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1. June 2000
Process Industry Practices
Piping
PIP PNC00004
Piping Flexibility Analysis Criteria
for ASME B31.3 Metallic Piping

2. PURPOSE AND USE OF PROCESS INDUSTRY PRACTICES
In an effort to minimize the cost of process industry facilities, this Practice has
been prepared from the technical requirements in the existing standards of major
industrial users, contractors, or standards organizations. By harmonizing these
technical requirements into a single set of Practices, administrative, application, and
engineering costs to both the purchaser and the manufacturer should be reduced. While
this Practice is expected to incorporate the majority of requirements of most users,
individual applications may involve requirements that will be appended to and take
precedence over this Practice. Determinations concerning fitness for purpose and
particular matters or application of the Practice to particular project or engineering
situations should not be made solely on information contained in these materials. The
use of trade names from time to time should not be viewed as an expression of
preference but rather recognized as normal usage in the trade. Other brands having the
same specifications are equally correct and may be substituted for those named. All
Practices or guidelines are intended to be consistent with applicable laws and
regulations including OSHA requirements. To the extent these Practices or guidelines
should conflict with OSHA or other applicable laws or regulations, such laws or
regulations must be followed. Consult an appropriate professional before applying or
acting on any material contained in or suggested by the Practice.
This Practice is subject to revision at any time by the responsible Function Team
and will be reviewed every 5 years. This Practice will be revised, reaffirmed, or
withdrawn. Information on whether this Practice has been revised may be found at
http://www.pipdocs.org.
© Process Industry Practices (PIP), Construction Industry Institute, The
University of Texas at Austin, 3208 Red River Street, Suite 300, Austin,
Texas 78705. PIP member companies and subscribers may copy this Practice
for their internal use.
Not printed with State funds

3. June 2000
Process Industry Practices Page 1 of 7
Process Industry Practices
Piping
PIP PNC00004
Piping Flexibility Analysis Criteria
for ASME B31.3 Metallic Piping
Table of Contents
1. Introduction..................................2
1.1 Purpose ............................................. 2
1.2 Scope................................................. 2
2. References ...................................2
2.1 Process Industry Practices ................ 2
2.2 Industry Codes and Standards .......... 2
3. General .........................................3
4. Flexibility ......................................3
5. Design Basis ................................3
5.1 Thermal Effects ................................. 3
5.2 Weight Effects ................................... 3
5.3 Friction Effects................................... 3
5.4 Wind Effects ...................................... 4
5.5 Seismic Effects .................................. 4
5.6 Pressure Relief Systems ................... 4
5.7 Water Hammer/Slug Flow ................. 4
5.8 Flanges .............................................. 4
5.9 Maximum Pipe Spans
and Deflections.................................. 4
5.10 Special Requirements........................ 4
6. External Load Limits on
Equipment.................................... 5
6.1 General.............................................. 5
6.2 Rotating Machinery............................ 5
6.3 Reciprocating Compressors .............. 5
6.4 Air Cooled Heat Exchangers ............. 5
6.5 Pressure Vessels and Heat
Exchangers........................................ 5
6.6 Tank Nozzle Loads............................ 5
7. Pipe Flexibility Requirements..... 6
8. Piping Flexibility Analysis
Record Keeping........................... 6

4. PIP PNC00004
Piping Flexibility Analysis Criteria for ASME B31.3 Metallic Piping June 2000
Page 2 of 7 Process Industry Practices
1. Introduction
1.1 Purpose
The purpose of this Practice is to outline the minimum basic requirements for
analyzing aboveground metallic piping systems for flexibility.
1.2 Scope
This Practice provides basic piping flexibility analysis, design conditions, and
documentation requirements for the flexibility analysis. All piping systems shall be
evaluated and, where appropriate, analyzed for applicable conditions in accordance
with ASME B31.3 (henceforth referred to as the Code) and this Practice.
2. References
When adopted in this Practice, the latest edition of the following references in effect on the
date of contract award shall be used, except as otherwise noted. Short titles will be used
herein when 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
• 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
• Welding Research Council (WRC)
– WRC 107 - Local Stresses in Spherical and Cylindrical Shells Due to External
Loadings
– WRC 297 - Local Stresses in Cylindrical Shells Due to External Loadings on
Nozzles - Supplement to WRC 107

