Pipe Stress Analysis: Preliminary Procedure

Model the piping system in CAEPIPE (either directly inside CAEPIPE, or by using one of SST’s data translators to import the piping model) and follow the steps shown in the CAEPIPE Tutorial to learn the basics of operating CAEPIPE to create and analyze a model and review its results. Once all the data is in, Analyze. Now, review Results. 

General Procedure 

From the thermal stress contour plot and the deflected shape for thermal load case, suitably route the pipe to make it more flexible and position axial restraints and/or intermediate anchors, if required, to direct thermal expansion/contraction away from critical locations such as equipment nozzles. Similarly, decide the types and locations of vertical supports based on the stress contour plot and deflected shape for sustained (= weight + pressure) load case as well as the deflected shape for operating load case (= sustained load + thermal load).

Step-by-Step Procedure

• Step 1

Review the thermal stress contour plot first. The plot is color-coded such that “blue” region denotes areas with the least stress ratios (where stress ratio equals to actual computed stress divided by material allowable tress), “green” region with higher stress ratios, “yellow” region with even higher stress ratios, and “red” region with the highest stress ratios. Intermediate areas between these distinct colors will be of “bluish-green”, “greenish-yellow” and “orange” colors. 

The goal will be to arrive at a layout that avoids “orange” and “red” zones in thermal stress plot so that there is sufficient thermal margin left for performing a detailed piping analysis when the layout is finalized at the 3D-design stage. You may wish to avoid even the “yellow” zone in the stress contour plot so as to provide additional thermal margin for future use. Since thermal stresses generated are directly dependent on how stiff” or “flexible” the layout is, in order to reduce thermal stresses, it may be necessary to make the layout flexible” (by including bends, offsets, loops etc.). So, the first step is to make sure thermal stress ratios remain within “blue to yellow” range and not get into “orange” and “red” zones. For more “flexible” layout, even “yellow” zone can be avoided. 

• Step 2

In case thermal stress ratios exceed “yellow” zone (and are in “orange” and “red” zones in one or more areas of the piping system), it is important to study the thermal case deformed shape provided by CAEPIPE in order to understand how the piping responds to “pure thermal” load. By studying such deformed shape, it is possible to arrive at a layout with appropriate bends, offsets and loops and/or with appropriately located axial restraints/intermediate anchors such that thermal stress ratios do not exceed “yellow” zone. This process may require you to perform several layout and/or restraint scheme iterations. 

• Step 3

After finalizing piping layout under Steps 1 and 2 for thermal loading, the next task is to support the system vertically to carry its deadweight under operating condition. In this connection, first review sustained stress ratio contour plot generated by deadweight and pressure for the system without any vertical supports excepting those provided by equipment nozzles, shown in color codes from “blue” to “green” to “yellow” 

to “red” (as in Step 2 above). 

Your goal is to arrive at a vertical support scheme consisting of 

(a) resting steel supports (b) rod hangers © variable spring hangers and (d) constant support hangers 

at appropriate locations (where such pipe supports can be attached to adjacent concrete/steel structures, platforms etc.) so that stress contour plot for sustained stress ratios avoids “orange” and “red” zones and remains within “blue to yellow” range. 

• Step 4

In case sustained stresses exceed “yellow” zone in one or more areas of the piping system, study the deformed shape for sustained load case in order to understand how the piping responds to its own deadweight: next, identify pipe locations in the 3D model where the pipe can be vertically supported by the support types listed above. Based on this information, it is possible to vertically support the piping such that 

sustained stresses do not exceed “yellow” zone. This step may require you to execute several iterations within CAEPIPE with several vertical support schemes. 

In case, resting steel supports are selected to provide vertical support for piping under sustained load, it is to be made sure that piping continues to rest on such steel supports even during operating condition (= weight + pressure + thermal) and does not lift off from these supports. If pipe lifts up at any of these resting supports during operating condition, then that support does not carry any pipe weight and hence will not serve its purpose. Similarly, at rod hanger locations, the tendency of piping should be to deform downward for operating load case, so that the rod hangers carry the pipe weight under tension. On the other hand, if pipe lifts up at any of the rod hangers, then that rod hanger goes into compression thereby not carrying the weight of the piping during operating condition. 

CAEPIPE displays the deflected shape of piping under operating load case too. By viewing this deflection from different directions, you can make sure that piping is resting on steel supports and/or piping is not deforming upward at rod hangers. 

• Step 5

You should perform Steps 1 to 4 for all piping systems of the project. Systems, for which the layout and support schemes are finalized, are ready for detailed analyses and stress report preparation. 

• Verification Step

Provide all additional input data into the models such as insulation thickness and density, corrosion allowance and mill tolerance of pipe sections, thermal anchor movements, seismic anchor movements, support conditions such as friction and gap, other loads such as wind, seismic and water/fluid hammer, multiple thermal and pressure cases, etc. and perform detailed analyses. It is most likely that the layout and support schemes (finalized during steps 1–4) meet all other pipe stress requirements (such as meeting nozzle allowable loads) and hardly require any further iteration(s).

Reference:

https://www.sstusa.com/pdfs/basic_pipe_stress_analysis_tutorial.pdf