Maxims and Instructions for the Boiler Room / Useful to Engineers, Firemen & Mechanics; Relating to Steam Generators, Pumps, Appliances, Steam Heating, Practical Plumbing, etc.

by N. Hawkins

BRACING OF STEAM BOILERS.

The material of a boiler being satisfactory and the plates being thoroughly and skillfully riveted there remains the important matter of strengthening the boiler against the enormous internal pressure not altogether provided for.

Fig. 42.

To illustrate the importance of attention to this point it may be remarked that a boiler eighteen feet in length by five feet in diameter, with 40 four-inch tubes, under a head of 80 pounds of steam, has a pressure of nearly 113 tons on each head, 1,625 tons on the shell and 4,333 tons on the tubes, making a total of 6,184 tons on the whole of the exposed surfaces.

Not only is this immense force to be withstood, but owing to the fact that the boiler grows weak with age—a safety factor of six has been adopted by inspectors, i.e., the boiler must be made six times as strong as needed in every day working practice.

Fig. 43.

Braces in the Boiler.—The proper bracing of flat surfaces exposed to pressure, is a matter of the greatest importance, as the power of resistance to bulging possessed by any considerable extent of such a surface, made as they must be in the majority of cases of thin plates, is so small that practically the whole load has to be carried by the braces. This being the case, it is evident that as much attention should be given to properly designing, proportioning, distributing and constructing the brace as to any other portion of the boiler.

All flat surfaces should be strongly supported with braces of the best refined iron, or mild steel, having a tensile strength of not less than 58,000 lbs. to the square inch. These braces must be provided with crow feet or heavy angle iron properly distributed throughout the boiler.

Fig. 44.

[Fig. 42] shows the method usually followed in staying small horizontal tubular boilers. The cut represents a 36-inch head and there are five braces in each head: two short ones and three long ones. The braces should be attached to shell and head by two rivets at each end. The rivets should be of such size that the combined area of their shanks will be at least equal to the body of the brace, and their length should be sufficient to give a good large head on the outside to realize strength equal to the body of the brace.

In boilers with larger diameters, 5 to 8 feet, stay ends are made of angle or T iron; by this arrangement the stays can be placed further apart, the angle irons very effectively staying the plate between the stays, and thus affording more room in the body of the boiler. The size of the stays have to be increased in proportion to the greater load they have to sustain. See [Fig. 43].

In a 66-inch boiler it is proper to have not less than 10 braces in each head, none under three feet in length, made of the best round iron one inch in diameter, with ends of braces made of iron 2¹⁄₂ × ¹⁄₂ inches with three pieces of T iron riveted to head above the tubes to which the braces are attached with suitable pins or turned bolts. See [Fig. 44].

Staying of Flat Surfaces.—When boilers are formed principally of flat plates, like low-pressure marine boilers, or the fire-boxes of locomotive boilers, the form contributes nothing to the strength, which must, therefore, be provided for by staying the opposite furnaces together. [Fig. 45] shows the arrangement of the stays in a locomotive fire-box. They are usually pitched about 4 inches from centre to centre, and are fastened into the opposite plates by screwing, as shown, the heads being riveted over. Each stay has to bear the pressure of steam on a square aa, and the sectional area of the stay must be so chosen that the tensile strength will be sufficient to bear the strain with the proper factor of safety.

Fig. 45.

If the spaces between the stays are too great, or the plate too thin, there is a danger of the structure yielding through the plate bulging outwards between the points of attachment of the stays, thus allowing the latter to draw through the screwed holes made in the plates.

In designing boilers with stayed surfaces, care should be taken that the opposite plates connected by any system of stays should, as far as possible, be of equal area, otherwise there is sure to be an unequal distribution of load in the stays, some receiving more than their proper share, and moreover, the least supported plate is exposed to the danger of buckling.