04g. Model Fluid / Gas Density in AutoPIPE


Comments, Questions, and Answers:

Question #1:

How to model fluid / gas density in AutoPIPE?

Answer:

The contents weight is based on the specific gravity of the contents relative to water, where water = 1.0 and gases have very low SG values. and both describe content weight being calculated from the contents' specific gravity relative to water.

Open the pipe properties dialog and enter the Specific Gravity value related to Water into the field:

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).

Question #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.

Question #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. 

Question #4:

In my pipe system I need to transport air with 15 psig and 100 C, Air density under these conditions is 1.91kg/m3. Is it correct if in the Pipe Properties I write 0.00191 in SG field?

Should I change something in Fluid Density Factor?
 
 

Answer:

As mentioned above, the software's Specific Gravity field is defined relative to water density:
 
Using:
SG = 0.00191
 
In regards to the fluid density factor, normally leave it at 1.0 unless you intentionally want to scale the fluid mass for a particular load case (refer to AutoPIPE help for details on this setting, then you must determine what the scale factor will be for your project). 
 

See Also

Modeling Approaches in AutoPIPE

Bentley AutoPIPE