Oak Grove ExConTec
Project Overview
The three-flight overland conveyor system at U.S. Steel Mining's Oak Grove Coal Mine has operated since 1974, delivering coal from the mine's slope belt to the Concord Preparation Plant near Oak Grove, Alabama — a total conveying distance of 8.3 kilometers. Originally designed for 771 t/h, actual throughput had grown to between 907 and 998 t/h, and projected increases in mine production required upgrading the system to deliver up to 1,270 t/h continuously.
Any upgrade had first to resolve persistent operational problems. Most critically, conveyor OL-2 experienced recurring slack belt conditions every time it stopped under load. Poor troughability of the high-tension multi-ply fabric belt — at only 914 mm width — caused poor idler roll contact when running empty and poor belt alignment on all three flights.
Dos Santos was contracted to thoroughly analyze the current system, document its operating limits, recommend improvements to the existing system, and design the upgrade to 1,270 t/h.
Field Monitoring and ExConTec Calibration
Dos Santos conducted a comprehensive field monitoring program alongside a detailed study of the system's performance, operating, and maintenance history. Initial ExConTec modeling produced acceptable results compared to field data, but with some behavioral discrepancies. The ExConTec program's built-in discretionary factors — Kx and Ky multipliers — allowed calibration of the model until it achieved very close correlation with field monitoring across all three conveyor flights.
With the calibrated ExConTec model, we could now predict the performance of the upgraded system with confidence — and evaluate multiple upgrade configurations side by side.
Upgrade Analysis and Configuration Selection
Using the calibrated ExConTec program, we performed complete analysis and preliminary design of multiple upgrade configurations, including:
- Upgrade of the existing head-end driven system with increased belt speed and motor power
- Tripper-type "Smart" booster drive scenarios at each flight
- Tripper-type "Natural" booster drive scenarios at each flight
A single strategically located booster drive per flight could mitigate maximum belt tensions — reducing belt strength requirements and permitting a better-troughing, more-trainable belt. However, ExConTec analysis revealed a critical finding: the "Smart" booster, with its added controls, actually caused tension aggravations and regenerative driving at the head end of OL-1 under certain transient loading conditions. The simpler "Natural" booster was the better solution.
Ultimately, for lowest initial cost, the upgrade configuration selected was the current head-end driven system with increased belt speed and secondary drive power on each flight. The OL-2 slack belt problem was resolved by appropriately increasing the take-up tension. Dos Santos also performed final and detailed design of all required modifications and structural reinforcements.
System Parameters — Upgraded Configuration
| Parameter | OL-1 | OL-2 | OL-3 |
|---|---|---|---|
| Arrangement | Wire rope type intermediate structure; belt turnovers | ||
| Material | Coal | ||
| Design Rate | 1,270 t/h (1,400 STPH) | ||
| Conveying Angle | −12° to +15° | −13° to +8° | −11° to +12° |
| Length | 2,812 m (9,226') | 2,628 m (8,622') | 2,904 m (9,526') |
| Net Lift | 23.5 m (77') | 19.2 m (63') | 23.5 m (77') |
| Max Lift | 73.1 m (240') | 52.4 m (172') | 37.8 m (124') |
| Belt Width | 914 mm (36") | ||
| Belt Speed | 3.81 m/s (750 FPM) | ||
| Primary Drive | 298 kW (400 HP) | ||
| Secondary Drive | 224 kW (300 HP) | ||
| Inspection Drive | 45 kW (60 HP) through reducer | ||
Key Highlights
- Birthplace of the ExConTec Computer Simulation and Analysis Program — developed specifically to meet the analytical demands of this upgrade
- Field calibration using Kx and Ky multipliers achieved close correlation across all three flights, validating the model for upgrade design
- ExConTec analysis revealed that a "Smart" booster drive would cause tension aggravations — a counterintuitive finding that saved the client from an inferior and more costly solution
- Slack belt problem on OL-2 resolved through accurately modeled take-up tension increase
- Capacity increased from 771 t/h original design to 1,270 t/h — a 65% throughput improvement
- Total conveying distance: 8.3 km across three flights