The Two Flue Boiler.—Fig. 31.
The Six Inch Flue Boiler.—Fig. 32.
THE HORIZONTAL TUBULAR STEAM BOILER.
The great majority of stationary boilers are cylindrical or round shaped, because—
1. The cylindrical form is the strongest.
2. It is the cheapest.
3. It permits the use of thinner metal.
4. It is the safest.
5. It is inspected without difficulty.
6. It is most symmetrical.
7. It is manufactured easier.
8. It resists internal strain better.
9. It resists external strain also.
10. It can be stayed or strengthened better.
11. It encloses the greatest volume with least material.
12. It is the result of many years’ experience in boiler practice.
13. It is the form adopted or preferred by all experienced engineers.
It follows, too, that the horizontal tubular boiler, substantially as shown in fig. 30, is the standard steam boiler; engineers and steam power owners cling with great tenacity to this approved form, which is an outgrowth of one hundred years’ experience in steam production.
In the plain horizontal tubular boiler shown in cuts, the shell is filled with as many small tubes varying from two inches to four inches in diameter as is consistent with the circulation and steam space. In firing this type of boiler the combustion first takes place under the shell, and the products, such as heat, flame, and gas, pass through the small tubes to the chimney, although in the triple draught pattern of the tubular boiler, the heat products pass, as will hereafter be explained, a second time through the boiler tubes, making three turns before the final loss of the extra heat takes place.
The illustrations on pages 78 and 80 exhibit the gradual advances to the horizontal tubular by the two-flued boiler (fig. 31) of the six flues (fig. 32) and of the locomotive Portable Boiler (fig. 33). The vertical or upright tubular boiler is but another modification of the horizontal tubular.
The Locomotive Portable Boiler.—Fig. 33.
In parts of the vertical boiler there is very little circulation and the corrosion on the inner side is such as to wear the boiler rapidly. In the ash pit, ashes and any dampness that may be about the place also causes rapid corrosion. The upper part of the tubes and tube sheet are frequently injured; for instance, if the tubes pass all the way through to the upper tube sheet, providing there is no cone top, when the fire is first made under the boiler, combustion at times does not take place until the gases pass nearly through the tubes. The water usually being carried below the tube sheet there is a space left above the water line, where there is neither steam nor water, and the heat is so great that the ends of the tubes are burned and crystalized, and the tube sheet is often cracked and broken by this excessive heat before the steam is generated. The first difficulty is experienced in “the legs” of the Portable Locomotive boiler—hence the general verdict of steam users in favor of the round shell, many-tubed boiler.
The Shell. This is the round or cylindrical structure which is commonly described as the boiler, in which are inserted the braces and tubes, and which sustains the internal strain of the pressure of the steam, the action of the water within, and the fire without.
The Drum. This part is sometimes called the dome, and consists of an upper chamber riveted to the top of the boiler for the purpose of affording more steam space.
The Tube Sheets. These are the round, flat flanged sheets forming the two ends of the boiler, into which the tubes are fastened.
The Manhole Cover. This is a plate and frame commonly opening inwards and large enough to admit a man into the interior of the boiler. These openings are sometimes made on the top and sometimes at the end of the boiler. Manhole openings in steam boilers should invariably be located in the head of the boiler, except in rare cases that may arise, when circumstances require it to be placed in the shell. The manhole, so placed, will not materially reduce the strength of the boiler, and from this position it can more readily be seen that the boiler is kept in proper condition. The proper sizes for manholes are 9×5 and 10×16, according to circumstances. These are amply large for general use and no material advantage is gained by increasing them.
The Hand Hole Plates. These are similar arrangements to the manhole cover, except as to size. They are made large enough to admit the hand into the boilers for the purpose of removing sediment and they are also used for the purpose of inspecting the interior of the boiler. Two are usually put in each boiler, one front and one in the rear.
The Blow Off. This consists of pipes and a cock communicating with the bottom of the boiler for the purpose of blowing off the boiler or of running off the water when the former needs cleaning.
THE TRIPLE DRAUGHT TUBULAR BOILER.—Fig. 34.
