[Trade Journal]
Publication: Chemical and Metallurgical Engineering
New York, NY, United States
vol. 31, no. 19, p. 729-733, col. 1-2
Manufacture of High-Voltage Insulators
By M. H. Hunt
Superintendent Westinghouse High-Voltage Insulator Co. , Emeryville, Calif.
Stimulated by the Rapid Expansion of the Electrical Industry, Ceramic Resources of the Pacific Coast Have Been So Developed as to Supply More Than 50 Per Cent of the Materials Required in the Production of Electrical Porcelain
CERAMIC manufacture has been developed most extensively in the region east of the Mississippi River. The reasons are logical. The tendency has been due to the centralization of population, the presence of skilled labor, cheap fuel and the proximity of raw materials, both domestic and foreign. The ball clays of Tennessee and Kentucky, the kaolins of Georgia, Florida and the Carolinas, the feldspars of eastern Canada and Maine, the silicas of Pennsylvania and Illinois—these are all well-known, standard materials, the properties of which are understood by manufacturer and potter alike.
Nevertheless the west coast, and especially California, is rapidly developing the resources of raw material in the region, and it is now possible to procure locally more than 50 per cent of the materials used in the manufacture of porcelain. This article has been written to give a brief exposition on the raw material and processes of insulator manufacture, with particular reference to operations at the Westinghouse company's plant at Emeryville, near the San Francisco Bay cities of Oakland and Berkeley.
RAW MATERIALS
The raw materials used in present-day method of manufacturing high-voltage porcelain are, mainly, ball clay, kaolin, feldspar and silica, together, at times, with small amounts of fluxes such as calcium, magnesium and barium carbonates, talc, barium sulphate and calcium fluoride, although the latter two are so little used as to be negligible in importance.
Ball clays, in general, are characterized by high plasticity, shrinkage and strength, and non-porosity when fired to temperature between 1,290 and 1,330 deg. C. They show a cream or tan color after firing, due to the presence of iron oxide. Ball clays are of sedimentary character, having been transported by the action of water and deposited on ocean bottoms where
there is little or no movement. Naturally they are extremely fine grained. The ball clays of Florida, Kentucky and Tennessee are obtained from the Tertiary formations. The function of ball clay in an electrical porcelain body is to impart plasticity, or what may be called working property, and to produce a strong body that will withstand the strains produced during handling and drying.
Kaolins are residual clays, found overlying the rock from which they are formed. They are the residue from the decay of almost pure feldspathic rock. In general, they are pure white in the crude state, and develop only a slight plasticity when mixed with water; they are weak, mechanically, after drying. They retain their white color on firing, but present a perfectly porous structure, in contrast to the vitreous structure of a ball clay. Kaolins are usually washed, before marketing, by a flotation process, which eliminates free silica and mica.
Nearly all porcelain bodies contain a mixture of these two types of clay, the quantity used of each depending upon the article being made. In the manufacture of insulators, it is general practice to mix them in about equal proportions. A high ball clay body gives excessive shrinkage and warping and produces a porcelain of poor color. A high china clay body has low plasticity, develops a weak structure, and therefore does not lend itself well to plastic body manufacture, whereas a mixture of the two produces a body with good workability.
Feldspars are of igneous origin and occur chiefly in granites. For ceramic use they must be almost free from impurities such as quartz, tourmaline, garnet. magnetite, beryl and mica. The function of feldspar in a porcelain body is to cement or fuse the whole mass into a thoroughly homogeneous and non-porous body. Good feldspars fuse to clear, colorless or milky glasses, almost free from specks, at temperatures varying between 1,275 and 1,295 deg. C. Electrical porcelain bodies are fired from 1,310 to 1,370 deg. C., which means that the feldspar has ample opportunity to exert maximum effect. Silica occurs in various forms, both crystalline and amorphous, nearly all of which are suitable for some type of ceramic manufacture. For electrical porcelain, silica ground from either quartzite rock or white sand is extensively used. It serves as a non-plastic during the early processes of manufacture, and as a skeleton during the firing, increasing the stiffness of the body and enabling it to retain its shape.
