Document DMbzd08J2QOyexBDJdyG3zz3O

608 CHAPTER 27 1953 Guide ,,X tubing that has been deoxidized is used. This tubing is available , in a variety of sizes and wall thicknesses. : EXPANSION AND FLEXIBILITY The increase in temperature of a pipe from room temperature to an oper ating steam or water temperature 100 deg or more above room tempera ture, results in an increase in length of the pipe for which provision must be made. The amount of linear expansion (or contraction in the case of refrigeration lines) per unit length of material per degree change in tem perature is termed the coefficient of linear expansion, or commonly, the coefficient of expansion. This coefficient varies with the material. The linear expansion of cast-iron, steel, wrought-iron, and copper pipe, the materials most frequently used in heating and ventilating work, canbe determined from Table 4. The three methods by which the elongation due to thermal expansion may be taken care of are: (1) expansion joints; (2) swivel, joints; (3) in- Fig. 1. Measurement op L on Various Pipe Bends herent flexibility of the pipe itself utilized through pipe bends, right-angle turns, or offsets in the fine. Expansion joints of the slip-sleeve, diaphragm, or corrugated types made of copper, rubber, or other gasket material are all used for taking up ex pansion, but generally only for low pressures or where the inherent flexi bility of the pipe cannot readily be used as in underground steam of hot water distribution lines. Swivel joints are used to some extent in low-pressure steam and hotwater heating systems, and in hot-water supply fines. Since swivel joints permit the expansive movement of the pipe by turning of threaded joints, which may ultimately result in a leak, it is preferable to provide sufficient flexibility without resorting to swiveling in the threads. Probably the most economical method of providing for expansion of piping in a long run is to take advantage of the directional, changes which must necessarily occur in the piping, and proportion the offsets so that sufficient flexibility is secured. Ninety-degree bends with long, straight tangents in either a horizontal or a vertical plane are an excellent means for securing adequate flexibility with larger sizes of pipe. When flexi bility cannot be obtained in this manner, it is necessary to make use of some type of expansion bend. The exact calculation of the size of ex pansion bends required to take up a given amount of thermal expansion is relatively complicated.1 The following approximate method; however, has been found to give reasonably good results and is deemed to be suf ficiently accurate for most heating installations. Fig. 1 shows several types of expansion bends commonly used for taking Pipe, Fittings, Welding 609 up thermal expansion. The amount of pipe, L, required in each of these bends may be computed from Equation 1. L = 6.16 -s/oa (1) where h - length of pipe, feet. D = outside diameter of the pipe used, inches. ' a = the amount of expansion to be taken up, inches. This formula, based on the use of mild-steel pipe with wall thicknesses 'not heavier than extra-strong, assumes a maximum:safe value of fiber stress of 16,000 psi. When square type bends are used, the. width of the bend should not exceed about twice the height, since for a given total length of pipe in the bend, the height of the bend becomes progressively, less with increase in width until the height approaches zero and no flexi bility exists. Actually, wide bends utilize to best advantage the inherent flexibility of the line, but such bends cannot be proportioned on the basis of Equation 1. For such applications, more accurate methods1 should be . employed. It is further assumed that the corners are made with screwed or flanged elbows or with arcs of circles having radii, five to six times the pipe diameter. Use of welding elbows with radii of times the pipe diameter will decrease the end thrusts somewhat, but will raise the fiber stress correspondingly. All risers must be anchored and safeguarded so that the difference in length when hot, from the length when cold, shall not disarrange the normal and orderly provisions for drainage of the branches. ; Proper anchoring of piping is especially necessary with light-weight radiators, to allow for freedom of expansion in order that no pipe strain will distort the radiators. .When expansion strains from the pipes are permitted to reach these light metal, heaters, they usually emit disturbing sounds. HANGERS AND SUPPORTS Heating system piping requires careful and substantial support. Where changes in temperature of the line are not large, such simple methods of support may be utilized as hanging the fine by means of rods or perforated strip from the building structure, or supporting it by brackets or on piers. When fluids are conveyed at temperatures of 150 F or. above, however, hangers or supporting equipment must be fabricated and assembled to permit free expansion or contraction of the piping. This can be accom plished by the use of long rod hangers, spring hangers, chains, hangers or supports fitted with rollers, machined blocks, elliptical or circular rings of larger diameter than the pipe giving contact only at the bottom, or trolley hangers. In all cases, allowance should be made for rod clearance to permit swinging without setting up severe bending action in the rods. For pipes of small size, perforated metal strip is often used. ' For hori zontal mains, the rod or strip usually is attached to the joists or steel work of the floor above. For long runs of vertical pipe subject to considerable thermal expansion, either the hangers should be designed to prevent ex cessive load on the bottom support due to expansion, or the bottom support should be designed to withstand the entire load. THREADING PRACTICE In all threaded pipe for heating and ventilating installations the American Standard taper pipe thread, ASA B2.1-1942 is used. This thread is cut