Published in the May 2005 American Longwall Magazine

Longwall mine operators, in an effort to keep pace with increasingly productive longwall units and eliminate downtime resulting from successive panels not being ready, have become progressively more innovative in improving productivity from continuous miner (CM) development sections.

Some operator’s methods include equipment innovations, section set-up alternatives and operations analysis. Although technological developments continue to enhance productivity in the coal industry, a focus on management items such as accountability, problem solving, and operations analysis seemed, surprisingly, to be more of a basis for improvement.

The two primary considerations in mine design that can significantly impact CM production are the number of entries being developed and the size of the pillars. Two-entry versus three-entry gate development is a key consideration. Several factors will drive this decision, including safety considerations, site-specific geologic and mine conditions, mining equipment, ventilation, and regulatory agencies.

Typically, two-entry development is only approved by the regulatory agencies in situations where it provides a greater degree of safety. This has been done exclusively in the western US where high cover (greater than 2000ft) and coal/rock outburst situations are more conducive to this yield pillar application. Three-entry design is also used in some western mines and throughout the remainder of the US.

The second critical consideration in mine layout is pillar sizing, particularly the length. Obviously, longer pillars result in fewer cross-cuts, thus decreasing the amount of time to develop the same length of gate entry. In western US operations where progressive sealing is sometimes employed to combat spontaneous combustion, a substantial cost savings can be realized with longer pillars, with the cost of each seal being in the range of $US7000.

Ventilation is a limiting factor in pillar length since regulations restrict the total depth an entry can be driven (typically 200ft) before connecting with an adjacent entry. Longer pillars also improve roof and rib control. However, a point of diminishing return will be reached since lengthening the pillar also increases the amount of car change time, which negatively affects productivity.

Some companies analyze this situation using time studies and a production model in an effort to optimize pillar sizing from the standpoint of productivity.

The bottom line for pillar sizing, though, is primarily going to be safety, and adequate sizing for roof and rib control will be the determining factor.

A third consideration critical to development layout is to reduce the ratio of total linear feet of gate entry development to linear feet of panel length. A ratio of four or less is thought to be optimal.

A combination of a reduction in the amount of coal mined in development panels and an increase in both the width of longwall faces and the length of the panels has decreased the amount of CM development required for each longwall ton. Operators persist, however, in pushing CM productivity to the limit by continuing to increase longwall productivity.

A wide variety of operating schedules is being utilized to help maximize CM productivity. The use of alternative schedules has allowed some companies to increase utilization of capital investments while reducing the number of hours worked by each employee, thus resulting in a safer, more productive operation.

Shift lengths will vary from eight to 13 hours depending upon the particular schedule and mine travel times. The disadvantage for the company is that additional personnel are required, which further worsens an industry-wide manpower shortage. The advantage for the employee is a work schedule allowing several consecutive days off between work days. See figure for one version of a “Weekend Warrior” schedule.

Other operations have experimented with alternative schedules, but after analyzing the results have determined that a conventional schedule working five to six production days per week is more cost effective in their particular situation.

Equally as important as the production schedule is the arrangement of idle, or preparation, shifts. It is critical for section belt and power advancements to be completed within the allocated timeframe (sometimes as little as two hours), and that all production equipment be serviced and ready for operation upon arrival of the production crew. This sets the tone for the entire shift, and its importance is crucial to optimizing productivity.

Time to first coal is an excellent statistic to track and rate the performance of idle shift crews, who will take just as much pride in this statistic as production crews take in their footage mined.

Other schedule changes include sending in a few personnel just prior to the regular shift starting time to perform required regulatory checks on the section and equipment. One company reported reducing the time to dump the first car from a 30-45-minute range down to 15 minutes following implementation of this change.

Preventive maintenance programs have steadily evolved over the years, with computer software packages substantially enhancing this development.

These programs have become an integral part of mine management and have substantially reduced the amount of unscheduled downtime by performing consistent servicing and component replacement prior to failure.

In regard to CM development, many companies focus on their mobile equipment, such

as cars/haulers, and on improving availability.

Some companies are also performing predictive maintenance as their preventive maintenance programs have improved to the point that only smaller gains are possible, and as predictive analysis technology has become more accurate, practical, and affordable for coal operations.

Companies have hired predictive maintenance engineers to concentrate on this emerging field. Vibration analysis and thermography of electrical motors and installations are two types of predictive maintenance tools. Test results allow necessary repairs/replacements to be scheduled rather than operating until failure and resulting in unplanned downtime. Conveyor belt systems are a common target for predictive analysis.

Thermography cameras have become much smaller in size and...click here to read on.