246 HYDRAULIC "EQUIPMENT 



body, the jets being approximately 90 apart. Such an arrange- 

 ment is shown in Fig. 135, this sketch representing a unit with 

 four wheels and eight jets, developing 10,500 H.P. under an 

 effective head of 380 feet at 200 R.P.M. 



Housings. The general type and construction of wheel 

 housings or casings for impulse turbines is illustrated in Fig. 134, 

 the best practice being to provide a separate housing for each 

 wheel to prevent interference from discharged water. The lower 

 part is usually made of iron castings and the upper housing or 

 cover of steel plate riveted into a cast-iron frame. This type of 

 housing for large units is claimed to be preferable to a housing 

 made entirely of cast iron, as it is lighter to handle and elimi- 

 nates any danger of breakage where the shaft of the runner passes 

 through the sides of the housing, and water leakage is prevented 

 by means of a centrifugal disc and water guard, which device in- 

 sures a frictionless packing. For small units, on the other hand, 

 the self-contained cast-iron housing is, as previously stated, to be 

 preferred. 



2. GOVERNORS J 



Before the advent of the automatic voltage regulator, close 

 speed regulation by water-wheel governors was of much greater 

 importance than now, for any departure from normal had an 

 immediate effect on the voltage. With the automatic regulator 

 in operation, reasonable changes in speed have no appreciable 

 effect on the voltage, but they do, of course, affect the frequency. 

 A slight variation in frequency, is to be expected, for like all gov- 

 ernors for prime movers, the water-wheel governor requires a 

 certain change in speed to ensure good governing. 



Factors Affecting Speed Regulation. While primarily the 

 regulation for speed originates with the governor, it also involves 

 the consideration of the pipe-line conditions and those devices 

 required for limiting the pressure rise therein, and besides the 

 effective flywheel effect of th,e rotating elements of the generator 

 and water wheel. 



Variations in the velocity of the water in the pipe line will 

 always occur, and every retardation in velocity of the moving 

 water column will bring about an increase in the pressure, in- 

 versely in proportion to the time occupied for a given change. It 



1 See also " Pipe Lines," " Water Hammer and Surge Tanks." 



