[Trade Journal]
Publication: Engineering
London, England
vol. 129, no. 3357, p. 633-635, col. 1-3
THE WORKS OF MESSRS. STEATITE AND PORCELAIN PRODUCTS, LIMITED.
Chemical Industries, Limited, and Falk, Stadelmann and Company, Limited, in this country, to build a modern factory for the manufacture of porcelain and steatite insulators. It was also decided to establish a high-tension and mechanical research laboratory to investigate the properties of these products.
The works at Stourport are the results of this policy, that location being chosen owing to its clear atmosphere and the fact that both rail and water transport facilities are conveniently available between it, the Stourbridge fireclay deposits and suitable coal supplies. The first unit of the factory, with the exception of the preparation building and silos, is a one-story structure and covers four acres, but a site of some 70 acres in all is available for extensions. The factory is so laid out in relation to the railway that the material passes smoothly through it from its receipt to its dispatch as a finished product. As cleanliness is an important factor in the manufacture of porcelain for insulating purposes, the whole of the internal steelwork is enamelled and, as will be seen, other precautions have been taken to prevent the intrusion of ferrous material into the mix.
The principal raw materials used in the manufacture of high-tension porcelain are china clay, plastic kaolin, quartz, and felspar. These are chiefly obtained from Cornwall and Dorset and are delivered by railway to an unloading bay on a level higher than the factory floor. Each consignment, after sampling, is unloaded by a short band conveyor, which is carried on a runway so that it can be brought over the wagons, into the storage bins. It passes from these bins into a disintegrator, where it is broken up to a standard degree of fineness and is then discharged into a silo, of which there are 22 in all, each with a capacity of 30 tons to 40 tons. The con-veyors used for the latter purpose are equipped with phosphor-bronze buckets, and the stoneware silos are fitted with ebonite mouthpieces. Further analyses of the materials are made at this stage, while, as an additional assurance that purity shall be maintained, there is no communication between the storage and preparation buildings, except over the silo floor. The raw materials from the silos are delivered to an automatic weighing machine. This has a capacity of 2 tons and will weigh to an accuracy of 0.01 per cent. It is controlled by a combination look, so that unless the exact quantity of each material required for the mix has been admitted, its contents cannot be discharged. Each charge is also tested to determine the relative fineness of its particles.
The contents of the weighing machine are discharged directly into one of four ball mills, which are built up of fin. steel and are lined with porcelain blocks 5 in. thick. Grinding is effected by flint pebbles, the size and weight of which are checked at intervals, and is carried out for a definite number of revolutions. Two motors are employed for driving the mills, one of 45 h.p. for starting, and a second of 15 h.p. for running. On the completion of the grinding process the material is blown by compressed air into porcelain-lined mixing vats, one of which is visible on the left of Fig. 1, whence it passes through a set of magnets and sieves, so that any iron or large pieces are removed. It is then pumped into a filter press, which is illustrated on the right of Fig. 1, where it is subjected to a standard pressure for three or four hours. These presses are arranged at such a height that a truck can be run underneath them and the finished cakes dropped, simply by operating a lever. The cakes are taken on these trucks to storage cellars, where they remain for three or four weeks in an atmosphere of constant humidity to increase their plasticity. They are next kneaded in kneading machines, to eliminate any entrapped air and to ensure a constant consistency. This process occupies from two to three hours, after which the resulting blocks are taken to the turning shop.
To manufacture a standard insulator the clay is cut into pieces from which a ball is made. The first, worker throws this ball into a sugar loaf and then beats the latter into a plaster mould of the rough shape of the insulator. This mould is handed to a second worker, who forms the clay with a shaped tool on which there is an arrangement for making the undercut. After being allowed to dry slowly, it is then passed to a third worker, who brings its underside to the exact final shape, while a fourth shapes the outside with a former and places it on a plaster bat, on which it is taken by an elevator to the drying ovens. As the size and shape of an insulator have an important bearing on its electro-mechanical properties, a sample product is cut into sections at least four times a day and its dimensions checked. This enables any necessary correction in the shaping tools to be made. It may be added that models of the insulators are cast in plaster in a special shop, reproductions being taken from these models for use in the factory.
