Common practices to simplify water distribution system model for hydraulic transient modelling

For a transient analysis, you'll generally want to model things as closely to the real system as possible. Bentley HAMMER tends to be more sensitive to modeling artifacts (things placed to "force" desired steady state/EPS results) as they can skew the complex interactions that occur during a transient simulation.

With that said, it is not uncommon to approximate boundary/connection points as either a fixed hydraulic grade, atmospheric discharge or a demand. It's important to have an understanding of how the wave reflection differs between these approaches so you know how this could potentially effect the overall transient envelope. It's also common to simplify a network down to focus on specific areas of interest for transient analysis such as large diameter mains. This can also potentially skew transient results but in some cases the results may not differ by a significant amount. An example of a study of this can be found in our official HAMMER training course on Learn.Bentley.com.

With that said, could you elaborate a bit more on this "feedermain" and exactly what "ties" to it? A diagram might help.

Thanks Jesse for the response. It is very helpful. I attached a sketch. I am trying to find way(s) to simplify the network for transient study. As you can see from the sketch, our work scope is only a small portion of the system (the system is slightly over 1000 nodes). In addition as you mentioned re steady state results, if I include the entire system in modelling, I am also including the uncertainty of the network steady state results. 

Shenh,

So to confirm, you are just looking to simplify the area indicated in the drawing as "there are a few pumps and tanks here" and the connection points just below it?

Do you have any specific concerns about modeling the entire network? (maybe some of them can be addressed?)

It's hard to say exactly what the best way is to simplify a given model without compromising accuracy of transient results. A good understanding of the system and some engineering judgment will be necessary.

If the majority of the system you want to simplify is the "network" part of the diagram, then it might be best to keep at least the interconnections shown below it (as they would probably have the biggest impact on transient response) and simplify the "network" part. Simplify it in such a way that the difference in how a wave reflects off your simplification (vs how it would interact and reflect with the "full" system) does not significantly effect the overall transient results such as the transient envelope (max/min HGL profile). Highly looped sections of a network that would otherwise dissipate the energy of a transient wave relatively quickly would probably have the least effect on the overall transient results, so focusing on simplifying that would be best.

If you have a skelebrator license, you can use that (it's located in the Tools menu). It automates the process of simplifying a network with hydraulic equivalency (steady state equivalent, at least) and will move demands, lump them together and come up with equivalent diameter pipes as appropriate.

Also if you're able to, I'd recommend the official HAMMER training courses in learn.bentley.com. Here's the outline for the course "Skeletonizing and Transients in Networks":

•Understand how to skeletonize a network down to a reasonable size without losing valuable network information and without changing the results
•Understand the different Skelebrator options you can use to reduce the size of a network
•Understand what a network is and the different network topologies
•Understand the importance of accurate elevations in networks