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Key takeaways
- B31.3 allows simpler methods for many lines, but some lines always need a formal computer analysis.
- Typical triggers are high or low temperature, large diameter, connections to rotating equipment, relief valves, vibration and offshore displacements.
- The critical line list is a project document, agreed early and updated as the design changes.
Why a list?
A medium-size plant has thousands of lines. Most of them are small, cool and well supported and will never be a problem. A few are hot, large, connected to sensitive equipment or exposed to dynamic loads. The critical line list identifies those few so the stress team analyses them formally, typically with software such as CAESAR II, AutoPIPE or ROHR2.
What B31.3 allows
B31.3 doesn’t require a formal analysis for every line. It accepts that a system needs no formal analysis if:
- it duplicates, or replaces without significant change, a system with a successful service record,
- it can readily be judged adequate by comparison with previously analysed systems,
- it is of uniform size, has no more than two points of fixation and no intermediate restraints, and passes the code’s simple approximate flexibility check.
Everything else needs a flexibility analysis by a simplified or a comprehensive method.
Typical criteria for formal analysis
Projects set their own criteria, but they usually include lines that are:
- above a design temperature limit (for example 150 °C or higher, lower limits for larger sizes),
- below a low temperature limit, including cryogenic and blowdown lines,
- connected to pumps, compressors, turbines and air coolers,
- connected to relief valves, where reaction forces occur,
- large diameter (for example NPS 16 and above) or thin walled,
- in two-phase or slug flow, or subject to vibration,
- made of non-metallic materials such as GRE,
- jacketed, or carrying high pressure,
- offshore lines between modules or subject to hull deflection.
Levels of analysis
Many projects use three levels:
- Visual or rule-of-thumb check by the layout designer.
- Simplified check using charts or hand calculations (such as guided cantilever methods).
- Formal computer analysis with a documented model and report.
The critical line list assigns a level to each line.
Onshore
Onshore the list is usually driven by temperature, rotating equipment and long hot rack runs.
Offshore
Offshore it adds motion, hull deflection, blast and compact layouts, so more lines end up on the list.
Code reference
ASME B31.3, para. 319.4.1 Conditions where formal flexibility analysis is not required.
ASME B31.3, para. 319.4.2 Formal analysis requirements.
What to take back to your desk
Make the critical line list a column in the line list, not a separate spreadsheet. When process changes a temperature, the stress requirement changes with it automatically.
Educational content. Always design to your project specification.
