BOILER AND MANAGEMENT 167 



17. Do not fail to empty the boiler every week or two 

 and refill with fresh water. 



18. Have your steam gauge tested at least twice a 

 year. 



BOILER INCRUSTATION* 



In all boilers after a period of use, there is deposited 

 upon the parts below the water level a scale or sediment 

 known as boiler incrustation. 



Cause of Scale. The formation of scale is due to 

 the impurities contained in the feed water. When impure 

 water is fed intp the boiler the impurity first manifests 

 itself in the form of scum on top of the boiling water. 

 The heavier particles of the scum slowly unite and sink 

 to the bottom where they first appear as mud. By con- 

 tinued exposure to high temperature, this mud gradually 

 forms into a hard impervious scale which usually con- 

 sists largely of lime. 



Objection to Scaler i. The excessive formation of 

 boiler scale is the immediate cause of most boiler explo- 

 sions. The scale acts as a non-conductor of heat, so that 

 in cases where the capacity of the boiler is severely taxed, 

 the metal becomes overheated, thus materially weakening 

 it. The scale is, therefore, not only dangerous, but by 

 overheating the metal, also materially shortens the life 

 of the boiler. 2. Another most serious objection to scale 

 is its wastefulness of fuel. This becomes evident when 

 we note that the heat before reaching the water must first 

 be conducted through a non-conducting layer of incrusta- 

 tion. 



Prevention of Scale. Since nearly all water used for 

 boilers is more or less impure, it is evident that to prevent 

 scale, boilers must receive frequent cleaning. How often 



