Regarding Pipe diameter in Water gems

Question

In WaterGems whenever I give a loop pipe network to Darwin designer in order to optimize the pipe diameter, it never gives results in decreasing order of the pipe dia, rather it is quite random. Can I know the reasons?? What could be the correct method? 
 For Example: 
 
 In the above pipe network (which I have made just for practice), when I use Darwin Designer for all the pipes the dia is in between 200-300mm but for red one it is 2500, if I manually decrease the dia of red marked pipe to 300 then even at that dia all other pressure criterias are fulfilled along with that of velocity.

Tom Walski · Answer

That is the nature of genetic algorithm optimization. It optimizes the objective function, usually to give the least cost solution, even if the solution doesn't always make sense. It is up to you as the engineer to review the solution and design something that will work in the long run. 
 
Remember that you entered some demands to server as the basis for the analysis. Are you 100% certain that the demand will occur? I can pretty much guarantee that the future will be somewhat different that your demand forecast. 
 
You also need to consider reliability. What happens when a pipe on the other side of the loop must be shut down for repair/maintenance? 
 
The "optimal" solution should be regarded as a good starting point for your design but as the design engineer you may need to modify it based on experience and engineering judgment. I've often said that the most important tool in design is located, not in the computer, between the engineer's ears.

tlfoster56 · Answer

You may want to look at the section titled Advanced Darwin Designer Tips in the WaterGEMS help section. It could be the way you have your data set up; the range of pipe sizes and rehab actions you are using; the design constraints, etc.

MPachlhofer · Answer

In addition to what Terry and Tom said you can also have a look at this wiki that we have on Darwin Designer for more tips, instructions on how to use it, and a free webinar on that goes over this information. 
 
 communities.bentley.com/.../6271.using-darwin-designer 
 
Regards, 
Mark

Jesse Dringoli · Answer

Check out the "tips" help topic that Terry mentioned, item # 7: 
 Designer keeps coming up with pipe sizes that change up or down in size. I wouldn't construct such a design; what can I do? 
 Darwin Designer applies a competent genetic algorithm to optimize the design. It does not require or have any domain-specific knowledge about the water system, which ensures it is a generic tool, but also causes some side-effect for some design cases-like giving up-or-down pipe sizes. In particular, the solutions are evaluated by comparing the fitness values of solutions. Darwin Designer will assume a pipeline with pipe sizes that go up and down (to meet required pressures as closely as possible) is better than one that has a constant size that exceeds the pressures at some locations, since there is no specific penalty assigned to the fitness of a solution that has pipes that change up and down in size. It is, therefore, up to you to control the eventual design and this can be done by different means, as follows:

The first means is simply to make manual adjustments to a design after Darwin Designer has finished, in order to clean up the design and make it a practical design. Cleaning up a design may technically move you away from the cheapest design, but an inexpensive design that won't be constructed is of little use. You may find that not much cleaning up is necessary. Quick edits to diameters or rehab actions like can be performed effectively in Darwin Designer by using a manual design run. 
 Another thing to consider when analyzing a Darwin Designer design is whether the chosen pipe sizes are a function of the lengths of pipe in your model.
 To better illustrate this concept, consider a run of four pipes in series, each with different lengths. For these four pipes, the controlling pressure is the downstream-most junction, and all intermediate junctions are well above the required pressure. Now, after Darwin Designer finishes designing the run of pipe, it selects the first pipe as a 16 in., the second as 12 in., the third as 16 in. and the fourth as 12 in. It is unlikely that this design would be constructed as-is, but if the pipes themselves represented sufficient length of pipe, then it may be practical to construct a portion of the pipeline as 16 in. and a portion as 12 in. If this is the case, then you need to look at the model to determine why Darwin Designer is changing the third pipe back up to 16 in. It may be that since the downstream-most junction is the only controlling node, that Darwin Designer is merely trying to achieve the right head-loss in the total pipe length, by choosing the length of pipe that should be 16 in. and the length that should be 12 in. Of course, it is still constrained by the individual pipe lengths in the model, but if they are different, the optimization algorithm will use this fact to its advantage. In this case, it may very well be that Designer is saying construct a total of 1500 ft. of 16-in. and 1000 ft. of 12-in. pipe, and not necessarily 850 feet of 16-in., 600 feet of 12-in., 650 feet of 16-in., and 400 feet of 12-in. pipe in sections. Use engineering judgment when analyzing the results. 
 
 Another means of achieving more constructible designs from Darwin Designer is to group in the same group pipes that would be constructed the same size. For example, a rising main would most likely be constructed a single size, and it would thus make sense to include all the model pipes that make up the rising main in the same design group. What you don't want to do by grouping pipes is artificially design the system even before you have had a chance to optimize it.