Thickness in 16ths of an Inch. 12345 96 102 108 114 120 7.7 7.3 16.2 14.6 13.3 12.2 11.2 10.4 9.7 9.1 8.6 8.1 32.4 29.2 26.5 48.6 43.7 39.8 64.8 58.3 53.0 81.0 72.9 66.3 97.2 87.5 79.5 113.4 102.1 92.8 24.3 8 129.6 116.7 106.1 9 22.4 20.8 19.4 18.2 17.2 16.2 15 4 14.6 33.7 31.3 29.2 27.3 25.7 24.3 23.0 21.9 44.9 41.7 38.9 36.5 34.3 32.4 30.7 29.2 56.1 52.1 48.6 45.6 42.9 40.5 38.4 36.5 67.3 62.5 58.3 54.7 51.5 48.6 46.1 43.8 78.5 72.9 68.1 63.8 60.0 56.7 53.7 51.0 97.2 89.7 83.3 77.8 72.9 68.6 64.8 61.4 58.3 145.8 131.2 119.3 109.4 101.0 93.8 87.5 82.0 77.2 72.9 69.1 65.6 10 162.0 145.8 132.6 121.5 112.2 104.2 97.2 91.1 85.8 81.0 76.8 72.9 11 178.2 160.4 145.8 133.7 123.4 114.6 106.9 100.3 94.4 89.1 84.4 80.2 12 194.4 175.0 159.1 145.8 134.6 125.0 116.7 109.4 102.9 97.2 92.1 87.5 13 210.7 189.6 172.4 158.0 145.8135.4 126.4 118.5 111.5 105.3 99.8 94.8 14 226.9 204.2 185.6 170.1 157.1 145.8 136.1127.6 120.1 113.4 107.5 102.1 15 243.1 218.7 198.9 182.3 168.3 156.3 145.8 136.7 128.7 121.5 115.1 109.4 16 259.3 233.3 212.1 194.4 179.5 166.7 155.6 145.8 137.3 129.6122.8 116.7 Rules governing Inspection of Boilers in Philadelphia. In estimating the strength of the longitudinal seams in the cylindrical shells of boilers the inspector shall apply two formulæ, A and B : A, B, { Pitch of rivets - diameter of holes punched to receive the rivets pitch of rivets = percentage of strength of the sheet at the seam. Area of hole filled by rivet X No. of rows of rivets in seam shearing strength of rivet = pitch of rivets thickness of sheet X tensile strength of sheet percentage of strength of the rivets in the seam. Take the lowest of the percentages as found by formulæ A and B and apply that percentage as the "strength of the seam" in the following formula C, which determines the strength of the longitudinal seams: C, Thickness of sheet in parts of inch X strength of seam as obtained by formula A or B x ultimate strength of iron stamped on plates internal radius of boiler in inches X 5 as a factor of safety = safe working pressure. Diameter of boiler, in... Safe Working Pressure with Longitudinal Seams 11/16 $4 13/16 21% 13/16 % 15/16 214 62 .60 7/16 .58 Diameter of boiler, in... Safe Working Pressure with Longitudinal Seams, 5/16 7/16 1/2 Double-riveted. Flues and Tubes for Steam-boilers.-(From Rules of U. S. Supervising Inspectors. Steam-pressures per square inch allowable on riveted and lap-welded flues made in sections. Extract from table in Rules of U. S. Supervising Inspectors.) T least thickness of material allowable, D= greatest diameter in inches, P = allowable pressure. For thickness greater than T with same diameter P is increased in the ratio of the thickness. 16 D = in. 7 8 9 10 11 12 13 14 15 17 18 19 20 21 22 23 T= in. .18 .20 .21 .21 .22 .22 .23 .24 .25 .26 .27 .28 .29 30 .31 .32 .33 P= lbs. 189 184 179 174 172 158 152 147 143 139 136 134 131 129 126 125 122 D = in. 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 T= in. .34 .35 .36 .37 .38 .39 .40 .41 .42 .43 .44 .45 .46 .47 .48 .49 .50 P= lbs. 121 120 119 117 116 115 115 114 112 112 110 110 109 109 108 108 107 For diameters not over 10 inches the greatest length of section allowable is 5 feet; for diameters 10 to 23 inches, 3 feet; for diameters 23 to 40 inches, 30 inches. If lengths of sections are greater than these lengths, the allowable pressure is reduced proportionately. The U. S. rule for corrugated flues, as amended in 1894, is as follows: Rule II, Section 14. The strength of all corrugated flues, when used for furnaces or steam chimneys (corrugation not less than 11⁄2 inches deep and not exceeding 8 inches from centres of corrugation), and provided that the plain parts at the ends do not exceed 6 inches in length, and the plates are not less than 5/16 inch thick, when new, corrugated, and practically true circles, to be calculated from the following formula: Tthickness, in inches; D= mean diameter in inches. Ribbed Flues.-The same formula is given for ribbed flues, with rib projections not less than 13% inches deep and not more than 9 inches apart. Flat Stayed Surfaces in Steam-boilers.