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Transcript for Hoover Dam Construction, segment 13 of 17
One of the most interesting and spectacular phases of the work was the fabrication and installation of the huge steel penstock pipes, forming the conduits for the power and pressure outlet systems. This work was performed by the Babcock and Wilcox Company of New York City, who erected a modern steel fabrication mill near the dam site to facilitate the undertaking. As the pipe units to be fabricated were of unprecedented size and weight, it was necessary to design, build, and install special machinery solely for this task. Provisions were made to manufacture pipe ranging in diameter from eight and one half to thirty feet from steel plate varying in thickness from five-eighths of an inch to two and three-quarters inches. As it was impossible to ship units of this size across country by rail, steel plate was brought from eastern rolling mills, and the entire process of manufacture, including rolling and assembling, was initiated and completed at the Boulder Dam plant.
First step in the fabrication process after the plates had been laid out to dimension was the shaping of the edges to insure the precision and accuracy of later steps in their manufacture. This work was done on a planing machine capable of handling a strip of steel fifty feet in length. The plates were then given an initial bend on a giant press operating at a pressure of three thousand tons. This initial bending was necessary to avoid damaging the rolls when the plates in the next step of fabrication were rolled into circular form. The plates entering into the manufacture of the thirty foot diameter penstock pipe were fabricated through the initial steps of manufacture in eleven foot widths. These plates were two and three-quarters inches in thickness and were rolled into circular form by being passed through forty inch vertical rolls until the desired degree of curvature had been obtained. One such plate represented a segment equal to one-third of the complete circumference of a finished pipe. Having been rolled to the correct degree of curvature, the three curved plates were joined to form a single ring, thirty feet in diameter and eleven feet long. Two such rings were then joined end to end to form a shop unit twenty-two feet long. All joints were made by electric welding, and in forming the longitudinal joints, an automatic welding machine traveling on a chassis supported in line over the joint was used.
As these pipe units were of a size never before assembled, it became necessary to design and build special machinery to accomplish many phases of the work. This was especially true in the fabrication of mitered rings to be later assembled into bend sections. The general usage of electric cutting and welding was applied not only to pipe sections but to the manufacture of other fabricated units as well.
A complete shop unit weighed from a hundred and fifty to a hundred and eighty-four tons, depending upon its design determined by its ultimate use upon installation in the power penstock system. Circumferential joints were made by rotating the rings, making up a shop unit beneath an electric welding machine suspended above the line of the joint to be welded. Every foot of welded joint was subjected to a searching examination by X ray and recorded on photographic film which exposed even the slightest imperfections in the continuity of the weld.
Samples of typically welded joints were subjected to severely rigid laboratory tests calculated to produce a condition of strain far in excess of that to be borne by the joint under actual usage. The discovery of even the slightest imperfection was sufficient cause for the rejection of a complete unit.
In marked contrast to the meticulous care and precision exercised throughout every phase of their manufacture was the actual size and weight of the pipe units themselves. Heavy-duty rigging of special design was required to handle the sections through the shop, and high capacity cranes were required to move them step by step through the progressive phases of their manufacture. To equalize the terrific internal strains introduced into the plates by bending and the additional temperature strains incurred during welding, individual pipe sections were subjected to a temperature of fourteen hundred degrees Farenheit in a gigantic annealing furnace. This temperature was induced not by the application of flame itself but through the circulation of super-heated gas. Shop processes were carried only as far as the production of the unit section. As these sections were to be joined to form a continuous penstock, provision was made to accurately accomplish this junction in the tunnels. To guarantee a satisfactorily tight field joint, the ends of the sections were machined on a mammoth vertical lathe operating across the thirty foot diameter of the pipe. After a final inspection, the pipe section was ready for installation. A modern streamlined train passing through one of the huge tubes affords an interesting gauge for the comparative size of the units.