11G STEAM BOILERS 



wherever such law is in existence. In localities having no 

 law governing this matter, the size of the safety valve may be 

 calculated by the accompanying formulas, which are based on 

 practice and recommended by leading authorities. 

 For natural draft, 



For artificial draft, 



1.406 w 



in which G = grate surface, in square feet; 



p = steam pressure, gauge, in pounds per square inch; 

 w = weight of coal burned per hour in pounds; 

 A = least area of safety valve, in square inches. 



Location of Safety Valve. The safety valve should be 

 placed in direct connection with the boiler, so that there can be 

 no possible chance of cutting off the communication between 

 them. A stop- valve placed between the boiler and safety valve 

 is a very fruitful cause of boiler explosions. Again, the safety 

 valve must be free to act, and to prevent it from corroding 

 fast to its seat, it should be lifted from the seat occasionally. 

 Care must be taken to prevent persons ignorant of the impor- 

 tance of safety valves from raising the blow-off pressure by 

 adding to the weights or increasing the tension of the sprir 

 To this end, the weights of lever safety valves are often loci 

 in position by the boiler inspector. 



Use of Fusible Plugs. Fusible plugs are devices placed 

 the crown sheets of furnaces or in similar places to obviate 

 danger from overheating through lack of water. The plug 

 often consists of an alloy of tin, lead, and bismuth, which melts 

 at a comparatively low temperature. In many localities, the 

 law requires that fusible plugs shall be attached to all high- 

 pressure boilers. 



Forms of Fusible Plugs. The fusible plugs in common 

 are shown in section in Fig. 2. They consist of brass or ir 

 shells threaded on the outside with a Standard pipe thi 

 The plugs have some form of conical filling, the larger end of 1 

 filling receiving the steam pressure. The conical form of the 



