Check-nuts e on the ends of the studs prevent the two ends of the pipe from being forced apart by the steam pressure. The nuts e are not intended ordinarily to be in contact with the flange; their distance from the flange is adjusted so that the proper expansion may occur. FIG. 2 In Fig. 2 is shown a corrugated expansion joint, which is sometimes used on large exhaust pipes. It consists of a short section of flanged corrugated pipe, usually copper, which is put in the steam pipe wherever necessary. The elasticity of this section, due to the corrugations, permits expansion and contraction. Expansion Bends.-The best way of allowing for expansion is by using Elbow Double bend Eye bend Retum bend expansion bends, or bent pipes; but the space they occupy often limits their use. The forms of bends more commonly used are shown in Fig. 3, the trade name being given below each bend. Where a bent pipe is used, the radius r of the bend should not be less than six times the diameter of the pipe, for wrought iron or steel; to secure the proper spring in bends used on long lines of piping, the radius should be greater than this. Bends of copper pipe may be of shorter radius, as copper yields more readily than iron or steel. Bends made from iron or steel pipe must be bent while red hot. Iron and steel pipe bends generally have iron flanges fastened on; copper bends either have composition flanges riveted and brazed on, or have steel flanges, the edges of the pipe being turned over. The piping is usually installed so that it is under a slight tension when cold; when filled with steam, the expansion of the pipes removes the tension, and there is no stress on the pipe except that due to the steam pressure. Arrangement of Piping. The pipes and fittings must be so proportioned as to permit of free flow of steam or water. Water pockets should be avoided; and where such pockets are unavoidable, they must be drained to free them from water. By-pass pipes should be arranged around feedwater heaters, economizers, pumps, etc. The system must be so designed as to give perfect freedom for expansion and contraction. Perfect drainage must be provided in order that all water of condensation shall be fully separated from the steam. Reliability is insured by careful design and superior workmanship, combined with the use of high-class materials and fittings and the judicious placing of cut-out and by-pass valves. Drainage is best effected by arranging the piping so that all the water of condensation will flow by gravity toward a point close to the delivery end of the pipe, and then providing a drip pipe at that point. A trap may be placed at the end of the drip pipe for Loop bend FIG. 3 automatic draining. BOILER FITTINGS SAFETY VALVES The safety valve is a device attached to the boiler to prevent the steam pressure from rising above a certain point. When steam is made more rapidly than it is used, its pressure must necessarily rise; and if no means of escape is provided for it, the result must be an explosion. Briefly described, the safety valve consists of a plate, or disk, fitting over a hole in the boiler shell and held to its place by a dead weight, by a weight on a lever, or by a spring. The weight or the spring is so adjusted that when the steam reaches the desired pressure the disk is raised from its seat, and the surplus steam escapes through the opening in the shell. G P B FIG. 1 Weight of Ball for Lever Safety Valve.-A simple diagram of a lever safety valve is shown in Fig. 1. The valve stem and the ball are attached to the lever at C and B, respectively, and the fulcrum is at F. Let d=FB distance from fulcrum to weight, in inches; c=FG=distance from fulcrum to center of gravity of lever, in inches; a=FC distance from fulcrum to center line of valve, in inches; Aarea of orifice beneath bottom of valve, in square inches; W= weight of ball P, in pounds; W1= weight of valve and stem, in pounds; W2 weight of lever, in pounds; p=blow-off pressure, in pounds per square inch. Then, if the position of the ball P on the lever is fixed, the required weight of the ball may be found by the formula W= a(pA-W1)-W2C EXAMPLE.-The area of the orifice is 10 sq. in., the distance from the valve to the fulcrum is 3 in., and the length of the lever is 32 in. The valve and stem weigh 5 lb., the lever weighs 12 lb., and the gauge pressure is 90 lb. What should be the weight W, if placed 2 in. from the end of the lever, assuming the lever to be straight? SOLUTION.-In this case, c=32÷2=16 in., and d=32-2=30 in. Then, substituting in the formula, Position of Ball for Lever Safety Valve. If the ball of a lever safety valve has a known weight and it is desired to find at what distance from the fulcrum it must be placed so as to give a required blow-off pressure, the formula to be used is in which the various letters have the same meanings as before. EXAMPLE. Suppose all the quantities to remain the same as in the solution of the preceding example, except that it is desired that the boiler should blow off at 75. lb. gauge pressure, instead of 90 lb. What will be the distance of the weight from the fulcrum? SOLUTION.