240 



HYDRAULIC EQUIPMENT 



long radius so as to change the direction of discharge from a ver- 

 tical to a horizontal plan. The section of the draft tube must also 

 be gradually increased from the discharge ring of the turbine 

 to the tailrace so as to gradually reduce the velocity of the water 

 from the turbine to the tailrace, and it is common practice to 

 gradually increase the section from the circular form at the tur- 

 bine to an oblong section at the end, the long axis being horizontal. 

 Good draft-tube design is fundamentally dependent upon the 

 proper elevation of the turbine above tail-water. The runner 



FIG. 131. Placing of Wooden Forms for Draft Tubes of Three 10,000 Horse- 

 power Turbines. 



should be so located that the total draft head at the top of the 

 tube (i.e., the static elevation of the runner above tail-water added 

 to the velocity head at the throat of the runner) is well within the 

 theoretical limits of a vacuum; namely, approximately 34 feet, 

 depending on the barometer reading. If not, the water column in 

 the draft tube will break, returning with a surge and causing water- 

 hammer. If, on the other hand, the vacuum in the draft tube is 

 near the breaking point, the continuity of the flow may be inter- 

 rupted at the discharge end of the water passages through the 

 runner, resulting in corrosion and pitting of the vanes. 



