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516 CHARTER 27 ,1948 Guide temperature 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.'arid copper pipe, the materials,most freq.uently used in heating and ventilating work, can be 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) Inherent flexibility-of the pipe itself utilized through pipe bends, right-angle'furris;7pr offsets ih the line. Expansion joints of the slip-sleeve, diaphragm, or corrugated .types made of copper, rubber, or other gasket material are all used for taking up expansion, but generally only for .low pressures or where the inherent' flexibility of the pipe cannot readily be used "ah in underground steam or hot water distribution lines.... ...... ,... Swivel joints are used .to some .extent in..low-pressute steam and hot- water heating systems, and in hot-water.supply lines. . Since swivel joints;' : permit the expansive movement of.the pipebyturningof threaded joiiitsij; i. which may ultimately result in,a leak, it is preferable"to. provide sufficient .. flexibility without resorting, tip swiveling, in the threads, j . . -. 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 complicated1,.. The following-approximate, method,, however, has been found' to give reasonably good results and is: deemed to.be sufficiently accurate for most heating.installations. . . Fig. 1 shows several types of expansion bends commonly-used for taking up thermal expansion. The .amount of. pipe, L, required in each of these bends may be-computed from Equation 1, l = 6.16 Ltd! where L = length of pipe, feet. D = outside diameter of-the pipe used, inches-' : A = the amount of expansion to be taken'iip, inches. d) 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 Ripe,'Fittings, Welding 517 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 flexibility exists.- Actually, wide, bends utilize to best advantage the .inherent flexibility of the line, but such bends cannot be proportioned on die basis of Equation 1-: For such' applications, more accurate methods1 should be employed. It is further assumed that the comers 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 1J4 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 allowvfor freedom .of. expansion in order that no pipe strain will distort they radiators^ " When \hxpansion-strains;:from the pipes are permitted to reach these light, metal heaters, they usually emit disturb ing sounds. IIAMJERS AND SUPPORTS y ... Heatmg systempipi.ngrequires cai^S'^d:lSffhstaif|^'%u^pof^Where changes in temperature of the line are not large, such simple methods of support may.be utilizedas hanging the line by means of rods or. perforated strip ffbm the'buildirig 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 cqntractibnf 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 horizontal mains, the rod orstrip usually is attached to the joists or steel Work-of-the floor above. - -For long runs of vertical ;pipe subject to con siderable thermal-expansion, either the hangers should-be designed to prevent-excessive 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, A.S.A. B2.1-1942 is used.. This thread-is cut with a;taper of 1 in 16 measured on the diameter of the pipe so as to secure a tight joint. The number of threads per inch varies with the pipe size. Threads for fittings are the-same, except that it is regular practice to furnish straight tapped couplings for Schedule 40 pipe`2 in. and smaller. For steam pressures in excess of 25 psi, it is recommended that taper-tapped couplings be used to obtain a tight joint. These may be secured by ordering line pipe* which is used for oil piping, the couplings of which are provided with taper-tapped.threads and may be used with regular mill-threaded standard weight pipe. .Thread lengths should be in accordance with A.S.A. B2.1. Right-hand threads are used unless