-Rule II., Section 6, of the rules of the U. S. Supervising Inspectors provides as follows: No braces or stays hereafter employed in the construction of boilers shall be allowed a greater strain than 6000 lbs. per square inch of section. Clark, in his treatise on the Steam-engine, also in his Pocket-book, gives the following formula: p = 407tsd, in which p is the internal pressure in pounds per square inch that will strain the plates to their elastic limit, t is the thickness of the plate in inches, d is the distance between two rows of stay-bolts in the clear, and s is the tensile stress in the plate in tons of 2240 lbs. per square inch, at the elastic limit. Substituting values of s for iron, steel, and copper, 12, 14, and 8 tons respectively, we have the following: FORMULE FOR ULTIMATE ELASTIC STRENGTH OF FLAT STAYED SURFACES. For Diameter of the Stay-bolts, Clark gives d' = .0024, in which d' diameter of screwed bolt at bottom of thread, P = longitudinal and P' transverse pitch of stay-bolts between centres, p = internal pressure in lbs. per sq. in. that will strain the plate to its elastic limit, s = elastic strength of the stay-bolts in lbs. per sq. in. Taking s = 12, 14, and & tons, respectively for iron, steel, and copper, we have In using these formulæ a large factor of safety should be taken to allow for reduction of size by corrosion. Thurston's Manual of Steam-boilers, p. 144, recommends that the factor be as large as 15 or 20. The Hartford Steam Boiler Insp. & Ins. Co. recommends not less than 10. Strength of Stays.-A. F. Yarrow (Engr., March 20, 1891) gives the following results of experiments to ascertain the strength of water-space stays: The above are taken as a fair average of numerous tests. Stay-bolts in Curved Surfaces, as in Water-legs of Vertical Boilers.-The rules of the U. S. Supervising Inspectors provide as follows: All vertical boiler-furnaces constructed of wrought iron or steel plates, and having a diameter of over 42 in, or a height of over 40 in. shall be stayed with bolts as provided by § 6 of Rule II, for flat surfaces; and the thickness of material required for the shells of such furnaces shall be de termined by the distance between the centres of the stay-bolts in the furnace and not in the shell of the boiler; and the steam-pressure allowable shall be determined by the distance from centre of stay-bolts in the furnace and the diameter of such stay-bolts at the bottom of the thread. The Hartford Steam-boiler Insp. & Ins. Co. approves the above rule (The Locomotive, March, 1892) as far as it states that curved surfaces are to be computed the same as flat ones, but prefers Clark's formulæ for flat stayed surfaces to the rules of the U. S. Supervising Inspectors. Fusible-plugs.-Fusible-plugs should be put in that portion of the heating-surface which first becomes exposed from lack of water. The rules of the U. S. Supervising Inspectors specify Banca tin for the purpose. Its melting-point is about 445° F. The rule says: All steamers shall have inserted in their boilers plugs of Banca tin, at least in. in diameter at the smallest end of the internal opening, in the following manner, to wit: Cylinder-boilers with flues shall have one plug inserted in one flue of each boiler; and also one plug inserted in the shell of each boiler from the inside, immediately before the fire line and not less than 4 ft. from the forward end of the boiler. All fire-box boilers shall have one plug inserted in the crown of the back connection, or in the highest fire-surface of the boiler. All upright tubular boilers used for marine purposes shail have a fusible plug inserted in one of the tubes at a point at least 2 in. below the lower gauge-cock, and said plug may be placed in the upper head sheet when deemed advisable by the local inspectors. Steam-domes.-Steam domes or drums were formerly almost universally used on horizontal boilers, but their use is now generally discontinued, as they are considered a useless appendage to a steam-boiler, and unless properly designed and constructed are an element of weakness. Height of Furnace.