This boiler, which is extensively used by the manufacturers of New England, is, as will be seen by the illustration, of the horizontal tubular class, and is essentially different from the well known type only in the arrangement of the tubes. The method secures the passage of the products of combustion through the same shell twice; forward through a part of the tubes, and backwards through the remaining ones. The manner of accomplishing this result can be best described by explaining how a common tubular boiler may be remodelled so as to carry out this principle.
Fig. 35.
A cylindrical shell, as shown in Fig. 34—of sufficient size to encircle about one-half of the tubes, is attached to the outside of the rear head below the water line, and extended backward to the back end of the setting. The encircled tubes are lengthened and carried backward to the same point; the extension is closed in and made to communicate with the boiler proper; the inner tubes emerge to the flue leading to the chimney and the old connection from the smoke arch is cut off. With this arrangement, the outer tubes of the boiler—a cluster on each side of the supplementary shell carry the products of combustion forward to the front smoke arch, and the inner tubes carry them backward to the chimney.
Fig. 35 exhibits the boiler in half section and shows the course of the heat products through one of the outer tubes and returning through the boiler by one of the inner cluster.
Fig. 36 (page 84) shows the boiler sectionally, over the bridge wall; the shaded tube ends exhibit the cluster which return the heat products to the rear of the boiler, after being brought forward by the two outer clusters which are left unshaded.
This arrangement of the tubes gives several advantages:
1. It enables an exceedingly high furnace temperature, without loss at the chimney.
2. By dividing the heat into these currents a more equal expansion and contraction is secured. This is an important point secured.
3. In this system the tubes are almost equally operative.
4. The extra body of water immediately over the furnace is both an element of safety and a reservoir of power.
5. The outlet for the waste products of combustion is found in this style of boiler in a more convenient position at the rear end of the boiler.
6. The boiler being self-contained, can be used in places where height of story is limited.
Fig. 36.
For one Horizontal Tubular Boiler 72 inches diameter 18 feet long for…………………of………
Type.
The boiler to be of the Horizontal Tubular type with all castings and mountings complete.
Dimensions.
Boiler 72 inches diameter and 18 feet long. Each boiler to contain 90 best lap welded tubes 31⁄2 inches diameter by 18 feet long, set in vertical and horizontal rows with a space between them vertically and horizontally of no less than one inch and one-quarter (11⁄4) except central vertical space, which is to be three inches (3). No tube to be nearer than two and one-half inches (21⁄2) to shell or boiler. Holes through heads to be neatly chamfered off. All tubes to be set by Dudgeon Expander and slightly flared at front end, turned over and beaded down at back end.
Quality and Thickness of Steel Plates.
Shell plates to be 1⁄2-inch thick of homogeneous steel of uniform quality having a tensile strength of not less than 65,000 lbs. Name of maker, brand and tensile strength to be plainly stamped on each plate.
Heads to be of same quality as plates of shell in all particulars 3⁄4-inch thick. Bottom of shell to be of one plate, and no plate to be less than 7 feet wide. Top of shell to be in three plates. All plates planed before rolling, and all joints fullered not caulked.
Flanges.
All flanges to be turned in a neat manner to an internal radius of not less than two inches (2) and to be clear of cracks, checks or flaws.
Riveting.
Boilers to be riveted with 3⁄4-inch rivet throughout. All girth seams to be double riveted. All horizontal seams to be double riveted. Rivet holes to be punched or drilled so as to come fair in construction. No drift pins to be used in construction of the boilers.
Braces.
All braces to be of the crowfoot pattern, one and one eighth (11⁄8) inch diameter and the shortest to be no less than four feet (4) long and of sufficient number for thorough bracing, and to bear uniform tension.
Manholes, Hand Holes and Thimbles.
