SUCTION TUBE 485 



than 2 feet per second. Also the area of the tube at its point of con- 

 nection to the turbine, should be as nearly as possible equal to the 

 discharge area of the runner so as to avoid loss by shock at the sudden 

 change of section. The area should then increase gradually to the open 

 end of the tube, the angle of flare not exceeding about 15 and decreasing 

 as the length of tube increases. This serves two useful purposes, since, 

 in addition to changing part of the kinetic energy of discharge into useful 

 pressure energy, it usually improves the speed regulation of the plant. 

 With quick regulation and a sudden closing of the turbine gates, the 

 momentum of the suction column may break the column and cause a 

 vacuum at the turbine. 1 Immediately this action is overcome, atmospheric 

 pressure forces water up the tube again, and this may strike the runner 

 with great force. Even a small change of load may set up such pulsa- 

 tions, which are detrimental to steady running and are reduced by the 

 use of a conical draught tube. 



When fitted to a pressure turbine, water will, in general, enter the 

 draught tube with a not inconsiderable velocity of whirl, and with a tube 

 of large diameter when working at part gate, an air core may be formed 

 in the tube when starting up the plant, and may exist for some consider- 

 able time before being expelled. The turbine then loses the advantage 

 of the tube to some extent. To obviate this, gates for throttling the 

 lower end of the tube have sometimes, been used. While advantageous 

 when starting the turbine, they are, however, not often fitted on account 

 of the expense. 



The lower end of a draught tube should always be bell-mouthed to 

 facilitate the escape of water. 



The draught tube is applicable to any type of pressure turbine, but 



1 Let the suction tube be parallel ; Z feet long ; dipping //^ feet below the surface in the 

 tail-race, and suppose air leakage increases the pressure at the top of the tube by the 

 equivalent of h a feet of water. 



Thus for separation due to downward momentum we must have 



34 - (I - h d ) - h a = ~ a 



32 



' = (34 + /id - // a ) - 32 feet per second per second. 



EXAMPLE. 



p a = 2 Ibs. per square inch, h a = 4*6 feet, I = 28 feet, /< d = 3 feet, 



32 



Thus o = j (37 - 4-(>) - 32 = 5 feet per second per second. 



If f = 8 feet per second, separation would take place if the gates were shut in less time than 

 1*6 seconds. Actually since the retardation is not uniform during a uniform closing of the 

 gates, but increases to a maximum at the instant of closing, this retardation would probably 

 be attained if the time of closing were less than three seconds. 



