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Colliery Engineering

March, 1933

Steel Props

In common with many other collieries in recent years, a good deal of attention has been devoted to roof support with steel. Many types of props have been experimented with, the one that has finally proved to give the best results, both as regards safety and economy, being shown in Fig. 16. This prop, manufactured by the company, consists of a length of H-section girder cut level at the top, which is covered by a stiff segmented capping piece; at the bottom the web is cut away to form an angle and the flanges bent over and welded as shown. The capping piece and the wedge-shaped portion are secured by oxyacetylene welds, the whole making a robust and serviceable job that entails no difficulty in handling and affords adequate support under all conditions. The freedom from projections is a decided point in its favour, and the prop can be erected and knocked out with the same ease as a wooden support. It is set in conjunction with corrugated steel straps, a thin wood capping piece being inserted between to form a cushion.

The angle of the point is determined by the hardness of the floor and varies from 450 to 800, the object being not to have too great penetration but simply sufficient to provide relief to the prop when heavy weighting of the roof takes place, and also to make the withdrawal of the prop a. comparatively easy matter, as compared with the withdrawal of a similar prop which has perfectly flat ends.

It will be obvious that this system of propping depends for its success on allowing the roof to yield slightly, and whereas this condition is in direct contradiction to what is considered to be the desirable practice so far as Scottish experience is concerned, there is no doubt that it gives the best results in the present case. This merely goes to prove once again, of course, that there are so many unknown variants in coal-mining practice that it is impossible to argue from the particular to the general, and a system which gives excellent results in one set of conditions may fail utterly in another set of conditions which are apparently similar.

At any rate, there can be no question of the success of this type of prop at the Consett collieries. The loss of individual props is negligible, and no face has collapsed since the system was introduced. The cost of equipping a 90-yard face with four rows of props, 2 ft. in length and 3 ft. apart, is approximately £60. This capital outlay is recovered on an average after about four months' work by the saving effected as compared with the cost of working with wooden supports. The cost for roof supports on conveyor faces, where nothing but steel straps and steel props are used, over a period of twelve months' working, is less than 1d. per ton, the replacement for lost material being less than 1 per cent.

These satisfactory results, particularly those relating to maintenance and replacement. are almost entirely due to the application of steel being carried out on a well defined system, which is recognised and acted upon by both workmen and officials. When it is decided to support a face by steel, a record is kept of the number of props and steel bars which are supplied for equipment. Thereafter a count is taken each week, and any shortage is noted, and reasons sought for such shortage. Unless the reasons are considered satisfactory, then the persons responsible for the shortage are dealt with in a suitable manner. Great care is exercised in seeing that the supports are set uniformly and as near as possible in well-defined lines; control of the roof is thus well maintained and the tendency for props and straps to be lost is considerably reduced. As a rule, both officials and workmen take a very intelligent interest in the general condition of the faces, and realise the value of the material and take a proper interest in its preservation.

Power Plants

The electrical requirements of the group are met by a central generating station at Chopwell Colliery, which runs in parallel with a second station at Derwenthaugh (six miles distant) where an extensive by-product coking plant is situated, and a third station at Templetown Coke Works near Consett. Thus the whole of the collieries and coke works are linked up, with the exception of Langley Park, which is outside the group. At the Chopwell plant the steam is generated in a battery of five 18,000-lb. capacity water-tube boilers — three Stirling and two Woodeson — and three Babcock & Wilcox waste-heat boilers fired by gases from a battery of beehive coke ovens. Apart from the waste-heat units the boilers are provided with Underfeed chain-grate stokers and water-sealed ash-handling plant — the latter being the first to be put to work in the country. The fuel used is coke ballast and low-grade small coal. The working pressure of the battery is 160 lb. per sq. in.

