Mill reports its high-value coal cutting machines and conveyors, as well as the underground environment, were brought under proper control with this combined PLC and SCADA solution. Likewise, because safety is the primary concern, particular efforts were made to monitor airflow, methane and carbon monoxide. Alarms are set to shutdown equipment and initiate evacuations at pre-determined levels.