This Practice provides minimum requirements for analyzing the flexibility of aboveground metallic piping systems.
This Practice describes the piping flexibility analysis parameters and applications, and documentation requirements.
Applicable parts of the following Practices and industry codes and standards shall be considered
an integral part of this Practice. The edition in effect on the date of contract award shall be used,
except as otherwise noted. Short titles will be used herein where appropriate.
2.1 Process Industry Practices (PIP)
– PIP PNFS0001 - Miscellaneous Pipe Support Details
– PIP RESE002 - Allowable Piping Loads on Rotating Machinery Nozzles
2.2 Industry Codes and Standards
American Petroleum Institute (API)
– API 618 - Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services
– API 661 - Air-Cooled Heat Exchangers for General Refinery Services
American Society of Civil Engineers (ASCE)
– ASCE 7 - Minimum Design Loads for Buildings and Other Structures
– NBC - The National Building Code of Canada
American Society of Mechanical Engineers (ASME)
– ASME Boiler and Pressure Vessel Code
– Section VIII - Pressure Vessels
– ASME B31.1 - Power Piping
– ASME B31.3 - Process Piping
Canadian Standards Association (CSA
Welding Research Council (WRC)
– WRC 107 (see WRC 537)
– WRC 537 - Precision equations and enhanced diagrams for local stresses in spherical and cylindrical shells due to external loadings for implementation of WRC Bulletin 107
– WRC 297 - Local Stresses in Cylindrical Shells Due to External Loadings on Nozzles - Supplement to WRC 107
3.1.1 All piping systems shall be evaluated and, if appropriate, analyzed for applicable conditions in accordance with ASME B31.3 and this Practice. The designer shall be qualified in accordance with the B31.3 Code para. 301.1.
3.1.2 The most severe, anticipated, coincident pressure and temperature conditions shall be considered to evaluate the flexibility and sustained load analyses for each anticipated operating condition. Design conditions (pressure and temperature) shall be set in accordance with ASME B31.3 paras. 301.2 and 301.3 with consideration to approved variations above same as set forth in ASME B31.3 para. 302.2.4. The sole uses for design conditions shall be in accordance with ASME B31.3 Appendix S, Example 1.
3.1.3 The flexibility analysis can require the combination of more than one load case to
determine the total displacement stress range.
3.1.4 Any computerized pipe flexibility calculations shall be performed using owner-approved software.
3.1.5 Piping systems shall be analyzed for expansion, contraction, differential settlement, relief valve reactions, and effects due to weight, wind, seismic, and other mechanical loading in accordance with ASME B31.3.
3.1.6 Expansion joints shall not be permitted unless approved by owner.
3.2 Analysis Parameters
3.2.1 Displacement Strains
188.8.131.52 The flexibility analysis for each stress range to be evaluated for each anticipated operating condition shall be based on the maximum operating temperature for that condition unless calculations are supplied to and approved by the owner that better predict for the pipe metal temperature for the condition.
184.108.40.206 Climatic effects shall be considered in determining the maximum differential temperature.
220.127.116.11 The metal temperature from the effect of solar radiation in the summer and the winter dry bulb design temperature should be used.
18.104.22.168 When more than one stress range is anticipated for a piping system with multiple operating conditions, as stated in Section 3.1.3 of this Practice, it may be necessary to determine the difference between displacement stress ranges (or compute the operating stress range, i.e., ASME B31.3 Appendix P). See ASME B31.3 Appendix S Example 3.
Comment: Cold branch includes cases dealing with parallel lines where at least one line (not always the same one) may be cold at any time. An example is three parallel pumps where one pump (not always the same one) is usually not in service.
3.2.2 Pressure, Weight, and Other Sustained Loads
22.214.171.124 The weight of piping, piping components, refractory lining, piping insulation, fluid transported, and fluid used for testing shall be considered.
126.96.36.199 Snow and ice loads shall be considered if specified by owner.
188.8.131.52 If piping lifts off a support during an ambient to operating condition flexibility (stress range) evaluation, the support shall either be removed for sustained load calculations or spring supports shall be considered.
See ASME B31.3 Appendix S Example 2 for the potentially multiple sustained load analyses required by Code for each anticipated operating condition.
184.108.40.206 The frictional resistance to thermal movement of the pipe shall be considered. The greater loads of those evaluated with friction and those evaluated without friction shall be used for reaction loads, flexibility based stress range analyses, and sustained load analyses and shall be documented.
220.127.116.11 Frictionless unrestrained movement of the piping system shall be assumed only if the entire system is supported by means of rod or spring hangers.
The wind loads on piping systems shall be determined in accordance with the procedure outlined in ASCE 7 or as specified by owner.
If specified by owner or required by jurisdiction, seismic loads on piping systems shall be considered.
3.2.6 Pressure Relief Systems
18.104.22.168 Pressure relief discharge piping shall be restrained to contain the thrust loads.
22.214.171.124 Forces and moments due to relief valve discharge may be calculated by any method approved by owner.
Comment: ASME B31.1, Appendix II is an example of a calculation method.
3.2.7 Water Hammer
126.96.36.199 Piping systems subject to water hammer shall be considered.
188.8.131.52 Forces due to water hammer shall be determined and suitable pipe restraints shall be provided.
184.108.40.206 External bending moments on flanges shall be considered.
220.127.116.11 External loads may be analyzed by the equivalent pressure method or other methods approved by owner.
18.104.22.168 Acceptance criteria shall be in accordance with owner’s requirements.
3.2.9 Maximum Pipe Spans and Deflections
22.214.171.124 For determining pipe spans, maximum sag deflections shall be limited to 16 mm (5/8 in).
126.96.36.199 Any deviations to Section 188.8.131.52 shall be approved by owner.
184.108.40.206 The increased stiffness of a piping systems caused by a refractory lining shall be considered when determining reaction loads.
220.127.116.11 To protect a piping system against collapse due to creep, the increased stiffness due to a refractor