VAULT
Turbines for Boulder Dam
An engineer who work on the construction of America’s most iconic dam describes the design of the large hydraulic turbines in this article from July 1934.
Written by Ireal A. Winter, U.S. Bureau of Reclamation
The Boulder power plant is located at the Boulder dam site on the Colorado River, approximately 25 miles southeast of Las Vegas, Nev. The power plant is immediately downstream from the dam and forms a U-shaped structure with the base of the “U” across the river on the downstream toe of the dam and one wing on each side of the river.
The main generating units will be installed in the wings of the power plant and the central or connection portion will contain station service units, sump pumps, machine shop, and control and auxiliary equipment. The power plant is designed for the ultimate installation of fifteen 115,000-hp turbines driving 50-cycle main generators of 82,500 kva capacity each, and two 55,000-hp turbines driving 60-cycle main generators of 40,000 kva capacity each. The initial installation comprises four 115,000-hp units and one 55,000-hp unit. The 82,500- kva units are designed for possible future operation at 60 cycles by changing the turbine runners.
Steel penstock headers 30 ft. in diameter installed in two of the 50-ft. diameter diversion tunnels and in the two main penstock header tunnels will conduct the water from the intake towers to the individual penstocks and to outlet valves. Each 30-ft. main penstock header connects with four branch penstocks, each 13 ft. in diameter, which will conduct water to the main turbines. The two 55,000-hp turbines, however, are connected to the downstream 13-ft. branch penstocks from the upper or penstock header tunnel on the Arizona side. The four initial 115,000-hp turbines are connected to branch penstocks from the upper and lower tunnels on the Nevada side of the river. A butterfly-type shut-off valve is installed at the inlets of the turbine casings.
HYDRAULIC DESIGN
The turbines as designed will have a scroll-case inlet velocity of 16 percent of the spouting velocity under the designed head. The cross-sectional area of the scroll case progressively increases around the turbine relative to the increment flow into the speed ring, producing in effect a negative acceleration. The guide case will consist of 24 fixed stay vanes in the speed ring, with 24 movable wicket-type gates for regulating the power output of the turbine. The velocity of the water at the tip of the wicket gates when the unit is operating at the point of best efficiency will be approximately 50 percent of the spouting velocity under the designed head.
The value of φ for the entrance edge of the runner blade at the center line of the distributor will be 67 1/2 for the 150-rpm runner and 81 for the 180-rpm runner. There will be 21 buckets to each runner with area and directional changes to produce a final discharge velocity at the throat of the draft tube of 25 fps. The energy in velocity head of the water delivered to the throat of the draft tube will be approximately 2 percent of the total energy available.
The turbine discharge to the tailrace will be through a flattened-type of elbow draft tube. The rate of negative acceleration of the draft-tube expansion in the elbow section will be 5 ft. per sec. per sec. The residual velocity at the draft tube orifice will be 5 ft. per sec when the turbine is operating at the point of maximum efficiency and at the rated head of 492 1/2 ft. The residual velocity represents a final rejection of one-tenth of one percent of the total available energy.
Water leakage around the shroud of the runner and between the crown plate of the runner and the turbine cover plate will be reduced to a minimum by the use of two seal rings of the nonlabyrinth type. When the unit is operating as a synchronous condenser the runner seal chamber will be supplied with penstock water through two 4-in. pipes capable of delivering about 3 cfs to each seal chamber. The sealing water under these conditions is essentially a cooling medium to prevent heating and seizing of the runner bands in case the stationary and revolving metals come in contact.
All projections and pockets in the runner seal chambers have been eliminated to reduce hydraulic losses to a minimum. The chamber between the crown of the runner and the bottom of the turbine cover plate is provided with a smooth plate deflector which will reduce materially the amount of water circulated by the runner crown and will also reduce the frictional load. This deflector apron forms a stationary channel for the leakage-water discharge near the center of the runner.
The electrical transmission requirements at the Boulder power plant are such that it may be desirable to operate the units as synchronous condensers. Special consideration had to be given to the problem of evacuating the draft tube when the unit is operating under these conditions, as the turbine runners are located below normal tailwater. This feature is considered essential as it is estimated that approximately 3,000 kw will be required to drive the turbine runner in dead water, as compared to approximately 500 kw with the runner case empty. It was considered highly undesirable to drive the unit at full-speed-no-load with water through the turbine gates because of the great waste of water and the possibility of increased pitting of the metal parts of the turbine under these conditions.
For motoring purposes the water in the draft tube will be depressed to a point 5 ft. below the bottom of the runner. The penstock butterfly valve will be closed and the turbine scroll case drained down to the bottom of the distributor plate. The depressing of the water level in the draft tube will be effected by means of compressed air supplied from the house-service air-compressing plant. Two compressors each delivering 350 cu ft. of free air per minute at 100 lb. gage pressure will be required to evacuate the unit in 7 1/2 min under maximum high tailwater conditions.
The elevation of the water surface in the draft tube will be controlled by means of a float-operated air valve with water-level connection to the roof of the draft tube in the horizontal discharge section. Pressure in the runner case and the top of draft tube will be transmitted to the automatic valve through a 3-in. pipe. The automatic float valve under these conditions will admit sufficient air to the turbine cover plate to maintain the water level at a predetermined point.
Provisions have been made for the admission of air through the cover plate at atmospheric pressure should the riverbed be eroded below the powerhouse. This air valve is of a conventional type, automatically operated from the turbine shifting ring, and will be provided with a pipe connection leading to the outside of the powerhouse to remove from the turbine pit the objectionable noise of the air intake.
The turbine casing will be filled by means of two internal differential needle valves, 12 in. in diameter, located on the penstock butterfly valve. The entrained air in the turbine scroll case will be discharged through two 4-in. atmospheric relief valves designed to close automatically when the air has been completely expelled from the highest point in the conduit.
Ireal A. Winter, U.S. Bureau of Reclamation
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