-Applying the formula 3X (75X10-5)-12X16 d= 83.1 =24.58 in. Roper's Safety-Valve Rules. Some inspectors of the United States Steamboat Inspection Service prefer to have lever safety-valve problems worked out by the rules that follow, known among American marine engineers as Roper's rules. Let A area of valve, in square inches; D=distance from center line of valve to fulcrum, in inches; L= distance of weight from fulcrum, in inches; P steam pressure, in pounds per square inch; I distance from fulcrum to center of gravity of lever, in inches. Then, the pressure at which the safety-valve will blow off is found by the formula If the distance L is known, the weight W to be hung on the lever is found by the formula APD-(wl+VD) W= L (2) The distance L from the fulcrum to the point at which the weight Wis hung is found by the formula L= APD-(wl+VD) W (3) Area of Safety Valve.-By area of safety valve is meant the area of the opening in the valve seat; that is, the area of the surface of the valve in contact with steam when the valve is closed. The size of the valve relative to the size of the boiler and the working pressure is prescribed by law in many localities, and must be made to conform to the law wherever such law is in existence. In localities having no law governing this matter, the size of the safety valve may be calculated by the accompanying formulas, which are based on practice and recommended by leading authorities. 1.406 w in which G=grate surface, in square feet; p=steam, gauge pressure, in pounds per square inch; A least area of safety valve, in square inches. Location of Safety Valve.-The safety valve should be placed in direct connection with the boiler, so that there can be no possible chance of cutting off the communication between them. A stop-valve placed between the boiler and the safety valve is a very fruitful cause of boiler explosions. Again, the safety valve must be free to act, and to prevent it from corroding fast to its seat, it should be lifted from the seat occasionally. Care must be taken to prevent persons ignorant of the importance of safety valves from raising the blow-off pressure by adding to the weights or increasing the tension of the spring. To this end, the weights of lever safety valves are often locked in position by the boiler inspector. FUSIBLE PLUGS Fusible plugs are devices placed in the crown sheets of furnaces, or in similar places, to obviate danger from overheating through lack of water. The plug often consists of an alloy of tin, lead, and bismuth, which melts at a comparatively low temperature. In many localities, the law requires that fusible plugs shall be attached to all high-pressure boilers. The fusible plugs in common use are shown in section, on the next page. They consist of brass or iron shells threaded on the outside with a standard pipe thread. The plugs have some form of conical filling, the larger end of the filling receiving the steam pressure. The conical form of the filling prevents it from being blown out by the pressure of the steam. Fusible plugs applied from the outside differ from those applied from the inside, as shown. Location of Fusible Plugs. In the absence of local laws, the following rules issued by the Board of Boiler Rules of the State of Massachusetts may be adopted. Fusible plugs must be filled with pure tin, and the least diameter shall not be less than in., except for working pressures over 175 lb., gauge, or when it is necessary to place a fusible plug in a tube, in which cases the least diameter of fusible metal shall not be less than in. The location of fusible plugs shall be as follows: In horizontal return-tubular boilers, in the back head, not less than 2 in. above the upper row of tubes and projecting through the sheet not less than 1 in. In horizontal flue boilers, in the back head, on a line with the highest part of the boiler exposed to the products of combustion, and projecting through the sheet not less than 1 in. In locomotive-type or star water-tube boilers, in the highest part of the crown sheet and projecting through the sheet not less than 1 in. Inside Type Outside Type In vertical fire-tube boilers, in an outside tube, placed not less than onethird the length of the tube above the lower tube-sheet. In vertical submerged-tube boilers, in the upper tube-sheet. In water-tube boilers of the Babcock & Wilcox type, in the upper drum, not less than 6 in. above the bottom of the drum and projecting through the sheet not less than 1 in. In Stirling boilers of standard type, in the front side of the middle drum not less than 6 in. above the bottom of the drum and projecting through the sheet not less than 1 in. In Stirling boilers of the superheated type, in the front drum, not less than 6 in. above the bottom of the drum, and exposed to the products of combustion, projecting through the sheet not less than 1 in. In water-tube boilers of the Heine type, in the front course of the drum, not less than 6 in., from the bottom of the drum, and projecting through the sheet not less than 1 in. In Robb-Mumford boilers of standard type, in the bottom of the steam and water drum, 24 in. from the center of the rear neck, and projecting through the sheet not less than 1 in. In water-tube boilers of the Almy type, in a tube directly exposed to the products of combustion. In vertical boilers of the Climax or Hazleton type, in a tube or center drum, not less than one-half the height of the shell, measuring from the lowest circumferential seam. In Cahall vertical water-tube boilers, in the inner sheet of the top drum, not less than 6 in. above the upper tube sheet. In Scotch marine-type boilers, in the combustion-chamber top, and projecting through the sheet not less than 1 in. In dry-back Scotch-type boilers, in the rear head, not less than 2 in. above the top row of tubes, and projecting through the sheet not less than 1 in. In Economic-type