128 STEAM BOILERS 



Sulphate of lime dissolves readily in cold water, but not in 

 hot water. It precipitates in the solid form when the water 

 is heated to about 290 F., corresponding to a gauge pressure 

 of 45 Ib. 



Chloride of sodium will not be precipitated by the action of 

 heat unless the water has become saturated with it. Since it 

 generally is present in but very small quantities in fresh water, 

 it will take a very long time for the water in a boiler to become 

 troublesome, and with the ordinary blowing down of a boiler 

 once a week or every 2 wk., there is little danger of the water 

 becoming saturated with it. Consequently, it is one of the 

 least troublesome scale-forming substances contained in fresh 

 water. 



Chloride of magnesium is one of the worst impurities in water 

 intended for boilers, for while not dangerous as long as the water 

 is cold, it makes the water corrosive when heated, and when 

 present in large quantities, it becomes dangerously corrosive, 

 attacking the metal of the boiler and rapidly corroding it. 



Organic mailer by itself may or may not cause the water to 

 become corrosive, but will often cause foaming; when it is 

 present in small quantities in water containing carbonate or 

 sulphate of lime, or both, it usually serves to keep the deposits 

 from becoming hard. 



Earthy mailer, like organic matter, is not dissolved in the 

 water, but is in mechanical suspension. It is very objection- 

 able, especially when the earthy matter is clay, and when other 

 scale-forming substances are present is liable to form a hard 

 scale resembling Portland cement. 



Acids, such as sulphuric acid, nitric acid, tannic acid, and 

 acetic acid, are often present in the feedwater. The sulphuric 

 acid is the most dangerous one of these acids, attacking the 

 metal of which the boiler is composed and corroding it very 

 rapidly. The other acids, while not so violent in their action 

 as the sulphuric acid, are also dangerous, and water contain- 

 ing any one should be neutralized when it must be used. 



Formation of Scale. The small solid particles, due to pre- 

 cipitation of substances in solution or matter in mechanical 

 suspension, remain for a time suspended in the water, especially 

 the carbonate of lime that for some time after precipitation 