One manhole in top of each boiler with heavy cast iron frame riveted on middle of centre plate; one manhole near the bottom of each front head; head reinforced with a wrought iron ring two inches (2) square, riveted to heads with flush countersunk rivets two inches (2) pitch and to have all the necessary bolts, plates, guards and gaskets; two six-inch thimbles riveted to top of each boiler, each to have a planed face; one heavy 6-inch flange on bottom of each boiler, 12 inches from back end to centre of flange. There must be two braces, one on each side of manhole in front head; also to have three braces opposite manhole on back head below tubes.
Lugs.
Four (4) lugs riveted on each side of boilers, of good and sufficient size, with six one-inch rivets in each lug.
Castings.
Each boiler to have a complete set of castings consisting of ornamental flush fronts containing tube, fire and ash-pit doors, and provide the best stationary grate bars as may be selected by buyer, with the necessary fixtures, all bearing bars, britching plates, dead plates, binder bars, back cleaning out doors with frames. Anchor bolts and buck stays. The fire door to contain adjustable air opening and to be protected with fire shields. One heavy cast iron arch over each boiler.
Testing.
Boilers to be tested with a water pressure of 200 lbs. per square inch and certificate of such test having been made shall be furnished with boiler. Test of boiler to be under direction of such steam boiler Insurance Company as may be selected by buyer.
Quality and Workmanship.
All boilers to be made in the best workmanlike manner and all material of their respective kinds to be of the best, and in strict accordance with specification.
Fittings and Mountings.
The boiler to be furnished with the following: One four inch heavy mounted safety valve. One six inch flanged globe valve. Two two inch best globe valves. Two two inch check valves. One eight inch dial nickel plated steam gauge. One low water alarm gauge. One set of fire irons for two boilers consisting of hoe, poker, slice bar and shovel.
Drawings.
All drawings furnished for masons in setting the boilers.
Duty of Boiler.
The boiler to develop 120 horse power and to work under a constant pressure varying from 125 to 150 lbs. to the square inch.
All rivets are to be 21⁄2 and 11⁄2 inch pitch. The pitch line of the rivets to be not nearer 11⁄8 inches to the edge of the sheet.
To be 8 lug plates for each boiler not less than 2 feet long, 8 inches wide, and one inch thick.
There shall be six 1 inch anchor rods running front to rear of each boiler, in the brick work.
These boilers and all their fronts, fittings and connections will be subject to the inspection of…………………
Something has been said under another heading of the nature and requisite quality of the materials entering into the structure of the boiler. Too much emphasis cannot be laid upon the necessity for the use of the very best iron and steel that can be manufactured, and the most skillful and thorough workmanship that can be performed in constructing the boiler.
It is becoming the practice, both for land and marine boilers, for boiler plate makers to furnish “test pieces” from each sheet or plate that goes into the construction of a boiler, and a sheet showing the tensile strength of each sheet or plate that enters into its make up.
But irrespective of this practice each plate entering into boiler construction will be found to have one of the following marks, which designate its quality and method of manufacture. The name “Charcoal Iron” is used because in its manufacture wood charcoal is employed instead of mineral fuel.
“Charcoal No. 1 Iron” (C. No. 1) is made entirely of charcoal iron. It has a tenacity of 40,000 pounds per square inch in the direction of the fibre. It is hard, but not very ductile, and should never be used for flanging.
“Charcoal Hammered No. 1 Shell Iron” (C. H. No. 1 S.), although not necessarily hammered, has been worked up before it is rolled into plates. It has a tenacity of 50,000 to 55,000 pounds per square inch in the direction of the fibre. It is rather hard iron, and should not be flanged. It is used for the outside shell of boilers.
“Flange Iron” (C. H. No. 1 F.), is a ductile material which can be flanged in every direction. It has a tenacity of 50,000 to 55,000 pounds per square inch along the fibre.
“Fire Box Iron” (C. H. No. 1 F. B.), is a harder quality, designed especially to withstand the destructive effect of the impinging flame, and is used for boxes and flue-sheets.
The letters in the brackets exhibit the plate stamp.
Cast iron and copper were used in an early day for steam boilers and the former is still extensively used for certain forms of low pressure steam heaters made for various purposes, such as green houses, etc.