The tests made on the raw materials, to determine their properties and to control their use in the porcelain body, have been incidentally enumerated in the above brief description. It will be noted that more reliance is placed upon the physical characteristics of the raw materials than upon chemical analysis. Experience indicates that it is undesirable to try to correlate plasticity and shrinkage, and even fusibility and color, very closely by means of a chemical analysis. Moreover, physical tests are easily and quickly made and interpreted.
In the manufacture of porcelain on the west coast, it has been found possible to make use of several Western materials. Feldspar of excellent quality is available, as well as silica. Numerous plastic fireclays occur, but no true ball clay has yet been developed. At the present time the demand for high-grade ceramic materials, especially clays, on the west coast would not justify mining operations on the scale necessary to produce at a reasonable price. For this reason the clays are at present imported from England.
BODY PREPARATION
It has long been recognized among ceramic engineers who have had the opportunity of experimenting with electrical porcelain bodies at temperatures between 1,250 and 1,370 deg. C. that the percentage composition only slightly affects the general properties of the porcelain so long as it is perfectly vitreous, workable and free from manufacturing strains; but it is equally true that the method of preparation does affect all the properties of the body, some beneficially, some adversely, and that a balance of properties must be sought and maintained.
The clays entering into the porcelain body are extremely fine grained, and it is necessary only to disperse thoroughly any aggregates of particles that may occur. This is done by placing the clays in a tank of water and agitating them thoroughly for several hours. However, the fineness of feldspar and silica depends upon how long and how carefully the miller has ground them, and in any case they are not fine grained enough to produce the beat quality of porcelain to suit the requirements of the Westinghouse company. Therefore it is necessary to regrind these materials; this is done in the ordinary ball mill, using a definite quantity of water. The working properties of the porcelain body and the characteristics of the finished product are vitally affected by this operation. The accompanying curves will show that the dielectric strength and resistance to impact are both increased. The experiments were all conducted in the type of mill used for the production of the commercial grades, but the time of grinding is not the same, although results can be correlated. The bodies were all fired in a commercial kiln.
There is a considerable amount of foreign material, mostly organic, in ball clays, even in those of the best quality. It is necessary to wash this out, but there is not enough of it to justify a separate operation. Therefore after the ball-mill charge has been dumped into the mixing tank, the mixture, which is in suspension in a large excess of water, is run through a series of screens, grading from coarse to extremely fine; all dirt is thereby eliminated. After passing through the screens, the body flows over a large magnetic separator, which removes any magnetic material picked up during transportation and handling, and is finally stored in large cisterns beneath the floor.
From these cisterns the body suspension is pumped into three 24x24-in., 72-chamber filter presses and the excess moisture eliminated, leaving a plastic mass, containing about 23 per cent water. From the filter presses the cakes of plastic body are passed through a special mill, which kneads and compacts them, producing a fairly homogeneous mass, which is stored in closed cellars for several weeks.
The question of aging has been much discussed, especially in reference to the effect it has on the quality of the product after firing. Undoubtedly the working properties of a body are changed in consequence of aging, but it has not been proved that the quality of the fired porcelain is improved thereby. Most porcelain factories in this country do not consider the benefits derived from aging sufficient to devote to it the large factory floor space necessary.
The aged clay is prepared for the insulator-forming process by pressing it through the ordinary pug mill, a machine that compacts the mass, works out air pockets and insures a uniform distribution of moisture, finally extruding it through a die. The principal factors entering into the successful operation of the pug mill comprise (1) the speed of the knives, their position and pitch, (2) the position of the push-out augers, and (3) the design and size of the die through which the body is extruded. A change in the character of the clays used or in the general body composition means a change in the design of the mill, which, for one thing, accounts for the reluctance shown by potters toward the use of raw materials of varying composition.
THE FORMING PROCESS
High-voltage insulators, in general, are formed either by one or by a combination of hot pressing, jiggering or casting, each process having its own particular field, which depends upon the size and shape of the insulator and the use to which it is to be put.
In the Westinghouse factory the hot-pressing method is used almost exclusively; it consists essentially of forcing a hot, revolving, metal tool into a mass of plastic clay, retained in a plaster mold, the tool forming one surface of the insulator, and the plaster mold the other. A number of factors enter into the success of this operation, such as the design of the press and tools, the heat applied to the tool, the moisture content of the plastic body and the general character of the body used, especially in reference to the kind and quantity of clay it contains. A small high-duty press is used for manufacturing small insulators. A large, massive press, the largest used for this type of work, turns out the larger size insulators at the rate of several hundred units per hour.
