WATER CONDUCTORS 125 



ator and the turbine gates close, there is an increase in pressure 

 in the penstock which tends to develop more horse-power, and 

 vice versa, when a load comes on the generator and the turbine 

 gates open, there is a drop in pressure in the penstock, tending 

 to decrease the output of the turbine. As the length of the pen- 

 stock for any particular installation is fixed, it is necessary to limit 

 the changes in velocity in the penstock, in order to give reasonably 

 good speed regulations. 



Excessive rises in pressure may be eliminated by the use of 

 pressure regulators, by-pass relief valves or surge tanks. After 

 the size of the penstock has been tentatively settled as most suit- 

 able for economical considerations, it must then be investigated 

 for speed regulation, and this may indicate that a larger pipe may 

 have to be used than consistent with the highest economy. (See 

 also Surge Tanks and Pressure Regulators.) 



Steel Pipe. These may be made of rolled steel plates, riveted 

 together, Fig. 60, or lap-welded, the latter only being used for 

 very high heads where the pressure is excessive and where the use 

 of the riveted construction would greatly increase the thickness of 

 the plate. In figuring the thickness of the plate, this should be 

 based not only on the pressure due to the net head but also on 

 the additional pressure caused by the water hammer. 



The formula for the strength of riveted steel pipe is 



27V 

 where 



t = thickness of plate in inches; 

 P = pressure in pounds per square inch; 

 d = diameter of pipe in inches ; 

 /= factor of safety, based on the ultimate tensile strength. 4 



is a factor generally used in this country. 

 T = tensile strength = 50,000 for mild steel; 



= 60,000 for wrought iron ; 



e = efficiency of riveted joint = approximately 0.60 for single 

 rivets and 0.70 for double rivets. 



Table XXXIII 1 gives the safe working heads and weights of 

 riveted steel pipes. 



1 Pelton Water Wheel Company. 



