THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CXIII. FRIDAY, APRIL 27, 1917. No. 2939. CoaLhandling Plant at By T. G. OTLEY and In connection with the methods of boiler operation and maintenance adopted and the efficiencies obtained at the power stations of the Rand power companies, the authors give the following particulars of the appliances used for supplying coal to the boilers, and for the removal of ashes: — Brakpan, The Brakpan station is so situated that the railway trucks can be shunted up an embankment direct into the overhead coal bunkers, the coal being dumped from hopper trucks of 40 and 50 tons capacity directly into the bunkers below. Fig. 1.—Coal Handling Plant at Simmerpan. Illi Illi II Illi Fig. 2.—Coal Bunkering Arrangements at Rosherville. Cross Section When the station was extended, the same system was adopted. The bunkers are divided by bulkheads into nine divisions, four being over the small boilers and five over the new and larger boilers. The capa- city of each of the four small bunkers is 120 tons, and of each of the five large bunkers 375 tons. This arrangement, which appears to be so simple on paper, is most inconvenient in practice. No very serious difficulties were met with before the station was extended, and when the coal consumption was only about 1,300 tons per week, but now that the con- sumption has risen to 6,000 tons and over, the draw- backs and difficulties are considerable, due principally to the fact that it is impossible to mix the coals to obtain the most efficient results. The effect of mixing upon efficiency will be dealt with later. It must be admitted that as far as the coal hand- ling costs are concerned the arrangement is a good one, but the approach to the coal bunkers is up a very steep embankment, which causes the shunting charges to be exceedingly heavy. An outside coal storage with elevators and conveyors would have reduced the shunt- ing, but somewhat increased the handling charges. The principal gain, however, would have been in effici- ency due to the possibility of mixing the coal, and the money value of this gain in efficiency would have paid a very handsome return on the additional capital cost involved by the outside coal storage and conveyors. Simmerpan. An outside coal storage arrangement (fig. 1) is pro- vided at this station, consisting of a concrete and steel bunker having a capacity of 3,240 tons. This bunker is at right angles to the centre line of the boiler houses. Gravity bucket conveyors run the full length underneath, and deposit the coal to any one of the three gravity bucket elevators and conveyors taking coal to the overhead bunkers of the three boiler houses. Several fillers are provided, so that alternate buckets can be filled with different coals, enabling the coals to be mixed to give maximum efficiencies. The arrange- ment has worked perfectly, and without trouble. All coals burnt at Simmerpan are either duff or a mixture of duff and peas. It is found that such coals can be burnt more efficiently when moistened, and this can be done very effectually either as the buckets empty themselves in the overhead bunkers or as they * From the Journal of the South African Institution of Engineers. Rand Power Stations.* YERNEY PICKLES. are being filled from the outside bunker. The results of wetting coal will be dealt with later. Rosherville. At Rosherville the bunkering arrangements (fig. 2) are generally similar to those at Simmerpan, except that the five staiths are in line with the centre lines of the five boiler houses, and that the outside storage is not in the form of a bunker, but as shown on fig. 3. Gravity bucket conveyors run the full length under- neath the coal staiths, and the elevators convey the coal into the overhead bunkers above each boiler house. The gravity bucket type of elevator and con- veyor in practice has been found so reliable that the consulting engineer for the Rosher- ville station deemed it unnecessary to have any considerable inside storage over the boilers. There is considerable difference of opinion among power station engineers as to the advantage or otherwise of large overhead bun- kers. In the authors’ opinion, the local condi- tions are the deciding factor. Where ground is dear and space conse- quently limited, it may be impracticable to have considerable outside storage, and in that case there is no option but to build bunkers over the boiler house of the necessary capacity, bearing in mind the possibilities of irregular supplies. On the other hand, where land is relatively cheap, as in South Africa, there is no doubt that an outside bunker is much the better arrangement, as not only can larger tonnages be more easily stocked, but the cost of the building steel work is largely reduced, and a lighter and cleaner boiler house results. The tendency in modern practice is, where possible, to store coal outside, and have only the minimum inside storage. In this case, the conveyors and