5. PIP PNC00004
June 2000 Piping Flexibility Analysis Criteria for ASME B31.3 Metallic Piping
Process Industry Practices Page 3 of 7
3. General
3.1 The designer shall consider the most severe anticipated coincident pressure and
temperature condition(s).
3.2 The flexibility analysis shall consider the most severe specified temperature
condition anticipated during start-up, normal operation, steam out/decoking,
depressuring, shutdown, defined upset, cold branch, and regeneration. Analysis of
this situation may require the combination of more than one load case to determine
the total displacement stress range.
Note: Cold branch shall include 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 normally shut down.
4. Flexibility
4.1 Any computerized pipe flexibility calculations shall be performed using owner-
approved software.
4.2 Piping systems shall be analyzed for expansion, contraction, differential settlement,
relief valve reactions, and effects due to weight, wind, seismic, and other mechanical
loading per the Code.
4.3 Expansion joints shall not be used unless approved by owner.
5. Design Basis
5.1 Thermal Effects
The analysis shall be for the maximum temperature differential. Climatic effects
shall be considered in determining maximum differential temperature. The metal
temperature from the effect of solar radiation in the summer and the winter dry bulb
design temperature should be used. Process conditions listed in Section 3.2 shall also
be considered.
5.2 Weight Effects
The weight of piping, piping components, refractory lining, piping insulation, fluid
transported, and fluid used for testing shall be taken into account in the analysis.
Snow and ice loads shall be considered when required by project specifications.
5.3 Friction Effects
The effect of frictional resistance to thermal movement of the pipe shall be
considered in the design of piping systems. Frictionless unrestrained movement of
the piping system shall be assumed only when the entire system is supported by
means of rod or spring hangers. The effect of friction shall not be added to the
analysis to reduce either piping loads or stresses.

6. PIP PNC00004
Piping Flexibility Analysis Criteria for ASME B31.3 Metallic Piping June 2000
Page 4 of 7 Process Industry Practices
5.4 Wind Effects
The wind loads on piping systems shall be determined by the procedure outlined in
ASCE 7 or as specified by owner.
5.5 Seismic Effects
Seismic loads on piping systems shall be considered when required by owner or by
jurisdiction, and as outlined in project specifications.
5.6 Pressure Relief Systems
The discharge piping shall be restrained to contain the thrust loads. Forces and
moments due to valve discharge may be calculated by any method acceptable to the
owner, such as ASME B31.1, Appendix II.
5.7 Water Hammer
Piping systems subject to water hammer shall be analyzed accordingly. Forces due to
water hammer shall be determined and suitable pipe restraints shall be provided.
5.8 Flanges
External bending moments on flanges shall be considered. External loads may be
analyzed by the equivalent pressure method or other methods acceptable to owner.
Acceptance criteria shall be in accordance with project specifications.
5.9 Maximum Pipe Spans and Deflections
In determining pipe spans, maximum sag deflections shall be limited to 5/8 inch.
Any deviations shall be approved by owner.
5.10 Special Requirements
5.10.1 The increased stiffness of piping systems caused by a refractory lining shall
be included in the piping flexibility analysis. The increased stiffness due to
refractory shall not be included in either the span calculations or the
sustained load case (to protect against incremental collapse due to creep).
5.10.2 Large diameter thin wall (D/T ≥ 100) lines shall be analyzed for crushing
loads at local stress points and reinforced as necessary.
5.10.3 When piping lifts off a support due to thermal movements, either that
support shall be removed for sustained load calculations or spring supports
shall be considered.
5.10.4 Attachments on welded ells or within two pipe diameters of the welds of an
ell decrease its flexibility. This reduced flexibility shall be considered in the
piping flexibility analysis. In the absence of better information, this
decreased flexibility may be simulated (in a computer analysis) by placing a
flange pair at the nearest weld. Although not addressed directly by
Appendix D of the Code, 45 degree ells shall have their flexibility reduced
by placing a flange pair at each end.

