Document 6Lp6pgb4bROqJRk7J0ewdG4g

688 CHAPTER 26 1957 Guide facilitate determination of the stress in an actual pipe. The following ex ample illustrates the computation of stress. Example: What is the force set up by a 1 in. schedule 40 steel pipe if the thermal expansion from a 100 deg temperature increase is fully-'Constrained? Answer: From Table 1, determine that for a 1" schedule 40 steel pipe, the metal area = 0.494 sq. inB. Then, from Table 20, dE = 195 psi, and from Equation 3 F = 195 X 0.494 X 100 = 9630 lbs. It is interesting to note that length does not enter into the determina tion of the constraining force. This is so because the stress is a unit length function and so is the expansion; therefore, they cancel each other. In other words, if a weight stretches a 10-foot wire 0.10 in., then that same weight would stretch a 100 foot wire one inch. In both cases the elongation was 1 part in 1200. The realization that thermal stress is independent of length is important, but often overlooked. Nonetheless, Table 20. Physical Properties fob Determining Thermal Stress (Temperatures between SB and 400 F) Steel Wrought Iron ' Cast Iron Brass or BBON8B Copper Aluminum Coefficient of Linear ExModulus of Elasticity, E, 6.5 30 195 6.8 6.95 9.85 28 12 190 71.4 14 138 9.3 13.5 16 10 149 135 Example: The coefficient of expansionfor steel = 0.0000085 in./in. and the modulus of elasticity =* 30,000,000 psi. Therefore aB = 195 psi. that concept explains the feasibility of utilizing the inherent flexibility of the pipe to take care of expansion. Regardless of the length of pipe between anchors, it is possible to take care of the expansion by putting the force of expansion into the pipe as internal stress. If the pipe is not straight, then the force of expansion will cause a bend ing moment. The pipe will then be under a cpmbined stress. The stresses will be longitudinal and transverse, and to a small extent radial. All of these stresses tend to fracture the pipe, and therefore must be added to make up what is called allowable combined stress. The Code for Pressure Piping, ASA B31.1, published by the ASME, sets up very definite limits for the allowable combined stresses for various pipe materials. For tem peratures below 400 F and for piping that is for neither district heating nor power, any standard pipe material may be used. For high temperature or high pressure work, some text on expansion and flexibility should be consulted. It is beyond the scope of this chapter to develop the theory of expansion caused by high temperatures. For the simpler problems encountered for temperatures 400 F and less, which the heating engineer is more apt to encounter, there are four methods of allowing for expansion. The first method is to use packless expansion joints. These joints in Pipe, Fittings, Welding 689 elude types of bellows expansion joints, rubber, corrugated copper and other metals. Rubber type joints are generally used in vacuum or low pressure steam lines. Maximum temperature is generally 180 F. If oil is present-in the line, rubber type joints will be attacked. Travel of these joints is up to 1 inch. The travel to be expected can be calculated by using Equation 2. The second type of expansion joint is called a slip joint. This joint allows for expansion by a sliding of a female member over a male member. The joint is kept tight by means of packing. The packing determines the limit of the temperature to which the joint may be subjected. The main disadvantage to this joint is that it must be continually inspected for a deterioration of the packing. By using double slip joints, the travel may go to several feet, but generally a single joint allows about a foot of ex pansion. It is common practice to use slip joints up to 250 psi. The most common method of allowing for expansion in heating systems is to use the swivel joint. This is the third method. The swivel joint was first used with screwed fittings, but it is also used now with welded fittings. With welded elbows, the swivel introduces torsional stress in the elbow and in the swing piece. This type of joint is adequate for taking up the expansion in a header, and preventing fracture of the riser or heating ele ment. The fourth method is to allow the flexibility of the pipe to absorb the stress of expansion. A technique used to reduce the stress in the ex panded pipe is to cold spring it before hooking it up. Cold springing can be used to absorb about one-half the stress. The technique is to install the pipe with an opposite stress. For example, if the pipe were to have a com pression of 2000 psi', then the pipe would'be installed with 1000 psi of ten sion. For refrigeration, the pipe would be cold sprung in compression. For conditions not requiring rigorous analysis, it is permissible to use expansion bends designed by means of Equation 4 vihere L = 6.16a/>A (4) L = length of pipe, feet. = O. D. of pipe, inches. A * deformation, inches (see Equation 2) fiber stress g 16,000 pounds per square inch. Equation 4 can be used for bends with two fittings, regular U-bend, and onset U-bend, as shown in Fig. 3. It gives the length of pipe, L, that