Hi,
I am working on a closed system with no downstream storage (no tank). I have learned that for a system with no downstream storge, I should create a scenario with pressure dependent demand alternative. I have followed the recommended steps. but unfortunately, even after applying PDD, I still have the same issue. before running PDD, the head ranges from -600000 ft to 16000000 ft but even after running PDD these values changes from -100 ft to 170000 ft at 90 gpm!
I am not sure what could be the issue. I would appreciate it if you could help me with that.
Best,
Fari
Hello,
You must be referring to the workaround mentioned in below article.
System head curves with no downstream storage
Could you please confirm if the pressure dependent demand function is configured to have no threshold limit and function type is set to power function? See: Using Pressure Dependent Demands
If required you may upload model files for our review.
Sharing model files.
Regards,
Sushma Choure
Bentley Technical Suppport
And make sure your calculation options are set to "Use PDD = True"
yeah, I checked both the threshold limit and function type, as well as calculation option in the scenario and they all seem to be right. When I run the model for ADD with PDD, this is how the system head curve looks like with the existing normal pump. But the weird thing is if I just change the maximum flow in the system head curve, the system curve changes! these two pictures show how the system curve changes at two different max flow (89 gpm and 90 gpm)., and as you can see, after applying PDD, the system head curve does not seem right to me! the head is changing from -100 ft to 1800 ft!
also, I wouldn't expect the system curve changes just by changing the pump definition, right? but by changing the pump definition (to for example the fire pump), the system curve changes!
Thank you so much for your help. Let me know if you need more clarification. I would be willing to share the model with you (but preferably not in a public forum) if necessary.
Thanks, Sushama for the response! I have checked both the threshold limit and function type, they seem to be right!
I have uploaded the model "Castle Hill PS Model with PDD 1005" for you to review.
Thanks for your help!
Hello Fari,
Can you confirm which scenario you are looking at?
In addition, there are valves in the model, which can cause some differences in the appearance of the system head curve. See this forum post for info. It has also been documented in the system head curve wiki.
Scott