I have been designing gravity networks using Bentley products for a few years and have always found issues with the automatic pipe sizing function choosing pipes sizes larger than they need to be.
For example, a have a pipe network running crown to crown with the usual default design constraints set up - pipe full design, single barrel, min v 0.75 m/s, etc.
If we take a pipe length leaving a T-junction (rational flow of 0.6 m3/s, slope 1 in 800). I click run and the network sizes that section at 1200mm dia (v=1.25m/s, d/D = 44%, Capacity Full % (for both design and full flow) = 41.6%).
This pipe clearly has spare capacity and can be reduced (Size is not influenced by the pipe sizes upstream of the junction as they are smaller (and also oversized)).
I now set this pipe length to 'do not design' and rerun the model with a 1000mm section size. Results are v=1.25m/s, d/D = 59.3%, Capacity Full = 67.1%. All the constraints are met and no error messages.
So why did the programme select the larger pipe? I've checked my conduit catalogue and all pipes diameters are available for design. The whole range of sizes are being used across the network so there is now problem there.
If anyone could let me know if there is a fix for this issue that would be great. I normally optimise the network by specifying local pipe constraints and limiting section sizes or in analysis mode. However it would be good if the programme could size the pipes correctly first time as our typical networks comprise of 1000+ pipes!
Cheers Dave
Hi Dave,
I suggest that you send this model to tech support so that they can help troubleshoot. You can create a 'service ticket' and attach your model here: appsnet.bentley.com/.../addcall.aspx
In the meantime there are some things you can check...
First, check the results in profile. Make sure that the d/D at both ends of the pipe is less that 100%.
Second, check the pipes downstream. One of the StormCAD pipe sizing rules is that a pipe can't be smaller than a pipe that is downstream.
Finally the design calculations are done using the 'Capacity Analysis' flow profile method, while the results you list above were probably generated using the 'Backwater Analysis' flow profile method. There is some more info in the help about the difference between these two methods, but it is possible that the Capacity Analysis says the pipe is running full while the 'Backwater Analysis' says it's not. Go into the Calculation Options and change the Flow Profile Method to 'Capacity Analysis' and check the 1000mm pipe results again.
Regards,
Mal
HI Mal,
Would you please explain more about Capacity Analysis and Backwater Analysis methods? how the StormCad deal with that? and when each method should be used?
Hi Mohamad,
The 'Capacity Analysis' is more simplified and the resulting hydraulic grade line is basically a straight line for a particular pipe.
The 'Backwater Analysis' does a gradually varied flow analysis where the hydraulic grade line is varies along the pipe (e.g. from critical depth to normal depth).
See attached screenshots for an example of the differences. Capacity Analysis is faster, but Backwater Analysis gives a more realistic hydraulic grade line.
Mal,
Thank you for your reply.
I have tried everything but there seems to be a glitch in the program. Maybe I have set too many constraints? I will submit the model for review.
Below is a profile with a 700mm pipe and 3 No. 800mm pipes following the stormCAD design run. All are around <40% full.
Here is the data for the first 800mm pipe.
I reduce one of the 800mm pipes to a 600mm. I also reduce all the 700mm upstream to 600mm. Here is the modified profile and data.
The calculation messages says 'conduit discharge is above design discharge' but that's not what the data says???
Dave
Mal, Dave ,
was this ever resolved ? we are having similar issue , we have already sent a service ticket but didn't get a response back yet.
appreciate any feedback.
regards.
JH