I've been trying to answer your basic question for 15 years. Before that there were about 3 mainframe answers and none of them were "affordable" and very few companies had more than one installed (we used MTRAN, STONER was the standard at pipeline companies, I know there was a third, but I don't remember what it was).
Today there are hundreds. A few years back I took one of the mainframe models and calibrated it to exactly match a pipeline snapshot. Then I took a different snapshot and fed in the rates and delivery pressures. The mainframe model matched the first set within 3 psi at every point in the first case (after tweaking pipe condition) and within 7 psi at every point in the second case (without any tweaking after the first case).
I've run these two cases in 15 models and had another 5 vendors run them in their labs. Until this year no one had ever come close. Finally, earlier this year Neotech in the UK was able to match every point in the first case within 5 psi and nearly every point in the second case to within 10 psi with their PipeFlo software. No one has ever come that close before and I was so impressed that I loaded a demo copy of the software, and I liked that enough to purchase the package (not cheep, but none of them are).
You need to do your own evaluation. Every package has their own quirks--some cosmetic and some serious. One package has a bias to always flow gas from low numbered nodes to high numbered nodes. Another doesn't let you control where line loops terminate. etc.
I've found that the pretty user interface is the least important feature of a model and the really important features (i.e., does it do the arithmetic correctly and repeatably?) can only be evaluated by doing a rigerous comparison to known data.
I like PipeFlo, but it may have some quirks that I have not yet discovered that will be disasterous in your application. Unless you do your own evaluation, you will only know there is a problem when the world crashes down around your ears at 2:00 am on a Monday morning before a board meeting at 9:00 am.
David Simpson, PE
MuleShoe Engineering
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