. American hydroelectric practice; a compilation of useful data and information on the design, construction and operation of hydroelectric systems, from the penstocks to distribution lines . riations to thenozzle openings given in the uppercurve. In the lower curve the speed fac-tor is expressed as a ratio betweenthe peripheral velocity of the wheeland the spouting velocity of thewater. Under such conditions satisfactory operation will be represented onlyby various nozzle or gate openings. The point X and the corresponding pointP represents the 100 per cent, nozzle opening of both the upper an
. American hydroelectric practice; a compilation of useful data and information on the design, construction and operation of hydroelectric systems, from the penstocks to distribution lines . riations to thenozzle openings given in the uppercurve. In the lower curve the speed fac-tor is expressed as a ratio betweenthe peripheral velocity of the wheeland the spouting velocity of thewater. Under such conditions satisfactory operation will be represented onlyby various nozzle or gate openings. The point X and the corresponding pointP represents the 100 per cent, nozzle opening of both the upper and lowercurves, and consequently the maximum power. If the power varies, as, ofcourse, it does on every system between maximum and zero, satisfactoryregulation can only be accomplished by proper change in the nozzle open-ing as the load varies. The point of position drops below P as the powerdemands decrease, or rises above P as the power demands increase. Ifthe load is entirely removed (cut-off) a maximum or runaway speed willresult as shown at ^max- In practice the runaway speed of a tangentialwheel is less than <^tooj;. = = V2^/i on account of nozzle and Fig. 9.—Relation of Resistance and Powerto Water Wheel Speed GENERAL SURVEY OF WATER-POWER ENGINEERING 27 The maximum speed for which a hydraulic unit should be designed de-pends to a great extent on the character of the hydroelectric wheels sometimes have a peripheral speed about 70 per cent, ofthe spouting velocity of the water and may attain 100 per cent, dependingon the pitch line used in measuring the peripheral velocity. In high-headplants with a peripheral velocity about 40 per cent, of the spouting velocity,the wheel at 100 per cent, excess speed may still have considerable is generally known that the peripheral speed of a water wheel bears acertain ratio to the spouting velocity of the water on any given head. Thisratio as a percentage rarely falls below 40
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