The generating plant consists of two 2,250-kVA. 1,500r.p.m. 3-phase 50-cycle turbo sets, one being for mixed pressure, and an English Electric 5,000-volt 3,000-r.p.m. high-pressure set of 2,500 kVA. capacity at 0.7 power factor. The mixed-pressure set derives its low-pressure steam from a Rateau accumulator which is fed by a steam winder, and, when it is running, by a standby Belliss set generating direct current. This latter equipment has a capacity of 200 kW. and generates current at 250 volts to supply the lighting needs of the colliery and Chopwell village. Normally, however, this duty is undertaken by a 200-kW. Bruce Peebles motor-converter set which runs off the 625-volt A.C. supply. A second converter set of 40 kW. capacity generates direct current at 110 volts for the pit bottom lighting and other auxiliary purposes.

Condensing water for the turbo sets is cooled in a Premier tower capable of handling 200,000 g.p.h. and by a spray pond of 250,000 g.p.h. capacity.

Particulars of the 5,000-volt overhead lines interconnecting the various collieries are shown on our opening sketch plan. The larger consuming points are connected by duplicate circuits, arid link air-break switches are provided at each substation so that a pit can be run on either line while the other can be made dead for repair work. As the duplicate lines are carried on double wooden poles pitched 7 ft. 6 in. apart, any work can be carried out on the dead line in absolute safety. Practically the whole of the motors, both at the surface and underground, are run on a 625-volt supply, a variety of transformers being provided for the necessary voltage conversion. Distribution and control is for the most part effected by Reyrolle and Metrovick ironclad switchgear.

As the overhead lines traverse a good deal of high ground trouble has been caused in the past through breakdowns induced by lightning surges. In order to safeguard the position Ferranti surge absorbers are being inserted at Medomsley Colliery — which occupies the highest position in the system — and it is anticipated that further trouble will at least be minimised. These units consist of a 100-amp. 12-kV. type "T" set for the protection of the main colliery substation and a 20-amp. type "C" set for the winding equipment. Power factor correction is effected by a static condenser situated at Eden Colliery, and by a number of synchronous motors connected at various other parts of the system.

The compressed-air requirements of the group are met by three 1,200 cu. ft. capacity electrically driven Belliss sets and several smaller units of 750 cu. ft. capacity. In addition a number of inbye units are in commission of from 200 to 500 cu. ft. capacity, and are either of Belliss or Sentinel manufacture.

Winding Equipment

As a rule steam winders have only been retained at those collieries where inferior coal is available, and most of the winders are of the electrical type. That at the Victory Pit is of special interest and indeed is unique in the country, since it incorporates a Williams-Janney variable-speed gear. At this pit the coal is drawn from the Brockwell Seam at a depth of 235 ft., part of the output also being derived from the Busty seam which is connected to the Brockwell by a cross-measure drift driven to the rise. The equipment has been designed to wind 1,000 tons per day in 14 hours, with a necessary margin for drawing the men, a certain amount of fireclay and lowering timber and supplies. The variable-speed equipment offered a much cheaper solution to this particular winding problem than the Ward-Leonard system, and the peak load that would have been involved with an induction motor rendered this alternative impracticable.

The infinitely variable speed is obtained by hydraulic (oil) pressure, the actual gear consisting of two units, each comprising a group of cylinders (Fig. 20) mounted on a shaft revolving in a fixed casing c and separated by a valve plate d. Oval-shaped ports are provided at the ends of the cylinders and are successively brought into communication with the two ports e in the valve plate as the cylinder barrel rotates. One unit, the A end, fulfils the functions of a variable delivery pump supplying fluid to the other, or B end, which acts as a motor. The pump end is driven at a constant speed in one direction by a 75-H.P. motor, and as the socket ring f, which is driven from the shaft through a universal joint, runs in roller bearings mounted in the tilting box g, means are provided whereby the stroke of the pistons may be varied. Consequently, the rate at which the fluid is delivered can varied as desired between the zero and full-capacity limits. The B end has a fixed stroke only, the tilting box being held rigidly in the casing; its speed, therefore, depends on the quantity of fluid delivered by the pump. Thus, by controlling the stroke of the pump, any speed can be obtained between zero and that of the full speed of the pump. The stroke controlling mechanism not merely alters the relative positions of the pistons but, incidentally, enables the motor to be run in the reverse direction with the same range of speed. The change from one speed to another is effected continuously, without any stops whatever, and the change from one direction to the other is made when the pump is in the "no stroke" position.

