TABLE 43.-COLLAPSING PRESSURE OF WROUGHT-IRON CYLINDRICAL TUBES IN LBS. PER SQUARE INCH, WHEN NOT MORE THAN THE THICKNESS OF THE PLATE FROM BEING A TRUE CIRCLE. 237 218 203 370 341 317 533 492 457 lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. Table 43 continued.-COLLAPSING PRESSURE OF WROUGHT-IRON TIRES. LENGTH OF TUBE AND THICKNESS OF PLATE. Diameter of TUBE, IN INCHES. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. lbs. 20 20 22 X X X X 2 22 22 22 22 24 24 26 26 26 26 26 28 28 28 28 28 X X X ХХХ 853 683 568 488 427 379 342 310 285 263 244 1080 864 720 617 540 480 432 393 360 332 309 1333 1067 889 762 667 593 534 485 445 410 381 432 346 295 247 216 192 173 158 216 134 124 588 471 393 336 294 262 236 215 197 181 168 768 615 512 440 384 342 308 279 256 237 220 972 778 648 556 486 432 389 354 324 299 278 1200 961 801 686 600 535 481 437 400 369 343 400 320 267 229 200 178 160 145 134 123 115 545 436 363 311 272 245 218 193 182 168 155 711 569 474 407 356 316 285 259 237 219 203 900 720 600 515 450 400 360 328 300 277 257 1111 889 741 635 550 494 445 404 371 342 318 369 295 246 210 185 164 148 134 123 114 105 500 402 335 287 250 223 201 183 168 154 144 656 525 437 374 328 291 263 238 219 201 187 830 664 553 474 415 369 332 302 277 255 237 1025 820 683 586 512 455 410 372 342 315 293 343 274 229 196 172 152 137 124 114 105 98 467 373 311 265 234 207 187 169 156 144 133 607 488 406 348 304 270 244 222 203 188 174 771 617 514 440 386 340 309 280 257 238 220 952 762 635 544 476 423 380 348 318 293 272 NOTE.-Factor of Safety. The working pressure should never exceed one-sixth of the collapsing pressure. This Table shows how weak long cylindrical tubes are, to resist external or collapsing pressure, and the necessity of strengthening the flue-tubes, of Lancashire and Cornish boilers, with strengthening rings of angle-iron, and also by using Adamson's flanged seams at the joint of each belt, or at least of alternate belts of plate, whereby the length of tube is practically reduced to the length between the strengthening rings. The Strength of Corrugated Furnaces with corrugations 1 inches deep, may be found by the following formula, given by Mr. Parker, of Lloyd's. Where T=thickness of plates in sixteenths of an inch, D=greatest 1000 × (T-2) diameter of furnace in inches, D A Safety-Valve should be capable of discharging considerably more steam than the boiler can generate, by the combustion of all the coal that can be burnt upon its fire-grate, to prevent the blowing-off pressure being materially exceeded, and the area should be proportional both to the fire-grate surface and to the pressure of steam. The lower the pressure the larger must the safety-valve be. When steam flows through an orifice with a square edge such as a safety-valve, its flow is considerably reduced, and the weight in lbs. of steam discharged per minute, per square inch of opening, corresponds nearly with three-fourths of the absolute pressure in the boiler, when that pressure is not less than 25 lbs., or 10 lbs. above the atmosphere. The area of opening requisite for the discharge of any given constant weight of steam, is in inverse ratio of the pressure; that is to say, it requires an orifice of three times larger area, to discharge steam of 30 lbs. pressure, than is required to discharge the same weight of steam per minute at 90 lbs. pressure. The opening for the escape of steam, through a conical valve with cone of 45°, is about one-third less than the lift. To find the proper area of a Safety-Valve, multiply the area in square feet of fire-grate surface, by one of the following multipliers, corresponding with the pressure at which the safety-valve is to blow off, and the product will give the area in square inches of that safety-valve; to which must be added the area of the wings of the valve, when the valve is constructed with wings. Pressure as shown by the steam gauge 10 lbs., constant multiplier 1'4 Direct Load upon the Valve.-When the valve is loaded by a weight or spring, placed direct upon the valve, without the intervention of a lever. To find the necessary weight in lbs. to attach, or the amount of tension to put upon the spring, to prevent the valve blowing off before the blowingoff pressure is reached, multiply the area of the valve in square inches by the pressure of steam in lbs. per square inch, and to the product add the weight of the valve. To find the pressure in lbs. per square inch, divide the load in lbs. upon the valve, by the area of the valve in square inches. Safety Valve with Lever.