[Trade Journal]
Publication: Ceramic Industry
Chicago, IL, United States
vol. 4, no. 5, p. 358-361, col. 1-3
Mechanical Innovations Cut Cost at West Coast Insulator Plant
Novel Methods of Handling Ware Prove Effective at
Plant Where Conveyors Are Used at Every Turn
R. H. HUNT, SUPT.
Westinghouse High Volicge Insulator Co., Emeryville, Cal.
THE DEVELOPMENT and commercial production of dependable high voltage porcelain insulators assures the rapid economic development of the hydroelectric power resources of the United States. Suspension insulators, which are generally used to insulate the transmission lines operating at the higher voltages, are made of electrical porcelain of high dielectric and mechanical strength.
Ceramic and electrical engineers of the Westinghouse Co. have developed and are supervising the commercial production of electrical porcelain used to insulate the highest voltage transmission lines in the country. The same quality of electrical porcelain is used by the Westinghouse Co. in the manufacture of insulators for lower voltage transmission.
The Westinghouse High Voltage Insulator Co., manufactures porcelain insulators at Derry, Pa., and at Emeryville, Cal. The manufacturing processes described in this article are common to both plants, but the equipment shown is that used at the new Emeryville Works, where insulators are manufactured for electric utilities in the Mountain and Pacific Coast States.
Use California Feldspar Of the three principal raw materials used in the manufacture of high voltage porcelain bodies, feldspar of excellent quality is obtained from California deposits.
The ground quartz, or flint, as it is commonly called, is obtained from Illinois and most of the clay is imported from England. There are several good clay deposits in the western states, but the present demand for high grade clays is not sufficient to permit the development of these deposits and the economic production of refined clays.
Feldspars are of igneous origin and occur chiefly in granites. For ceramic use they must be almost free from impurities such as tourmaline, garnet, magnetite, beryl and mica. Good feldspars fuse to clear, colorless or milky glasses at temperatures varying between 1,275 and 1,295 deg. C.
The flint 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 assisting in retaining its shape.
Kaolin, or china clay and ball clay are about equally proportioned for insulator manufacture. Too much ball clay would result in a body having excessive shrinkage and warping. A high china clay body would have low plasticity and when dried would not be strong enough to handle. The proper mixture of these two clays with the feldspar and flint results in a strong, plastic, workable body of high dielectric and mechanical strength after firing.
Ball clays, in general, are characterized by high plasticity, shrinkage and strength, and non-porosity when fired to temperatures between 1,290 and 1,330 deg. C. They are extremely fine grained. 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 can be economically handled during insulator manufacture.
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. Kaolins are usually washed, before marketing, by a flotation process, which eliminates free silica and mica.
The raw materials are unloaded and distributed direct from cars into six storage bins, each of 200 tons capacity. Smaller bins are provided for storage of the plaster of paris used in the molds and for clays and oxides used in preparing the glazes. The arrangement and subdivision of the raw material bins permit the separate storage of each shipment of material until the properties of the material have been checked and the shipment approved.
Grind Flint and Spar Commercial flint and feldspar are received ground so fine that for the most part they will pass thru a 200 mesh lawn sieve, but to produce the best electrical porcelain they are ground wet in a ball mill before they are mixed with the clays. The working and firing properties of the body and the characteristics of the finished product are vitally affected by the proper control of this grinding operation. The accompanying curves show the increased dielectric strength and resistance to impact resulting from proper grinding. The non-plastic materials are ground in a 6 foot by 8 foot ball mill. The commercial clays are extremely fine grained and it is only necessary to properly proportion and mix them in double paddle blungers until all lumps are disintegrated and the fine particles suspended in the water. After the non-plastic materials are ground to the proper fineness they are forced from the ball mill into the blunger by compressed air and there mixed with the clays which have been previously suspended in water.
The blunged body or "slip" flows by gravity into either one of two intermediate storage cisterns, each of which will hold two blunger charges. From these cisterns the "slip" flows thru vibrating lawns of 100 and 140 mesh, then thru a magnetic separator into storage cisterns, each of 16 blunger charge capacity. While in storage the "slip" is continually agitated by motor driven paddles to maintain a homogeneous mixture and uniform specific gravity.