supporting structure, and the small coal bunkers and chutes connecting them to the stoker hoppers, can be suspended from the roof principals. A further advantage of outside storage with minimum inside bunker is that one can draw upon any particular coal at any time with the certainty that it will reach the boiler to meet sudden changes of load, whereas, with large Overhead bunkers the choice of coals is somewhat restricted. The first four boiler houses at Rosherville have only very limited inside storage; the fifth boiler house is provided with an overhead bunker capable of holding 800 tons. As the boilers in this house are steamed steadily and continu- ously,- there is no very great disadvantage in the overhead bunker, although it is not necessary, and certainly added considerably to the capital cost of the buildings, and interferes with the natural lighting. Vereeniging. At Vereeniging the outside coal staiths are very similar to those at Rosherville, being arranged in line with the centres of the two boiler houses. Bunkers are also arranged over the boiler houses, each of a capacity of 1,000 tons. In this case, also, they could have been done without quite well, since they interfere some- what with the light, and added considerably to the capital costs of the buildings. Conveyors, Elevators, and Weighing Machines. Experience with the gravity bucket elevators and conveyors has been almost uniformly satisfactory. They have proved most reliable in service, and the maintenance costs are not excessive. The most impor- tant factor in keeping maintenance costs down is to keep the conveyor clean and well lubricated; the auto- matic oiling device, while quite pretty in theory, does not work well in practice, and it is much better to put the conveyors in charge of a white man, with natives to keep clean and lubricate by hand. Even when well lubricated, the bushes and axles wear in course of time. The life of the bushes appears to be about four years’ almost continuous running. The material from which they are made is usually too soft. Conveyors with case-hardened bushes have been recently fitted, which, after several months’ continuous running, show no signs of wear. The total operating and maintenance cost of all the conveyors and eleva- tors on the system is given in the following table, and averages 0-780d. per ton delivered into overhead bunkers. Operating and Maintenance Cost of Gravity Bucket Conveyors, 1915. • Number of conveyors ................. 12 Total length ..................... 7,210 ft. Tonnage handled per annum (1915) . 828,7 3 tons. Operating and maintenance cost per ton 0'780d. Weighing of Coal. To weigh coal correctly in such quantities as are used in large power stations is a much more difficult problem than one would imagine, and a very considerable error in stock can be made when tonnages in the region of 1,000 to 1,400 tons per day are handled. The importance of correctly weighing coal will be realised when it is understood that the knowledge of the boiler house efficiency is wholly dependent upon correct returns. At Brakpan and Simmerpan no weighing machines are used. The coal is measured by levelling the bunkers, which are calibrated to definite volumes per foot depth. If the bunker capacity is relatively small, this method of measurement is capable of giving accu- rate results, and it is impossible for an accumulative error to be made. At Simmerpan the maximum stock carried is about 3,500 tons, and at Brakpan about 2,500 tons. At Rosherville the stock frequently reaches 12,000 tons, and has on occasion reached between 15,000 and 16,000 tons. When this occurs, considerable expenses are incurred in trimming and handling, and it is very difficult to estimate whether the stocks are in accordance with the book figures. At Vereeniging the maximum stock carried is about 7,000 tons. Blake - Denison continuous weighing machines are provided at Rosherville and Vereeniging, and with great care and frequent calibration, and a knowledge of the bias of the machines, a fairly reliable return can be- obtained. It ‘may be said that the methods adopted for weighing coal in power station I ■ ,r Fig. 3.—General View of Rosherville Plant, with Outside Storage. work leave room for considerable improvement. The weighing of coal is a fundamental measurement, and the more accurately it is effected, the more confidence one has in the station returns. The stocks at Rosherville and Vereeniging are checked at the end of each quarter by a careful survey. It is not unusual to find an error of between 1,000 to 2,000 tons either plus or minus. This would represent a continuous error of between 1 and 2 per cent. Only a few of the Transvaal collieries weigh the duff or by-product coal which they send to the power companies. The bulk of the coal is delivered in 40- or 50-ton hoppers filled right up. Frequent check tests have shown that the weights of coal delivered agree fairly well with those on the weigh bill.