-Recent practice in the United States makes the height of furnace much greater than it was formerly. With large sizes of anthracite there is no serious objection to having the furnace as low as 12 to 18 in., measured from the surface of the grate to the nearest portion of the heating surface of the boiler, but with coal containing much volatile matter and moisture a much greater distance is desirable. With very volatile coals the distance may be as great as 4 or 5 ft. Rankine (S. E., p. 457) says: The clear height of the "crown "or roof of the furnace above the gratebars is seldom less than about 18 in., and often considerably more. In the fire-boxes of locomotives it is on an average about 4 ft. The height of 18 in. is suitable where the crown of the furnace is a brick arch. Where the crown of the furnace, on the other hand, forms part of the heating-surface of the boiler, a greater height is desirable in every case in which it can be obtained; for the temperature of the boiler-plates, being much lower than that of the flame, tends to check the combustion of the inflammable gases which rise from the fuel. As a general principle a high furnace is favorable to complete combustion. IMPROVED METHODS OF FEEDING COAL, Mechanical Stokers. (William R. Roney, Trans. A. S. M. E., vol. xii.)-Mechanical stokers have been used in England to a limited extent since 1785. In that year one was patented by James Watt. It was a simple device to push the coal, after it was coked at the front end of the grate, back towards the bridge. It was worked intermittently by levers, and was designed primarily to prevent smoke from bituminous coal. (See D. K. Clark's Treatise on the Steam-engine.) After the year 1840 many styles of mechanical stokers were patented in England, but nearly all were variations and modifications of the two forms of stokers patented by John Jukes in 1841, and by E. Henderson in 1843. The Jukes stoker consisted of longitudinal fire-bars, connected by links, so as to form an endless chain, similar to the familiar treadmill horse-power. The small coal was delivered from a hopper on the front of the boiler, on to the grate, which slowly moving from front to rear, gradually advanced the fuel into the furnace and discharged the ash and clinker at the back. The Henderson stoker consists primarily of two horizontal fans revolving on vertical spindles, which scatter the coal over the fire. Numerous faults in mechanical construction and in operation have limited the use of these and other mechanical stokers. The first American stoker was the Murphy stoker, brought out in 1878. It consists of two coal magazines placed in the side walls of the boiler furnace, and extending back from the boiler front 6 or 7 feet. In the bottom of these magazines are rectangular iron boxes, which are moved from side to side by means of a rack and pinion, and serve to push the coal upon the grates, which incline at an angle of about 35° from the inner edge of the coal magazines, forming a V-shaped receptacle for the burning coal. The grates are composed of narrow parallel bars, so arranged that each alternate bar lifts about an inch at the lower end, while at the bottom of the V, and filling the space between the ends of the grate-bars, is placed a cast-iron toothed bar, arranged to be turned by a crank. The purpose of this bar is to grind the clinker coming in contact with it. Over this V-shaped receptacle is sprung a fire-brick arch. In the Roney mechanical stoker the fuel to be burned is dumped into a hopper on the boiler front. Set in the lower part of the hopper is a "pusher" to which is attached the "feed-plate" forming the bottom of the hopper. The "pusher," by a vibratory motion, carrying with it the feed-plate," gradually forces the fuel over the "dead-plate" and on the grate. The grate-bars, in their normal condition form a series of steps, to the top step of which coal is fed from the "dead-plate." Each bar rests in a concave seat in the bearer, and is capable of a rocking motion through an adjustable angle. All the grate-bars are coupled together by a "rocker-bar." A variable back-and-forth motion being given to the "rocker-bar," through a con necting-rod, the grate-bars rock in unison, now forming a series of steps, and now approximating to an inclined plane, with the grates partly overlapping, like shingles on a roof. When the grate-bars rock forward the fire will tend to work down in a body. But before the coal can move