Stress analysis is a complex task and in any process unit, there are a huge number of lines exist which run from one location to another. Analyzing all lines will take a lot of time which in turn will increase the engineering time and corresponding cost. So every engineering organization in this field has set up some guidelines for deciding which lines are to be stress analyzed using pipe stress analysis software (Caesar II, Autopipe, Caepipe, START-PROF, or Rohr II).
What is a Critical Line List or Stress Critical Line List?
A Stress Critical Line List or SCLL is a listing of all critical lines requiring attention from a piping stress engineer during the piping design phase. It is the piping stress engineer’s responsibility to prepare one Critical Line List (CLL) by isolating the non-stress critical lines from the stress critical line based on the criteria provided in the stress analysis specification. Piping Critical Line List is also known as Flexibility Log in some EPC organizations or design consultancies.
All those critical lines are then categorized into some number of stress systems so that the stress system numbers can be easily found from the generated critical line list. The number of stress critical lines in each stress system is decided based on the engineering judgment of the stress engineer. The stress Critical Line List is an important deliverable from the piping stress team. The main inputs required from the stress critical line list preparation are:
- Process Line List
- Process P&ID or PEFS, and
- Flexibility Specification or Stress Analysis Design Basis.
Ideally, the critical line list should be updated as and when the process line list is updated. But to reduce frequent changes many organization prepares only three revisions of the stress critical line list. Those:
- Preliminary Stress Critical Line List during the start of the project upon receipt of the first set of line lists and P&ID
- Advanced Critical Line List just before 60% model review and
- Final Stress Critical Line List after 90% model review.
Types of Stress Critical Lines
Stress Critical Lines are normally divided into a few groups for deciding critical lines. Those are:
- Equipment Critical Lines: Lines connected to Rotary Equipment and Critical Static equipment fall in this category. For example, lines connected to pumps, compressors, and turbines are by default stress critical and require analysis using the software.
- Support Critical Lines: Lines for which engineered supports are required fall in this category. Example: Supports requiring spring hangers, Pipes with SS or SDSS material where normal CS supports can not be used, etc.
- Relief Critical Lines: Pipes experiencing relief loads come into this category. For example, the line connected to pressure safety valves, rupture disk, etc.
- Material Critical Lines: Pipes made from SDSS and non-metals like GRE, FRP, Aluminum alloy, etc. fall in this category.
- Service Critical Lines: Piping systems carrying category M fluid service, hazardous fluid service, severe cyclic condition, etc. fall into this category.
- Temperature Critical Lines: Lines carrying fluids having high temperature comes into this group.
The basis for deciding Stress critical lines
The main factors which decide stress critical lines for preparing a critical line list are as follows:
- Line design/operating/upset temperature
- Equipment connection
- Pipe and Equipment material
- Pipe condition
- Pipe thickness
- Design/Upset pressure
- Fluid Category
- Severe cyclic condition
Every organization has its own guidelines and the guidelines vary from project to project. The following write-up will provide a few criteria for deciding stress critical lines. This is only an idea of how differentiation occurs. The user is requested to check project-specific documents for use in any project. Mostly the critical lines for which stress analysis is to be performed by formal computer analysis consist of the following lines:
- All Pump (Centrifugal-API/ANSI, gear pump, Screw pump) suction and discharge piping (4 inches and larger).
- Centrifugal Compressor inlet and outlet piping.
- Lines to and from steam generators.
- Reciprocating pump and compressor suction and discharge piping.
- Piping requiring expansion joints or other proprietary expansion devices.
- Steam and Gas Turbine inlet and outlet piping.
- Air Cooler inlet and outlet piping (3 inches and larger).
- Process Heater inlet and outlet piping
- Lines carrying category M fluid service as per ASME B31.3.
- Piping subjected to high cyclic temperature conditions.
- All jacketed lines.
- Lines that require nozzle load compliance as stipulated per applicable codes or equipment Vendor allowable (Heat exchanger, Pressure Vessel Connected systems).
- Lines subject to dynamic loading (relief lines, line with large pressure drop at control valves, surge pressure, slug flow, churn, two-phase flow, water hammer, flashing, etc.)
- All Fiberglass, aluminum alloy, refractory, or elastomer-lined piping.
- All piping systems connected to FRP, plastic, glass-lined steel, or brittle equipment
- Lines subjected to non-thermal movements (Expected differential settlement between structures, structure-equipment, etc., process equipment growth, header growth, tower growth, or other significant displacements, etc.)
- All lines 8” and larger operating above 150 deg. C (300 deg. F) and greater.
- All lines 20” and larger operating above 80 deg. C (200 deg. F) and greater.
- All lines 36” and larger.
- All lines operating below -45 deg. C (-50 deg. F) which requires special “cold” supports.
- All plastic-lined piping systems. Special attention shall be given to adding enough additional supports to limit the external forces and moments in the flange connections to avoid an extra risk of flange leaks.
- Lines with special design requirements
- All Safety pressure-relieving systems 4 inches and larger (not including thermal reliefs)
- Lines judged by the lead piping engineer/stress engineer as not having sufficient inherent flexibility
- In addition, the piping effects of other conditions such as temperature gradients that could cause thermal bowing or where piping is connected to equipment with significant thermal growth may warrant detailed computer analysis.
- For thin wall piping, if the D/T ratio exceeds 100, the following requirements are applicable:
- The design and support of piping systems using this specification should be reviewed by a stress engineer. Support and spans of thin wall piping systems are not covered by current Project practices and therefore must be designed for each application.
- Stub-in connections per 304.3.2 thru 304.3.4 of ASME B31.3, are not allowed for run pipe with D/T greater than or equal to 100 and the branch diameter is greater than one-half of the header diameter.
- Lines connected to non-ferrous equipment.
- Underground process lines with more than a 30-degree difference between design and ambient temperature.
- All vertical lines, connected to vertical vessels that require pipe supports or guides from that vessel.
- All lines, 4 inches and larger subject to external pressure or vacuum conditions.
- All lines, subject to vibration, as specified by Process, due to high-velocity flow, high-pressure drop, water hammer, or mixed phase flow.
- All lines that are connected to equipment constructed of thermoset or thermoplastic materials or that are glass, refractory, or elastomer lined.
- All pressure containing non-metallic lines.
- All flare line headers
- Lines for which an Alternative Leak Test has been specified.
Many organizations have the practice of dividing these critical lines into three groups based on their criticality:
- Highly critical lines or group C1 lines: Must be reviewed thoroughly using stress analysis software.
- Moderately Critical lines or group C2 lines and
- Lower critical lines or group C3 lines
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