Modeling Large Development with Multiple Connections.

Question

I have read Scott Kampa's article on modeling a connection of a new system to an existing system by use of reservoirs and pumps. It was a very helpful and informative article. However, he states to not connect the existing system in two locations as this may provide false data. I am currently modeling a 180 acre future multi use development in which we will have to make multiple connections to the existing system in order to meet the demands on the new system and to loop the network. Am I misinterpreting what Scott is saying or how can I model a system that will be looped and have multiple connections since this a very large scale system?

Jesse Dringoli · Accepted Answer

Hello Chad, 
 Here is a link to the article you mentioned: 
 Modeling a Connection to an Existing System 
 As the article explains, multiple connections (using the fake pump and reservoir technique) may not be valid, because the measurements you used to develop the pump curve at the connection point were likely not taken at the same time. In the model, both connections will be flowing at the same time. So, the pressure at one connection point would likely drop as a result of water flowing in from the other connection. 
 Now, if you have a situation where the upstream system is large and will not experience a significant change in pressure as a result of multiple connections flowing at the same time, or if there are separate, disconnected systems upstream of each connection point, modeling multiple connections may be fine. Of course, they would also need to all be flowing in a single direction at all times, since the pump approach would not support reverse flow. 
 I have updated the Support Solution article to include these added details.

MPachlhofer · Answer

Hi Chad, 
 You're not misinterpreting what Scott had written and the reasons for not modeling more than one connection to an existing system using the pump approximation method are explained in the assumptions and limitations section of the article . You can model one of the connections using the pump approximation method, but for the other connections you need to model you should model them all the way back to the source at least by obtaining a skeletonized data on the existing system. The skeletal model must begin at the real water source(s), such as the pump or tank, which will serve as the primary water source(s) for the new extension pipes. It should be calibrated using the results of fire hydrant flow tests, especially the tests conducted near the location where the new extension will tie in. You may need to call the municipality to obtain basic information on the existing system. Also, you should keep in mind that modeling all the way back to the source will always be the most accurate way to model a connection to an existing system. 
 Regards, 
 Mark

Tom Walski · Answer

When you are working with a 1800 gpm demand and multiple interconnections, you really can't separate the development site from the rest of the system and expect to get accurate results. The interactions are just too complex to get by with some of these shortcuts you can use in other situations. 
 
I suggest you get together with the City water distribution engineers and discuss how to model this. Ideally you would get a copy of the City model (at least this pressure zone) and build your model on top of it. Or you can get their distribution maps and build very simple skeletal model of the system. You need to model back to some real boundary condition. 
 
I'm trying to think how you might get by with some shortcut. One, somewhat far fetched, test might be to run (assuming you have 6 connection points) 6 simultaneous flow tests. You couldn't run all the hydrants wide open (unless you have a very strong system) without lowering the pressure too much. So, if your demand is 1800 gpm, you could run 300 gpm at each flowed hydrant. This isn't as good as running a model of the full pressure zone but at least it gives you an idea of what a 1800 gpm demand will do to the City system. 
 
Good luck.