Throttle control valves are one feature within the software that may or may not get used as much by different modelers as other valve types, but are another tool in the modeler’s toolbox that can be used. Due to some recent support calls that identified a few repeated questions regarding Throttle Control Valve use, we wanted to point out a few things regarding TCVs that may or may not be obvious to all users to help avoid confusion when they are used. These key points are intended to help clarify information to help users best use this feature within the software.
First, let’s start with a description of the Throttle Control Valve (TCV) from the software help file to describe what they are:
A TCV may be used to simulate a partially opened valve by adjusting the minor loss coefficient. They are normally used to increase or decrease flows or to control pressures in the system. A TCV can either have a Minor Loss vs. %Open curve associated with it or not. Whether a curve is associated or not depends on what the Setting value means to InfoWater.
Specify the Valve Type as Throttle Control in the Type field of the Modeling Data section of the Model Explorer – Attribute Tab.
- Setting – %Open (with curve identified), K value (no curve)
- Diameter – Diameter of valve, in. (mm)
- Curve (Optional) – Minor Loss vs. %Open Curve ID
Note: For TCVs please do not enter any data to the PID, UCL and LCL fields.
In addition here is the help description regarding the type of curve a TCV uses which is a Minor Loss vs %Open (Control Valves – Motorized Throttled)
For Motorized Throttled Valves (MTVs), a Minor Loss Coefficient Curve consists of a collection of points defining the minor loss coefficient K (Y-axis) as a function of the percentage (degree) opening setting (X-axis). It provides the capability to model valves with unique headloss characteristics such as cone and butterfly valves.
A sample table and curve are provided below. Either a general (multi-purpose) type of curve or a specific MinorLoss vs % Open curve will work as long as the X values are percent open and the y values are the minor loss K values.
Lastly, here are a few key notes about Throttle Control Valves to assist users in better understanding key features regarding this model element.
- Throttle Control Valves use a setting which is the percent open for the valve. The TCV setting is expected to range from 0-100. (Note this is different from how pump speed percentage is used which is expressed as a decimal from 0-1).
- A percent open of zero as a valve setting for a TCV will not directly make the valve have zero flow.
- This is due to the way TCV’s were developed in EPANET. In EPANET the TCV does not have a curve, but simply assigns a minor loss K based on the setting. This means that the percent open is not used in EPANET and that at zero percent open the model will simply assign the K value to the valve. Even when the largest k value you can enter (9999999999.00) is used as the K for 0 percent open, the valve will still generally have a small flow. This is due to the apparent intended purpose of TCV’s to be used more as throttling a valve’s flow rather than closing a valve off completely.
- This means the only way to completely shut off flow in a TCV is to change its status to “closed”. To make it active again the valve would have to be assigned a new setting. Be careful not to set the valve to “Open” as this will make it act like an open pipe. Note: One can also put open/closed controls on a pipe up or downstream of the valve as well to completely shut it off flow to a valve and avoid the specific requirements of closing and making a valve active.
- Note: This may be of importance if you use TCV’s to model isolation valves, which is something we see periodically in Technical Support. Make sure to close the valve rather than set it to zero percent open if the user wants to truly “close” the valve in the model.
- The key to understanding flow through a TCV is to understand how the headloss is calculated by the software:
- The valve setting (which ranges from 0-100) is used to identify the minor loss K to use for the valve based on the curve identified. If no curve is assigned, then the model assumes the setting is the K value to use. Once the K is known, the model will calculate a flow that will induce a headloss through the valve to match the calculated head difference on both sides of the valve.
- Key points in how the headloss is calculated and how it impacts model results:
- The headloss through the valve is calculated by the following formula: HL = K*(V^2/2g), where HL is headloss in feet, K is the minor loss coefficient, V is the velocity through the valve in ft/s and g is gravity at 32.2 ft/s^2.
- Given that 2*32.2 ft/s^2 is 64.4 ft/s^2 is the divisor to the square of the velocity, headloss through the valve would need to be roughly 8 ft/s to get a value close to 1 in the V^2/2g term. Thus, unless the K is large, or the valve is small (increases the velocity) the TCV will potentially do little to throttle the flow (i.e. induce headloss to limit the flow) as the headloss will be small unless the velocity is very high.
- In essence, the model will push water through the valve until the headloss equals the head difference up and downstream of the valve. This can potentially lead to large flows if the valve is very large ( i.e. low velocity) or the K is very small, so be aware of this when you use this type of valve and make adjustments as needed to get the valve working as desired. Understanding the headloss and how it is calculated for a TCV is the key to understanding how a TCV controls the flow within the model. For lower flows through the valve either increase the K, lower the valve diameter, or decrease the setting used. If flows are still too large at the desired setting, increasing the K values in the curve or simply reducing the valve diameter may be the only way to reduce the actual flow through the valve as the only way to reduce the flow is to increase the headloss.
We hope this information will help our users to make better use of TCVs in their models and help clarify a few things that may not have been completely apparent from the current information in the help files for TCVs.
If you find you need further assistance with TCVs please don’t hesitate to contact us at email@example.com.