Comments, Questions, and Answers:

Item #1:

How to model fluid / gas density in AutoPIPE?

Answer:

Using AutoPIPE V8i 09.06.xx.xx, a user can input the contents density value on the Pipe Properties dialog screen / Input grid:

Note: As mentioned in AutoPIPE help, Specific gravity is relative to density of Water taken at temperature of 40 deg F and atmospheric pressure (14.7 psi).

Also important to consider that fluid density changes with temperature.

AutoPIPE has a feature on the Load> Static Analysis grid where the user can provide a "Fluid Density Factor" per each analysis set. This factor is multiplied by the "Specific gravity of contents" value for all pipe properties in the model for that analysis set. 

Define separate analysis sets for each temperature case and modify the fluid density factor as needed.

Note: Can the fluid density be zero (0.00) or blank? Yes, this is OK. One reason to set the density factor = 0 would be to analyze the piping system in a installed but not operating condition. Another words, this would help users to analyze their model with different states of fluid (no fluid [installed], high temperature, high pressure, etc.) at the analysis level without having to change individual content specific gravity. The 0.0 value of fluid density factor is inclusive of 0.0 value and it does not cause any known problems with any AutoPIPE calculations.

Lastly, be sure to confirm each Static Analysis Set load case dialog for the state of the Contents Checkbox (see help for details on this feature).

Item #2:

How do consider the weight of the fluid in the system, If the service is steam?

Answer:

Yes, even a gas has a molecular weight that can be considered in the analysis. Suggest entering a small value in the "Specific gravity of contents" field on the Pipe Properties dialog screen. Then be sure that the Static Analysis set "Density factor" is 1.00 as needed.

Item #3:

In static stress analysis, if the medium is steam, does the medium fluid density need to be modified? The software defaults to 1.00. When establishing the pipeline model, the medium specific gravity has been modified, but does it still need to be modified manually again during the static stress analysis?

Answer:

To accurately mimic the fluid in the pipe or surrounding the pipe, each individual setting must be manually adjusted as needed:

1. Fluid transient - Fluid / Density
2. Pipe Properties - Specific gravity of contents
3. Hydrotest - Specific gravity of fluid
4. Wave Load - Water density
5. Buoyancy Load - Specific gravity of water
6. Modal Analysis - Fluid density factor
7. Analysis Set - Fluid Density Factor

Read AutoPIPE's help to fully understand what these settings are and how they will affect the analysis. 

See Also

Modeling Approaches in AutoPIPE

Bentley AutoPIPE