In selecting a boiler, the most efficient design will be found to be that in which the greatest amount of shell surface is exposed to direct heat. It is the direct heating surface that does the bulk of the work and every tendency to reduce it, either in the construction or setting of the boiler, should be avoided. The smaller the amount of surface enclosed by or in contact with the setting, the better results will be obtained.
A boiler with a bad circulation is the bane of an engineer’s existence. Proper circulation facilities constitute one of the chief factors in the construction of a successful and economical boiler. In tubular boilers the best practice places the tubes in vertical rows, leaving out what would be the centre row. The circulation is up the sides of the boiler and down the centre. Tubes set zig-zag to break spaces impede the circulation and are not considered productive of the best results.
The surface from which evaporation takes place should be made greater as the steam pressure is reduced, that is to say, as the size of the bubbles of steam become greater. To produce 100 lbs. of steam per hour at atmospheric pressure this surface should not be less than 732 square feet, which may be reduced to 146 square feet for steam at 75 lbs. pressure, and to 73 feet for steam at a pressure of 150 lbs. It is for this reason that triple-expansion engines can be worked with smaller boilers than are required with engines using steam of lower pressure. The amount of steam space to be permitted depends upon the volume of the cylinders and the number of revolutions made per minute. For ordinary engines it may be made a hundred times as great as the average volume of steam generated per second.
A volume of heated water in a boiler performs the same office in furnishing a steady supply of steam as a fly-wheel does to an engine in insuring uniformity of speed; hence the centre space of a boiler should be ample, in order to take advantage of this reserve force.
Steel for boilers is always of the kind known as low steel, or soft steel, and is, properly speaking, ingot iron, all of its characteristics being those of a tenacious, bending, equal grained iron, and quite different from true steels, such as knife blades, cutting tools, etc., are composed of. Steel is rapidly displacing iron in boiler construction, as it has greater strength for the same thickness, than iron; and, except in rare instances, where the nature of the water available for feed renders steel undesirable, iron should not be used for making boilers, careful tests having shown it to be vastly inferior to steel in many important features.
Good boiler steel up to one-half inch in thickness should be capable of being doubled over and hammered down on itself without showing any signs of fracture, and above that thickness it should be capable of being bent around a mandrel of a diameter equal to one and one-half times the thickness of the plate, to an angle of 180 degrees without sign of distress. Such bending pieces should not be less in length than sixteen times the thickness of the plate.
On this test piece the metal should show the following physical qualities:
Tensile strength, 55,000 to 65,000 pounds per square inch.
Elongation, 20 per cent. for plates three-eighths inch thick or less.
Elongation, 22 per cent. for plates from three-eighths to three-fourths inch thick.
Elongation, 25 per cent. for plates over three-fourths inch thick.
The cross sectional area of the test piece should be not less than one-half of one square inch, i.e., if the piece is one-fourth inch thick, its width should be two inches; if it be one-half inch thick, its width should be one inch. But for heavier material the width shall in no case be less than the thickness of the plate.
It has been found that the addition of about three per cent. (3.16 to 3.32) of nickel to ordinary soft steel produces most favorable results; thus it has been shown by Riley that a particular variety of nickel steel presents to the engineer the means of nearly doubling boiler pressures without increasing weight or dimensions.
In a recent experiment made with Bessemer steel rolled into three-fourths inch plates from which a number of test specimens were cut, the elastic limit was respectively 59,000 pounds and 60,000 pounds. The ultimate tensile strength was 100,000 pounds and 102,000 pounds, respectively. The elongation was 151⁄2 per cent. in each specimen, and the reduction of area at fracture was 291⁄2 per cent. and 261⁄2 per cent. respectively. These figures show that the elastic limit and ultimate tensile strength was raised by the nickel alloy to almost double the limits reached in the best grades of boiler plate steel, and the elongation was reduced to a scarcely appreciable extent.
The experiment had for its object, the reproduction, as nearly as possible, of the alloy used in the nickel steel armor plate made at Le Creusot, France, and the result was reported to the Secretary of the Navy at Washington. The new plate showed a percentage of 3.16 nickel, as against 3.32 for the imported plate.
