Pipe Stress Analysis Procedure

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Input required for pipe stress analysis

Piping stress analysis needs various types of input data. This input data should come through Engineering Design Basis for Piping Stress Analysis. It includes :  

  1. Applicable Codes and Standards.
  2. Applicable Company or Project Specifications.
  3. Analysis Softare and its Version to use : E.g. Caesar II Ver 5.0
  4. Installation Temperature for cold and hot lines.
  5. Design line in years e.g. 10 Years
  6. Maximum Full Displacement Cycles for Design Life : e.g. 7000
  7. Solar temperature for uninsulated open gas line e.g. 70 deg C.
  8. Wind load criteria i.e. Load Profile, Min. height and min. OD.
  9. Nozzle Loading Verification Guidelines.
  10. Flange Leakage Check Guidelines for Relief Valves.
  11. Stress Analysis Criteria.

Preparation of 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.

Synopsis

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.

Pipe Stress Analysis Procedure

Piping Stress Analysis in a typical engineering project is carried out using softwares like CAESAR II. The procedure adopted normally is as follows :

  1. Based on stress design criteria, stress engineer prepares stress critical lines list.
  2. He/she gives it to layout engineer.
  3. Layout engineer makes sure that lines are routed in 3D Model assuring enough supporting structure is available for the line.
  4. Layout engineer marks logical supports on line in 3d Model at all locations where it is possible to provide a support.
  5. Layout engineer then extracts isometrics, check it routing point of view with all components placed as per PID.
  6. He then issues isometrics to stress engineer.
  7. Layout engineer keeps track of isometrics issued to stress in a copy of stress critical lines list by adding extra columns to track stress progress.
  8. Stress engineer marks node numbers on stress isometrics.
  9. He/she then inputs the line data in CAESAR using classic piping input spreadsheet of CAESAR II.
  10. He/she adds preliminary supports based on judgement and experience.
  11. He/she then runs the static analysis.
  12. He/she then checks the stress reports for :
    1. Excessive displacements.
    2. Nodes exceeding allowable stresses.
    3. Excessive loads in equipment nozzles.
    4. Excessive loads on dynamic equipments like pumps, compressors, turbines etc.
  13. If stress engineer finds everything ok, he finalises supports and give back a copy of stress isometrics with support markup to layout engineer to incorporate those supports in 3d Model.
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