Pyrex Glass in Chemical Industry, Suspension Insulators Discussed

POSSIBILITIES OF PYREX GLASS IN CHEMICAL INDUSTRY

Investigator Sees Tremendous Possibilities for Boro-silicate Glass in Chemical Industries. Industrial Uses of Pyrex Developed Within Last Year. A Most Suitable Substitute for Porcelain.

The following paper was presented at the Richmond meeting the American Institute of Chemical Engineers, December 8, 1922, by A. E. Marshall, a consulting chemical engineer of Baltimore, and is reprinted from Chemical and Metallurgical Engineering:

Anyone who has undertaken construction work on chemical plants, whether for the manufacture of mineral acids or less corrosive products, will admit that no available material gives satisfactory service under all conditions.

Materials in general use have specific advantages for specific work, and the chemical engineer utilizes a number of different materials in one piece of construction in order to develop maximum durability under varying conditions. Incidentally, durability is in many cases synonymous with resistance to corrosion.

Glass, because of its insolubility in acids, has always found uses in the chemical industry, but its limitations in the way of temperature resistance have restricted the field of application.

Ordinary glass, if desired to withstand temperature changes, has to be made into chapes [sic] shapes with thin walls, and such shapes when thin enough to survive slight heat shocks are much too thin to stand up under ordinary plant usage, or in many cases to survive rough handling during erection.

The desire to make use of the non-corrosive properties of glass has led to its being tried out under a variety of plant operating conditions, but the inherent distadvantages [sic] disadvantages of ordinary glasses so far outweigh the useful features that it is difficult to point to successful applications in plant-scale work except under special circumstances.

I remember making a survey some years ago of materials used in the construction of gas-conveying lines from hydrochloric acid pots and muffles. One plant at the time of my visit was using 15-in. slip joint glass pipes on the pot line. Very good results were being obtained because of the thorough cooling of the gas in its passage through the thin-walled pipes to the absorption tower. Later inquiries indicated rather heavy breakage during the winter months, the cause being ascribed to leakage of melted snow through the roof and onto the pipes, or to the considerable difference in temperature between the atmosphere and the gas inside the pipes.

There was a purely local reason for the use of glass on this line. The chemical plant adjoined a glass factory, and the glass pipes were thin cylinders from the plate glass department.

When wire glass was introduced it seemed to offer decided possibilities to the chemical industry, and it was tried out for various uses without any great measure of success. I was very much interested in sulphuric acid concentrators at that time, and I substituted some wire glass plates for the acid-proof cover slabs on a cascade concentrator. The glass cracked in the course of a few hours and thereafter final collapse was merely dependent on the time required for the acid fume to attack the wire reinforcement.

Other engineers have endeavored to utilize glass in various ways, and where temperature resistance and mechanical strength have not been essential, the material has proved satisfactory. I have in mind, as an instance, the glass-packed Gay-Lussac and absorption towers introduced in England about 1909 by Carmichael. These towers, usually of square section, are packed will annealed plate glass sheats [sic] sheets set on edge and spaced fairly closely. Each successive layer of packing is placed at right angles to the row below, thus giving excellent surface contact and good gas distribution.

Mention has been made of attempts to use, and the actual use of, ordinary glass in plant construction, because such attempts afford evidence of a desire on the part of the chemical industry to utilize the valuable non-corrosive properties of glass.

With the introduction of Pyrex as a laboratory material and as a domestic utility in the form of baking ware, the possibilities of using glass in plant contsruction [sic] construction assumed a more promising outlook. Chemical manufacturers tried out Pyrex baking ware for small-scale chemical operation, drawn Pyrex tubes were used in Hart nitric acid condensers, and many other minor uses were discovered for the standard shapes which were being produced for the laboratory or the home.

In December of last year the author suggested to the Corning Glass Works the desirability of gathering together these sporadic developments and investigating the possibilities of producing a line of Pyrex products designed for the use of the chemical industry. The field appeared promising, and a decision was reached to establish an industrial Pyrex department, the author being retained as consulting engineer.