too far the bars rock back to the stepped position, checking the downward motion, breaking up the cake over the whole surface, and admitting a free volume of air through the fire. The rocking motion is slow, being from 7 to 10 strokes per minute, according to the kind of coal. This alternate starting and checking motion is continuous, and finally lands the cinder and ash on the dumping-grate below. Mr. Roney gives the following record of six tests to determine the comparative economy of the Roney mechanical stoker and hand-firing on return tubular boilers, 60 inches x 20 feet, burning Cumberland coal with natural draught. Rating of boiler at 12.5 square feet, 105 H. P. Three tests, hand-firing. Three tests, Stoker. Evaporation per pound, dry 10.36 10.44 11.00 coal from and at 212° lbs H.P. developed above rating, % 5.8 13.5 68 11.89 12.25 12.54 54.6 66.7 84.3 Results of comparative tests like the above should be used with caution in drawing generalizations. It by no means follows from these results that a stoker will always show such comparative excellence, for in this case the results of hand-firing are much below what may be obtained under favorable circumstances from hand-firing with good Cumberland coal. The Hawley Down-draught Furnace.-A foot or more above the ordinary grate there is carried a second grate composed of a series of water tubes, opening at both ends into steel drums or headers, through which water is circulated. The coal is fed on this upper grate, and as it is partially consumed falls through it upon the lower grate, where the combustion is completed in the ordinary manner. The draught through the coal on the upper grate is downward through the coal and the grate. The volatile gases are therefore carried down through the bed of coal, where they are thoroughly heated, and are burned in the space beneath, where they meet the excess of hot air drawn through the fire on the lower grate. In tests in Chicago, from 30 to 45 lbs. of coal were burned per square foot of grate upon this system, with good economical results. (See catalogue of the Hawley Down Draught Furnace Co., Chicago.) Under-feed Stokers.-Results similar to those that may be obtained with downward draught are obtained by feeding the coal at the bottom of the bed, pushing upward the coal already on the bed which has had its volatile matter distilled from it. The volatile matter of the freshly fired coal then has to pass through a body of ignited coke, where it meets a supply of hot air. (See circular of The American Stoker Co., New York, 1898.) SMOKE PREVENTION. A committee of experts was appointed in St. Louis in 1891 to report on the smoke problem. A summary of its report is given in the Iron Age of April 7, 1892. It describes the different means that have been tried to prevent smoke, such as gas-fuel, steam-jets, fire-brick arches and checker-work, hollow walls for preheating air, coking arches or chambers, double combustion furnaces, and automatic stokers. All of these means have been more or less effective in diminishing smoke, their effectiveness depending largely upon the skill with which they are operated; but none is entirely satisfactory. Fuel-gas is objectionable chiefly on account of its expense. The average quality of fuel-gas made from a trial run of several car-loads of Illinois coal, in a well-designed fuel-gas plant, showed a calorific value of 243,391 heat-units per 1000 cubic feet. This is equivalent to 5052.8 heat-units per lb. of coal, whereas by direct calorimeter test an average sample of the coal gave 11,172 heat-units. One lb. of the coal showed a theoretical evaporation of 11.56 lbs. water, while the gas from 1 lb. showed a theoretical evaporation of 5.23 lbs. 48.17 lbs. of coal were required to furnish 1000 cubic feet of the gas. In 39 tests the smoke-preventing furnaces showed only 74% of the capacity of the common furnaces, reduced the work of the boilers 28%, and required about 2% more fuel to do the same work. In one case with steam-jets the fuel consumption was increased 12% for the same work. Prof. O. H. Landreth, in a report to the State Board of Health of Tennessee (Engineering News, June 8, 1893), writes as follows on the subject of smoke prevention: |