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Answer: The pressure and temperature elongation settings found in the Design Request (DR) definition form are used to control how pressure and thermal loads are used in the design (the base analysis results are not affected by the elongation settings, only design result sets are impacted). Figure 1 shows the Design Request DR form where elongation settings can be specified.
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For each design check stress category (except Pressure), the elongation settings are used to provide control for how pressure and temperature loads are included (or not) in the analysis design results. The following behavior should be expected:
- With Pressure/Temperature elongation Disabled, the Reference Pressure/Temperature loads are NOT included in the design analysis.
- With Pressure/Temperature elongation Enabled, the Reference Pressure/Temperature loads ARE included in the design analysis.
Note that the elongation settings DO NOT apply to the following situations (Figure 2 shows the non-linear static load case form, highlighting the reference pressure and temperature drop down menu):
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- Base analysis result sets. The Base analysis result sets includes all loads specified in the load case, irrespective of the elongation settings.
- Pressure/temperature load patterns directly added to the load case (i.e. in addition to Reference Pressure or Reference Temperature) are not Figure 1 shows the Design Request form where elongation settings can be specified.
Figure 1: Design Checks tab has the elongation settings, allowing separate treatment for each Design Stress Category (except pressure which is not editable and defaults to include pressure elongation/exclude temperature elongation)
Pressure and temperature loads can be included in the analysis to generate internal forces and moments. The elongation settings are used to provide control for each design stress category on how those loads are included in the analysis.
For the elongation settings to be applied as intended in the design check, the pressure and temperature patterns of interest must be specified in the Reference Pressure and Temperature fields. Adding pressure or temperature loads directly to the load case bypasses the elongation setting and the corresponding loads will be included in analysis. Figure 1 shows the non-linear static load case form and the different ways pressure and temperature patterns can be added.
Figure 1: Non-linear static load case form indicating Reference condition fields.
- . Adding pressure or temperature loads directly to the load case bypasses the elongation setting and the corresponding loads will always be included in the analysis.
Figure 2: Non-linear static load case form indicating Reference condition fields.
Effect of Restraint Condition
An advantage of enabling Elongation settings, is CSiPlant to automatically account for the degree of restraint in the system. By analyzing the Reference Pressure/Temperature loads acting on the piping model flexibility, the appropriate longitudinal stress effect is captured. If there is sufficient flexibility, Poisson and Thermal elongation can freely occur without generating internal stress which leaves only Pressure Thrust/End Cap forces. If there is sufficient restraint, Poisson and Thermal elongation are restrained and the corresponding Elongation forces are calculated, while Pressure Thrust/End Cap forces are minimized.
Implication on Design
Different design codes cater to different types of Piping system. Some are buried, some are not. Others request axial stresses from pressure are scaled with the axial SIF, some require only the non-pressure related axial stresses are scaled by the axial SIF and others do not use axial SIF. Clearly, the implications on design depend to some degree on the Design code specified on the DR settings. The following is a summary of the implications on design.
Elongation Disabled
- As no elongation loads are considered in analysis, a manual calculation is necessary to include the longitudinal pressure/temperature stresses if requested by code.
- Unless specifically directed by the design code, manually calculated longitudinal stresses will be combined with other axial stress the following
Elongation Enabled
- The design results automatically include the effects of pressure/temperature loads from analysis.
- A manual stress check is not performed if the corresponding elongation setting is enabled. This avoids the potential of double counting the longitudinal stress due to pressure or temperature.