STEAM BOILERS 129 



floats on the surface of the water. These particles will gradu- 

 ally settle on the plates, tubes, and other internal surfaces. A 

 large part of the impurities will be carried by the circulation 

 of the water to the most quiet part of the boiler and there 

 settle and form a scale. In a few weeks, if no means of pre- 

 vention are used, the inner parts of the boiler will be covered 

 with a crust from rj to -| in. in thickness. 



Danger of Scale. A scale & in. or less in thickness is 

 thought by many to be an advantage, as it protects the plates 

 from the corrosive action of acids in the water. When, how- 

 ever, the scale becomes J in. thick or more, heat is transmitted 

 through the plates and tubes with difficulty, more fuel is 

 required, and there is danger of overheating the plates. The 

 chief danger from a heavy incrustation is the danger of over- 

 heating the plates and tubes; it also prevents a proper examina- 

 tion of the inside of the boiler, since it may hide a dangerously 

 corroded piece of plate or a defective rivet head that would 

 otherwise be discovered. 



Scale Containing Lime. The carbonate of lime forms a 

 soft, muddy scale, which when dry, becomes fluffy and flour- 

 like. This scale may be easily swept or washed out of the 

 boiler by a hose, provided it is not baked hard and fast. A car- 

 bonate scale is much harder to deal with when grease is allowed 

 to enter the boiler. The grease settles and mixes with the 

 floury scale, making a spongy crust that remains in contact 

 with the plates, being too heavy to be carried off by the natural 

 circulation of the water. The sulphate of lime forms a scale 

 that soon bakes to the plates. 



Kerosene as Scale Remover. Some substances seem to 

 soften and aid in detaching scale. Of these, kerosene oil has 

 met with much favor. Its action appears to be mechanical 

 rather than chemical, the oil penetrating or soaking through 

 the scale and softening and loosening it. It is somewhat 

 useful, too, in preventing the formation of scale, enveloping 

 the fine particles of the scale-forming substances that, after 

 precipitation, float on the surface of the water for a little 

 while. It seems that this prevents the particles from adher- 

 ing firmly to one another and to the metal when they finally 

 settle. 



