Pipe Stress Critical Lines List


Stress critical line list is developed by Stress Engineer based on stress design criteria for the particular project. A typical Stress Design Criteria specifies following types of lines to be critical from stress point of view.

The Critical line list should include following lines:

  • 4” to 14” : Operating Temperature beyond 175°C;
  • 16” to 18” : Operating Temperature beyond 120°C.
  • 20” and above.
  • If pipe stress calculations compel the national approval or client authorities, this shall be implemented by the procedures as required by the respective authorities.
  • 3” and above sized process lines connected to rotating Equipments.
  • Piping fastened to air cooled Heat Exchangers, Pulsating equipments, inlet and outlet of boilers and Heaters.
  • Unacceptable stresses and reactions caused by external displacements.
  • Lines of cycling services eg. Batch process or regenerations.
  • Locations of snubbers, spring type support, expansion joints.
  • Specific piping for slug flow/water hammer/ double phase flows.
  • 4” and above diameter flare lines, Relief loaded lines.
  • 8” and above lines with operating temperature above 200° C, in case of toxic services.
  • Glass reinforced plastic piping.
  • Steam outlets, jacketed piping.
  • Cryogenic lines below -46° C.
  • Lines connected to Solenoid Pumps (X’mas trees) and well heads.
  • Lines in which bolts joints do not comply with ASME B16.5/B16.47
  • Piping’s in, in and on derrick and flare structures.

Prior to the computerized analysis the following minimum analysis shall be carried out manually

  • Lines with operating temperatures 100° C (210° F) to 175° C (350°F) of pipe sizes 4” to 14”.
  • Calculations of forces, moments and stresses caused by the expansion strains shall be formally analyzed by means of analytical and chart method
  • All critical lines must be highlighted in both project line list and PEFS/ P&ID’s. The integrity of non-critical lines must be ensured by the piping stress engineer, whereas the non-critical lines shall be flexible or natural flexibility shall be achieved by pipe work configuration.


Critical lines include piping systems of 4” to 14” with temperature above 175°C, 16” to 18” with operating temperature of 120°C. Whereas the non-critical piping system come under pipe sizes 4” to 14” with operating temperature of 100° C to 175°C.

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