This TechNote will describe steps to model an existing valve, such as a flow control valve (FCV) or pressure sustaining valve (PSV) as a throttle control valve (TCV) in Bentley HAMMER to model such valves throttling during the transient even to meet their pressure or flow setpoints.
Bentley HAMMER has the ability to model a number of valve types, but most are not capable of throttling dynamically in a transient simulation. For instance during the initial conditions (steady state) a FCV will throttle to only allow the desired flow through. In the transient simulation, the valve is assumed to stay at a fixed position, matching the initial conditions. In order to simulate dynamic throttling during the transient simulation, you would need to use a transient Operating Rule (pattern of valve position) with trial and error, to model any impact from modulation of such valves.
If you are trying to model the transient response to a FCV, PRV or PSV fully closing (or opening), then you can simply use the Operating Rule in the properties of the valve to configure the pattern of closure. Look at the "results" section of the valve to see the calculated relative closure in the initial conditions, to use as the starting relative closure on your operating rule pattern. If this result is showing as N/A, be sure to enter a minor loss coefficient.
First, consider if you need to model these effects. In most cases, the throttling of a FCV, PRV or PSV would not be significant in a transient simulation.
Note that as of HAMMER V8i SELECTseries 4 (08.11.04.50), a new option has been added for PRVs to dynamically throttle during the transient simulation, by way of an Opening Rate Coefficient. See: Using Modulating PRVs This may be expanded to PSVs and FCVs in a future update.
In the meantime, the steps below can be used to convert existing valves, such as a flow control valve (FCV) or pressure sustaining valve (PSV), to equivalent TCVs with operating rule configured to transition as the valve would, to simulate throttling. Here is an outline of the steps, with details further below:
1) Set up and compute initial conditions to simulate the starting steady state conditions2) Record the corresponding discharge coefficient of any active valves3) Set up and compute initial conditions to simulate the ending steady state conditions (after the transient event, where the valve positions may be different)4) Record the corresponding discharge coefficient of any active valves5) Convert the valve to a TCV and configure it with a pattern that modulates from the initial discharge coefficient to the final discharge coefficient.
Note: For clarity, the steps below will assume that an FCV is the valve that is throttling open or closed in the simulation. In addition, the valves that are not throttling will be assumed to be PRVs. It is important to note that though the simulation is assuming that it is the FCV that is throttling, since the PRV may open or close during the transient simulation based on the settings of the PRV, operating rules will need to be created for these as well.
To start, it is recommended that you do a Save As on the model to assure that you don't lose the original data. Next, calculate the initial conditions of the model (usually steady state) as it is originally designed, with the original FCV and PRV currently in the model, and with the FCV set to Active. Then, go to Calculation Options and open the properties for the transient calculation options. Now, find the property field "Specify Initial Conditions?" and set this to True.
Back in the drawing, highlight the FCV. Next, go to Tools > Copy Initial Conditions. This will open a new dialog. Choose the "Selection" radio button and select Okay.
Now, open the FCV properties. You will find a new result field called "Discharge Coefficient (Transient)", under the "Transient (Physical)" section:
Right-click on the attribute name and select "Units and Formatting." Change the precision value to 10 and select Okay. Now, copy the value to Notepad or Excel.
Note: You will want to take special care when keeping track of this value to make sure that you don't inadvertently use the wrong discharge coefficient.
Next, you will need to morph the FCV into a TCV. This can be done by selecting the TCV from the list of elements. Place the crosshairs over the FCV and left-click on the drawing pane. You will get a message asking if you want to morph the FCV into a TCV. Choose Yes.
Most of the physical properties will be retained, including the Operating Rule. Paste the value for the discharge coefficient taken from the previous step into the TCV attributes "Discharge Coefficient (Fully Open)" and “Discharge Coefficient (Initial)."
Note: Even though the discharge coefficient obtained from the FCV is when the FCV is active, or throttling, this value is used for "Discharge Coefficient (Fully Open)" since it represents the 0% relative closure in the operating rule being used.
With this done, now the discharge coefficients for the other valves in the model must be obtained. For the sake of clarity, we are assuming that other valve or valves are PRVs. Set the initial status of the TCV representing the FCV to Active. Next, open the properties for the transient calculation options and change "Specify Initial Conditions?" to False again. Next, compute initial conditions.
Now, change "Specify Initial Conditions?" back to True and then go to Tools > Copy Initial Conditions. When the new dialog opens, this time choose "All" and select Okay.
Copy the calculated discharge coefficients for any PRV in the model. Now, set the initial status of the new TCV to Closed. Change "Specify Initial Conditions?" to False again and compute initial conditions. Next, return "Specify Initial Conditions?" to True. Go to Tools > Copy Initial Conditions once more. Again, select "All" and then select Okay. Copy the calculated discharge coefficient for each PRV. Change "Specify Initial Conditions?" to False once more.
Next, morph each PRV into a TCV. Make sure that the new TCVs are initially active (since the PRVs they represent are active). Paste the value for the discharge coefficient when the first TCV representing the FCV is closed into the TCV attribute "Discharge Coefficient (Fully Open)" and Discharge Coefficient (Initial)."
Now you need to set up operating rules for these new TCVs to simulate the change in the discharge coefficient as the first TCV opens. This will take some trial and error and still might not get the exact results you would expect. You will need to use the valve characteristic curve for the PRV to find the relative closure for a given discharge coefficient. The valve characteristic curve for a given valve type can be found in the Help documentation under "Closing Characteristics of Valves". The time entered will coincide with the time that the TCV representing the FCV takes to open.
Once completed, make sure the first TCV (representing the FCV) is closed and compute the model. You may need to make adjustments to the hone the results.
If you want to have the FCV throttle between two Active settings (in other words, never fully closed, only allow less flow), you will need to also identify the discharge coefficient for this second setting before morphing the FCV into a TCV.
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General HAMMER V8i FAQ