Can Darwin Designer help design the number of parallel pipes to use or an optimal design layout?
The use of the zero diameter option in the cost/properties table (along with a zero cost) can be used to give Darwin Designer the freedom to essentially remove a pipe as a possible option in the design (make it inactive during the calculations). This means you can for example lay out two pipes in parallel and Darwin can help determine if it is optimal (based on contraints, cost, etc) to have two parallel pipes or one single pipe. This is demonstrated in the Quick Start Lesson called "Darwin Designer to Optimize a Pipe Network", under Help > WaterGEMS Help > Contents > Quick Start Lessons.
You can also use the zero diameter option to help in optimal design layout, whereby more pipes are being sized than are necessary to service all demands. For example, a certain segment of piping (for example around a loop) could be omitted from the design (zero diameter/inactive state with zero cost) if optimal.
In these cases, it is important that you only assign the cost/properties with the zero diameter option to the set of pipes that are not essential. For pipes that are essential for service and that must be sized, define and use a pipe-option group that contains no zero diameter option.
For more information, see the Help topic called "Costs/Properties Tab".
Forum Thread - Darwin Designer