Stress critical line list

Stress Critical line list

A piping critical line is a line for which a flexibility review is required to be carried out by a Piping Stress Engineer due to temperature, weight, supporting arrangement, external loading, lines connection to strain sensitive equipment, vibrations, etc.

The criticality level will dictate the method of analysis for each of the lines.

Level 3 – Stress Critical lines – Comprehensive Computer Analysis Required
Level 2 – Non Stress Critical lines – Simplified or Computer Analysis Required
Level 1 – Non Stress Critical lines – No Analysis Required

Stress Critical Piping Lines

Lines classified as stress critical “Level 3” should undergo comprehensive computerized stress analysis in accordance with the following criteria.

3” NPS and larger piping and design temperature ≥ 80 deg.C or ≤ 0 deg.C
6” NPS and larger piping all lines

Comprehensive analysis should be also required for the following lines

3” NPS and larger piping connecting to rotation machines
Piping connected to load/stress-sensitive equipment
3” NPS and larger piping connecting to low allowable load nozzles (Air fin coolers, PCHE’s etc.)
2” NPS or larger lines on pipe rack with branch connections to modules. (Expansion loop is preferred if the line is running on long pipe rack or long straight pipe)
Jacketed piping systems
Tie-in connection piping 3” NPS and larger for offshore facilities
Lines 3" NPS and larger with a wall thickness larger than 10% of the outside pipe diameter. (Typical water injection, gas injection and flowline circulation piping)
Thin wall piping of 20" NPS and larger with wall thickness less than 1% of the outside pipe diameter (Typical gas turbine power generator exhaust piping)
All piping expected to be subjected to vibration due to internal and external loads such as pressure transients, slugging, and vortex shedding induced oscillation, high gas velocities, FIV, and acoustic vibrations of the pipe wall membrane.
Suction and discharge piping to and from reciprocating pumps and compressors
Piping that requires support for occasional loadings (e.g., seismic, wind, steam out, steam tracing)
Lines subjected to impacted forces caused external loads such as changes in flow rates, hydraulic shock, liquid or solid slugging and flashing
All 3” NPS and larger piping connected to pressure relief valves and rupture disks
All blowdown piping 2" NPS and larger excluding drains
All piping along the flare tower
Piping subjected to mixed phase flow (liquid and vapor)
Piping requiring proprietary expansion devices (e.g., bellows expansion joints)
All piping above 3" NPS likely to be affected by movement of connecting equipment, structural deflection and hull deflection (hogging and sagging)
GRE piping 2" NPS and larger
All production and injection manifold piping up to the riser hang off
Lines with oblique connection to the header (ex: 45 degree branch connection to flare header)
Firewater lines (Main pipe ring and deluge)
Lines subjected to vacuum conditions
Lines subjected to severe cyclic temperature conditions (e.g. mole sieve regeneration)
Lines designated as high pressure as per ASME B31.3 Chapter IX
Seawater Lift piping systems
Other lines requested by the project owner, class society, the project spec or responsible pipe stress analyst to be categorized as critical

Non-Stress Critical Piping Lines

Lines that do not fall under stress critical group should be categorized as non-stress critical. These are further divided into two levels, Level 2 and Level 1. The support span for noncritical lines should be based on the piping support standards.

Level 2 Non-critical lines

The lines that fall under this category require pipe stress engineer review and approval. For these lines a simplified analysis should be carried out using graphical methods, span charts, expected thermal growth, and other accepted industry methods. The final piping isometrics drawings should be reviewed and approved by the pipe stress engineer. At the discretion of the pipe stress engineer, if a non-critical line warrants rigorous computer analysis then those lines can be upgraded to Level 3. Some of the typical Level 2 lines are:

All large bore lines operating at ambient temperatures & not covered in Level 3
Large bore utilities lines operating at ambient conditions
Lines that require pipe support load estimation and flange evaluation

Level 1 Non-critical lines

The lines that fall under this category require no formal stress analysis. The lines are usually field routed by experienced site engineers. These lines will undergo a visual check using engineering judgment and available support charts. Some of the typical Level 1 lines are: