intake structure, and a traveling bridge of like design is installed in the hoist house of the outlet works intake structure. Each hand-racked bridge has a span of 17 feet and the bridge trucks operate on 10-inch I-beams. There are two 1-1/2-ton-capacity electric hoists which are operated together on either bridge. Each hoist has a hand-racked trolley and is equipped with an electric brake and a mechanical load brake. One hoist has a hoisting speed of 30 feet per minute and a lift of 24 feet. The other hoist has a hoisting speed of 18 feet per minute and a lift of 18 feet. The single-speed hoists are controlled from a pendant pushbutton station. Threephase, 440-volt, 60-cycle power is supplied through a flexible power cable from plug receptacles located on the walls of the intake structures. Each hoist is equipped with a block-actuated limit switch to limit the upward travel of the hook. The traveling cranes are installed in the intake structures for handling the fixed-wheel gate hoist stems. The cranes were designed with a factor of safety of not less than 5, based on the ultimate strength of the material and the rated capacity of the unit. 119. Draft Tube Bulkhead Gate Gantry Crane. - A 5-ton-capacity, four-wheel, hand-pushed gantry crane with an electric hoist is installed on the transformer deck of the powerplant and is used for raising, lowering, and transporting the draft tube bulkhead gates during erection and servicing of the turbines. The hoist is mounted on the operator's platform and is a commercial type, basemounted electric hoist of standard manufacture. The single sheave block supports a lifting eye and is suspended from two parts of 9/16-inch-diameter, 6 by 37 wire rope reeved two parts single. The hoist has a total lift of 45 feet, and is powered by a 7-1/2-horsepower motor which drives the hoist at 18 feet per minute. A block-actuated limit switch prevents overtravel in the hoisting direction. The hoist is controlled from a pushbutton station mounted on the top handrail of the operator's platform. Three-phase, 440-volt, 60-cycle power is supplied to the hoist from plug receptacles, located on the transformer fire walls, through a flexible power cable. The gantry structure was designed according to the Navy Department's standards of design for structural steel, using a base stress of 14, 000 pounds per square inch. Computation of stresses took into account all dead and live loads and windloads. A factor of safety of not less than 5, based on the ultimate strength of the material and the rated capacity of the crane, was used in the design of mechanical parts. A concrete counterweight, attached to the downstream side of the crane, provides a stability factor of over 2, based on the rated capacity of the crane. CHAPTER VIII-- Design--SWITCHYARD A. Structural Design 120. Location and General Description. The location of the 69- and 115kilovolt switchyard is approximately 500 feet east of the powerplant at the toe of the dam, as shown on figure 6. The broad, silty flood plain of the old river channel downstream from the dam did not provide an adequate switchyard area above maximum tailwater elevation within economic limits of distance and cost of controls. Therefore, a peninsula embankment, extending downstream from the toe of the dam, was constructed for the switchyard. The fill for this embankment varies from 12 to 22 feet in depth, and is comprised of sand, gravel, and cobbles, corresponding to zone 3 dam materials, compacted in 12-inch layers by crawler-type tractors. The switchyard grade elevation provides an 11-foot 6-inch freeboard above maximum tailwater. 121. Concrete Foundations. The pad and pedestal type foundations were designed so that the toe pressures do not exceed 2, 500 pounds per square foot and so that the factor against overturning is not less than 1.5. Slab foundations are 24 inches thick and were designed so that the average pressures do not exceed 2, 000 pounds per square foot. To provide for possible settlement, anchor bolts are projected sufficiently to permit leveling of equipment and steel structures. 122. Steel Structures. - The switchyard structures are of galvanized steel, field assembled with bolts. The structures were designed to withstand the loads, as shown on the design drawings, imposed by tension buses, line or transformer circuit conductors, and ground wires; the loads from supported electrical equipment; and the loads due to wind and structure weight. 123. Cable Duct. A cable duct 627 feet long, with access openings every 75 feet, extends from the powerplant to the switchyard. The top of the cable duct is exposed, except for the two crossings under the powerplant access road. The portions of the duct under the access road were originally designed for an H 15 loading with 25 percent impact, and later redesigned for a 60-ton trailer required to transport heavy prefabricated steel manifold sections. Figure 122 shows the cable duct. B. Electrical Design 124. Requirements and General Description. The switchyard is composed of 115-, 69-, and 12.5-kilovolt structures constructed within a fenced area located approximately 500 feet east of the powerhouse and across the tailwater channel. The elevation of the switchyard is approximately 5, 400 feet above sea level. The general arrangement is shown on figure 123, and electrical connections, circuit destinations, and principal equipment ratings and procurement data are shown on figure 124. 125. General Design. - The buswork is of the rigid type design. The 115and 69-kilovolt installations provide for a main and transfer bus scheme, and the 115kilovolt yard has a bus-tie breaker in the present installation. Aluminum tubing and cables were used for the buswork. Some of the general arrangement installations for the switchyard and for the takeoffs on the transformer deck are shown on figures 125 through 129. The cables are run in a cable duct from the powerplant to the switchyard as indicated on figure 130. The cable tray arrangement is similar to that shown for type A cable duct on figure 131. The cable trenches that run through the switchyard are shown on drawing 456-D-1219*, which figure also shows the cable tray arrangement. The cable duct from the powerplant to the switchyard is equivalent to a shallow tunnel with manholes located at convenient intervals. The cable trench in the switchyard has removable aluminum plate covers for its entire length. 126. Insulation, Coordination, and Lightning Protection. The electrical insulation and protective devices were selected and coordinated to provide a safe margin of insulation strength above the maximum abnormal voltages permitted by the protective equipment during lightning, switching, and short-circuit surges. The overhead *Not included. |