ASME B31.5-2019 pdf – Refrigeration Piping and Heat Transfer Components.
519.1.1 Objectives. Piping systems shall be designed to have sufficient flexibility to prevent thermal expansion from causing any of the following:
(a) failure of piping or anchors from overstress or over- strain
(b) leakage at joints
(c) detrimental distortion of connected equipment (pumps, turbines, or valves) resulting from excessive thrusts and moments
519.1.2 Expansion Strains. Expansion strains may be taken up in two ways, either primarily by bending or torsion, in which case only the extreme fibers at the critical location are stressed to the limit, or by axial compression and tension, in which case the entire cross-sectional area over the entire length is substantially equally stressed.
(a) Bending or torsional flexibility may be provided by the use of bends, loops, or offsets; or by swivel joints, ball joints, corrugated pipe, or expansion joints of the bellows type permitting angular movement. Suitable anchors, ties, or other devices shall be provided as necessary to resist end forces from fluid pressure, frictional, or other resistance to joint movement and other causes.
(b) Axial flexibility may be provided by expansion joints of the slip-joint or bellows types. Pipe running from anchors to the joints must be guided where necessary to keep the pipe from bowing because of end forces originating in the joint from fluid pressure, friction, and deformation of the bellows. Anchors must be adequate for these forces plus the force arising from friction in the guides. For design and selection of expansion joints of the bellows type, reference to the standards of the Expansion Joint Manufacturers Association is recommended.
519.2 Concepts
Concepts peculiar to piping flexibility analysis and requiring special consideration are explained in the following paragraphs.
519.2.1 Stress Range. As contrasted with stresses from sustained loads (such as internal pressure or weight), stresses caused by thermal expansion in systems stressed primarily in bending and torsion are permitted to attain sufficient initial magnitude to cause local yielding or creep. The attendant relaxation or reduction of stress in the hot condition leads to the creation of a stress reversal when the component returns to the cold condition. This phenomenon is designated as self-springing of the line and is similar in effect to cold springing. The amount of self-springing depends on the initial magnitude of the expansion stress, the material, the temperature, and the elapsed time. While the expansion stress in the hot condition tends to diminish with time, the arithmetic sum of the expansion stresses in the hot and cold conditions during any one cycle remains substantially constant. This sum, referred to as the stress range, is the determining factor in the thermal design of piping.
519.2.3 CoLd Spring. Cold spring is recognized as beneficial in that it serves to balance hot and cold stresses without drawing on the ductility of the material, for which reason it is recommended in particular for materials of relatively low ductility. In addition, it helps assure minimum departure from as-erected hanger settings. Inasmuch as the life of a system under cyclic conditions depends primarily on the stress range rather than the stress level at any one time, no credit for cold spring is given for stress range calculations. In calculating end thrusts and moments where actual reactions at any one time rather than their range are considered significant, cold spring is credited. (See para. 519.4.6.)
519.2.4 LocaL Overstrain. All the commonly used methods of piping flexibility analysis assume elastic behavior of the entire piping system. This assumption is sufficiently accurate for systems where plastic straining occurs at many points or over relatively wide regions, but fails to reflect the actual strain distribution in unbalanced systems where only a small portion of the piping undergoes plastic strain, or where, in piping operating in the creep range, the strain distribution is very uneven. In these cases, the weaker or higher stressed portions will be subjected to strain concentrations due to elastic follow-up of the stiffer or lower stressed portions. Unbalance can be produced
(a) by use of small pipe runs in series with larger or stiffer pipe with the small lines relatively highly stressed
(b) by local reduction in size or cross section, or local use of a weaker material
(c) in a system of uniform size, by use of a line configuration for which the neutral axis (actually, the wrench axis) is situated close to the major portion of the line with only a very small portion projecting away from it absorbing most of the expansion strain.