312 



APPLIED SCIENCE 



A lever safety valve (Fig. 165) consists of a disk, a stem, and 

 a lever with a weight hung on the end. The weight keeps 

 the valve in its place until the steam pressure under the valve 



f overcomes that of the weight 



ft H and some of the steam es- 



capes. If it were not for the 

 safety valve, boiler explosions 



would be much more 

 quent. 



fre- 



(Copyrighted by Millers Falls Co.) 



FIG. 165. Safety Valve. 



353. Construction of 

 Safety Valves. Calculations 

 for lengths of arms and 



weights required for any boiler pressure are obtained from the 

 formulas for levers, taking into account the weight of the 

 lever and valve. 



The center of gravity is the point at which the lever and 

 valve attached to it will just balance over a balancing bar 

 (bar with .knife edge). The fulcrum is at the center of the 

 pivot on which the lever works. 



Where F = the fulcrum on which the lever works. 

 W = weight of ball in pounds. 

 g = distance in inches from fulcrum to center of 



gravity. 



VI = weight of valve and lever in pounds. 

 L = distance between ball and center of fulcrum in 



inches. 

 I = distance between fulcrum and center of valve in 



inches. 



P = boiler pressure per square inch. 

 A area of safety valve in square inches. 



Then 



