PlantFlow FAQ

Index:

1. Can you give me some convergence tips?
2. Can PlantFlow model heavily fouled piping?
3. What is best approach to analyze complicated heat transfer problems?
4. How do I analyze my long distance gas pipeline as an isothermal system?
5. The results for my long pipeline appear incorrect. Why?
6. I am modeling a campus distribution system (both supply and return piping) and want to model building loads by specifying a flow and a pressure drop through a building. Can I do this by inputting a variable user loss with one flow and one constant pressure drop?
7. My system is unstable and will not solve. Why?
8. What does the warning message E72-17 "Consumptions at all inlets/outlets" mean?
9. How can I print or save the graphics to my word document?
10. My compressor model is not converging and giving the warning message "negative temperatures found" and no results?

Question 1: Can you give me some convergence tips? 

Answer: See the list below for tips:

• You should always have small pressure and flow tolerances.
• Smaller relaxation factor will help convergence, but will take more iterations.
• When no heat transfer is used, disabling tee losses will help convergence.
• Using property table might speed iterations for gas mixtures.
• Non-convergence may occur when "Include losses in T-junctions" is checked because the program has difficulty resolving a flow direction in a loop and corresponding tee losses.
• Use the Restart File option
• Also check the PlantFlow help topic "Tips for Convergence and Instability Problems".
• It is important to give reasonable boundary conditions and/or flow directions, which are the main cause of non-convergence.  

Question 2: Can PlantFlow model heavily fouled piping?

Answer: Yes PlantFlow can simulate heavily fouled piping by the following:

Model the clean pipe in model #1 then file > save as model to different filename, model #2. In model #2, change the pipe roughness. Modify > Pipe properties, enter a higher Roughness value (when using General flow equation).

If the pipe is heavily fouled such that the bore has reduced significantly then Under Modify > Pipe properties, change Schedule to NS and enter user defined thickness to define a new smaller pipe inside diameter and also update the roughness to an appropriate value e.g. see typical roughness table in Help.

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Question 3: What is best approach to analyze complicated heat transfer problems?

Answer: Regarding the iteration process, the program analyzes the simpler isothermal model to get better initial flow estimates for use in more complicated thermal analysis. This method will have better chance of convergence, especially since we cannot set individual flows in different pipes (one initial flow). 

Question 4: How do I analyze my long distance gas pipeline as an isothermal system?

Answer: The option under Tools > Model Options > General "Consider Temperature effects" = OFF and set the initial gas temperature in the fluid properties dialog. If heat transfer effects are considered then the compressor and boundary conditions defined determine the temperatures used in the calculations.

Note: The base temperature used in General Model Options refers to temperature to use for standard flow units sm3/sec, which means std. M3/sec at 101 kpa and 15 degC.

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Question 5: The results for my long pipeline appear incorrect. Why? 

Answer: When analyzing long pipelines with heat transfer, the accuracy depends on the pipe length. In this case you need to add intermediate points. You can do so using Tools > ModelOptions > Edit and set to A(Auto) and use 5 as L/D ratio. You can study sensitivity of result to L/D ratio by using smaller lengths. 

Question 6: I am modeling a campus distribution system (both supply and return piping) and want to model building loads by specifying a flow and a pressure drop through a building. Can I do this by inputting a variable user loss with one flow and one constant pressure drop?

Answer: You will need to specify either a fixed pressure drop or fixed flow, it may unstable to have a variable user loss with fixed flow and pressure drop.

Fixed pressure drop
Either connect the supply and return with user loss element and specify fixed pressure drop or k-factor.

Fixed flow
PlantFlow does not have fixed flow element so recommend a disconnect between supply and return lines and enter the same known consumption for both inlet and outlet Ctrl points (no known pressure).

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Question 7: My system is unstable and will not solve. Why? 

Answer: Your model appears like a single system but the program sees at least 3 separate systems for example, each is not connected to the other system. Attached is a test model showing 1 separate system and the model listing shows 21 inlets and 4 outlets control points but are all known consumptions but no known pressure in this system hence the program cannot solve it. At least one known pressure must be specified.  

Question 8: What does the warning message E72-17 "Consumptions at all inlets/outlets" mean? 

Answer: This means there is one redundant consumption in the system. You should not specify known consumption all inlets and outlets. At least one of these consumptions should be unknown.

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Question 9: How can I print or save the graphics to my word document?nbsp;

Answer: Your printing graphics options are:

• File > print > graphics > Printer - send the current plot to the printer
• File > print > graphics > File (this will save a PLT i.e. HPGL file which can be opened in MS PowerPoint)
• File > print > graphics > DXF (save the model as a 2D DXF file which can be opened in AutoCAD R14 or higher)
• Alt + Print screen to screen capture the current PlantFlow window to the clipboard which can be pasted as a picture into MS Word or a graphics program. Then cropped, modified and annotated.   

Question 10: My compressor model is not converging and giving the warning message "negative temperatures found" and no results?

Answer: Your model appears to have difficulty converging since there are different fluid phases in the solution. Since your model is a relatively simple system, model and analyze the suction system up to the compressor in model #1. Create a model #2, which includes the compressor and discharge piping e.g. using copy > paste, copy the compressor plus some suction and discharge pipe as shown in your model and solve the simplified system given the inlet conditions from the suction side. Enter the compressor inlet results from model #1 and analyze the compressor/discharge in model #2.

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