7. PIP PNC00004
June 2000 Piping Flexibility Analysis Criteria for ASME B31.3 Metallic Piping
Process Industry Practices Page 5 of 7
5.10.5 Branches at angles other than 90 degrees are not addressed by Appendix D
of the Code. For flexibility purposes, testing has shown that these branches
act like unreinforced connections. In the absence of applicable data or
rigorous analysis, these branches may be modeled by doubling the default
stress intensification factor for unreinforced fabricated tees.
5.10.6 Tees with aspect ratios of 3:4 have been found to be non-conservative. These
tees shall have their stress intensification factor increased by 25%.
5.10.7 Piping systems supported primarily by rod hangers shall accommodate the
rod’s rotation and the consequent load impact on near by nozzles.
5.10.8 Unless otherwise approved by owner, use of cold spring for piping systems
that connect to rotating equipment (compressors, turbine pumps) shall not be
allowed.
6. External Load Limits on Equipment
6.1 General
Loads imposed on equipment by the piping shall not exceed the lesser of that
allowed by the equipment manufacturer or that listed in the applicable references in
this Practice unless otherwise approved by owner.
6.2 Rotating Machinery
Allowable nozzle loads for rotating machinery shall be in accordance with
PIP RESE002.
6.3 Reciprocating Compressors
Reciprocating compressor piping shall be analyzed in accordance with API 618.
6.4 Air Cooled Heat Exchangers
Nozzle loads shall comply with the load criteria of API 661 unless otherwise stated
in project specifications.
6.5 Pressure Vessels and Heat Exchangers
Where appropriate, nozzle loads shall be evaluated in accordance with WRC 107 and
WRC 297, as applicable. If the nozzle/vessel geometry is outside the geometry limit
of WRC 107 or WRC 297, other acceptable client-approved local stress analysis
methods may be used; extrapolation of the curves in these documents is not
acceptable. Allowable stresses shall be based on ASME Boiler and Pressure Vessel
Code, Section VIII, Division 1 or 2, as applicable.
6.6 Tank Nozzle Loads
Nozzle loads shall be evaluated per owner’s requirements.

8. PIP PNC00004
Piping Flexibility Analysis Criteria for ASME B31.3 Metallic Piping June 2000
Page 6 of 7 Process Industry Practices
7. Pipe Flexibility Requirements
7.1 All piping systems shall be reviewed to identify those that may be outside the
requirements of Section 7.2 and that may still require flexibility analysis.
7.2 A formal flexibility analysis shall be performed on all of the following piping
systems (see Code, Paragraph 319.4.2):
a. All lines connected to load/stress-sensitive equipment
b. All process, regeneration, and decoking lines to and from steam generators and
fired heaters
c. Process lines to and from centrifugal compressors, turbo-expanders, and
blowers
d. All working fluid lines to and from turbines
e. Suction and discharge lines to and from reciprocating pumps and compressors
f. Lines NPS 3 and larger to and from centrifugal pump nozzles
g. Lines NPS 4 and larger to air cooled heat exchangers
h. All relief systems, whether closed or relieving to atmosphere, with
considerations for attached or detached tail pipes
i. All piping requiring proprietary expansion devices (e.g., bellows expansion
joints)
j. Lines NPS 3 and larger subject to significant differential settlement of vessels,
tanks, equipment, or supports
k. Lines subjected to mixed phase flow (liquid and vapor) and lines identified as
severe cyclic or vibrating service
l. Jacketed piping
m. Piping as required by applicable codes and standards (e.g., Code Category M)
n. All plastic lined piping systems
7.3 Figures 1 and 2 provide a guide to evaluate situations not covered in Section 7.2.
7.4 Analysis for other materials shall be per agreement with owner.
8. Piping Flexibility Analysis Record Keeping
8.1 The designer shall assign calculation numbers and store the flexibility analysis files.
8.2 When the project has been completed, the designer shall furnish all stress
calculations and documentation as required by owner.

9. PIP PNC00004
June 2000 Piping Flexibility Analysis Criteria for ASME B31.3 Metallic Piping
Process Industry Practices Page 7 of 7
Figure 1. Guide for Analysis of Carbon Steel or Low and Intermediate Alloy Steel
Figure 2. Guide for Analysis of Stainless Steel Pipe
Notes:
1. Lines in this category may be analyzed by the use of engineering experience or approximate methods.
2. Lines in this category require formal analysis. Analysis may be done by approximate or comprehensive
methods. Documentation is required.
3. Lines in this category require a comprehensive analysis (typically by computer). Other methods may be
used with owner’s approval. Documentation is required.