In the drying ovens the insulators are exposed to moist air, so that they dry from the inside outwards, the humidity being gradually reduced and the temperature gradually increased. This ensures against the formation of cracks, which might not otherwise be noticed until the electrical tests were made. The next process is glazing, which is effected on an automatic glazing machine. This is arranged so that only the part which is to be glazed receives a coating of highly refractory felspathic mixture, and that this coating is uniform. The exact composition of this glaze and its expansion relative to the body have an important bearing on the ultimate mechanical strength of the insulator. The insulators are now ready for firing, but before describing the furnaces used for this purpose it may be mentioned that to prevent direct contact with the flames, the insulators are enclosed in saggars of highly refractory fire clay. These saggars are formed in plaster and cast-iron moulds by a process similar to that used in making the insulators themselves, and are fired in the top chambers of the same round kilns, in which the large pieces such as bushings are treated.
The firing equipment consists of a tunnel kiln, the charging end of which is illustrated in Fig. 3, and two round kilns, which are used for the purpose already mentioned. Most of the high-tension products are, however, dealt with in the former kiln, which is 320 ft. long, and will accommodate 50 trucks simultaneously, one truck being charged per hour. The saggars are brought up to a low-level loading platform by a continuous conveyor, from which they are transferred to the trucks. The firing operation occupies 72 hours, at the end of which the insulators are first examined for mechanical or glazing faults, and are then transferred to the routine testing room. The kiln is fired by clean producer gas of constant composition at constant pressure. This gas is generated in two rotary producers, which are situated in a separate building, and after cleaning and scrubbing enters the kiln through 56 burners, each of which can be adjusted separately, so that the temperature and condition of the gas at any point can be regulated. The temperature is controlled by five Platinum-rhodium pyrometers and a Fery optical pyrometer, while Seger cones and test cups are placed on each truck; frequent analyses of the gas are also made.
In the routine testing room the insulators are tested for 15 minutes at their flash-over voltage, while all the steel and malleable iron fittings are subjected to a load 50 per cent. above normal before assembly. After assembly each insulator is tested mechanically to 25 per cent. above its working load on the machine illustrated in Fig. 6, and is then subjected to an impulse voltage of 200 kv. and a high-frequency test of 150 kv. at a frequency of from 120,000 to 150,000 cycles. Finally, each insulator is tested at its flash-over voltage for 15 minutes. In addition, a proportion of each batch is sent to the research laboratory and tested as described below.
Before dealing with this part of the factory, however, a short description may be given of the methods adopted in manufacturing steatite insulators. Steatite is a combination of magnesium oxide and silica in varying proportions, and, though it is found in many parts of the world, practically the only deposit from which insulators of suitable electrical properties can be made is in Bavaria. It has a much higher tensile and bending strength than porcelain, and has been largely used in the manufacture of the solid core and stick insulators employed in railway work, as well as for insulators for high-frequency telephony and in the production of pressed articles of high accuracy and strength. Moreover, when finely powdered, it can be pressed in a dry state to give an article which is strong enough to be dropped on to a band conveyor, and this property is made full use of at Stourport. The preparation of the raw steatite follows the same lines as those detailed in the case of the porcelain. The material, after being separated, dried, disintegrated and ground, is passed to the pressing machines. One of these machines, which were designed by the Steatite Magnesia Company of Berlin, is illustrated in Fig. 2. Its operation is entirely automatic, and by the use of specially designed tools and dies a large variety of articles can be manufactured on mass production lines. In this connection an important point is that on firing at 1,450 deg. C., steatite only contracts 8 per cent., as against 18 per cent. for porcelain, so that high accuracy and speed of assembly are possible, the necessary tolerance being only 2 per cent., while with porcelain 5 per cent. must be allowed. The articles after pressing are sprayed with a leadless glaze, endue then fired in a tunnel muffle kiln. The present capacity of this department is upwards of 200,000,000 pieces per annum.