Before entering into a description of the present state of development, it is necessary to set forth the essential characteristics of Pyrex and the difference in its properties and ordinary glass.

WHAT PYREX IS

Pyrex is a low-expansion boro-silicate glass of simple chemical composition, containing no metals of the magnesia-lime-zinc group and no heavy metals.

A comparison of the linear expansion coefficients of Pyrex and a number of materials is given in Table I and, as will be seen, Pyrex has a smaller coefficient than porcelain, ordinary glass or any of the usual metals.

The low coefficient of expansion introduces a marked distinction from ordinary glass whether of the lead or lime-soda type.

TABLE I - LINEAR EXPANSION COEFFICIENTS

(Per. Deg. C.)

Pyrex Glass.................0.0000032

Porcelain.....................0.0000036

Hard glass...................0.0000077

Soft glass............0.0000085

Cast iron..........0.0000102

Wrought iron...............0.0000119

Portland Cement..........0.0000120

Copper........................0.0000167

Brass (66-Cu-34Nz).....0.00000190

Zinc............................0.0000258

Lead...........................0.0000276

The melting point of glass is not particularly valuable as an engineering consideration, as there is usually a fairly wide range between the initial softening point and final melting point. In the case of Pyrex, the softening point is about 800 deg. C., but the material will soften slightly, especially under pressure, if maintained for a long time above 600 deg. C.

In connection with the softening point it is useful to remember that devitrification, which is a serious factor in the use of some materials, does not affect Pyrex in its working range.

Acid resistance is not usually given much thought in the case of glass, the general assumption being that all glasses are equally resistant. Bulletin 107 of the Bureau of Standards gives considerable data on the acid-resisting qualities of Pyrex and other glasses, and is well worth study. Researches have also been conducted in the Corning laboratories on the resistance of Pyrex to prechloric, phosphoric, constant boiling hydrochloric and concentrated sulphuric acids under a variety of conditions.

The action of hydrochloric and sulphuric acid is very slight, constant boiling hydrochloric acid attacking Pyrex at a rate of 0.000006 gram per square centimeter per hour. Concentrated sulphuric acid in four hours at the fuming temperature shows an attack of 0.000002 gram per square centimeter per hour. Both figures relate to initial surface attack, as after the lapse of a few hours a state of practical stability is reached.

Work on the coefficient of heat transference is being carried out at Corning, but has not been completed. Preliminary results on the relative efficiencies of Pyrex, porcleain [sic] porcelain and stoneware indicate Pyrex and porcelain as equal, whereas stoneware shows about one-half the Pyrex value.

It can be said that while Pyrex is superficially a glass, its physical characteristics justify consideration from the engineering standpoint as a special and distinct material adapted to a variety of uses to which ordinary glass cannot be applied.

DEVELOPMENT OF INDUSTRIAL SHAPES

Proceeding from a consideration of useful properties to the development of definite industrial shapes, it is obvious that the upper limit of size is a factor of great importance. If the sizes had to be restricted below the usual standards in other materials, then the field of application would also be restricted. It was not practicable to pick out very large pieces and concentrate on them, for the reason that failure in manufacture might be caused by lack of dexterity in handling such large shapes. The history of most plant materials, stoneware, fused sicica [sic] silica, high silicon irons, etc., has been one of gradual enlargement of product. In the case of Pyrex the existing laboratory and domestic shapes could be used as a starting point and a stage selected which would represent useful commercial products without going out to sizes which would call for the introduction of new methods of handling.

The starting points selected were therefore an 18-in. evaporating dish, a 6-in. bore socket pipe 31 in. over all, and a cylindrical pot 12 in. in diameter by 20 in. high.

It was expected that difficulties would develop in the manufacturing process; but contrary to expectations, production was worked out with nothing more than the usual minor troubles.

With proof that manufacture was possible, it was then necessary to test out the product under working conditions. These tests were made on single pieces in various plants, and following satisfactory reports, distribution was started on a small scale. A demand for other forms of Pyrex equipment, and as a consequence new items have been added in the last few months, while others are approaching the production stage.