-The centre of gravity of the lever, is the point at which it will balance, when placed upon a knite-edge. In Fig. 145 F is the fulcrum, or joint where the lever is fixed, V is the centre of the vaive, W is the weight. The best angle for the seat of the valve is 45°; the width of mitre should not exceed inch; the lift of the valve should not exceed inch; the distance between the fulcrum and the centre of the valve, should equal the diameter of the valve; the pivot should bear upon the valve considerably below the level of the valve-seat. When a weight is used the total length of lever should equal one-third the diameter of the boiler; when the lever is held down by a spring-balance, the distance between the fulcrum and the centre of the valve should equal the diameter of the valve, and the distance between the fulcrum and the spring-balance, should equal as many times the diameter of the valve, as there are square inches in its area. Safety-Valve Loaded by a Lever and Weight.-When a lever and weight are employed to load a valve, it is necessary to find the resistance due to the weight of the lever and the valve. This may be ascertained by securing the valve to the lever with a piece of wire, and attaching a spring balance directly over the centre of the valve, which will give the load due to the weight of the valve and the action of the lever. This result divided by the area of the valve in square inches, will give the pressure in lbs. per square inch, at which the steam will raise that valve. To calculate the action of the lever when the above method cannot be employed, Approximate Rule: Multiply the weight in lbs. of the lever, by the distance between the fulcrum and the centre of gravity, and divide the product, by the distance between the fulcrum and the centre of the valve; which will give the approximate resistance in lbs. due to the action of the lever, to which result add the weight of the valve and pivot. To find the pressure in lbs. per square inch, at which the valve will begin to blow off : 1. Multiply the weight in lbs. of the ball, by the distance in inches it is placed from the fulcrum. 2. Multiply the weight in lbs. of the lever, by the distance in inches between the centre of gravity and the fulcrum. 3. Multiply the weight in lbs. of the valve, by the distance in inches between the centre of the valve and the fulcrum. 4. Multiply the area of the valve in square inches, by the distance in inches between the centre of the valve and the fulcrum, then add together the first 3 products, and divide the sum by the 4th product. To find the position of the weight on the lever, so that the safetyvalve will blow off at a given pressure : 1. Multiply the weight in lbs. of the lever, by the distance in inches between the centre of gravity and the fulcrum. 2. Multiply the weight in lbs. of the valve, by the distance in inches. between the centre of the valve and the fulcrum. 3. Multiply the area of the valve in square inches, by the pressure of the steam in lbs. per square inch, and multiply the product by the distance in inches between the centre of the valve and the fulcrum; then add together the first two products, and subtract the sum from the 3rd product, and divide the remainder by the weight of the ball in lbs. To find the weight to place on the lever, so that the valve will blow off at a given pressure:-Multiply the area of the valve in square inches, by the required pressure of steam in lbs. per square inch, from which result deduct the weight of the valve and action of the lever in lbs. ; then multiply by the distance from the fulcrum to the centre of the valve in inches, and divide the product by the distance in inches, between the fulcrum and the point of the lever at which the weight is placed. PROPORTIONS OF STEEL SPRINGS. Spiral Springs.-The proportions of spiral springs for safety valves loaded with direct springs, may be determined by the following rules :The internal diameter of the coil, should equal 4 times the thickness of the steel of which the spring is composed. The lift of safety valves for all sizes, may be taken at one-tenth part of an inch. The compression or extension of the spring, to produce the initial load, should be forty times the lift of the valve, or 4 inches for all sizes of valves with the above lift. To find the diameter of round steel, or side of square of square steel, for springs:-* The Author is indebted for the above rules for safety-valve springs, and for some of the information on safety-valves to a report on safety-valves in the Transactions of the Institution of Engineers and Shipbuilders of Scotland. |