In a pressure turbine the guides and wheel must be completely filled with water, as must also the draught tube, for efficient operation. If draught tubes are much more than twenty-five feet long, it is hard to keep a solid column of water from turbine to tail-water in each, and if this is not done a part of the head of water becomes ineffective. As pressure turbines are employed almost exclusively at electric stations with low heads of water, it is frequently impossible to locate such stations above the possible level of tail-water in times of flood if horizontal wheels direct-connected to generators are employed.
Fig. 22.—Power-house at Garvin’s Falls on the Merrimac River.
If turbines with vertical shafts are to be used, a power-station may be so located or constructed that all the electrical equipment will be above the highest known water-mark. With vertical shafts, connecting wheels, and generators, the main floor of an electric station may be located above the crest of the falls where the power is developed instead of at or near their base.
Fig. 23.—Power-house No. 2 at Niagara Falls.
By far the most important examples of electric stations laid out on this plan are those at Niagara Falls, where there are four such plants. Two of these generating plants, with an aggregate capacity of 105,000 horse-power, stand a mile above the falls, and are supplied with water through a short canal from Niagara River. Beneath each of these two stations a long, narrow wheel pit has been excavated through rock to a depth of 172 feet below the level of water in the canal. Both wheel pits terminate in a tunnel 7,000 feet long that opens into the river below the falls.
In this wheel pit the tail-water level is 161 feet below that of the water in the canal, and 166 feet below the floor of the power-station. Water passes from the canal down the wheel pits to the wheels near the bottom through steel penstocks, each seven feet in diameter, and a vertical shaft extends from each wheel case to a generator in the station above.
Locations like that at Niagara give great security against high water and washouts, but are seldom adopted because of the large first cost of plant construction. With heads of water from several hundred to 2,000 feet the loss of a few feet of head reduces the available power to only a very slight extent, and impulse wheels are usually employed. Draught tubes are not available to increase the heads at such wheels, and any fall of the water after it leaves the wheels does no useful work.