boilers, in the rear head, above the upper row of tubes. In cast-iron sectional heating boilers, in a section over and in direct contact with the products of combustion in the primary combustion chamber. In other types and new designs, fusible plugs shall be placed at the lowest permissible water level, in the direct path of the products of combustion, as near the primary combustion chamber as possible. CONNECTION OF STEAM GAUGE A steam gauge should be connected to the boiler in such a manner that it will neither be injured by heat nor indicate incorrectly the pressure to which it is subjected. To prevent injury from heat, a so-called siphon is placed between the gauge and the boiler. This siphon in a short time becomes filled with water of condensation, which protects the spring of the gauge from the injury the hot steam would cause. Care should be taken not to locate the steam-gauge pipe near the main steam outlet of the boiler, as this may cause the gauge to indicate a lower pressure than really exists. In locating the steam gauge, care must also be taken not to run the connecting pipe in such a manner that the accumulation of water in it will cause an extra pressure to be shown. BLOW-OFFS For the double purpose of emptying the boiler when necessary and of discharging the loose mud and sediment that collect from the feedwater, every boiler is provided with a pipe that enters the boiler at its lowest point. This pipe, which is provided with a valve or a cock, is commonly known as the bottom blow-off. The position of the blow-off pipe varies with the design of the boiler; in ordinary return-tubular boilers, it is usually led from the bottom of the rear end of the shell through the rear wall. Where the boiler is fitted with a mud-drum, the blow-off is attached to the drum. Blow-Off Cocks and Valves.-While in many boiler plants globe valves are used on the blow-off pipe, they have the disadvantage that the valve may be kept from closing properly by a chip of incrustation or similar matter getting between the valve and its seat, with the result that water may leak out of the boiler unnoticed. Plug cocks packed with asbestos are widely used, the asbestos packing obviating the objectionable features of the ordinary plug cock. Gate valves are also used to some extent, but have the same disadvantage as globe valves. In the best modern practice, the blow-off pipe is fitted with two shut-off devices. The one shut-off may be an asbestos-packed cock and the other some form of valve, or both may be cocks or valves, the idea underlying this practice being that leakage past the shut-off nearest the boiler will be arrested by the other. Protection of Blow-Off Pipe.-When exposed to the gases of combustion, the bottom blow-off pipe should always be protected by a sleeve made of pipe, by being bricked in, or by a coil of plaited asbestos packing. If this precaution is neglected, the sediment and mud collecting in the pipe, in which there is no circulation, will rapidly become solid. The blow-off pipe should lead to some convenient place entirely removed from the boiler house and at a lower level than the boiler. Sometimes it may be connected to the nearest sewer. many localities, however, ordinances prohibit this practice; the blow-off is then connected to a cooling tank, whence the water may be discharged into the sewer. FURNACE FITTINGS In Bridge Wall.-The bridge, also termed the bridge wall, is a low wall at the back end of the grate; it forms the rear end of the furnace and causes the flame to come into close contact with the heating surface of the boiler. It is usually built of common brick and faced with firebrick. The passage between the bridge and the boiler shell should not be too small; its area may be approximately one-sixth the area of the grate. The space between the grate and the shell should be ample for complete combustion, and the distance between the grate and the boiler shell may be made about one-half the diameter of the shell. Fixed Grates.-The grate, which is nearly always made of cast iron, furnishes a support for the fuel to be burned and must be provided with spaces a FIG. 1 for the admission of air. The area of the solid portion of the grate is usually made nearly equal to the combined area of the air spaces. The common type of fixed grate is made of single bars a, Fig. 1, placed side by side in the furnace. The thickness of the lugs cast on the sides of the bars determines the width of the open spaces of the grate. It is the general practice to make the thickness across the lugs twice the thickness of the top of the bar. For long furnaces, the bars are generally made in two lengths of about 3 ft. each, with a bearing bar in the middle of the grate. Long grates are generally set with a downward slope toward the bridge wall of about in. per ft. of length. For the larger sizes of anthracite and bituminous coal, the air space may be from to in. wide, and the grate bar may have the same width. For pea and nut coal, the air space may be from to in., and for finely divided fuel, like buckwheat, rice, bird's-eye, culm, and slack, air spaces from to in. may be used. The grate bar shown in Fig. 2, and known as the herring-bone grate FIG. 2 bar, has in many places superseded the ordinary grate bar, because it will usually far outlast a set of ordinary grate bars. Herring-bone grate bars can be obtained in a great variety of styles and with different widths of air spaces. They are usually supported on cross-bars, and, like many other forms of grate |