This article contains FAQ related to Design Request elongation settings.

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FAQ

How do pressure and temperature elongation settings affect the results in CSiPlant?

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 DR 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)

Elongation Settings

For each design check stress category (except Pressure), the elongation settings are used to provide control for how Reference 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):

• 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). 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 otherwise by design code (some codes absolute different stress components), manually calculated longitudinal stresses have the following sign convention:
  • Internal pressure causes a positive average hoop stress, which generates a longitudinal contraction due to Poisson effect.  When the contraction is restrained, a positive (tensile) longitudinal stress develops.
  • Internal pressure causes a positive (tensile) longitudinal stress due to End-cap/Pressure Thrust.
  • External pressure causes a negative average hoop stress, which generates a longitudinal elongation due to Poisson effect.  When the elongation is restrained, a negative (compressive) longitudinal stress develops.
  • External pressure causes a negative (compressive) longitudinal stress due to End-cap/Pressure Thrust.
  • Operating Temperature greater than ambient causes a longitudinal elongation due to Thermal expansion. When the elongation is restrained, a negative (compressive) longitudinal stress develops.
  • Operating Temperature less than ambient causes a longitudinal contraction due to Thermal expansion. When the contraction is restrained, a positive (tensile) longitudinal stress develops.

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.