TURBINES 



235 



with eddies possessing high velocities and very low static pressure, 

 in which oxygen is liberated from the water. This oxygen is 

 believed to be in the nascent state and rapidly attacks the sur- 

 face of the metal, forming an oxide coating, the greater part of 

 which is rapidly washed away by the water. When once the 

 depth of this pocket is increased by corrosion, it is natural that, 

 due to the greater area exposed, the pitting action should con- 

 tinue at an accelerated rate until the vane is entirely eaten 

 through." 



Gate Mechanism: For controlling the flow of reaction tur- 

 bines there are two principal types of gates in use, the cylinder 

 gate and the wicket or swivel gate. The latter, Figs. 127 and 128, 



FIG. 127. Typical Arrangement of Gate Mechanism for a Small Vertical 



Reaction Turbine. 



offers decided advantages of the two. Wear and tear is greatly 

 reduced for small fractional loads due to better flow conditions, 

 resulting in higher efficiencies than can be obtained with cylinder 

 gates. 



As previously stated, the exposed or so-called " outside ;> 



