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What is the purpose of the "Valve Type" field in the properties of a TCV? When is this used and what are the assumptions behind it? Do I need to use this dropdown field when I need to model a Globe, Butterfly, Gate, Ball or Needle valve?
The selection made from the "Valve Type" field for a TCV (butterfly, globe, etc) allows the program to relate "relative closure" (%) to a discharge coefficient, based on the entered "fully open discharge coefficient". This feature is only used when the Valve Coefficient Type is set to Valve Characteristic Curve type. (this type allows you to specify closure, whether initial or in a pattern/control, based on a "%open" valve, which may be more convenient or meaningful than a headloss or discharge coefficient)
So, if you need to model one of the valves in the dropdown, like a Globe, Butterfly, etc, this option could be used, but is not required. You could simply select Headloss coefficient or Discharge coefficient as the coefficient type and enter the corresponding value to the position being modeled. Or, you could go into further detail by entering the "fully open discharge coefficient" and selecting respective type in the Valve Type dropdown. You'll then be able to enter the position of your valve (either initial or on a relative closure pattern for EPS) in terms of percent closed, and the program will determine the discharge coefficient to use based on the fully open discharge coefficient along with a typical Globe valve relationship (see chart below) for that particular percent closure value.
This applies to steadystate/EPS in WaterCAD and WaterGEMS as well as a transient simulation in HAMMER. The initial percent open and corresponding discharge coefficient will be used by HAMMER as the starting, initial conditions. You can then configure a transient Operating Rule to change the position (relative closure) over the course of the transient simulation. You would set the initial relative closure equal to the position in the initial conditions.
This chart is based on Fok, A.T.K., “A Contribution to the Analysis of Energy Losses in Transient Pipe Flow”, Ph.D. Thesis, University of Ottawa, 1987.
T/Tc is time over time to fully close (so 0.5 would mean 50% closed/"stroke") and A/Ao is area over full area (which can be correlated to discharge coefficient). Basically it shows the relationship between stroke and discharge coefficient. Meaning, one particular valve will have a different flow control characteristic when closed half way versus another type of valve.
If your particular valve doesn't align with these relationships, you always have the option to choose "user defined" as the valve type, then enter your own relationship between %closed and %discharge coefficient. Or, you could of course enter valve positions the traditional way using headloss coefficient or Discharge coefficient instead of %closed.
The curves in the chart above have the functional form:
1 – Yk ...
where needle valves have k = 2.0; circular gate valves, k = 1.35; and globe valves k = 1.0;
or
(1 – Y )k ...
where for ball valves, k = 1.35; and butterfly valves, k = 1.85.
More information can be found at the following paper:
Fok, A.T.K., “A Contribution to the Analysis of Energy Losses in Transient Pipe Flow”, Ph.D. Thesis, University of Ottawa, 1987.
Note: most valve manufacturers can provide the discharge coefficient(s)
Modeling Reference - Valves