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AFT Product Tips

4 minutes reading time (805 words)

Introducing the New Arrow XTS Module!

AFT has some good news for engineers who analyze gas transients. AFT is introducing the new AFT Arrow Extended Time Simulation (XTS) Add-on Module to help engineers better calculate compressed gas flow rate changes in piping systems. This time-based simulation software allows engineers who are designing or troubleshooting industrial compressed gas systems to easily model gas transients. Users may be familiar with the AFT Fathom XTS module, and the AFT Arrow XTS module now brings the same transient capability to Arrow.

  Types of Transients

The Arrow XTS module can capture a wide range of transient events, including tank charging and blowdowns, compressor/fan speed changes, valves opening and closing, and many more. The XTS module can also capture many passive transient events in a system including relief valves, check valves, and control valves. Combined, these transient events allow the user to capture most real operational events their system may encounter. 

The same transient event types and logic found in Fathom XTS, AFT Impulse, and AFT xStream are also found in Arrow XTS. Users can initiate transient events based on simulation time, or other conditions in the model. They can also define a single event that happens at the chosen junction or dual events that happen at the chosen junction. Figure 1 below shows how a tank isolation valve closes when the system pressure at Pipe 2 exceeds 500 psig.

Figure 1: Valve Transient Tab showing a valve set to close when the pressure at Pipe 2 exceeds 500 psig.

  XTS Transient Fundamentals

Both Fathom XTS and Arrow XTS simulate a transient event by running a series of steady-state snapshots of the system with changing process conditions. Essentially, a standard Arrow solution is found for every time step in the transient simulation.

For example, if the transient event is a valve closing linearly over 10 seconds, and the XTS time step is 1 second, the XTS simulation will effectively run the Arrow model 11 times: once with the valve fully open (the initial steady-state system), once with the valve 90% open, with the valve 80% open, and so on, until the last time step is run with the valve closed. 

Figure 1 below shows this example scenario in XTS. The flowrate through the valve is only updated at each time step, resulting in linear changes between time steps.

Figure 2: Mass flowrate through a valve as the valve closes. Flowrate values are only updated at each time step - or once per second in this example.

If the transient simulation continues past the valve closure, the system conditions will not change because the model inputs are no longer changing.

An exception to this series of steady-state snapshots is the Finite Tank option in the Tank junction. Arrow XTS integrates the flowrate into or out of the tank for each time step to determine how the tank pressure and mass of gas change during the transient event. This capability allows Arrow XTS to capture tank charging and blowdown events.

  AFT Arrow XTS vs. AFT xStream

In the spring of 2021, AFT introduced a new software called AFT xStream, meant to simulate dynamic gas transients in pipe networks. While both xStream and Arrow XTS capture transients in gas systems, they each take a different approach. 

Arrow XTS captures the transient event by running a series of steady-state snapshots, as discussed above. Thus, unless model inputs are changing (valve position, compressor speed, etc.), the Arrow XTS results will not change.

xStream captures the transient event by tracking acoustic waves moving through the system. Thus, it can capture pressure, velocity, and temperature perturbations traveling along pipes even after transient events stop. 

These differences mean that the two products have different applications. Arrow XTS is best for capturing longer-duration events where system conditions are changing relatively slowly. xStream is best for capturing short-lived transients and gas hammer pressure effects in a system. 

For users familiar with the difference between the Fathom XTS module and our AFT Impulse software, the Arrow XTS module and xStream have the same differences, but for compressible flow.

  XTS Interface

Let's take a look at some of the features and tools in the Arrow XTS module!


Activate the XTS module using the Add-on Modules button near the status light in the lower right corner, or from the Modules panel in the Analysis Setup window:


Define transient events on the Transient tab in the Properties window for each junction:
View each time step's output for each pipe and junction in the Output window:
Quickly graph selected output from the Output window:
Generate more detailed graphs in the Graph Results window:
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Sunday, 23 January 2022

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