Document z84KNJm05yzwoBaqvN4y2Yg7

638 CHAPTER 27 1955 Guide used to produce acceptable welds. Welding processes and procedure are described in various publications. Welding application requires the same basic knowledge of design as do the other types of assembly, but, in addition, requires a generous knowl edge of welding principles and particularly the welding qualities of metal, its reaction to extremely high temperatures, and the ability to select and use only, the best quality welding rods. This requirement applies equally to employer and employee, with the employer accepting all of the respon sibility. Thus the employer should select his welding mechanics with good judgment, provide them with first-class equipment and tools, arrange for their training and use of acceptable workmanship standards* and at regular intervals subject their work to prescribed tests: Rules for fusion welding "of pipe joints, the qualification of welding operators, welding procedures, and testing are contained in the Stand ard Manual on Pipe Welding* and in publications of other groups.1' * * In general, the wall thickness and chemical analysis of the pipe are the governing factors, not the working pressure. There are a number of safety codes which govern the installation of welded piping in many cities and states. A complete line of manufactured steel welding fittings is now available, and a dimensional standard has been prepared under the procedure of the American Standards Association, (see Table 10) to unify heretofore diver gent dimensions for the same type welding fittings produced by different manufacturers. Standard dimensions for steel butt-welding elbows, tees, caps, and lapped-joint stub ends are given in Table 10. Dimensions for eccentric and concentric reducers are not .shown in Table 10, but are in cluded in the standard. Steel butt-welding fittings have beveled ends. For wall thicknesses less thanKe in., the .bevel may be either 37H deg. or cut square. For wall thicknesses of Kg in. to and including H in. the bevel is 37K deg. For walls larger than K in. but not greater than 1% in., the bevel is Ushaped with a slope of 20 deg. above the Kg in. radius fillet. For walls greater than lK in., the welding ends are prepared as agreed between the manufacturer and the purchaser. Flange fittings may be welded from sizes H to 24 in. inclusive (see Table 12). Socket-welding fittings also are commercially available. These fittings have a machined recess for inserting the pipe which is attached by a fillet weld between the pipe wall and socket end. Use of socket-welding fit tings generally is restricted to nominal pipe sizes 3 in. and smaller in which range commercial fittings are available. This type of fitting has gained rapid acceptance owing to its ease of installation, low cost, and ability to make a pressure tight joint without weakening the pipe, as is the case with threading. Dimensions for socket-welding fittings, in accordance with ASA Standard B16.11-1946, are given in Table 13. EXPANSION AND FLEXIBILITY Changes in temperature cause a change in dimensions of any matterThe metals that are used for pipe have the same characteristics in that they expand with increasing temperature. If the metal is constrained so Pipe, Fittings, Welding 639 that it cannot expand, then an internal compressive stress is set up ih.tbe material. This stress can be calculated, just as any other stress, .by using Hooke's Law. Where F eEA L <1) A = Force of constraint, pounds. e -- Deformation, inches. E -- Modulus of elasticity,'pounds per square inch. A -- Area of metal, square inches. L = Length of metal, inches. . . This equation is valuable in determining the internal stress, of the pipe mid also the constraining force on the anchors. This equation assumes that Table 19. Thermal Stresses due to Total Constraint TEMPER ATURE* Change, F Dbg ' Steel . Wrought Iron 20 3900 3800 40 7800 7600 60 11700 11400 80 15600 15200 100 19500 19000 For temperatures between 32 and 400 F. Thermal Stresses, psi Cast Iron Brass or Bronx 1428 2856 4284 5712 7140 2760 5520 8280 11040 13800 Copper 2980 5960 8940 11920 14900 Aluminum 2700 5400 8100 1QS00 13500-. the stress is less than the yield strength of the metal. In other words, the metal will return to its original dimension when the temperature-returns to its original level. Materials such as cast "iron are not ductile, hence the ' yield strength is approximately equal to. the ultimate strength. These materials; have ex cellent compressive strength but poor tensile strength. For.;thiS;.reasqn, it is difficult to bend these materials appreciably. Cast.iron,, for example, could be put in tension and, from Table. 19, it is seen that for a temperature change of 100 deg., a compressive stress of 7140 .pin woiild.be developed. If the pipe were straight, this stress would be handled easily. If, however, the stress caused a bending moment, then fracture might occur. The values of stress found in Table 19 were determined by using Equa tion 1 and substituting Equation 2 for e Where e = aLAt (2) a = coefficient of linear expansion, inches per inch. At = temperature change, Fahrenheit degrees. This gives Equation 3 F -- aEAAt (3) Values for a and E are given in Table 20 for various pipe materials, to