If the pipes are carrying steam with minimum loss, then for same r, i, and p, the loss of pressure L for pipes of different diameters varies in versely as the diameters. The general equation for the loss of pressure for the minimal loss from friction and radiation is The loss of pressure for pipes of 1 inch diameter for different absolute terminal pressures when steam is flowing with minimal loss is expressed by the formula L = Cl1r, in which the coefficient C has the following values: In order to find the loss of pressure for any other diameter, divide the loss of pressure in a 1-inch pipe for the given terminal pressure by the given diameter, and the quotient will be the loss of pressure for that diameter. The following is a general summary of the results of Mr. Rudiger's investigation: The flow of steam in a pipe is determined in the same manner as the flow of water, the formula for the flow of steam being modified only by substituting the equivalent loss of pressure, divided by the density of the steam, for the loss of head. The losses in the flow of steam are two in number-the loss due to the friction of flow and that due to radiation from the sides of the pipe. The sum of these is a minimum when the equivalent of the loss due to friction of flow is equal to one fifth of the loss of heat by radiation. For a greater or less loss of pressure-i.e., for a less or greater diameter of pipe -the total loss increases very rapidly. For delivering a given quantity of steam at a given terminal pressure. with minimal total loss, the better the non-conducting material employed, the larger the diameter of the steam-pipe to be used. The most economical loss of pressure for a pipe of given diameter is equal to the most economical loss of pressure in a pipe of 1 inch diameter for same conditions, divided by the diameter of the given pipe in inches. The following table gives the capacity of pipes of different diameters, to deliver steam at different terminal pressures through a pipe one half milə long for loss of pressure of 10 lbs., and a mean value of ƒ 0.0175. Let W denote the number of pounds of steam delivered per hour: Resistance to Flow by Bends, Valves, etc. (From Briggs on Warming Buildings by Steam.)-The resistance at the entrance to a tube when no special bell-mouth is given consists of two parts. The head v2 + 2g v2 is expended in giving the velocity of flow; and the head 0 505 in over 2g coming the resistance of the mouth of the tube. Hence the whole loss of head at the entrance is 1.505 This resistance is equal to the resistance of a straight tube of a length equal to about 60 times its diameter. v2 2g The loss at each sharp right-angled elbow is the same as in flowing through a length of straight tube equal to about 40 times its diameter. For a globe steam stop-valve the resistance is taken to be 11⁄2 times that of the right-angled elbow. Sizes of Steam-pipes for Stationary Engines.-Authorities on the steam-engine generally agree that steam-pipes supplying engines should be of such size that the mean velocity of steam in them does not exceed 6000 feet per minute, in order that the loss of pressure due to friction may not be excessive. The velocity is calculated on the assumption that the cylinder is filled at each stroke. In very long pipes, 100 feet and upward, it is well to make them larger than this rule would give, and to place a large steam receiver on the pipe near the engine, especially when the engine cuts off early in the stroke. An article in Power, May, 1893, on proper area of supply-pipes for engines gives a table showing the practice of leading builders. To facilitate com parison, all the engines have been rated in horse-power at 40 pounds mean effective pressure. The table contains all the varieties of simple engines, from the slide-valve to the Corliss, and it appears that there is no general difference in the sizes of pipe used in the different types. The averages selected from this table are as follows: Diam. of pipe, in.... 2 22 66 3 3 91 96 4 43 5 6 7 8 9 10 100 126 156 225 306 400 506 625 116 143 206 278 366 463 571 121 150 216 294 384 486 600 in. of steam-pipe area. d = diam. of pipe in inches. Av. H.P.of engines.. 25 39 56 77 The factor .1375 in formula (1) is thus derived: Assume that the linear velocity of steam in the pipe should not exceed 6000 feet per minute, then pipe area cyl. area X piston-speed ÷ 6000 (a). Assume that the av, mean effective pressure is 40 lbs. per sq. in., then cyl. area x piston-speed × 40 + 33,000 horse-power (b). Dividing (a) by (b) and cancelling, we have pipe area H.P.=.1375 sq. in. If we use 8000 ft. per min. as the allowable velocity, then the factor.1375 becomes .1031; that is, pipe area H.P. = .1031, or pipe area X 9.7 = horse-power. This, however, gives areas of pipe smaller than are used in the most recent practice. A formula which gives results closely agreeing with practice, as shown in the above table is Horse-power = 6d2, or pipe diameter =, Diam. of pipe, înches.. Vel. 4000.. ................. DIAMETERS OF CYLINDERS CORRESPONDING TO VARIOUS SIZES OF STEAMPIPES BASED ON PISTON-SPEED OF ENGINE OF 600 FT. PER MINUTE, AND ALLOWABLE MEAN VELOCITY OF STEAM IN PIPE OF 4000, 6000, AND 8000 FT. PER MIN. (STEAM ASSUMED TO BE ADMITTED DURING FULL STROKE.) 4 42 5 6 10.3 11.6 12.9 15.5 12.6 14.2 15.8 19. 14.6 16.4 18.3 21.9 80 100 125 180 11 12 18 14 28.4 31.0 33.6 36.1 34.8 37.9 41.1 44.3 40.2 43.8 47.5 51.1 606 718 845 981 66 6000.. 