Reference is derected [sic] directed to the 72-liter capacity retorts, designed for distillation and reaction work, various sizes of potes up to 9-gal. capacity, and large separatory funnels of 8-gal. capacity as articles now available.

Pyrex glass is also being produced in sheets 14x18 in., and it is expected that much larger sheets can be made if there is the necessary demand.

In addition to the above items a 25-in. diameter dish of 30-liter capacity is about ready for distribution, 12-in. socket pipes will follow in a few weeks, and other shapes of equivalent size, such as a drier tray, cascade dishes, etc., will be put in production as soon as a demand is assured.

Incidentally the use of Pyrex in forms developed especially for the chemical industry has created interest in the possibilities of equipment made from Pyrex tubing. Condensing equipment made up of "S" bends, pipe lines with socket or butt joints, and many similar uses are becoming standardized.

A PYREX DENITRATING TOWER

An interesting piece of Pyrex equipment was constructed recently for the du Pont company. This was in the form of an experimental denitrating tower 6 in. in diameter by 8 ft. high. All parts, including distributor, inlet and outlet connections, were Pyrex. It is understood that the tower has given satisfactory service, despite rather strenuous conditions.

The use of Pyrex in actual plant work has developed some interesting sidelights, not only on applications but on a phase of extensive cost, created by a lack of standardization. This point will be discussed later.

It had been expected by users that there would be a higher handling breakage with Pyrex dishes than with porcelain. Experience has been entirely to the contrary, and the reason apparently lies in the province of psychology. One plant, which is now completely equipped with Pyrex dishes, reports handling breakage as nil, and states that the men treat the dishes as glass, setting setting them down with care and so obviating breakages through dropping. It has been the experience of every user of Pyrex, irrespective of the shape of the article, that transparency creates care in handling. It is a natural assumption that an opoque [sic] opaque object will withstand shocks and that a transparent one will not, so a transparent material which is at least equal to stoneware and porcelain in mechanical strength has a much better chance to survive at the hands of a workman. Transparency has other advantages which are not psychological. The first and most important is that a transparent article cannot have blowholes or other hidden flaws. Even strains can be detected by use of a special polariscope, this being one of the routine tests to which every piece of Pyrex equipment is subjected in the Corning factory.

Next comes the question of cleanliness. Opaque equipment may or may not be clean, but transparent equipment always supplies its own positive answer on this point.    Finally there is the feature of controlling reactions through direct observation.

Another interesting feature which has been brought forward by users is in relation to the process of manufacture. Pyrex is to a certain extent competitive with chemical stoneware. The process of making stoneware is quite lengthy, the time required being about two months. Breakage of a special piece of stoneware, provided there is no duplicate in the plant store room or at the stoneware manufacturer's works, means a long delay in starting up after the shut-down. Providing a mold exists for the piece of Pyrex, manufacture can be compjleted [sic] completed in three days as a minimum, although factory conditions might necessitate a dlay [sic] delay of a few additional days on account of prior routing of work.

Touching on the phase of excess costs of construction materials, it seems very desirable to emphasize the lack of standardization in the chemical industry. Molds are costly, whether they are intended for stoneware, silica, Pyrex or other materials. The manufacture of the equipment has to charge up his mold cost to the user, and there seems to be too great a demand for "specials" - which may vary only 1/8 in. from a stock mold. Some concerted effort to standardize shapes would cheapen the products, and, of equal importance, would enable producers to carry representative stocks. A striking example of the lack of standardization is shown by the Corning Glass Works list of sight glass molds. Continuous efforts have been made to keep down the number of sizes, but the success can be judged when it is shown that between the range of 2 ½ in. diameter by ¼ in. thickness to 8 ½ x ¾ in. it has been necessary to provide forty-one molds.

In the case of plant equipment, a further effort is being made to work out shapes which will suit a variety of uses, and it is hoped that a full measure of co-operation will be extended by plant managers and engineers, through the use of stock rather than special molds.

