Boiler and setting designed by the Hartford Boiler Insurance Co., illus- Tubular boiler constructed by the Bigelow Company, illustrated and de- Table of measurements for setting return tubular boilers with arch fronts . 176 Setting of the Lancashire boiler, illustrated and described Cross section of the setting of 4 feet Cornish boiler, illustrated and de- Table for reducing to standard the observed evaporation of a boiler Standard for the evaporative efficiency of a boiler recommended by the Formula for working pressure on tube plates Rules for inspection of boilers; Rules for determining working pressures of Formula for working pressure per square inch of circular boilers with the longitudinal joints welded or made with a butt strap Formula for working pressure per square inch of corrugated furnaces; Rules for finding the strength of the joints of cylindrical superheaters; Regulations as to feed water, boiler pressure, vertical tube boilers, steamers Regulations of wood work, position of boilers, boiler plates, auxiliary or 233 PART I. THE CYLINDRICAL SHELL OF A CIRCULAR BOILER. The strength of the shell of a cylindrical boiler to resist a pressure within it, is in exact proportion to the diameter of the boiler and the thickness of the plate of which it is formed. For instance, take three cylindrical boilers each made of inch plate, the first one 2 feet 6 inches in diameter; the second twice that, or 5 feet in diameter; and the third twice that again, or ten feet in diameter; and if the 2 feet 6 inch boiler is fit for a safe working pressure of 180 lbs. per square inch, then the 5 foot boiler will be fit for exactly one-half that amount, or 90 lbs. per square inch, and the ten foot boiler will be fit for half the working pressure of the five foot boiler, hence we have: Diameter of boiler shell. Thickness of plate. Relative working pressure. 2 feet 6 inches. inch. 180 lbs. per square inch. The same principle applies to the thickness of the plate, for instance, if we take two cylindrical boiler shells, each 5 feet in diameter, the first one made of plate inch thick, and the second twice that, or 1 inch thick, then if the first is equal to a safe working pressure of 90 lbs. per square inch, then the second is equal to a safe working pressure of twice as much, or 180 lbs. per square inch, providing, of course, that the rivetted seams are of equal strength in each case, and that both boilers are allowed the same margin for safety, hence we have: 1 These principles (namely, that the strength of a boiler is, all other things or elements being equal, in direct proportion to its diameter, and also in direct proportion to its thickness) afford us a groundwork upon which we may lay down rules for determining, by calculation, the strength of the solid part* of any boiler shell, and the bases of these calculations are as follows: If the shell plate of a cylindrical boiler is inch thick, there is one inch section of metal to be broken before the boiler can be divided into two pieces, that is to say there is inch on each side of the shell, as shown in Fig. 1, and the two together will make 1 inch. If we take a Fig. 1. ring an inch broad, as, say, at A in Fig. 2, we shall obviously have a section of 1 square inch of metal to break before the ring can be broken into two pieces. The next consideration is what is the average strength of a plate of boiler iron. Now suppose we have a strip of boiler iron 2 inches wide and inch thick, or, what is the same thing, a bar of boiler iron 1 inch square, and that we lay it horizontally and pull its ends apart until it breaks, how many lbs. will it bear before breaking? Now for our present purpose we may assume this to be 47,040 lbs., and if this number of lbs. be divided by the diameter of the boiler in inches, it will give the bursting pressure in lbs. for any square inch in the ring, or any other square inch in the cylindrical shell of the boiler. *In the case of the rivetted joints or seams other considerations come in, as will be shown hereafter. |