Niagara Falls 100.000-Hp. Development

Niagara Falls 100,000 Hp. Development 8] once known and gave a rough idea of the limitations of speed and load for each unit. There was available for development 4,400 cubic foot-seconds (123 cu.m, per second), and the problem was to determine the number and size of units which would most efficiently develop that amount of water. Consideration of this feature and the rough limitations set by the specific speed led to the selection of three units as the proper number to use. The number of units being fixed, the next thing was the point of maximum efficiency for each single unit, which was set at the point of use of 1,500 cubic foot- seconds (42 cu.m, per second). The experience of the waterwheel builders was that in order to arrive at this result it would be necessary to design the wheels for a maximum output of from 10 to 12 per cent in excess of the point of maximum efficiency. The builders fur- ther agreed that with the hydraulic efficiency naturally to be expected this maximum output would be 37,500 hp. The combination of this maximum output with the specific speed and periodicity of the current to be TO OTHER PLANTS IN THE NIAGARA FALLS SY TSM generated fixed the speed of the unit at 150 r.p.m per minute. Having’ determined the over-all characteristics of the installation, the first minor problem to obtain considera- tion was that of the proper shape of the draft tube. The ideal draft tube would, of course, be a straight tube of uniformly enlarging cross-sections, the expan- sion angle being in the neighborhood of 4 deg. This ideal shape of draft tube is impracticable in most instal- lations on account of the prohibitive cost of the excava- tion required. It was necessary, therefore, to deter- mine some form of tube which would accomplish the same purpose in a much shorter vertical distance. The company’s engineers designed and developed an apparatus for testing the regain efficiency of a series of model tubes, and, after a long series of experiments on many different models, decided on the White hydrau- cone and a somewhat similar form of tube designed by the I. P. Morris department of the William Cramp & Sons Ship & Engine Building Company as offering the best practical solution of the problem. Hydraulic Design and Efficiency of Units and Plant Large and Easy Waterways Provided—Two Types of Draft Tubes Selected—New Method of Testing Wheels By N. R. Gibson Hydraulic Engineer Niagara Falls Power Company EFFICIENCY was the underlying principle upon which the design of Station No. 3 extension was based, and to this end hydraulic losses were reduced to a minimum by providing large and easy waterways and by special attention to the design of some details which frequently are not so carefully considered. For the nominal velocities in the various waterways, the follow- ing were chosen: _______________________________________ __________ Canal ................................. Forebay .............................. Through racks......................... Penstock entrances................... Penstocks .............................. Entrances to turbine casing's..,...... Top of draft tube....................... Exist from draft tube................. Tailrace ............................... Ft. Per Second 4.5 3.5 1.5 1.8 3.0 8.0 17.3 20.0 5.0 to 4.5 _______________________________________________ With these velocities the resulting hydraulic losses from forebay to tailwater, exclusive of turbine effi- ciency, has been found to be approximately 2 ft. at nor- mal load, or less than 1 per cent total average head available. In collaboration with the designers of hydraulic tur- bines, considerable attention was given to the designs of draft tubes, and after experimenting with about twenty-five draft tube models of various designs two types were finally adopted and constructed in the power- house foundations. The right hand view of Fig. 7 shows the hydraucone regainer patented by W. M. White of Milwaukee; the left view shows the spreading draft tube patented by Lewis F. Moody of Philadelphia. The efficiencies of these two tubes, as shown by Fig. 4, were practically the same, and both gave results which were far in excess of the efficiency of any other model of equal dimensions. The effect of these draft tubes is to maintain flow parallel to or radially from a central axis, with a grad- ual diminution of velocity until the velocity of discharge is finally reached. Such designs allow free play for the whirls in the water as it leaves the runner until the velocity of whirl has been greatly reduced. Large losses are thus prevented which occur when the direction of flow is changed, as in a bent tube, before the velocity of the whirl has been reduced. The passages are also designed so that the hydraulic friction losses through- out the tube are reduced to a minimum. So far as can be ascertained, the adoption of these draft tubes has been fully justified by the results attained, such as the direct gain in efficiency and the freedom from excessive vibration of the machinery. The efficiencies of the hydraulic turbines received particular consideration, but no guarantees were ex- acted from the manufacturers. Instead, each manu- facturer agreed that “In lieu of any guarantees of efficiency the contractor will use its best engineering talent and skill in the design and construction of the turbine, to the end that the highest attainable efficiency may be secured. It is expected, but not guaranteed, that the combined efficiency of the turbine and generator will be as high as 90 per cent.”