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TCV in WaterCAD

Ref :- http://communities.bentley.com/products/hydraulics___hydrology/f/5925/t/62107 

Hi
Regarding Victor's question,
Assume a pipeline with a TCV. Coefficient Type is 'Discharge Coefficient' (Cv).

initial Cv= 0.25

Fully Open Cv: 0.5

Valve Type is Globe

Pattern (Valve Settings) is Fixed.

Question): Run the model. How Q and Relative Closure of the TCV are calculated? What is the role of Cv in calculation of Q and Relative Closure? I know how a TCV operates and how Headloss, Cv and relation between relative closure and opening area in various kinds of valves are considered. Then my question is about how of calculating Q and Relative Closure in TCV? I mean when the software knows Cv initially, Cv fully open,diameter, valve type and other hydraulic characteristics of the model, How does it do to calculate a) Flow and B) relative Closure and C) relation between relative Closure and Cv?

Answer): My interpret is, the software uses energy equations for each pipe (start and stop point of each pipe) to show what the amount of Q is. Relative Closure is equal to 1-[Cv (initial)/Cv (fully open)]. With this parameters (Q, V, D, Cv, Relative Closure) all equations can be solved. Is my interpret correct?

  • Hoshi,

    The flow is determined based on the flow entering the valve and the discharge coefficient at the time of the calculation, which is based on the valve settings pattern (time vs. multiplier) that you'd create and assign to the valve. Yes, the flow is determined using the energy equations from the junction to junction and yes, your understanding of the way the flows are calculated is correct.

    The "Discharge coefficient" is also known as a "valve coefficient" or "Cv", as you know, which is defined as: Flow / (Pressure Drop) ^ 0.5. Cv is converted to a minor loss coefficient in order to determine that. There are numerous coefficients used to describe the operation of a valve (example). The one used in HAMMER, WaterCAD, and WaterGEMS is the commonly used valve coefficient Cv, defined as:

    Q = Cv (pressuredrop)^0.5.

    Another way to express it and consider specific gravity is:

    Cv = Q (specific gravity / Pressuredrop)^0.5

    You can also refer to this wiki, which explains how headlosses are determined for the TCV with the different initial status settings.

    communities.bentley.com/.../26424.how-are-headlosses-determined-for-tcv-s-with-the-different-initial-status-settings

    This wiki about TCV field assumptions gets into details about the discharge coefficient and valve function:

    communities.bentley.com/.../15981.valve-type-field-assumptions-and-use-with-a-tcv

    Regards,
    Mark

    Mark

  • Regarding how it calculates the relative closure: this is based on the valve type, initial Cv and fully open Cv. Behind the scenes, the Valve Type represents a curve of relative closure vs. relative discharge coefficient. You can read more about this in the second article that Mark mentioned.

    Regarding how it calculates the flow - this is indeed where energy balance comes into play. In a case where you have a downstream boundary condition, the program iterates to balance energy across the network based on the physical characteristics, demands, that assumed boundary HGL, and the relationship between flow and headlos that the TCV's Cv represents. The more flow the more headloss, but the boundary condition needs to be satisfied. The program will settle on a balanced solution and you'll see the corresponding flow and headloss across the TCV.

    In case where you only have demands downstream of the TCV, it's easier to solve - the flow through the TCV is simply the sum of the downstream demand, and the headloss across the TCV will correspond to that flow, given the Cv.


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.