233 



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FIG. 125* Single-runner Vertical Turbine with Cast-iron Spiral Casing and 

 Steel-lined Concrete Draft Tube. (I. P. Morris Company.) 



than formerly, especially for low and medium heads, while cast 

 steel is not considered a very desirable metal from considerations 

 of corrosion, on account of the unavoidable roughness of the sur- 

 face. 



Damage to turbine runners may be caused by both corrosion 

 and erosion, the two being of an entirely different nature. Mr. 

 H. B. Taylor thus explains their difference as follows: " Erosion is 

 entirely a mechanical action, while corrosion or pitting, is the 

 result of chemical action. The abrasive action of foreign sub- 

 stances in the water has the effect of first polishing the vane sur- 

 faces, and eventually cutting away the metal until the vanes 

 are worn entirely through. The eroded parts are, therefore, 

 smooth and can be readily distinguished from the pitted marks 

 which result from corrosion. 



"It has been demonstrated that corrosion is primarily a ques- 

 tion of design and it has been clearly shown in practice where 

 sharp curves are resorted to, where contraction is not sufficient, 



