Wind analysis is performed based on Client/ITB requirement. Wind load is an occasional load which normally occurs less than 20% of plant operating time. There are two methods for wind analysis-Static and Dynamic. In this article I will try to explain the static method of wind analysis using Caesar II of COADE Inc following Pressure Vs Elevation Profile.
Criteria for selection of lines for Wind Analysis:
Criteria should be mentioned in ITB document. As a guideline the following can be followed after verification from client:
- Lines with outside diameter 10” and larger (including insulation) running on 10 m and above.
- Steam / Flare header on the pipe rack.
- Other lines considered important as per stress engineer’s decision.
However if lines are covered by some shelter or other structures then wind analysis can be ignored for those lines.
Data Required for Wind Analysis:
For wind analysis you must have following data from the client.
- Wind shape factor: Normally for pipe elements the data varies from 0.6-0.8. Check in ITB what value it says to use.
- Pressure Vs Elevation Profile: Sometimes client provides this profile directly and sometimes provides equation and data to calculate the profile. A typical wind profile will be shown in diagram while explaining the steps required while analyzing using Caesar II.
- Elevation of the line under analysis. If HPP elevation is other than 0 you have to reduce HPP from line global elevation to get actual elevation.
What to check:
As per code B 31.3 we have to check code compliance of the calculated stress (Sustained +Wind). The allowable stress for wind analysis is 1.33 times Sh values. However sometimes client requires to check the nozzle loading in Operating+ Wind cases (W+T+P+Win) for static equipment. Normally client does not require wind load checking for rotating equipment.
Steps for Static Analysis in Caesar II:
Most of the steps are mentioned in attached images. All are self explanatory. However if you face any problem in understanding please reply in comments section.
- Model the piping system under analysis from piping isometric drawings.
- Enter elevation of the first node in global coordinates.
- Click on Wind/Wave check box on Caesar II Spreadsheet and mention wind shape factor as shown in Fig. 1
- Now run the analysis and go to the load case editor and select Pressure Vs Elevation as shown in Fig. 2
- In next step enter the pressure vs elevation profile in consistent unit and enter wind direction cosines as shown in Fig. 3. Normally wind analysis is performed considering wind flow from North, South, East and West direction. Accordingly Enter +1 or -1 in X or Z direction. Wind analysis is generally not considered in vertical direction.
- Refer Fig. 4 and prepare the highlighted load cases additionally for wind analysis. Load cases for L17 to L20 are for code compliance checking and load cases from L5 to L8 for support and Nozzle load checking.
- Refer Fig 5 and make the combination method as scalar or absolute for the shown load cases.
- In the final stage run the analysis and check results. If failing make suitable adjustments to qualify the same.