Safety Instrumented Systems / Intrinsic Safety / Asset Management / SCADA / Fieldbus

Intrinsic Safety Aids Cost-Effective Coal Mining

Daw Mill Colliery Implements Intrinsically Safe (IS) PLCs and IS Ethernet and Supporting Components to Fuel U.K. Power Utilities

By Jim Montague

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.

Also Read: Workforce Safety Is Top Priority for Mining Companies

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.

Control Needs Communications

The mine's PLC-controlled machinery includes coal-face shearing machines, ventilation and conveyors. The coal face at Daw Mill is presently 10 km from the control room, so communications have to be fast and effective, and fiberoptic IS Ethernet is used throughout the mine. One unique difference in mining, compared to manufacturing, is that its "underground plant" is under continuous movement and redeployment. When a coal seam reaches a boundary, all of the mining equipment may be moved to a position several kilometers away, and require totally different operating conditions. (Figure 2).

Even though Daw Mill has two main shafts, one for the miners and one for equipment, it's actually a drift mine with coal transferred to the surface by inclined conveyors which have powerful, PLC-monitored motors to cope with weight and loading changes.

For example, within the recent project period, miners noticed that a motor was drawing higher than normal current, but examination of the fine resolution data showed this was caused by external effects, rather than imminent motor failure. The fault proved to be a belt scraper that was pressing on the belt, causing it to slip, which explained the high current. Similarly, some high-resolution, archived operating data continually updated from the IS PLCs to the historian server recently diagnosed a blocked air pipe, which could have caused a £500,000 shutdown, but was instead cleared in five minutes once it was identified.

Besides these immediate cost savings, Daw Mill adds its IS PLCs and IS Ethernet systems deliver real-time, easily accessed information and useful insights for running the mine day to day. Its engineers add their PLCs and communications system has become the backbone for the colliery's operations because data required underground is readily available.

n fact, UK Coal has already taken the IS PLCs and IS Ethernet systems and capabilities developed for Daw Mill, and is using them extensively at six other collieries. Finally, the mine's IS Ethernet infrastructure gives it the option of adding wireless access points to connect to IS mobile computers; cameras for operations, safety and security; RFID for tracking assets and people; and voice over Internet protocol (VoIP)  telecommunications in the future.