# Thread: Modeling normally open Isolation Valves -> What is the best type of valve to use

1. ## Modeling normally open Isolation Valves -> What is the best type of valve to use

While not every modeler prefers to model normally open valves in the model as valves, there are a significant portion of modelers that do.

For those that do choose to model normally open isolation valves as true valves in the model, the question is often raised "what is the best model valve type to use to model normally open isolation valves?"

Most people use Throttle Control Valves (TCVs) to model normally open isolation valves because these tend to be very stable in most models and don't create difficulties in the solution method.

However, in support we have received several cases in the last year where people have tried to use general purpose valves (GPVs) for this purpose and when they do, find that they often cannot get the model to converge. The reason for this is that GPVs use a headloss vs. flow curve. When many GPV's are used in a model (even two in series or two in a loop equation), the model can have difficulty converging due to model instability associated with the GPVs. When there are multiple GPVs in a loop, it is difficult for the model to converge head and flow as every time the flow changes this changes the headloss which causes another check of the flow balance, etc which leads to another change in the headloss equations, etc, etc, etc. A single GPV's here and there in a model is fairly stable, but multiple GPVs in a loop are problematic and are known to cause instability in EPANET.

If you use GPV's to model Isolation valves this will create generally thousands of GPV's in a model which creates a very difficult solution for EPANET to solve. To rectify the instability, simply convert all the isolation valve GPV's to Throttle Control Valves (TCV's). TCVs are the typical way that isolation valves are modeled when they are modeled as valves rather than junctions. These TCVs are much more stable especially when two or more of the valves are associated with any pipe loop in the model. Generally once all of the GPV's in a model are converted to TCVs the model will easily converge with a green light.

Example of this issue can be seen in this thread:-> http://forums.innovyze.com/threads/3...-at-0-00-00-hr

So if you decide to model all isolation valves in your model, know that TCV's are greatly preferred to GPV's in order to have good model convergence and avoid conditions which lead to model convergence instability. General purpose valves should be avoided for use as modeling normally open isolation valves as they tend to cause model instability when a high number of them are used in a hydraulic model.

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