ASME B31.8-2012

832.3 Flexibility Requirements

(a) Piping systems shall be designed to have sufficientflexibility to prevent thermal expansion or contractionfrom causing excessive stresses in the piping material,excessive bending or unusual loads at joints, or undesir-able forces or moments at points of connection to equip-ment or at anchorage or guide points. Formalcalculations shall be performed where reasonable doubtexists as to the adequate flexibility of the system. Seepara. 833.7 for further guidance.

(b) Flexibility shall be provided by the use of bends,loops, or offsets, or provision shall be made to absorbthermal changes by the use of expansion joints or cou-plings of the lip joints type or expansion joints of thebellows type. If expansion joints are used, anchors orties of sufficient strength and rigidity shall be installedto provide for end forces due to fluid pressure and othercauses.

(c) In calculating the flexibility of a piping system,the system shall be treated as a whole. The significance

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of all parts of the line and all restraints, such as rigidsupports or guides, shall be considered.

(d) Calculations shall take into account stress intensi-fication factors found to exist in components other thanplain straight pipe. Credit may be taken for the extraflexibility of such components. The flexibility factors andstress intensification factors shown in Table E-1 maybe used.

(e) Properties of pipe and fittings for these calcula-tions shall be based on nominal dimensions, and thejoint factor E shall be taken as 1.00.

(f) The total range in temperature from minimumdesign temperature to the maximum design tempera-ture shall be considered in all expansion stress calcula-tions, whether piping is cold-sprung or not. Shouldinstallation, start-up, or shutdown temperatures be out-side of the design temperature range, the maximumpossible temperature range shall be considered. In addi-tion to the expansion of the line itself, the linear andangular movements of the equipment to which it isattached shall be considered.

(g) Flexibility calculations shall be based on the mod-ulus of elasticity corresponding to the lowest tempera-ture of the operational cycle.

(h) In order to modify the effect of expansion andcontraction, runs of pipe may be cold-sprung. Cold-spring may be taken into account in the calculations ofthe reactions, provided an effective method of obtainingthe designed cold-spring is specified and used.

832.5 Modulus of Elasticity

The modulus of elasticity for carbon and low alloysteel at various temperatures is given in Table 832.5-1.Values between listed temperatures may be linearlyinterpolated.

ASME B31.8-2012

Table 832.5-1 Modulus of Elasticity forCarbon and Low Alloy Steel

Temperature, Modulus of Elasticity,°F (°C) psi � 106 (GPa)

−100 (−73) 30.2 (208)70 (21) 29.5 (203)200 (93) 28.8 (198)300 (149) 28.3 (195)400 (204) 27.7 (191)500 (260) 27.3 (188)

be affected by aspects of pipeline construction, supportdesign, soil properties, and terrain. Part 833 is applicableto all steel pipingwithin the scope of B31.8. For purposesof design, this Code recognizes two axial restraint condi-tions, “restrained” and “unrestrained.” Guidance in cat-egorizing the restraint condition is given below.

(b) Piping in which soil or supports prevent axial dis-placement or flexure at bends is “restrained.” Restrainedpiping may include the following:

(1) straight sections of buried piping(2) bends and adjacent piping buried in stiff or con-

solidate soil(3) sections of aboveground piping on rigid

supports(c) Piping that is freed to displace axially or flex at

bends is “unrestrained.” Unrestrained piping mayinclude the following:

(1) aboveground piping that is configured toaccommodate thermal expansion or anchor movementsthrough flexibility

(2) bends and adjacent piping buried in soft orunconsolidated soil

(3) an unbackfilled section of otherwise buriedpipeline that is sufficiently flexible to displace laterallyor which contains a bend

(4) pipe subject to an end cap pressure force

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ASME B31.8-2012

833.7 Flexibility Analysis for Unrestrained Piping

(a) There is no need for formal flexibility analysis foran unrestrained piping system that

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(1) duplicates or replaces without significantchange a system operating with a successful record

(2) can be readily judged adequate by comparisonwith previously analyzed systems

(3) is of uniform size, has no more than two pointsof fixation, no intermediate restraints, and falls withinthe limitations of the following empirical equation

DY

(L − U)2≤ K

whereD p nominal outside diameter of pipe, in. (mm)K p 0.03, for U.S. Customary units (208, for SI units)

listed in the equation aboveL p developed length of piping between anchors,

ft (m)U p straight line separation between anchors, ft (m)Y p resultant of total displacement strains, in. (mm),

to be absorbed by the system

NOTE: No general proof can be offered that this empirical equa-tion always yields conservative results. It is not applicable to sys-tems used in severe cyclic conditions. It should be used withcaution in configurations such as unequal leg U-bends havingL/U > 2.5; or nearly-straight “saw-tooth” runs; or where i ≥ 5 dueto thin-walled design; or where displacements not in the directionconnecting anchor points constitute a large part of the total dis-placement. There is no assurance that terminal reactions will beacceptably low even if a piping system falls within the limitationsof para. 833.7(a)(3).

(b) Any piping system that does not meet one of thecriteria in para. 833.7(a) should undergo a flexibilitystress analysis by a simplified, approximate, or compre-hensive method as deemed appropriate.