The industrial use of Pyrex is spreading into many fields, and while not a strictly chemical application, decided interest attaches to the development by the research laboratories of the Corning Glass Works of Pyrex high-tension insulators.

The generally accepted causes of failure of porcelain high-tension insulators are: (1) change in structure of porcelain with time and absorption of moisture; (2) breakage due to thermal changes; (3) flaws in the porcelain body, causing dielectric and other failures; (4) failure due to expansion of cements used in attaching hardware to the insulator; (5) mechanical weaknes [sic] weakness. These failures may be generally classified as failure due to the properties of the insulator and failure due to design.

Pyrex glass seems to have ideal properties for an insulator, for it apparently is not subject to any of the intrinsic weaknesses of the porcelain insulator. It does not change in structure, can be inspected for any defects, thus assuring a uniform product, has a sufficiently high dielectric strength, a great resistance to thermal changes (its thermal expansion coefficient being lower than that of porcelain) and in addition is not heated by direct solar radiation as much as porcelain. Consequently if a Pyrex glass insulator could be built free from the design defects of the porcelain, there is no question that it would be a better insulator.

The construction of the Pyrex insulator gives a cement-free all-metal and glass insulator and the design is one of great resistance to tension, as the material of the insulator is largely under compression.

Considerable data have been collected showing the relative properties of Pyrex.

Heating glass and porcelain insulators by solar radiation shows a rise in temperature above air temperature of a porcelain insulator of 39.5 deg. C., and for a Pyrex insulator of 10.8 deg. C. This is a mean of ten observations in both cases.

These data show the increase in temperature due to solar radiation to be 3.65 times as much for the porcelain as it is for the glass. The glass is transparent and the porcelain absorbs the heat radiation. This proves that in service the glass is not subjected to anything like as severe heat changes as porcelain.

In a report on dielectric strength tests made by one of the large manufacturers of electrical equipment it was stated that unit No. 1 punctured at 143 kilovolts, unit No. 2 punctured at 160 kilovolts, and unit No. 3 punctured at 140 kilovolts.

"The tests on Pyrex suspension insulators show quite conclusively that the material has good characteristics. The uniformity of electrical puncture is a distinct advantage over porcelain."

It is evident that Pyrex glass has very good dielectric properties, more than ample for the service, as well as being uniform.

Tensile strength tests made by Cornell University, Sibley College, show:

No 1. 22,600 lb ... No break

No 2. 23,000 lb ... Pin broke in holding device

No 3. 19,500 lb ... Pin broke in holding device, reaction broke insulator

No 4. 22,750 lb ... Yielding, no break

No 5. 23,600 lb ... Cracked

No 6. 20,600 lb ... Click, no break

No 7. 21,800 lb ... Click, cracked

No 8. 22,000 lb ... No break

No 9. 17,000 lb ... No break

22,500 lb ... Clicked

No 10. 25,460 lb ... Brode stud, flange, shattered by reaction, head O.K.

Since the better grades of porcelain insulators will not stand over 10,000 lb. load as a maximum, and many break below that, it is evident that the glass insulator has a strength for superior to anything yet developed. This property should allow for longer spans, and in many cases where two strings of porcelain are used in parallel one of Pyrex will be ample to carry the load.

Pyrex glass has a great resistance to water absorption and surface attack, and is therefore durable under long exposures to severe atmospheric conditions, as for instance around chemical plants.

It is probable that glass insulators will not be as attractive to birds and spiders for nesting sites, since they will not have the dark shadows that exist in a string of porcelain insulators.

The chief advantage of the Pyrex insulator are the fact that it does not absorb heat as does porcelain; has no cement in its construction gradually to absorb water and expand, ultimately fracturing the insulator; and has a mechanical strength that will average twice as high as any porcelain insulator yet on the market.

This paper describes in sufficient detail the present status of industrial Pyrex, and carries a suggestion of future possibilities. It is believed that Pyrex will in a short time occupy a definite place along with the other materials used for plant construction. It is not a universal panacea for construction ills, but, like everything else in our practice, it demands thought in its application and care in its use.