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

Shop Names for Boiler Braces.—1. Gusset brace (fig. 47). 2. Crowfoot brace. 3. Jaw brace (fig. 44). 4. Head to head brace (fig.50). These shop terms refer to braces used in the tubular form of boiler.

A Stay and a Brace in a steam boiler fulfil the same office, that of withstanding the pressure exerted outward of the expanded and elastic steam.

Socket Bolts are frequently used instead of the screw stay between the inside and outside plates that form the centre space. Socket bolts are driven hot the same as rivets.

The method of bracing with T bars is considered the best; the bars make the flat surface rigid and unyielding even before the brace is applied. The braces should be spaced about 8 inches apart on the T bar and 7 inches from the edge of the flange T the bar should be 4″ × 4¹⁄₂″ T iron and riveted to the head or flat surface with ¹¹⁄₁₆″ rivets spaced 4¹⁄₂ inches apart.

Hollow Stay Bolts are used in locomotive fire boxes to show when fracture has occurred by permitting an escape of steam or water.

The flange of a boiler head ¹⁄₂″ thick will amply support 6 inches from the edge of the flange.

A radius of 2 inches is ample for bend of flange on the head. The lower braces should be started 6 inches above the top row of tubes. Braces should be fitted so as to have a straight pull, i.e. parallel with the boiler shell. The heads of the boiler should be perfectly straight before the braces are fitted in place. Gusset brace plates should not be less than 30 inches long and 14 inches wide. Braces are best made of 1 inch O iron of highest efficacy with tensile strength of not less than 58,000 lbs. to the square inch.

The riveted stay shown in Fig. 51, consists of a long rivet, passed through a thimble or distance piece of wrought iron pipe placed between plates, to be stayed together, and then riveted over in the usual manner.

An ingenious device is in use to show when a bolt has broken. A small hole is drilled into the head, extending a little way beyond the plate, and as experience shows that the fracture nearly always occurs next to the outside plate, that is the end taken for the bored out head: when the bolt is broken the rush of steam through the small hole shows the danger without causing serious disturbance.

Even where the best of iron is used for stay bolts they should never be exposed to more than ¹⁄₁₀th or ¹⁄₁₂th their breaking strength.

The stays should be well fitted, and each one carefully tightened, and, as far as possible each stay in a group should have the same regular strain upon it—if the “pull” all should come on one the whole are liable to give way.

Dimensions and Shape of Angle and T Iron.

The condition of a boiler can be learned by tapping on the sheets, rivets, seams, etc., to ascertain whether there are any broken stays, laminated places, broken rivets, etc.

Fig. A represents the method of preparing testing pieces of boiler plate, for the machines prepared specially to measure their elongation before breaking, and also the number of pounds they will bear stretching before giving way. Fig. B exhibits the same with reference to the brace and other O iron.

RULES AND TABLES
FOR DETERMINING AREAS AND CALCULATING THE CONTENTS OF STEAM AND WATER SPACES IN THE STEAM BOILER.

In order to ascertain the number of braces, which are necessary to strengthen that part of the boiler head, which is not stayed by the tubes, it is first necessary to know its area; the part to be stayed is a segment of a circle.

The length of the segment is measured above the top row of the tubes, and its height or width is equal to the distance from the top of the tubes to the top of the boiler shell.

Since, however, part of this segment is braced by the boiler shell, and also by the top row of the tubes, it has been generally agreed that the length of the segment should be measured two inches above the tubes, and the height or width, should be measured from a line, drawn two inches above the tubes, to a point within three inches from the top of the boiler shell, as shown in the illustration by the dotted line. Thus, referring to Fig. D, the length of the segment is equal to l, and the height is equal to h.

Rule. The area of a segment may be obtained, very approximately, by dividing the cube of the width (or height) by twice the length of the chord, and adding to the quotient the product of the width into two-thirds of the chord.

Example. If we suppose the height h of the segment in Fig. D to be equal to 18 inches, and the length l to be equal to 48 inches, we have

18³ ÷ (48 × 2) + (48 × ²⁄₃ × 18) = 60.7 + 576.0 = 636.7 square inches.

In order to calculate the contents of the steam and water spaces of a boiler, the same rule, as above, may be employed. The volume of the steam space may be readily obtained by the above rule, taking the distance from the water level to the top of the shell for the height, and the diameter of the shell, measured at the water line, for the length of the segment lines.

The area of the segment thus found, expressed in square inches, divided by 144, and multiplied by the length of the boiler in feet, is equal to the steam space, in cubic feet, this result is slightly reduced by the space occupied by the braces.

In order to find the volume of the water space, it is first necessary to find the total area of the boiler head, and this minus the area of the segment above the water line, is equal to the area of the segment below the water line. From this must also be subtracted the combined cross sectional area of the tubes.

Thus, the rule for finding the volume of the steam space in cubic feet.

1. Find the area of the segment of the boiler head, above the water line, in square inches.

2. Divide this by 144, and multiply the quotient by the length of the boiler in feet.

To find the volume of the waterspace in cubic feet.

1. Find the area of the boiler head in square inches.

2. Multiply the square of the outside diameter of one tube by .7854, and multiply this by the number of tubes, and add to the product, the area of the segment above the waterline.

3. Subtract 2 from 1, and divide the remainder by 144.

4. Multiply the quotient by the length of the boiler in feet.

To find the number of braces, necessary for the flat surface above the tubes.

1. Find the area of the segment of the boiler head, which is to be braced, in square inches.

2. Multiply the area, thus found, by the steam pressure in pounds per square inch.

3. Multiply the cross sectional area of one brace by the number of pounds, which it is allowed to carry, per square inch of section.

