Professor Rankin’s Rule.—Multiply the number of pounds of water evaporated per hour by .006, and the product will be the area in square inches of the valve.
The U. S. Steamboat Inspection Law requires for the common lever valve one square inch of area of valve for every two square feet of area of grate surface.
United States Navy Department deduced from a series of experiments the following rule: Multiply the number of pounds of water evaporated per hour by .005, and the product will be the area of the valve in square inches.
Rule adopted by the Philadelphia Department of Steam Engine and Boiler Inspection:
1. Multiply the area of grate in square feet by the number 22.5. 2. Add the number 8.62 to the pressure allowed per square inch. Divide (1) by (2) and the quotient will be the area of the valve in square inches. This is the same as the French rule.
The maximum desirable diameter for safety valves is four inches, for beyond this the area and cost increase much more rapidly than the effective discharging around the circumference.
There should not be any stop valve between the boiler and safety valve.
The common form of safety valve is shown in [Fig. 96].
Here the load is attached to the end B of the lever A, B, the fulcrum of which is at c. The effective pressure on the valve, and consequently the blowing off pressure in the boiler can be regulated within certain limits, by sliding the weight W along the arm of the lever. In locomotive engines, as well as on marine boilers, the weight would on account of the oscillations, be inadmissible and a spring is used to hold down the lever.