DRYING AND INSPECTION
The drying process is divided into two stages : The first, a preliminary drying in the plaster mold; the second, a final drying after the unit has been trimmed and smoothed into its final shape. In the preliminary drying the plaster mold absorbs a small quantity of water from the surfaces of the plastic clay and a slight contraction results. During this period of half an hour or more, evaporation has taken place from the upper surface of the insulator to harden it sufficiently to allow the shape to support its own weight, after which it can be taken out by inverting the mold and is then supported on a pallet. The surface in contact with the mold is usually slightly roughened, due to contact with the plaster, and must be cut away to a depth of approximately 1/16 in. This operation is done by revolving the unit on a vertical spindle, while an operator skims the surface with a small wire tool. The tie wire groove is cut in this operation, and the unit is sponged before it is taken off the machine and placed on a truck. An inspector passes on all the work done in this department. The material is then dried finally as described under the photograph of the drier on page 728 and inspected once more before being glazed.
GLAZING
The glaze is applied to insulators by dipping or by spraying. Dipping is more generally used, because it is quickly accomplished with simple apparatus; moreover, it lends itself readily to the numerous sizes and shapes that comprise a day's output. The operation consists of immersing the insulator, unfired, for a couple of seconds, in a tub of glazing compound, suspended in water. A coating of approximately 1/32 in. thickness deposits on the insulator, which, in the firing operation, fuses to a clear, dark-colored glass.
Insulator glaze is usually a natural clay, which fuses to a glass at the temperatures used. Manganese dioxide, red oxide of iron and chromium oxides are frequently added in small amounts to intensify the color produced.
FIRING
The firing of electrical porcelain follows the same principles that are recognized in the burning of any ceramic ware. The operation can be divided into four general stages: (1) Expulsion of the water held in the pore spaces of the ware; (2) expulsion of the water of crystallization of the clay; (3) oxidation of organic material, and (4) hardening or vitrification.
The evaporation of the pore water takes place from atmospheric temperatures up to approximately 400 deg. F. The water of crystallization begins to be given off at approximately red heat, which is about 930 deg. F. The oxidation period is between 1,290 and 1,470 deg. F. Vitrification takes place between various temperatures, depending on the composition of the porcelain body, the fineness of particles present and the rate of temperature increase. The feldspar in the body begins to sinter at about 2,000 deg. F., which can be considered the beginning of the vitrification period, but does not produce its maximum effect until the neighborhood of 2,350 deg. F. is reached.
The insulators are placed in the kiln in saggers—round or square fireclay boxes. The purpose of these is to protect the insulators from the direct impingement of the combustion gases during firing and to afford a means of loading the kilns. The number of insulators in a kiln depends upon their size, and varies ordinarily from 2,500 to 7,000. The kilns are fired by gas and fuel oil, the gas being used in the early part of the burn, when low temperatures are desired. The firing period lasts about 68 hours.
INSPECTION AND TESTING
After the kilns are unloaded, each insulator is given a visual inspection before it is tested, principally for cracks, the threaded pin-holes being checked by the use of a steel gage. An insulator that shows a crack must be discarded. Cracks and other irregularities on the surface of insulators collect dirt and so decrease insulating efficiency.
The electrical testing of an insulator consists of applying a sufficient voltage to it to cause arcing over its surface. This arc is sustained for from 5 to 10 minutes. Insulators with flaws or imperfections will puncture—they will cease to arc over. The current will pass to ground through the place of weakness. When this happens, the unit is taken off the rack and the test continued.
The voltage required for testing depends upon the shape of the unit, which, of course, controls the arcing distance over its surface. Test losses in modern insulator works should be less than 2 per cent, although it is not considered poor operation for this loss to amount to 5 per cent. Two testing outfits are in use in the Westinghouse factory. One is a 300-kva., 250,000-volt, sixty-cycle transformer, regulated by a 69-kva., 220-volt induction regulator. The other is a 200,000-cycle, 250-kilovolt, high-frequency transformer. Test voltages are indicated on either a crest voltmeter or ratio voltmeter.
After passing the first electrical test, those insulators requiring hardware are assembled with cement and steam-set, after which they are given the final or assembled tests and are ready for packing and shipping.