4. Divide product 2 by product 3, and the result is the number of braces, required for the head.

Table No. 1 gives the total area in square inches. No. 2, areas to be braced. No. 3, number of braces of one inch round iron required, allowing seven thousand five hundred pounds per square inch of section at one hundred pounds steam pressure.

Table No. 3 will be found of more practical use than Table 2, for it gives directly the number of braces required in any given boiler, instead of the area to be braced. It was calculated from Table 2. The iron used in braces will safely stand a continuous pull of 7,500 pounds to the square inch, which is the figure used in computing the foregoing table. A round brace an inch in diameter has a sectional area of .7854 of an inch, and the strain that it will safely withstand is found by multiplying .7854 by 7,500, which gives 5,890 pounds as the safe working strain on a brace of one-inch round iron.

In a 60-inch boiler, whose upper tubes are 28 inches below the shell, the area to be braced is, according to table 2, 930 square inches. If the pressure at which it is to be run is 100 pounds to the square inch, the entire pressure on the area to be braced will be 93,000 pounds, and this is the strain that must be withstood by the braces. As one brace of inch-round iron will safely stand 5,890 pounds, the boiler will need as many braces as 5,890 is contained in 93,000, which is 15.8. That is, 16 braces will be required. The table is made out on the basis of 100 lbs. pressure to the square inch, because that is a very convenient number.

Table No. 1. TOTAL AREA ABOVE TUBES OR FLUES.

(Square Inches.)

Table 2. AREAS TO BE BRACED. (Square Inches.)

Table 3. NUMBER OF BRACES REQUIRED, AT 100 LBS. PRESSURE.

In Table 2 this calculation has been made for all sizes of boilers that are ordinarily met with. The area to be braced has been calculated as above in each case, the two-inch strip above the tubes, and the three-inch strip around the shell being taken into account. As an example of its use, let us suppose that upon measuring a boiler we find that its diameter is 54 inches, and that the distance from the upper tubes to the top of the shell is 25 inches. Then by looking in the table under 54″ and opposite 25″ we find 714, which is the number of square inches that requires staying on each head.

BOILER TUBES.

Table.

Dimensions of Lap Welded Boiler Tubes.

The above is the regular manufactures’ list of sizes and weights.

Note.

Boiler tubes are listed and described from the outside diameter. This should be noted, as gas-pipe is described from the inside diameter. Thus a 1-inch gas-pipe is nearly 1¹⁄₄ outside diameter while a 1-inch boiler tube is exactly one inch. Another difference between the two consists in the fact that the outside of boiler tubes is rolled smooth and even; gas-pipe is left comparatively rough and uneven.

When the boiler tubes are new and properly expanded there is a large reserve or surplus of holding power for that part of the tube sheet supported by them, this has been proved by experiment made by chief engineer W. H. Stock, U. S. N., as shown in the following

Table of Holding Power of Boiler Tubes.

Mr. C. B. Richards, consulting engineer at Colt’s Armory at Hartford, Conn., made some experiments as to the holding power of tubes in steam boilers, with the following results: The tubes were 3 inches in external diameter, and 0.109 of an inch thick, simply expanded into a sheet ³⁄₈ of an inch thick by a Dudgeon expander. The greatest stress without the tubes yielding in the plate was 4,500 pounds, and at 5,000 pounds was drawn from the sheet. These experiments were repeated with the ends of the tubes which projected through the sheet three-sixteenths of an inch, being flared so that the external diameter in the sheet was expanded to 3.1 inches. The greatest stress without yielding was 18,500 pounds; at 19,000 pounds yielding was observed; and at 19,500 pounds it was drawn from the sheet. The force was applied parallel to the axis of the tube, and the sheet surfaces were held at right angles to the tube axis.

Note.

When the tube sheet and tube ends near the sheet become coated with scale or the tubes become overheated, the holding power of the tubes becomes largely reduced, and caution must be used in having the tube ends re-expanded and accumulated scale removed.

Note 2.—In considering the stress or strain upon the expanded or riveted over ends of a set of boiler tubes, it may be remembered that the strain to be provided against is only that coming upon tube plate, exposed to pressure, between the tube ends—the space occupied by the tubes has no strain upon it.

The gauge to be employed by inspectors to determine the thickness of boiler plates will be any standard American gauge furnished by the Treasury Department.

All samples intended to be tested on the Riehle, Fairbanks, Olson, or other reliable testing machine, must be prepared in form according to the following diagram, viz.: eight inches in length, two inches in width, cut out their centres as indicated.

Portions of the Marine Boiler which Become Thin by Wear.

These are generally situated, 1st, at or a little above the line of fire bars in the furnace; 2d, the ash pits; 3d, combustion chamber backs; 4th, shell at water line; 5th, front and bottom of boiler.

The thinning can usually be detected by examination, sounding with a round nosed hammer, or drilling small holes in suspected parts not otherwise accessible for examination.

EXAMPLES OF CONSTRUCTION AND DRAWING

The small table above is of use in this and the four succeeding pages; in all places in the drawings where “d” is used it indicates the diameter of the rivet; “t” means the thickness of the plate; “p” stands for pitch. The table also shows the proportion of rivet to the plate—thus, a ¹⁄₄-inch plate requires a ⁹⁄₁₆ rivet, etc.

It is recommended, in view of the increased disposition on the part of official examiners to test the applicant’s knowledge of drawing, for any one interested, to redraw to a full size all the rivets, plates, and methods of joining the two contained on pages 113-116.

The figures 53 to 60 will be understood without much explanation.

In figures 53 and 54 the cup head, the conical head and pan head rivets are shown.

Figs. 55 and 56 exhibit the details (and drawings) of single and double riveting. Where the cut reads p = (2¹⁄₂)d, it means that the distance from the centre of one rivet to the centre of the next shall be 2¹⁄₂ the diameter of the rivet, see example, page 115.