STEAM BOILERS 127 



perfectly smooth and of the proper bore. As in course of time 

 they become clogged with lime, the capacity of the injector 

 decreases until, finally, it refuses to work at all. If the water 

 used is very bad, it becomes necessary frequently to cleanse 

 the tubes of the accumulated lime. This may be done by 

 putting the parts in a bath consisting of 1 part of muriatic 

 acid to 10 parts of water. The tubes should be removed from 

 it as soon as the gas bubbles cease to be given off. 



Advantages and Disadvantages of Injectors. The advan- 

 tages of the injector as a boiler feeding apparatus are its cheap- 

 ness, as compared with a pump of equal capacity; it occupies 

 but little space; the repair bills are low, owing to the absence 

 of moving parts; no exhaust piping is required, as with a 

 steam pump; it delivers hot water to the boiler. The dis- 

 advantages of the injector are that it will not start with a 

 steam pressure less than that for which it is designed, and 

 that it will stand but little abuse, being poorly adapted for 

 handling water containing grit or other matter liable to cut 

 the nozzles. 



INCRUSTATION AND CORROSION 



Incrustation. Broadly speaking, any deposit that is formed 

 on the plates and tubes of a boiler is termed scale, or incrusta- 

 tion; it is caused by impurities that enter with the water and 

 that are left behind in the boiler when the water is evaporated. 

 In passing through the soil, water dissolves certain mineral 

 substances, the most important of which are carbonate of 

 lime and sulphate of lime. Other substances frequently pres- 

 ent in small quantities are chloride of sodium, or common 

 salt, and chloride of magnesium. It also often contains other 

 troublesome substances. 



Impurities in Feedwater. Some of the more common impu- 

 rities found in feedwater, together with their properties, are 

 given in the following paragraphs: 



Carbonate of lime will not dissolve in pure water, but will 

 dissolve in water that contains carbonic-acid gas. It becomes 

 insoluble and is precipitated in the solid form when the water 

 is heated to about 212 F., the carbonic-acid gas being driven 

 off by the heat. 