The research laboratory is equipped with apparatus, which allows both the electrical and mechanical properties of the insulators to be exhaustively examined. It includes a high-tension room, mechanical laboratory and fog room, together with an open-air testing field where endurance tests can be made under normal atmospheric conditions. The high-tension room is 4,300 sq. ft. in area and 45 ft. high, and is lighted entirely by artificial light. It is equipped with a single-phase 500-kv.-a. alternator, from which a supply is given at normal frequency and 6,000 volts to the two 450-kv.-a. testing transformers illustrated in Fig. 4. When these two units are supplied in parallel a pressure of 1,000 kv. between poles can be obtained, and thus can be increased to 1,200 kv. to earth by using the normal cascade connection. To allow pressures how 50 kv. to be measured accurately the generator voltage can be reduced to 3,000 volts by using an auto-transformer, while smooth and accurate regulation is ensured by employing a direct-coupled exciter, which, in its turn, is supplied from a battery. This equipment is connected to a bus-bar from which the high-tension rectifier, fog room, mechanical laboratory and open-air testing field are supplied, interlocks being provided so that not more than one of these circuits can be fed simultaneously. The isolating switches are, mounted on the roofs of the buildings. Voltages at high frequencies are obtained from a Tesla transformer, which forms another item in the laboratory equipment. This enables the conditions, which occur in high-tension lines, when parts of the system are caused to oscillate at their natural frequency by lightning surges, to be simulated. In addition, an impulse plant, in which two high-voltage condensers are charged by an unsmoothed unidirectional current, enables the effect of a direct lightning stroke to be imitated.
For this purpose the condensers, are charged in parallel through high resistances, and are discharged on to the object under test in series. In this way, it is stated, pressures exceeding 1,800 kv. have been obtained. The unidirectional current for charging the condensers is obtained from a Delon mechanical rectifier, which allows a voltage double the peak voltage of the transformer supplying the current for rectification to be obtained. The unidirectional current for charging the condensers is obtained from a Delon mechanical rectifier, which allows a voltage double the peak voltage of the transformer supplying the current for rectification to be obtained. The above tests can be applied to insulators under normal conditions or under artificial rain, while, in heavy fog, mist, salt spray and artificial fouling by solid matter. The mist is obtained by suspending the insulators for a considerable time in a refrigerator and then removing them to the higher temperature of the fog room.
The high-tension insulators made by the firm are of two types. In the first the pin is secured in the porcelain by cement, while in the latter, which is the type which is being used on the Grid, a spiral spring of high-tensile steel wire is placed on the head of the pin, and is forced through the neck of the hollow cylindrical space in the head of the insulator so that it can expand in the larger cavity into which that neck opens. The spring is retained in position by a serrated collar and clip, and after its insertion the neck is filled with a lead alloy, so that no relative movement of the two parts is possible. and a good distribution of the mechanical and electrical stresses is obtained. The tension on the pin is converted into a uniform compression over the whole of the interior of the insulator and the mechanical strength of the combination is higher than that attainable with cement, while tests show that the pin will shear before it can he drawn out of the porcelain.
For testing these insulators a 30-ton testing machine, which we illustrate in Fig. 5, has been installed at Stourport. This has been specially designed for the purpose, and can also be used for compression tests. In addition, the tensile strength of test pieces is ascertained on a 10-ton machine, which is equipped with a number of auxiliary parts so that torsion and bending tests can be made on all types and sizes of insulators. Another interesting machine is illustrated in Fig. 7. This has been designed for carrying out vibration tests on insulator posts in accordance with the specification of the Central Electricity Board. As will be seen, a spring-loaded rod is attached to the top of the insulator string, and is moved backwards and forward by an eccentric which is driven by a small motor. Torsional tests are carried out in a similar way, while endurance tests on isolating switches are effected on a machine on which five columns of insulators are mounted. Four of these carry a standard isolating switch contact and the fifth a rotating switch arm. The latter is driven by compressed air, and can be adjusted for speed of operation and contact. pressure. The central column is therefore subjected to impact torsional forces, and the outer to impact bending and vibration stresses as in service. A bus-bar rune over all these machines so that they can be employed for electro-mechanical as well as mechanical tests.