66 8000. ............... 2 216 3 31 Horse-power, approx. 66 6000.. 66 8000.. Horse-power, approx......... Formula. Area of pipe = For piston-speed of 600 ft. per min. and velocity in pipe of 4000, 6000, and 8000 ft. per min. area of pipe respectively .15, .10, and .075 X area of cylinder. Diam. of pipe respectively .3873, .3162, and .2739 x diam. of cylinder. Reciprocals of these figures are 2.582, 3.162, and 3 651. The first line in the above table may be used for proportioning exhaust pipes, in which a velocity not exceeding 4000 ft. per minute is advisable. The last line, approx. H.P. of engine, is based on the velocity of 6000 ft. per min. in the pipe, using the corresponding diameter of piston, and taking H.P. = (diam. of piston in inches)2. Sizes of Steam-pipes for Marine Engines.-In marine-engine practice the steam-pipes are generally not as large es in stationary practice for the same sizes of cylinder. Seaton gives the following rules: Main Steam-pipes should be of such size that the mean velocity of flow does not exceed 8000 ft. per min. In large engines, 1000 to 2000 H.P., cutting off at less than half stroke, the steam-pipe may be designed for a mean velocity of 9000 ft., and 10.000 ft. for still larger engines. In small engines and engines cutting later than half stroke, a velocity of less than 8000 ft. per minute is desirable. Taking 8100 ft. per min. as the mean velocity, S speed of piston in feet per min., and D the diameter of the cyl., Stop and Throttle Valves should have a greater area of passages than the area of the main steam-pipe, on account of the friction through the circuitous passages. The shape of the passages should be designed so as to avoid abrupt changes of direction and of velocity of flow as far as possible. Area of Steam Ports and Passages= Area of piston X speed of piston in ft. per min. (Diam.) X speed 6000 = 7639 Opening of Port to Steam.-To avoid wire-drawing during admission the area of opening to steam should be such that the mean velocity of flow does not exceed 10,000 ft. per min. To avoid excessive clearance the width of port should be as short as possible, the necessary area being obtained by length (measured at right angles to the line of travel of the valve). In practice this length is usually 0.6 to 0.8 of the diameter of the cylinder, but in long-stroke engines it may equal or even exceed the diameter. Exhaust Passages and Pipes.-The area should be such that the mean velocity of the steam should not exceed 6000 ft. per min., and the area should be greater if the length of the exhaust-pipe is comparatively long. The area of passages from cylinders to receivers should be such that the velocity will not exceed 5000 ft. per min. The following table is computed on the basis of a mean velocity of flow of 8000 ft. per min. for the main steam-pipe, 10,000 for opening to steam, and 6000 for exhaust. 4 = area of piston, D its diameter. Bursting-tests of Copper Steam-pipes. (From Report of Chief Engineer Melville, U S. N., for 1892.)-Some tests were made at the New York Navy Yard which show the unreliability of brazed seams in cop. per pipes. Each pipe was 8 in. diameter inside and 3 ft. 15% in. long. Both ends were closed by ribbed heads and the pipe was subjected to a hotwater pressure, the temperature being maintained constant at 871° F. Three of the pipes were made of No. 4 sheet copper ("Stubbs" gauge) and the fourth was made of No. 3 sheet. The following were the results, in lbs. per sq. in., of bursting-pressure: The theoretical bursting-pressure of the pipes was calculated by using the figures obtained in the tests for the strength of copper sheet with a brazed joint at 350° F. Pipes 1 and 2 are considered as having been annealed. The tests of specimens cut from the ruptured pipes show the injurious action of heat upon copper sheets; and that, while a white heat does not change the character of the metal, a heat of only slightly greater degree causes it to lose the fibrous nature that it has acquired in rolling, and a serious reduction in its tensile strength and ductility results. All the brazing was done by expert workmen, and their failure to make a pipe-joint without burning the metal at some point makes it probable that, with copper of this or greater thickness, it is seldom accomplished. That it is possible to make a joint without thus injuring the metal was proven in the cases of many of the specimens, both of those cut from the pipes and those made separately, which broke with a fibrous fracture. Rule for Thickness of Copper Steam-pipes. (U. S. Supervising Inspectors of Steam Vessels.)-Multiply the working steam-pressure in lbs. per sq. in. allowed the boiler by the diameter of the pipe in inches, then divide the product by the constant whole number 8000, and add .0625 to the quotient; the sum will give the thickness of material required. EXAMPLE.-Let 175 lbs. working steam-pressure per sq. in. allowed the 175 X 5 boiler, 5 in. = diameter of the pipe; then +.0625.1718+ inch, thickness required. 8000 Reinforcing Steam-pipes. (Eng., Aug. 11, 1893.)-In the Italian Navy copper pipes above 8 in. diam. are reinforced by wrapping them with a close spiral of copper or Delta-metal wire. Two or three independent spirals are used for safety in case one wire breaks. They are wound at a tension of about 11⁄2 tons per sq. in. Wire-wound Steam-pipes.