226 HYDRAULIC EQUIPMENT 



Over-speed. Due to the action of the governor the normal 

 speed of the turbine is usually maintained constant under oper- 

 ating conditions. If the load changes, however, take place with- 

 out a corresponding regulation of the admitted quantity of water, 

 the speed will necessarily vary, increasing as the load decreases 

 and vice versa. If the load should suddenly drop off with the 

 gates wide open and remain so for some reason or other, the speed 

 will rise considerably, sometimes resulting in disaster to the 

 direct-connected generators, and these should, therefore, always 

 be designed safely to withstand such runaway speeds of the 

 water wheels. These depend to a great extent on the hydraulic 

 development and the type of wheel used. For high-head plants, 

 where impulse wheels are used, the over-speed should preferably 

 be estimated at 100 per cent of the normal speed. For low heads 

 with reaction turbines, when the same are working at the most 

 efficient speed, and the head is constant, the over-speed may be 

 from 50 to 80 per cent above the normal speed. Under low-head 

 conditions with a wide variation in the head and with wheels 

 designed for an intermediate speed to work under these different 

 conditions, a runaway speed of up to 200 per cent may then be 

 realized under the maximum head. The above values are only 

 general, and it is most desirable that in all cases a detail analysis 

 is made, based on test data for the particular type of wheel which 

 is to be used, considering the extreme range of heads and the other 

 conditions under which the wheel is anticipated to operate. 



To prevent dangerous over-speeds several types of over-speed 

 devices are being used. One of these consists of a fly-ball mechan- 

 ism, independent of the turbine governor, driven from the shaft 

 of the unit which, in the event of excessive speeds, by means of 

 control valves admits water behind the piston in an auxiliary 

 cylinder on the governor. This causes it to move in such a man- 

 ner as to overcome the oil pressure in the control element of the 

 governor and shut down the unit. 



Mechanical Designs. Reaction type: There is a very great 

 variety of turbine designs and while overlapping to a certain degree, 

 each has its particular field of application. For example, the units 

 may be horizontal or vertical, the latter being now almost entirely 

 used for low and medium-head installations. As a fact, in about 

 90 per cent of the large installations built during the past two or 

 three years the turbines have been of the vertical-shaft type. The 



