| Product(s): | HAMMER |
| Version(s): | 08.11.XX.XX and higher |
| Area: | Modeling |
Problem
How can I model a "dynamic" air valve such as A.R.I. D-070, which does not start closing until all air has been released?
Solution
Update 2023-Nov-28: Starting with version 23.00.00.19, a dynamic air valve type will be available in the air valve properties. Details on this feature will be added here soon.
The following article will address a workaround to simulate a Dynamic Air Valve (DAV), which works by opening vents at sub-atmospheric pressure, but does not start closing until all the air in the system is expelled (so some water discharge occurs).
This workaround is based on an observation method, where the formation and collapse times of the air pockets are tracked. From these observations, a manual pattern for closing the valve is developed to emulate the operation of this particular type of valve.
To be able to simulate a Dynamic Air Valve you can use the combination of a TCV valve (for closure of the air valve) and a Discharge To Atmosphere (D2A) component (to model the air intake and release) as shown in the image below:
First, in order to establish the pressure vs. outflow relationship at the D2A element, you can use the standard orifice equation. For example use a known diameter, hydraulic grade/pressure head and Discharge Coefficient data to calculate the corresponding Discharge value to input into the D2A element. For this, the headwater elevation can be based on a quick steady state run to see a typical pressure head (with the D2A set to initially closed) and then you would enter this headwater (expressed as pressure head above the D2A elevation) as the "pressure drop (typical)" and the calculated flow as the "flow (typical)" into the D2A properties. As an example, you could use Bentley FlowMaster to perform the calculation as indicated in the figure below, for a 50 mm diameter air valve opening, with a typical pressure of 14 m:
For the TCV the "Status (Initial)" can be set to "Inactive" to model it as fully open, for the initial run where you will first observe the time when the valve should start to close, and set the coefficient type appropriately along with the fully open discharge coefficient or "minor loss coefficient" to establish the fully open characteristics. See more on TCV valve modeling here: Modeling Reference - Valve Closure
For the D2A, set the "Discharge Element Type" to "Valve" and initial status to closed (to model the air valve initially closed.) Initially you can enter an arbitrary Time to Start and Time to Fully Open value of 5 seconds (these times will be changed later based on the time the pressure drops to zero), these settings can be observed in the following images:
Next you will observe the time at which the pressure drops to zero and the air valve would open and admit air into the system. To do this, you must add the elements (TCV and D2A) as report points in Calculation Option > Base Calculation Options > Report Points Collection, then create a profile from the water source to the D2A element to be able to visualize the profile results. Next, compute the transient simulation and analyze the profile created in the Transient Results Viewer tool and also the Time History of our D2A element, to find the time when the pressure drops to zero. This will be the new value for Time to Start Operating in the D2A and for Time to Fully Open you can enter 0.10 seconds for example, to make it instantaneous (otherwise enter the time it takes for your DAV to fully open when the pressure reaches zero.)
You can then compute the model again and perform the same analysis as the last paragraph but this time using Air/Vapor Volume indicators at the endpoint adjacent to the D2A, to be able to find the time when the air pocket is fully expelled and reaches a volume of zero. This will indicate the time when the DAV valve should start closing:
In the above example the air is fully expelled at 12.50 seconds. To model the valve starting to close at this time, go to the TCV properties > Operating Rule > Edit and create a new "Operational (Transient, Valve)" pattern and configure as follows:
This pattern models the DAV valve starting in the fully open position, then beginning to close at 12.5 seconds (the time when we observed the air pocket being fully expelled.) Between 12.5 sec and 17.5 seconds, the valve is closing (a 3 second closure is assumed) and water will be discharging out of the DAV (D2A element), and the DAV will be fully closed at 17.5 seconds.
Consider that you would typically be most interested in the transient that may occur when the DAV valve is fully closed.
The procedure explained above will simulate an air valve that opens when the pressure drops to zero, but does not start closing until all the air is expelled from the system. More information on how the air flow is modeled using the D2A element can be found in the following article: Modeling Reference - Discharge To Atmosphere
The Idea Portal can be used to provide a suggestion for adding this type of valve to the air valve element in a future version. See: Model a 'dynamic combination air valve'