BOILERS AND GENERATION OF STEAM 309 



349. Testing Boilers for Defects. Boilers are tested in 

 two ways: (1) by hydraulic pressure, and (2) by the hammer 

 test. The hydrostatic test consists in filling the boiler with 

 water and then exerting by means of a boiler test-pump (Fig. 

 163) one-half more pressure than the boiler is expected to 

 carry. For instance, if it is expected to carry 100 Ibs. pressure, 

 it is tested up to 150 Ibs. The hammer test is made by going 

 over the boiler and tapping it with a small hammer. An 

 experienced ear can tell by the sound of a blow and by the 

 feel of the 

 iron whether a 

 weakness has 

 developed . 

 Corrosion and 

 strains from 

 expansion and 

 contraction 

 are liable to 

 cause a de- 

 crease in the 



,, ,. FIG. 163. Boiler Test-Pump, 



strength of a 



steam boiler. Corrosion, which may be either internal or 

 external, is the wasting away of the material of the boiler 

 by pitting, grooving, etc. Internal corrosion is mainly caused 

 by the action of oxygen, minerals, or acids in the water. 

 External corrosion takes place generally through rusting and 

 from the action of sulphur in the fuel. Under certain con- 

 ditions this sulphur attacks the metal when the boiler is 

 "starting up" or " cooling down," a time at which the gases 

 are much reduced in temperature. 



Boilers should be fed with hot water. Cold water tends to 

 reduce the temperature of the water already in the boiler, 



