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AFT Product Tips

2 minutes reading time (407 words)

It's Only Logical!

Many engineers are familiar with how “Add/Or” logic works in computer coding or in control systems, which are used often to control transient events such as valve closures and pump start-ups. “Or Logic” determines the conditions that prevent a specific event from happening unless one event OR another event happens. “And Logic” determines the conditions that prevent a specific event from happening unless one event AND another event occur at the same time. It is possible to create this type of logic model within AFT Impulse and AFT Fathom XTS by creating a sub-model within your workspace. This sub-model would not really be constructed in a real sense; it merely expands the number and type of conditions that can be used to trigger the initiation of the transient.

I created the discussed logic models based off of the Pump Startup with Event Transient example found in the Help Files of AFT Impulse. In the example a valve is set to open if the pressure at the outlet of pipe is too great. But what if the valve also needs to open if the flow in a different pipe exceeds the desired flow rate? This can be done, but you have to create a logic model within the same scenario.

The following images show an overview of the “logical sub-model” required to model the type of logic you desire. This sub-model will have different configurations for “Or Logic” and “And Logic”. The “Or Logic” sub-model consists of parallel valves and the “And Logic” model consists of valves in series. On the boundaries there are assigned pressure junctions with a small pressure difference between them. This is required to introduce flow into the system. In both models the valves are originally closed and open depending on the system’s conditions. If one opens flow is introduced into the “Or Logic” model and if both open flow is introduced into the “And Logic” Model. This new flow is then used to determine if conditions have been met for another transient event to begin in the main model.

Overall

Overall "Or Logic" Model

Overall

Overall "And Logic" Model

Using this method to model different types of logic can be a powerful tool when recreating custom controllers. Other variations can be made to this type of model so that more than two events could be linked or a combination of “And Logic” and “Or Logic” could be created depending on the complexity of your controller.

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Tuesday, 13 April 2021

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