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 software (Caesar II, Autopipe, Caepipe or Rohr II).
The basis for deciding Stress critical lines
The main factors which decide stress critical lines are as follows:
- Line design/operating/upset temperature
- Equipment connection
- Pipe and Equipment material
- Pipe condition
- Pipe thickness
- Design/Upset pressure
Every organization has their 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 consists 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 classified as category M 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 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
- Moderately Critical lines or group C2 lines and
- Lower critical lines or group C3 lines
Few more useful Resources for You…