114 STEAM BOILERS 



Let d = F B = distance from fulcrum to weight, in inches; 

 c = F G = distance from fulcrum to center of gravity 



of lever, in inches; 

 a = F C = distance from fulcrum to center line of 



valve, in inches; 

 A =area of orifice beneath bottom of valve, in square 



inches ; 



W = weight of ball P, in pounds; 

 Wi = weight of valve and stem, in pounds; 

 Wt = weight of lever, in pounds; 



p = blow-off pressure, in pounds per square inch. 

 Then, if the position of the ball P on the lever is fixed, the 

 required weight of the ball may be found by the formula 

 a(pA-Wi)-Wic 



~7~ 



EXAMPLE. The area of the orifice is 10 sq. in., the distance 

 from the valve to the fulcrum is 3 in., and the length of the 

 lever is 32 in. The valve and stem weigh 5 lb., the lever 

 weighs 12 lb., and the gauge pressure is 90 lb. What should 

 be the weight W, if placed 2 in. from the end of the lever, 

 assuming the lever to be straight ? 



SOLUTION. In this case, c = 3=lG in., and d = 32 2 = 30 in. 

 Then, substituting in the formula, 



Position of Ball for Lever Safety Valve. If the ball of a 

 lever safety valve has a known weight and it is desired to find 

 at what distance from the fulcrum it must be placed so as to 

 give a required blow-off pressure, the formula to be used is 



in which the various letters have the same meanings as before. 

 EXAMPLE. Suppose all the quantities to remain the same 

 as in the solution of the preceding example, except that it is 

 desired that the boiler should blow off at 75 lb. gauge pressure, 

 instead of 90 lb. What will be the distance of the weight 

 from the fulcrum ? 



