INSPECTOR’S RULES RELATING TO BRACES IN STEAM BOILERS, ALSO TO BE OBSERVED BY ENGINEERS.

Where flat surfaces exist, the inspector must satisfy himself that the spacing and distance apart of the bracing, and all other parts of the boiler, are so arranged that all will be of not less strength than the shell, and he must also after applying the hydrostatic test, thoroughly examine every part of the boiler.

No braces or stays employed in the construction of marine boilers shall be allowed a greater strain than six thousand pounds per square inch of section, and no screw stay bolt shall be allowed to be used in the construction of marine boilers in which salt water is used to generate steam, unless said stay bolt is protected by a socket. But such screw stay bolts, without sockets, may be used in staying the fire boxes and furnaces of such boiler, and not elsewhere, when fresh water is used for generating steam in said boiler. Water used from a surface condenser shall be deemed fresh water. And no brace or stay bolt used in a marine boiler will be allowed to be placed more than eight and one-half inches from centre to centre, except that flat surfaces, other than those on fire boxes, furnaces and back connections, may be reinforced by a washer or T iron of such size and thickness as would not leave such flat surface unsupported at a greater distance, in any case, than eight and one-half inches, and such flat surface shall not be of less strength than the shell of the boiler, and able to resist the same strain and pressure to the square inch, and no braces supporting such flat reinforced surfaces, will be allowed more than 16 inches apart.

In allowing the strain on a screw stay bolt, the diameter of the same shall be determined by the diameter at the bottom of the thread. Many State laws and City ordinances allow a strain of seven thousand five hundred pounds per square inch of section on good bracing without welds. The following table gives the safe load of round iron braces or stays.

DIAMETER OF BRACE.

Tensile
strength per
square inch of
section allowed		1⁄2″5⁄8″3⁄4″7⁄8″1″11⁄8″11⁄4″11⁄2″13⁄4″2″
5000 98115332208300639274970613688351202615708
60001178184026503607471259647363106021443118849
70001374256730924209549769588590123691683721991
75001472275033134509589074559204132531803923562

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.

Fig. 51.

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.

Fig. 52.

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.

Fig. B.

[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.

Fig. C.Fig. D.

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.)

Height from
tubes to
shell.		DIAMETER OF BOILER IN INCHES.
36424854606672
15389
16419
17458526
18 566620667
19 608667720
20 650714770824
21 756824882
22 808878937
23 9309961059
24 98210561121
25 103711161184
26 1090120912521324
27 1145123413161394
28 129113811465
29 135214451536
30 141415111608
31 15761674
32 16411746
33 1818
34 1896

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

Height from
tubes to
shell.		DIAMETER OF BOILER IN INCHES.
36424854606672
15206
16235
17264297
18 331365396
19 316404439
20 401444483519
21 485528568
22 526574618
23 620668714
24 667720769
25 714772825
26 761824882937
27 809877940998
28 9309981061
29 9831056 1124
30 1037 1115 1187
31 1174 1252
32 1234 1317
33 1382
34 1447

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

Height from
tubes to
shell.		DIAMETER OF BOILER IN INCHES.
36424854606672
153.5
164.0
174.55.0
18 5.66.2 6.7
19 6.26.9 7.5
20 6.87.5 8.2 8.9
21 8.2 9.0 9.6
22 8.9 9.810.5
23 10.511.312.1
24 11.312.213.1
25 12.113.114.0
26 12.914.015.015.9
27 13.714.916.016.9
28 15.816.918.0
29 16.717.919.1
30 17.618.920.2
31 19.921.3
32 21.022.4
33 23.5
34 24.9

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.