Configuring TCV relative closure patterns for EPS

  Applies To 
  Product(s): WaterGEMS, WaterCAD
  Version(s): V8i, CONNECT Edition
  Area:  Modeling
  Original Author: Scott Kampa, Bentley Technical Support Group

Problem

Is it possible to set up a control to change the relative closure of a valve over the course of an Extended Period Simulation (EPS)?

Solution

Option 1: Operational Controls

You can set a control through the controls manager (Components > Controls). For a TCV valve, currently only the TCV Status (active, inactive or closed) or the TCV Setting (headloss coefficient, K) are supported actions. If you need to change the setting on a TCV based on a tank, this method must be used, and use Headloss Coefficient as the "Coefficient Type" for the TCV.

If you do not have the headloss coefficient for the valve, it possible to derive it. First, use the fully open discharge coefficient along with the user-defined valve characteristics curve to determine the exact discharge coefficients that correspond to a range of relative closures (If you have the manufacturers discharge coefficient, you can use that as well). Then, you would take the valve diameter along with the discharge coefficients and use an equation to convert to a headloss coefficient (K). Here’s the equation with US units:

K = 39.693 * D^4/Cv^2

Note that these controls are intended for an Extended Period Simulation in WaterCAD, WaterGEMS or HAMMER, and do not apply during a Transient simulation in HAMMER. For HAMMER transient simulations, use the transient Operating Rule.

Another option is to set up a standard steady state simulation where you change the relative closure or discharge coefficient setting for your TCV and observe the calculated headloss coefficient. This is found in the Results section of the TCV properties: "Headloss coefficient setting (calculated)".

Option 2: Pattern

However, if you want to adjust the relative closure of a TCV based on time, you can create a pattern. Go to Components > Patterns and create a new pattern for "Valve Relative Closure." Set the starting relative closure to be the initial position of the valve. Then create the pattern that will be used by the program to adjust the relative closure of the TCV. 

Note that this type of pattern works in conjunction with "Valve Characteristics Curve" selected as the "Coefficient type" for the TCV, and the pattern that you enter would be selected in the field called "Pattern (Relative Closures)".

If you are using "Headloss Coefficient" or "Discharge Coefficient" as the Coefficient Type, your pattern needs to be entered under the "Valve Settings" category in the Pattern manager, and selected in the "Pattern (Valve Settings)" field in the valve properties. In this case, the values entered in the pattern are multipliers, which multiply against the initial headloss coefficient (which is calculated, in the case of using the "discharge coefficient" type). So, a value of "1" on the pattern will cause the program to use the original initial headloss coefficient at that time. Therefore in order to close the valve using this method, you would need to use a very high multiplier in the pattern, such as 999999999999. So, in a situation where you need to open and close a valve based on time, it would be better to use the Valve Characteristics Curve type, or logical controls.

Steps to Configure Valve Characteristics Curves

When using the Valve Characteristics Curve Coefficient Type to set specify valve position as a percentage, there are four main steps that occur:

1) Enter the fully open discharge coefficient. First, you must enter a discharge coefficient that represents the loss through the valve in the fully open position, corresponding to a Relative Closure of 0%. See section below for how to calculate this if you do not know it.

2) Select or configure the Valve Type. Select one of the default options from the valve type dropdown (see article in the "see also" section for more on this) or select "user defined and configure a user define relationship between "relative closure" and "relative discharge coefficient". 

"Relative closure" can be interpreted as the "stroke" of the valve and is what you enter in the pattern and initial percent closed. "Relative discharge coefficient" represents the discharge coefficient corresponding to a given relative closure, expressed as a percentage of the fully open discharge coefficient. For example if the fully open discharge coefficient is 1.0 and you would like it to be 0.5 when the valve is 25% closed, then on the Valve Characteristics curve table, a value of 25% for the relative closure column would have a corresponding value of 50% for the relative discharge coefficient.

3) Enter your initial relative closure percentage in the TCV properties. Again, this is correlated to headloss by way of the Valve Type.

4) Enter and select your "Pattern (Relative Closure)" in the TCV properties. The Starting Relative Closure should be equal to the initial relative closure, and the pattern defines how the valve changes positions (stroke) over time. As this changes, the discharge coefficient changes based on the relationship defined by way of the Valve Type. You can view the corresponding headloss coefficient, discharge coefficient and total headloss for each timestep by looking at the Results section of the TCV properties or by graphing these attributes.

What if I want to use valve Characteristics Curve but don't know the Discharge Coefficient?

If you only know the Headloss Coefficient (K) of the valve in the fully open position but want to use the "Valve Characteristics Curve" Coefficient Type, you can calculate it based on one of these methods:

1) Set the coefficient type to headloss coefficient, set the state to active, set the correct valve diameter, set the headloss coefficient to the "K" that you know and compute initial conditions. In the Results section of the TCV properties, the corresponding calculated discharge coefficient will be displayed. You can then switch the coefficient type to Valve Characteristics Curve and enter that number in the fully open discharge coefficient field.

2) Calculate the discharge coefficient using the following equation:

US Units

Cv = ((39.693 * d^4) / K)^0.5

Where:

Cv = discharge coefficient (cfs/ftH20^0.5)
d = diameter (ft)
K = Headloss/Minor Loss coefficient

SI Units

Cv = ((1.22.D^4)/K)^0.5

Where:

Cv = discharge coefficient (m³/s/Kpa^0.50)
d = diameter (m)
K = Headloss/Minor Loss coefficient

Note: for SI units, WaterCAD, WaterGEMS and HAMMER accept a unit of m³/s/M H2O^0.50 - a unit conversion factor of 3.1316 can be used to multiply the end result in the kpa unit to achieve the unit required by the program.


If you do not know the headloss coefficient, you could approximate with a K of 1.0, try a sensitivity analysis, or calculate the discharge coefficient based on an assumed headloss:

Cv = ((39.693 * d^4) / [Hl/V^2/2g])^0.5

Where:

Cv = discharge coefficient (cfs/ftH20^0.5)
d = diameter (ft)
K = Headloss/Minor Loss coefficient
V = Velocity (ft/s)
g = gravity (32.17 ft/s^2)


See Also

Valve Type field assumptions and use with a TCV

What is the "Discharge Coefficient"?

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