-The system instituted by the British Admiralty of winding all steam-pipes over 8 in. in diameter with 3/16-in. copper wire, thereby about doubling the bursting-pressure, has within recent years been adopted on many merchant steamers using high-pressure steam, says the London Engineer. The results of some of the Admiralty tests showed that a wire pipe stood just about the pressure it ought to have stood when unwired, had the copper not been injured in the brazing. Riveted Steel Steam-pipes have recently been used for high pressures. See paper on A Method of Manufacture of Large Steam-pipes, by Chas. H. Manning, Trans. A. S. M. E., vol. xv. Valves in Steam-pipes.-Should a globe-valve on a steam-pipe have the steam-pressure on top or underneath the valve is a disputed question. With the steam-pressure on top, the stuffing-box around the valve-stem cannot be repacked without shutting off steam from the whole line of pipe; on the other hand, if the steam-pressure is on the bottom of the valve it all has to be sustained by the screw-thread on the valve-stem, and there is danger of stripping the thread. A correspondent of the American Machinist, 1892, says that it is a very uncommon thing in the ordinary globe-valve to have the thread give out, but by water-hammer and merciless screwing the seat will be crushed down quite frequently. Therefore with plants where only one boiler is used he advises placing the valve with the boiler-pressure underneath it. On plants where several boilers are connected to one main steam-pipe he would reverse the position of the valve, then when one of the valves needs repacking the valve can be closed and the pressure in the boiler whose pipe it controls can be reduced to atmospheric by lifting the safety-valve. The repacking can then be done without interfering with the operation of the other boilers of the plant. He proposes also the following other rules for locating valves: Place valves with the stems horizontal to avoid the formation of a water-pocket. Never put the junction-valve close to the boiler if the main pipe is above the boiler, but put it on the highest point of the junction-pipe. If the other plan is followed, the pipe fills with water whenever this boiler is stopped and the others are running, and breakage of the pipe may cause serious re sults. Never let a junction-pipe run into the bottom of the main pipe, but into the side or top. Always use an angle-valve where convenient, as there is more room in them. Never use a gate valve under high pressure unless a by-pass is used with it. Never open a blow-off valve on a boiler a little and then shut it; it is sure to catch the sediment and ruin the valve; throw it well open before closing. Never use a globe-valve on an indicator-pipe. For water, always use gate or angle valves or stop-cocks to obtain a clear passage. Buy if possible valves with renewable disks. Lastly, never let a man go inside a boiler to work, especially if he is to hammer on it, unless you break the joint between the boiler and the valve and put a plate of steel between the flanges. A Failure of a Brazed Copper Steam-pipe on the British steamer Prodano was investigated by Prof. J. O. Arnold. He found that the brazing was originally sound, but that it had deteriorated by oxidation of the zinc in the brazing alloy by electrolysis, which was due to the presence of fatty acids produced by decomposition of the oil used in the engines. A full account of the investigation is given in The Engineer, April 15, 1898. The "Steam Loop" is a system of piping by which water of condensation in steam-pipes is automatically returned to the boiler. In its simplest form it consists of three pipes, which are called the riser, the hori zontal, and the drop-leg. When the steam-loop is used for returning to the boiler the water of condensation and entrainment from the steam-pipe through which the steam flows to the cylinder of an engine, the riser is gen erally attached to a separator; this riser empties at a suitable height into the horizontal, and from thence the water of condensation is led into the drop-leg, which is connected to the boiler, into which the water of condensa tion is fed as soon as the hydrostatic pressure in drop-leg in connection with the steam-pressure in the pipes is sufficient to overcome the boiler-pressure. The action of the device depends on the following principles: Difference of pressure may be balanced by a water-column; vapors or liquids tend to flow to the point of lowest pressure; rate of flow depends on difference of pressure and mass; decrease of static pressure in a steam-pipe or chamber is proportional to rate of condensation; in a steam-current water will be car. ried or swept along rapidly by friction. (Illustrated in Modern Mechanism, p. 807.) Loss from an Uncovered Steam-pipe. (Bjorling on Pumping. engines.)-The amount of loss by condensation in a steam-pipe carried down a deep mine-shaft has been ascertained by actual practice at the Clay Cross Colliery, near Chesterfield, where there is a pipe 71⁄2 in. internal diam., 1100 ft. long. The loss of steam by condensation was ascertained by direct measurement of the water deposited in a receiver, and was found to be equivalent to about 1 lb. of coal per I.H.P. per hour for every 100 ft. of steam-pipe; but there is no doubt that if the pipes had been in the upcast shaft, and well covered with a good non-conducting material, the loss would have been less. (For Steam-pipe Coverings, see p. 469, ante.) |