Displacement stress

To assure that a system meets these requirements, the computed displacement-stress range Se shall not exceed the allowable stress range [Eqs. (10-93) and (10-94)], the reaction forces [Eq. (10-105)] shall not be detrimental to supports or connected equipment, and movement of the piping shall be within any prescribed hmits. 

Displacement Stresses Stresses may be cousideredproportioual to the total displacement strain only if the strains are well distributed and not excessive at any point. The methods outhued here and in the code are apphcable only to such a system. Poor distribution of strains (unbalanced systems) may result from ... 

As contrasted with stress from sustained loads such as internal pressure or weight, displacement stresses may be permitted to cause hm-ited overstrain in various portions of a piping system. When the system is operated initially at its greatest displacement condition, any yielding reduces stress. When the system is returned to its origin condition, there occurs a redistribution of stresses which is referred to as self-springing. It is similar to cold springing in its effects. 

The allowable stress range for displacement stresses and permissible additive stresses shall be as specified in Eqs. (10-93) and (10-94) for systems primarily stressed in bending and/or torsion. For pipe or piping components containing longitudinal welds the basic allowable stress S may be used to determine S. (See Table 10-49, Note 13.)... 

R = range of reaction forces or moments (derived from flexibility analysis) corresponding to the full displacement-stress range and based on E, Ibf or in lbf (N or N mm)... 

The minimum value for/is 0.15, which results in an allowable displacement stress range, SA, for an indefinitely large number of cycles. 

When the computed stress range varies, whether from thermal expansion or other conditions, SE is defined as the greatest computed displacement stress range. The value of N in such cases can be calculated by eq. (Id)... 

Ne = number of cycles of maximum computed displacement stress range, SE... 

Nj = number of cycles associated with displacement stress range, S r = S /S ... 

S = any computed displacement stress range smaller than SE... 

Restraint Flexibility. If restraints are not considered rigid, their flexibility may be considered in determining displacement stress range and reactions. 

Average axial stresses (over the pipe cross section) due to longitudinal forces caused by displacement strains are not normally considered in the determination of displacement stress range, since this stress is not significant in typical piping layouts. In special cases, however, consideration of average axial displacement stress... 

The allowable displacement stress range, S, and permissible additive stresses shall be as specified in para. IP-2.2.10(d) for systems primarily stressed in bending and/or torsion. 

The stress intensification factors in Appendix D of ASME B31.3 have been developed from fatigue tests of representative piping components and assemblies manufactured from ductile ferrous materials. The allowable displacement stress range is based on tests of carbon and austenitic stainless steels. Caution should be exercised when using eqs. (la) and (lb) (para. IP-2.2.10) for allowable displacement stress range for some nonferrous materials (e.g., certain copper and aluminum alloys) for other than low-cycle applications. 

The range of bending and torsional stresses shall be computed using the reference modulus of elasticity at 21°C (70°F), E , except as provided in para. IP-6.1.3(b)(2)(d), and then, combined in accordance with eq. (17) to determine the computed displacement stress range, S , which shall not exceed the allowable stress range, S,4, in para. IP-2.2.10(d). 

Ratio of lesser computed displacement stress range, S to maximum computed stress range, SE (/ = 1, 2,. ..) ... 

Allowable displacement stress range S. The computed displacement stress range Se in a piping system shall not exceed the allowable displacement stress range S. ... 

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