Automated Weigh System for Coal Mine Passes the Most Stringent of Reliability Tests

Just outside Tuscaloosa, AL, reside some of the highest-grade coal mines in America. The Blue Creek Coal Mine No. 5 is owned by Jim Walter Resources, Inc. It produces low sulfur medium volatile coal for both utility consumption and for export to metallurgical markets worldwide. At 2,200 feet deep, it also happens to be the deepest vertical shaft coal mine on the North American Continent.

The mine also represents some serious challenges for electrical and mechanical engineers. One particular challenge involves the process of removing, weighing and transporting the coal ore to the surface. Since the mine opened in 1978, it has had no less than four weighing systems installed. Only one has been able to meet the stringent requirements of this demanding mining operation, the SIWAREX® M weighing system from Siemens.

A Tour of the Mine - To get to the area where the reciprocating feeder, weighing and load cells are located, you have to first take the single elevator to the bottom of the shaft. That's nearly a half-mile, straight down on a fast moving elevator. The ears "pop" as the air pressure changes during the deep descent.

Even Dante with his description of Hell could not fathom this cruel environment. Except for the inside of the control room, everything is covered with black crust. A penetrating dampness hangs in the air. This is the load cell's home. It is where it must perform its job 24-hours a day (three shifts), generally six days per week. It has to accurately weigh out 25-ton loads for each of the skips as they take turns in a seesaw motion carrying coal ore to the surface. It does this for about 2 million net tons of coal a year.

At 2,200 feet deep, the Blue Creek Coal Mine No. 5 owned by Jim Walter Resources, Inc., is the deepest vertical shaft coal mine on the North American Continent. Under the severest of conditions, reciprocating feeders and weigh\load cells load two skips around the clock, weighing out 25-ton loads for each of the skips as they take turns in a seesaw motion carrying coal ore to the surface. It takes just over 100,000 weighing cycles annually to bring enough ore to the surface for a production schedule of two million net tons of coal.

Accuracy and durability of equipment have plagued the weighing operation over the years. The problem was not solved until about one year ago when the mine removed its third system and replaced it with the SIWAREX M weighing system. Some of the earlier scale systems started having problems within the first month of operation. These problems ranged from inaccurate readings and lost calibration accuracy to total failure.

The load cells are hermetically sealed in casings of stainless steel that are impervious to outside elements-even for the harshest conditions. As coal ore is deposited inside the weigh cell, a compensation arm inside the load cell triggers a phototransistor switch, activating an electromagnetic coil. This causes a corresponding voltage change across a connected load resistor. The change is sent to a processing module that interfaces with a programmable controller.

Configuration software provided by Siemens runs on standard PC platforms. Working with the data provided by the weigh scale module, the software provides for easy setup of such functions as tare (container weight compensation), gross weight, net weight, scale standstill messages, calibration, automatic zero offset, coarse/fine feed signals, limit value generation, tolerance monitoring and filter parameters. The weigh scale modules allow calibration to be controlled with 20-bit sampling that provides an accuracy of 0.05 percent with 65,000 division resolution, ± 524,000 parts. The system has an EU rating of Class III, 6000d.

All the parameters are configured and stored in the Siemens SIMATIC(r)505 programmable logic controller (PLC). The weigh scale modules snap on a standard DIN rail inside the control cabinet just below the PLC. There are two modules, one for each load cell. The modules can be replaced in just a few minutes. This requires no re-calibration of the scales because the parameters are downloaded from memory.

The modules come standard with an RS232 port for connecting to a printer, PC or host. They also have a 20 mA connector for interfacing a third-party human machine interface that the control room uses to report the activity being transmitted from the trip stations and the weigh areas.

There are also status LEDs on the modules that indicate system errors, operation errors, calibration mode, scales standstill, 0.25e zero and scales adjusted status. Additional LEDs show the 24V power supply operation, the data activity of the three digital inputs and four digital outputs, as well as the analog output and the load cell connection status.

The control room on the surface for the hoist exchanges information with the control room at the weigh stations over Ethernet. In this manner, the technician at the bottom of the shaft and the technician at the surface level know what is happening in either area. Alarms and interlocks built-into the respective PLC system programming provide virtually trouble free automated operation.

Information from the two SIWAREX weigh scale modules is processed through a PLC. Data provided by the weigh scale module includes tare (container weight compensation), gross weight, net weight, scale standstill messages, calibration, automatic zero offset, coarse/fine feed signals, limit value generation, tolerance monitoring, and filter parameters. As a load setpoint is reached, the PLC signals the feeder to stop, the gate for input into the weigh cell is closed, and then a signal is sent to the weigh cell to open a door, dumping the weigh cell load into the trip unit.

After almost one full year of operation, the weigh system is operating accurately and with zero downtime. Since the initial calibration and other parameters were set during commissioning, no further re-calibration or other adjustments have been necessary. There has been neither control module failure nor electro/mechanical failure of the load cells.

Coal mines are among the harshest industrial environments in existence. It is not a place for products untested for the most extreme boundaries. Here the help of an experienced and proven manufacturer can make a world of difference in operational success.