When the materials are of best quality, then there only remains to rivet and stay the boiler. Riveting is of two kinds, single and double. Fig. 37 shows the method of single riveting, and Figs. 38 and 39 show the plan and cross-section of double riveted sheets.
Fig. 37.
Double Riveting consists in making the joints of boiler work with two rows of rivets instead of one—nearly always, horizontal seams are double riveted as well as domes where they join upon the boiler. Usually all girth seams,—those running round the body of the boiler, are single riveted. The size of the rivets is in proportion to the diameter of the boiler, being 5⁄8, 3⁄4 and 7⁄8 as required in the specification.
Rivet holes are made by punching or drilling, according to the material in which they are made. In soft iron and mild steel they may safely be punched, but in metal at all brittle the holes should be drilled.
Fig. 38.
Rivets are driven by hand, by steam riveting machines or by an improved pneumatic machine which holds the sheet together and strikes a succession of light blows to form the head of the rivet while hot. Rivets are made both of iron and steel, and there are certain well-known brands of such excellent quality that they are almost exclusively used in boiler work.
A place where skill is shown in boiler construction is in laying out the rivet holes, with a templet, so that the sheets come exactly together with the holes so nearly opposite that the dreaded drift pin does not have to be used.
In these figures the letters P and p refer to the “pitch of the rivets,” i.e., the part from centre to centre, and the dimensions given at the sides indicate the amount of lap given in inches and tenths of inches—the diameter of the rivet (1″) is also shown, and the turned over portion of the shank of the rivet is shown by dotted lines.
Fig. 39.
No riveted boiler work can be considered fairly proportioned unless the strength of the plate between the rivets is fully equal to the strength of the rivets themselves. A margin (or net distance from outside of holes to edge of plate) equal to the diameter of the drilled hole has been found sufficient.
Rivets should be made of good charcoal iron or of a very soft mild steel, running between 50,000 and 60,000 pounds tensile strength and showing an elongation of not less than ninety per cent. in eight inches, and having the same chemical composition as specified for plates.
A long rivet, holding thick plates together, is rarely tight except immediately under the head. The heads are set and the centre cooled before the hole is properly filled. If it is a very long rivet there is a chance of the contraction fracturing the head of the rivet. In the Forth Bridge, which is made of very heavy plate girders, the rivets, first carefully fitted, were driven tight into the holes, the burr around the holes were removed, and the ends of the rivets heated to a sufficient degree to enable them to be closed over.
A simple mathematical deduction shows that a circle seam has just one-half the strain to carry as a longitudinal seam, under the same pressure and with the same thickness of metal, hence the custom of single riveting the former and double riveting the latter, or longwise seams.
Different Modes of Riveting.
In fig. 41 may be seen an example of zig-zag riveting.
Fig. 41.
Caulking.—By this is meant the closing of the edges of the seams of boilers or plates. In preparing the seams for caulking, the edges are first planed true inside and outside; and after the plates have been riveted together, the edges are caulked or closed by a blunt chisel about 1⁄4-inch thick at the edge, which should be struck with a 3 or 4-lb. hammer; sometimes one man doing the work alone and sometimes one holding the chisel and another striking.
Fullering a boiler plate is done by a round-nosed tool, while caulking is executed by a sharper instrument.
The thinnest plate which should be used in a boiler is one-fourth of an inch, on account of the almost impossibility of caulking the seams of thinner plates.
It is a rule well known to all practical boiler makers that the thinner the metal (compatible with due strength) the longer the life of the boiler under its varying stresses and the better the caulking will stand.
Hitherto there has been some prejudice against steel rivets, and while this may have some foundation when iron plates are used, it is certainly baseless when steel plates are concerned. The United States government has clearly demonstrated this. All the ships of the new navy have steel boilers, riveted with steel rivets, and an examination of the character of the material prescribed and the severity of the tests to which it is subjected show that these steel-riveted steel boilers are probably the best boilers ever constructed.
United States Government Requirements for Boiler Rivets.
