128 WATER CONDUCTORS AND ACCESSORIES 



A study must be made of the entire penstock from the head- 

 gates to the turbine casing, and the exact drop in pressure cal- 

 culated at each section under the most severe conditions, which 

 possibly would occur when a turbine unit is running light, and a 

 short circuit occurs on the generator, in which case the turbine 

 gates open wide very quickly, and there is a tendency to accelerate 

 the water in the various sections of the pipe line. 



There may be some section in a long penstock where the 

 water column below this section has sufficient head to accelerate 

 the lower column quicker than the water column above may be 

 accelerated. This may cause a break in the water column at the 

 section in question, and a considerable vacuum, which is very 

 likely to collapse the penstock. To prevent this air vents (see 

 page 157) may be provided at the points along the pipe line where 

 dangers are expected, as whenever the pipe greatly increases its 

 slope or rate of fall. The amount of air which must be admitted 

 to keep the pressure from going below a certain given value must 

 be such as will, at the given pressure, replace the water which 

 has run away from the section. 



On account of the uncertainty of the calculation of the col- 

 lapsing strength of a riveted steel pipe, and in order to provide a 

 margin of safety, it would seem to be the best practice to pro- 

 vide against any excess of external over the internal pressure 

 at any point in the pipe line, rather than attempt to compute 

 the collapsing pressure. . The critical points subject to a deficient 

 internal pressure can best be located by drawing a hydraulic 

 gradient under conditions of accelerated or retarded flow in the 

 pipe line. 



For a more complete treatise on this important subject, the 

 reader is referred to an article by Enger and Seely, in "Engineering 

 Record "for May 23, 1914. 



Expansion joints are not usually employed in this country, and 

 if the pipe is carefully laid and buried or kept with water flowing 

 at all times, are not required except in special cases. Whether 

 the pipes are buried or not, they should be carried on concrete 

 piers. Heavy anchorage blocks should be inserted at all vertical 

 and horizontal bends, and with considerable temperature varia- 

 tions, expansion joints should in such instances be provided to 

 take care of the expansion and contraction of the pipe. While the 

 stress may be well within the elastic limit of the pipe material, 



