Why is that there is a big discrepancy between the AutoPIPE Hanger Analysis calculated values and the values that are actually on the spring cans that have been in service for many years.
Highly recommend to review AutoPIPE online help mentioned at the top of Spring Hanger Selection WIKI page for details on how the program selects a spring hanger.
Reasons why the results of the Spring Hanger selection report does not match Static Analysis results:
1. Is it a Hot load or Cold load Spring hanger analysis?
Depending on the type of spring hanger analysis being performed, the reported hanger cold / hot load will be a linear interpolated from the spring hanger manufacture's literature. Again, the Spring Rate and Cold load will be the only values imported into the model from the spring hanger selection report. When the actual analysis is performed using these ascertained values, the results may differ from the value of the hanger selection report. This is not uncommon.
Based on the Spring hanger report below for Point J34, one can clearly deduce that the spring was analyzed using the Hot load procedure. AutoPIPE's hanger selection calculated a hot load (37566 lbs) and the travel distance (-4.6 in.). These values where used to interpolate a cold load (35145 lbs) from the spring manufacturer's table. The program then transferred the cold load (35145 lbs) and spring rate (524.8) to be used for other analysis.
Note: AutoPIPE's Spring hanger selection procedure may report various spring sizes starting with the longest range and highest spring rate and finishing with the shortest range and lowest spring rate that meets the criteria. For each node point, 1 to 5 different springs may have been chosen. These reported springs are just a suggestion. As mentioned previously, the selected spring settings are then used by the software to perform all remaining analysis. Based on these analysis results, these values may require additional changes or refining. Suggest trying each of the different spring options at each node point recommended by the spring hanger selection report to see which is the best fit.
After completing the preliminary analysis phase, one would then contact a spring hanger manufacturer and provide them the final Spring Rate, Cold Load, and Travel distance for each undesigned spring. The spring manufacturer will validate the spring choice or present alternate possibilities. Consider these alternatives in your analysis and continue with revisions until the analysis results are acceptable.
2. Does the Spring hanger analysis settings match the Static Analysis settings in the model?
The Spring hanger analysis dialog has a number of analysis settings (ex. Design operating cases, Include pipe content weight, Include cutshort in design, Gaps/Friction/Soil) that should match Static analysis settings. Furthermore, additional Non-Linear analysis settings are considered when Spring hanger analysis option Gap/Friction/Soil checkbox is enabled. All of which must be specified and compared to the Static Analysis settings Non-Linear Analysis settings. Any discrepancies across all of these settings would help answer why there are differences between the hanger report and static analysis results.
3. Does Anchor have any axis direct / rotation Release for hanger selection?
On the Anchor dialog, users can release anchor stiffness settings in different directions (ex. X, Y, Z, Rx, Ry, Rz) for hanger selection only. This release of anchor stiffness will not be accounted for in the static analysis / results. Review AutoPIPE help for details of using this feature.
In conclusion, the final spring that best fits the criteria may be the same or vastly different from that mentioned on the spring hanger selection report. Ultimately, if you disagree with the Spring Hanger selection, override the automated values with your own to be used during the actual stress analysis.
Spring Hanger Selection