They are subjected to the most severe hammer tests, such as flattening out cold to a thickness of one-half the diameter, and flattening out hot to a thickness of one-third the diameter. In neither case must they show cracks or flaws.
Kind of Material.—Steel for boiler rivets must be made by either the open-hearth or Clapp-Griffith process, and must not show more than .035 of one per centum of phosphorus nor more than .04 of one per centum of sulphur, and must be of the best quality in other respects.
Each ton of rivets from the same heat or blow shall constitute a lot. Four specimens for tensile tests shall be cut from the bars from which the lot of rivets is made.
Tensile Tests.—The rivets for use in the longitudinal seams of boiler shells shall have from 58,000 to 67,000 pounds tensile strength, with an elongation of not less than 26 per centum; and all others shall have a tensile strength of from 50,000 to 58,000 pounds, with an elongation of not less than 30 per centum in eight (8) inches.
Hammer Test.—From each lot twelve (12) rivets are to be taken at random and submitted to the following tests:
Four (4) rivets to be flattened out cold under the hammer to a thickness of one-half the diameter without showing cracks or flaws.
Four (4) rivets to be flattened out hot under the hammer to a thickness of one-third the diameter without showing cracks or flaws—the heat to be the working heat when driven.
Four (4) rivets to be bent cold into the form of a hook with parallel sides, without showing cracks or flaws.
Surface Inspection.—Rivets must be true to form, free from scale, fins, seams and all other unsightly or injurious defects.
In view of the fact that the government is using many hundred tons of these rivets, shown by the records of the tests to be vastly superior to any iron rivet made, in all the essentials of a good rivet, it would seem that it would benefit the boiler maker, the purchaser of the boiler and also the maker of the rivet by adopting a standard steel rivet to be used in all steel 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.
The absolute stress or strain on a flat surface of a steam boiler, which is carried by the stays, can be easily determined by a simple rule:
Choose 3 stays as A B C in Fig. 46, measure from A to B in inches, and from A to C. Multiply these two numbers together and the result is the number of square inches of surface depending upon one bolt for supporting strength.
Example.
Suppose the stays measure from center to center 5 inches each way with steam at 80 lbs., then
5 × 5 = 25 × 80 = 2,000 lbs. borne by 1 stay.
The pressure on the surface does not include the space occupied by the area of the stay bolt, hence, to be absolutely correct that must be deducted.
Fig. 46.
The flat ends of cylindrical boilers are, especially in marine boilers, stayed to the round portions of triangular plates of iron called gusset stays. These are simply pieces of plate iron secured to the boiler front or back, near the top or bottom, by means of two pieces of angle iron, then carried to the shell plating, and again secured by other pieces of angle bar. This arrangement is shown in Fig. 47.
Fig. 47.
Palm Stays.—These are shown in Fig. 48, and are often used in the same position as a gusset stay; that is, from the back or front end of the boiler to the shell plates; they are sometimes used to stay the curved tops of combustion chambers.
Fig. 48.
The two opposite ends are also stayed together by long bar stays, running the whole length of the boiler, it is dangerous, however, to trust too much to the latter class of stays; for, in consequence of the alternate expansion and contraction which takes place every time the boiler is heated and cooled, they have a tendency to work loose at the joints; and if the portion of the boiler in which they are situated should happen to be hotter than the outside shell, they have a tendency to droop and are then perfectly useless.
Fig. 49.
In addition to palm and gusset stays, there are in use riveted or screwed stays, as shown in Fig. 49.
This would not answer in furnaces, owing to the burning off of the heads, hence driven stays are used there.
Fig. 50.
These screwed stays, shown in Fig. 50, are used (in marine and similar boilers) between the combustion chamber back and boiler back and also between the sides of the combustion chambers.
The general plan is to have a large nut and washer inside and outside the boiler with the outside washer considerably larger than the inside, so as to hold more efficiently the back and front ends together.
In marine boilers it is customary to place the stays 15 to 18 inches apart for ease of access to the parts of the boiler, and to make them of 21⁄4 to 21⁄2 inch iron of the best quality.
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.