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 :
- Applicable Codes and Standards.
- Applicable Company or Project Specifications.
- Analysis Softare and its Version to use : E.g. Caesar II Ver 5.0
- Installation Temperature for cold and hot lines.
- Design line in years e.g. 10 Years
- Maximum Full Displacement Cycles for Design Life : e.g. 7000
- Solar temperature for uninsulated open gas line e.g. 70 deg C.
- Wind load criteria i.e. Load Profile, Min. height and min. OD.
- Nozzle Loading Verification Guidelines.
- Flange Leakage Check Guidelines for Relief Valves.
- 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.
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 :
- Based on stress design criteria, stress engineer prepares stress critical lines list.
- He/she gives it to layout engineer.
- Layout engineer makes sure that lines are routed in 3D Model assuring enough supporting structure is available for the line.
- Layout engineer marks logical supports on line in 3d Model at all locations where it is possible to provide a support.
- Layout engineer then extracts isometrics, check it routing point of view with all components placed as per PID.
- He then issues isometrics to stress engineer.
- 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.
- Stress engineer marks node numbers on stress isometrics.
- He/she then inputs the line data in CAESAR using classic piping input spreadsheet of CAESAR II.
- He/she adds preliminary supports based on judgement and experience.
- He/she then runs the static analysis.
- He/she then checks the stress reports for :
- Excessive displacements.
- Nodes exceeding allowable stresses.
- Excessive loads in equipment nozzles.
- Excessive loads on dynamic equipments like pumps, compressors, turbines etc.
- 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.