The connecting rods h have spherical ends carried in spherical seatings, both in the pistons and socket rings; both the pump and motor shafts are supported at the inner ends by roller bearings fitted into the valve plate, while the outer ends run in bushed bearings carried in the main casing.

Control is effected by a conveniently situated handwheel, which operates the tilting box in the A end. The movement of the handwheel is about seven-eighths of a revolution to each side of the "off" position, so that control is easy and positive in all conditions. Mechanical brakes are provided, but do not need to be made use of when retarding or lowering loads down the pit.

The 7-ft. drum is driven through single-reduction helical gearing having a ratio of 84 to 1, the motor being a 625-volt 75 H.P. machine having a speed of 360 r.p.m.

The graphs in Fig. 19 show the characteristics when winding coal and stone, the input curve of the 100-H.P. 3-phase winder — which would have been necessary without the variable-speed gear — being also shown by way of comparison. It will be seen that the energy consumption is 0.33 kWh. when raising coal (net load 15.38 cwt.) and 0.37 kWh. when raising stone (net load 20.18 cwt.), thus showing savings of 30 and 38 per cent. on the energy consumption needed by the plain 3-phase equipment.

In addition to the reduced energy consumption and the reduction in the peak loads, the equipment has proved to be mechanically reliable in every way, and repair and maintenance costs have been of a negligible order. The saving in capital expenditure compared with a converter set is, of course, considerable, and the running costs are naturally much smaller.

Three other winders of this type are at work at the company's Hunter, Medomsley Main Coal and Langley Park Kaysburn pits. the makers in each case being Vickers Limited.

At Medomsley Colliery, Busty Pit, the winding is accomplished by a Ward-Leonard set incorporating a flywheel balancer. The depth of this shaft is 480 ft. and the coal is wound from three separate levels corresponding to the Towneley, Busty and Brockwell seams. An 11-ft. cylindrical drum is provided and is driven through 7 to 1 reduction gearing by a 325-552 H.P. 500-volt D.C. motor. The flywheel converter set operates on the 5,000-volt supply and has a speed range of 585 to 500 r.p.m., the flywheel weighing 6 tons. An exactly similar unit is in operation at Eden Colliery, winding over 1,000 tons per day.

Central Workshops

The centralised workshops at Bradley are particularly well equipped and enable much of the mechanical equipment of the colliery group to be manufactured on the spot, and practically no repair work is sent out at all. In the engineering shop proper there are six lathes — the largest having 16-in, centres — two drilling machines, slotting machines, &c., while the smiths' shops contain ten hearths and two steam hammers. In addition there is a fully equipped electricians' shop which possesses every facility for rewinding motors, &c., and an electrical testing chamber. There is also a pump testing chamber in which reconditioned pumps are thoroughly tried out for performance before being sent out to service again. A sawmill and joiners' shop are also, of course, available and supply the requirements of the collieries as regards tubs, sheave frames, sleepers and the many other items of woodwork.

All the company's scraper chain conveyors, liquid starters, centrifugal pumps, and smaller haulages, are built up in these shops from raw materials supplied from the steelworks. All the transformer and lubricating oil is also brought here for clarification, a De Laval separator having been installed for this purpose.

Other Activities

Apart from the normal work of producers of coal, the company owns and operates four batteries of by-product coke ovens, situated at Derwenthaugh, Templetown, Consett Fell and Langley Park, which have a carbonising capacity of approximately 29,000 tons of coal per week, and in addition a battery of beehive coke ovens situated at Chopwell Colliery. Electrical energy is produced in bulk to serve the power requirements of the collieries and other works and, in addition, practically the whole of the local villages and neighbourhood are supplied with current for lighting and domestic use. From the Derwenthaugh coke-oven plant gas is supplied in bulk to the Newcastle & Gateshead Gas Company. Since the war this company has built over 1,500 new workmen's dwellings of an excellent type and the condition of the colliery villages, equipped with their institutes and recreation grounds and other social amenities, indicate the great interest the directors of the company take in the welfare of their workpeople.

Drawings and Photographs accompanying the article