December
1947
Equipment and Design The application of pure science to industry can readily be seen in today's commercial equipment.
by Charles Owen Brown
I
τ is often a very instructive lesson to follow the history of equipment design from the origin, in a pure scientific research, to commercial production equipment. The pure science work, of course, precedes the commercial applications by many years; just how many is indicated by a new type of valve, recently an nounced, which forms the basis for this story. The evolution could be described from little steel test tubes to huge vessels and back to small valves. Prettsure equipment At the turn of the century P. W. Bridgman decided to continue the work of Amagat in the physics of high pressures. His first task was to devise fundamentals, joints, packings, vessels, and closures which would withstand very high pressures. His am bitions embraced stresses many times higher than any pressure previously used, and his ingenuity and ability were sufficient to carry the work far beyond his original plans. After completing the basic pieces of a high pressure system, Bridgman next devised instruments and auxiliaries with which to follow the work. Of all these examples of brilliant design, including the high pressure window for reaction vessels and the electrical resistance method of measuring high pressures, Bridgman gives most credit to the gasketed joint he devised based upon the principle of an un supported area. Of this extremely simple device (Figure 1) Bridgman writes in his book1, "Finally, there is my own work RETAINING NUT
Figure 1.
B r i d g m a n ' s Gasketed J o i n t
done at Harvard University since 1906. A good part of the re mainder of this book is to be devoted to it, so that no more need be said here, except to give -the one characterization that all this work grew out of the development of a packing technique which makes it possible to reach without leak any pressure allowed by the mechanical strength of the wall of the containing vessel." This packing technique is so simple it can be accurately shown in the pipe connection sketch of Figure 1. Packing nut A is screwed into a machined and threaded opening in the wall of vessel V. This nut must always have an opening in the center 1 Bridgman, P. W., "Physics of High Pressures," p. 14, New York, Macmillan Co., 1931. 77 A
creating the unsupported area C. Even when merely closing an opening with a plug, the nut is actually a ring. The gasket must be totally confined under the nut and above a piece completely closing opening B. The gasket area B-C is never larger than the area of metal in the nut or plug. Internal pressure forcing the bottom piece (which closes the opening) upward compresses the gasket proportional to the whole area Β divided by the gasket area B-C. Usual and convenient design permits a gasket pressure of 5 to 8 times the unit working pres sure without special effort. The first commercial equipment to use the same design em ployed in Bridgman's 20- to 25-cc. vessels was at relatively low pressures in the eliptieal manhole cover used in most boilers. The eliptical shape and low pressures permitted the cover to be in serted in the boiler from the outside, using the small diameter, then a 90° turn, and it covered the opening completely. Al though limited to moderate pressures, this joint has only two pieces, the cover and the gasket. Under very high pressures this eliptical type would not be practical, because, for maximum strength, the vessel must be round; then three pieces are re quired—the cover, the gasket, and the nut. It is fortunate that this type of closure is self-sealing by the internal pressure, be cause the screwed nut retaining the gasket on large diameter, high pressure vessels is a very heavy affair with enormous thread friction. It would be most difficult to tighten this nut against the pressure as ordinary joints are tightened. In Bridgman's design all nuts and bolts, when used, are positioned and tightened be fore any pressure is applied to the system. The nut, or retaining ring, with bolts, gasket, and cover for a vessel 48 inches in inside diameter will weigh about 18 tons. These large closures have been made up to 60 inches in inside diameter, and trouble has never been known in this type of joint. Quite as remarkably, these massive closures are easily opened and yet are leakproof. In the whole world there are probably less than 350 heavy vessels using this closure, although an important industry is based on it. This condition is soon to change, because of the rather belated appreciation of the advantages by the Crane Company now using this joint, as a means of closing the opening in a valve body. This application, although not so small as Bridgman's laboratory vessels, is still only a few inches in diameter but will place tens of thousands of these joints in'service. The Crane Company describes Figure 2. C r a n e C o m this old design as follows: p a n y ' s Leakproof B o n n e t Joint "The bonnet joint is inside the
Equipment and Design
^AlOR-AM** \ 0 lOUjf REQUIREMENTS
valve, sealed with a wedge-shaped seal ring. Fluid pressure in valve automatically keeps bonnet joint tight. No bolts to retain pressure. Leakage? is impossible." The method of applying this joint to an O. S. and Y. valve is shown in Figure 2. The space saved by using this joint on a 1500-pound 8-inch steel gate valve is 4s/, inches in length and 11 inches in height; the weight saving is over 15%. Hudroavnatian
unit
A portable hydrogénation unit has been developed in the instrument shop of Bristol Laboratories, Inc., Syracuse, Ν. Υ., supervised by Ernie Smith. The design has been described by Douglas E. Cooper and is shown in Figure 3. The assembly is completely portable except for a standard electrical cord, which supplies heat, power, and control energy. Space is provided for storage of liners, tools, and other accessories.
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Figure 3. Portable Hydrogénation Unit Most of the equipment is standard. The bomb shaker assembly is model 406-OIC of American Instrument Company, also supplying the Brown temperature controller and bomb tools. The truck is a Lewis-Shepard model SM-3060. The standard hydrogen cylinder is held upright by a 14-inch length of 10-inch steel pipe with L-strap braces bolted to the truck platform. Storage for liners is in the box supporting the Brown instrument, and other tools arc kept in the box at the end of the truck. When in use the truck is anchored and leveled by hand screws at each corner, guided by two spirit levels placed at right angles. Gage faces an; fitted with Plexiglas. This assembly is compact and conveniently moved from one laboratory to another, ready to be put to work without loss of time.
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NEW ENGLAND TANK & TOWER CO. 83
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78 A
