STEAM BOILERS 



117 



filling prevents it from being blown out by the pressure of the 

 steam. Fusible plugs applied from the outside differ from 

 those applied from the inside, as Fig. 2 clearly shows. 



Location of Fusible Plugs. According to the rules issued by 

 the Board of Boiler Rules of the State of Massachusetts, 

 fusible plugs must be filled with 

 pure tin, and the least diameter 

 shall not be less than $ in., 

 except for working pressures 

 over 175 lb., gauge, or when it 

 is necessary to place a fusible 

 plug in a tube, in which cases 

 the least diameter of fusible 

 metal shall not be less than 

 f in. The location of fusible p IG 



plugs shall be as follows: 



In horizontal return-tubular boilers, in the back head, not 

 less than 2 in. above the upper row of tubes and projecting 

 through the sheet not less than 1 in. 



In horizontal flue boilers, in the back head, on a line with 

 the highest part of the boiler exposed to the products of com- 

 bustion, and projecting through the sheet not less than 1 in. 



In locomotive-type or star water-tube boilers, in the highest 

 part of the crown sheet and projecting through the sheet not 

 less than 1 in. 



In vertical fire-tube boilers, in an outside tube, placed not 

 less than one-third the length of the tube above the lower 

 tube-sheet. 



In vertical submerged -tube boilers, in the upper tube-sheet. 



In water-tube boilers of the Babcock & Wilcox type, in the 

 upper drum, not less than 6 in. above the bottom of the drum 

 and projecting through the sheet not less than 1 in. 



In Stirling boilers of standard type, in the front side of the 

 middle drum, not less than 6 in. above the bottom of the drum 

 and projecting through the sheet not less than 1 in. 



In Stirling boilers of the superheated type, in the front 

 drum, not less than 6 in. above the bottom of the drum, and 

 exposed to the products of combustion, projecting through the 

 sheet not less than 1 in. 



