226 STEAM TURBINES 



blades and the casing. The former may vary from J to 1 in. 

 or more from the high-pressure to the low-pressure stage; 

 but the tip clearance must be kept between a few hundredths 

 and a few thousandths of an inch. 



Stripping of Blades. The stripping of the blades is one of 

 the troubles to which turbines are subject. It may be due 

 to the interference of the stationary and the movable blades, 

 or to the rubbing of the blades against the shell or the rotor. 

 In either of these cases the existing clearances are reduced, 

 by wear of the parts, shifting of the rotor, or unequal expansion 

 of the rotor and the casing, until the blades touch and tear 

 one another loose. The same result will occur if some foreign 

 solid, as a stray nut or bolt, is carried along with the steam 

 into the turbine. If a turbine is started too quickly, without 

 being properly warmed up, the sudden unequal expansion set 

 up in the heavy casing and the lighter rotor may cause the 

 blades to come in contact and be stripped. Stripping is claimed 

 by some engineers to be more common in turbines in which the 

 blades are not supported at their outer ends. To prevent it, 

 therefore, shroud rings and metal lacings are applied to the 

 blades at their outer ends, by some manufacturers of steam 

 turbines. 



Erosion of Blades. As there are no valves, pistons, or piston 

 rings in the turbine to be maintained free from leakage, about 

 the only thing that can affect the steam consumption is the 

 condition of the blades. The blades of steam turbines are 

 subjected to the cutting action of steam flowing at high veloci- 

 ties, and often carrying water particles with it. This cutting, 

 or erosion, wears away the edges and surfaces of the blades. 

 From the data available, it appears that the erosion is very 

 slight if the steam is dry or superheated, no matter what 

 velocities are used; but if the steam is wet, erosion will take 

 place, and it will be greatly increased if the velocity of the 

 steam is high. The horsepower is not affected to any great 

 extent by blade erosion, according to the results of experience. 

 In the case of a 100-H. P. De Laval turbine, the steam inlet 

 edges of the blades were worn away about A in-, yet the 

 steam consumption was only about 5% above that with new 

 blades. 



