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. Vl = weight of valve and lever in pounds. L = distance between ball and center of fulcrum in inches. / = distance between fulcrum and center of valve in inches. P = boiler pressure per square inch. A - area of safety valve in square inches.

Then W = A X P X l - (Vl x g) / L and L = A X P X l - (Vl x g) / W

Fig. 165.   Safety Valve.

(Copyrighted by Millers Falls Co.)

Fig. 165. - Safety Valve.

Example. - At what distance from the center of fulcrum must a weight be placed, if the boiler pressure is 100 lbs., weight is 16 lbs., area of valve is 3 sq. in., and valve and lever weigh 16 lbs., center of valve is 2 1/2 in. from fulcrum, and center of gravity is 12 in. from fulcrum?