The refined slip is then pumped into 24 inch by 24 inch, 72 leaf filter presses on the floor level where, by carefully controlled pressure, the water is forced out thru filter cloth and the slip reduced to a plastic body with 23 per cent, water content. The filtering pressure is gradually raised to 100 pounds at the end of the three hour filter period. After the filter cakes have been removed from the press the body is given a preliminary pugging and stored in large closed concrete cellars, each of fifty tons capacity.
After removal from the storage cellars the body is again mixed in the pug mills and, thru dies of proper dimensions and shapes, extruded in columns from which blanks of clay are obtained of the weight required for the type of insulator to be made.
The ware is carried thru the dryer on trucks suspended from three overhead tracks. The drying final operation requires from 24 to 60 hours, depending on the size and shape of the ware. This drying is carried on by circulating air under very accurately controlled temperature and humidity conditions, the temperature being gradually increased and the humidity decreased as the drying progresses.
The dried insulator parts are then glazed. Those surfaces on which a cementing bond is required for the assembly with other porcelain or metal parts are covered with particles of ground body, which during firing are bonded to the porcelain by the fused glaze.
The Emeryville Works has at present four kilns of the potters, up-draft type, each 15% feet inside diameter. The kilns are equipped with combination burners and may be fired with either oil or gas. The insulators are placed in saggers which are stacked to a height of twelve feet in the kiln.
The control of the firing cycle and final temperature is one of the most important operations in insulator man-ufacture. The temperature control during the early stages of firing is indicated by the use of electrical recording pyrometers. Pyrometric cone plaques of four cones each placed in all parts of the kiln indicate the temperature during the final stages of firing and assist in obtaining the desired firing cycles.
The temperature of the kiln is gradually increased to cone 10 (1330 deg. C.) during a firing period of from 55 to 70 hours. After firing the kiln is cooled slowly to prevent internal strains which might result in the cracking of the porcelain.
After the inspection of the insulators drawn from the kiln they are subjected to a rigorous five minute 60-cycle flashover test to eliminate parts having mechanical imperfections. The voltage of the 300-kva., 250,000-volt testing transformer used for the 60-cycle flashover and puncture tests is controlled from a panel, from which the operator has an unobstructed view of all tests and high voltage equipment. The uniform regulation of voltages up to 250,000 is accomplished thru a 69-kva., 220-volt induction regulator with 100 per cent. regulation. The test voltages are indicated at the control panel both by a crest voltmeter operated from the condenser bushing of the transformer and from a ratio voltmeter operated from a special voltmeter winding in the transformer.
The tested parts of multipart pin type insulators are then assembled and suspension insulator parts are assembled with hardware, mechanically mixed neat portland cement being used. Resilient compounds are used to cushion mechanical stress which might be established as the result of unequal expansion or contraction when the component parts of units are subjected to sudden or extreme temperature changes. Care in the assembly and the accurate control of conditions during the setting of the cement are vital factors in determining the resistant of insulators to the severe temperature change stresses to which they are subjected in service.
After proper setting of cement the units are cleaned, and the suspension insulators assembled in strings of six units each are tested in the pneumatically operated and controlled testing machine, shown in an accompanying illustration. A mechanical load approximately 40 per cent. of the ultimate strength of the insulators is applied before the units are subjected to the high frequency flashover test voltage obtained from the 200,000-cycle, 250,000-volt high frequency testing transformer.
The assembled insulators are then tested under a 60-cycle flashover voltage, and after final inspection crated for shipment. The smaller one piece insulators used on distribution lines operating at voltages of 15,000 and below are packed in individual fiber board shells in heavy wooden boxes. The larger pin type and suspension insulators are packed in open crates of three or six units each.
The completeness and capacity of the test equipment installed affords means of conducting the most searching routine and design tests on the insulators manufactured. Sphere spark-gaps of 25 cm. and 37.5 cm. diameter are available for the accurate calibration of meters and for research work.
A 50,000-1b. Olsen testing machine is used for combined mechanical and electrical design tests and for ultimate mechanical strength tests on samples from standard production.
The Westinghouse Co. manufactures suspension insulators having mechanical strengths of from 11,000 to 30,000 pounds. Samples of production are also tested for dielectric strength by immersing the insulators in oil and applying an increasing 60 cycle voltage until puncture occurs. Porosity tests under pressures of 5,000 lb. per sq. in. or more are made in a high pressure container.
