Technical Market Support » General
Queensland Rail (QR) transports coal from Queensland coalmines to the coal ports in bottom dump rail wagons. Some coals have a propensity to be "sticky" which hinders their unloading and jackhammers are needed to vibrate the wagons to facilitate unloading. During 2003 and 2004, twenty six percent of the trains unloaded at the RG Tanna Coal Terminal in Gladstone required jackhammering, which cost the port approximately two percent of annual port throughput. This equates to 800,000 tonnes a year of under-utilised port capacity. The consolidating force applied to the coal during loading had previously been identified as a significant contributor to determining the strength of the consolidation and the position in the wagon where it occurred.
The objectives of this project were to determine methods for reducing loading forces and to modify the loading method used at a mine to improve wagon unloading at the port.
To enable loading forces at existing loadouts to be measured QR outfitted a new VSA wagon with a grid of 10 accelerometers fitted to its front slope sheet, and on and rear the front doorset. This unit was used to measure five loadings at one loadout and a sixth loading at another loadout. The loadings at the first loadout gave different force profiles, which suggested that at this loadout, operator technique could control the force profile. In line with the laboratory findings one of the profiles gave what was considered to be the desired force profile where the position of first impact was high on the slope sheet. The force profile from a different loadout gave a significantly different force profile.
A pilot scale train loadout and model wagon was used to investigate loading forces. The model wagon had the same slope sheet angle and door width as current wagons. Accelerometers mounted on the front slope sheet and on the wagon door were used to measure the loading force profiles. Tests were conducted with the loadout's slidegate at heights between 4 meters and 7 meters "above rail" and hence replicated the drop heights of most of the loadouts employed in central Qld. The major findings from this part of the work were:
- Loading forces increase with the drop height from the loadout slide gate to "rail".
- Initial contact at the top of the slopesheet instead of the middle of the slopesheet resulted in a less force being registered on the wagon door than when the middle of the slopesheet was hit.
- Missing the front slopesheet so that the position of first impact was onto the wagon door significantly increased the force registered in this position.
- The speed and the direction of opening of the slidegate did not significantly alter the force profile but they can influence the opportunity of placing the coal onto the top of the slopesheet.
An audit was performed on the loading of three trains. These audits examined different loading methods whereby the position of first impact of the coal during loading was on the middle of the front slope sheet, the top of the front slope sheet, or over the first doorset of the wagon. These audits showed that when during loading the coal missed the front slopesheet and hit over the front doorset the resultant jackhammering time increased. A second consolidation force was introduced by having a varying train speed. This was due to consolidation as the result of compaction at stoped or slow speed. The effect was most noticeable when the train halted partway through the loading of a wagon.
The amount of jackhammering required during the unloading of a train provided a simple method for quantifying unloading performance. Information was collected initially for between May and October 2003. Six coal products from two mines (approximately 1.7 million tonnes of coal) were loaded during this time. These trains were generally comprised either of 86 VSA wagons or 44 VSA and 56 aluminium wagons (of varying vintages). This data was used to obtain statistics on the jackhammering requirements of each of the six coal products carried in the trains of different wagon makeup. Once sorted in this manner it was easy to see that the different coal products had different unloading characteristics and that the trains comprised entirely of VSA wagons had superior unloading characteristics to trains that also contained aluminium wagons. These wagons of all these trains were loaded so the position of first impact of the coal was generally onto the middle of the front slopesheet of the wagon.
In late 2003 QR's coal loading requirements with respect to bogie overload criteria resulted in a change in the loading method. Overloaded wagons present significant operational issues at loadouts lacking facilities to remove excess coal and the loadout operators adopt a conservative approach to wagon loading that put less coal into each wagon and hence reduced the risk of overloads. This meant that the loading method was modified so that the point of first impact of the coal during loading was no longer onto the front slopesheet but was on the front doorset of the wagon. The trains loaded by this new method between January and June 2004 (carrying approximately 1.9 million tonnes of coal) required more jackhammering than those loaded with the previous method.
This finding was consistent with the laboratory tests, which found that loading the coal directly onto the wagon door significantly increased the force registered in this position. Due to operational constraints it was not possible to implement the preferred loading method, of hitting high on the slopesheet (identified during laboratory testing). Against this background the following project recommendations are made.
The different coal products give different unloading performance. Improved coordination between the mines and QR to maximise the proportion of the sticky coals that are carried in the trains comprised of only VSA wagons offers the most potential benefit to improve unloading performance in the short term. For this to work each product railed would need to be considered as a separate client and where possible products that give poorest unloading performance would be assigned trains comprised of only VSA wagons.
Regardless of the type of loadout that is used the challenge is to maintain, or improve on the efficiencies that fast loading rates return to the coal producers whilst reducing the impact that sticky coal has on unloading rates. Future loadouts should be designed not just to have a fast loading rate but also to minimise loading forces. It is envisaged that some version of a chute loader will continue to be the preferred design in the immediate future. Currently there would appear to be a preference to install these loadouts with high drop heights but QR should consider implementing guidelines with respect to maximum allowable drop height.