AFT Blog

Welcome to the Applied Flow Technology Blog where you will find the latest news and training on how to use AFT Fathom, AFT Arrow, AFT Impulse, AFT xStream and other AFT software products.

Is This Valve Choked?

Under sub-sonic gas flow conditions, loss models can use the familiar K, Cv or resistance curve methods. At choked flow, however, the dynamics of compressible flow require a different method to characterize the loss, which AFT Arrow users will see as the optional 'CdA' input value on the valve and other junctions specifications window. The product of coefficient of discharge and effective flow area under choked conditions, AFT Arrow can directly calculate the CdA for some cases, such as at the discharge from a pipe. For other components, such as valves, determining these values is most commonly done through flow testing....

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Adjusting Ambient Pressure

We're not talking about literally adjusting atmospheric pressure here, rather, the ambient reference pressure used in AFT software. The default value is 1 standard atmosphere, but can be changed as desired. Open System Properties either from the Analysis menu or by double clicking the System Properties check box in the status bar, and select the System Data tab. Enter the desired value in the 'Atmospheric Pressure' field and select the appropriate pressure unit in the adjacent unit drop down box. Gauge pressures, NPSHA and cavitation calculations will reflect this specified atmospheric pressure. Note that this does not change the value of...

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Calculating Results in Pipes and Junctions Not in Service

AFT flow modeling software allows you to easily change system line-up by closing junctions and pipes. How you take a portion of a system out of service affects what calculated results are available in output. Let's take the case where we have a valve, pipe, heat exchanger, pipe and valve in sequence. To close one of the valves we can either open its specifications window, select the Optional tab and check 'Special Condition / Closed' or simply click the valve junction to select it then click the Special Condition button on the toolbar. A red "X" will appear next to the...

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Applying a Default Unit System

AFT software supports ISO and English units and provides a variety of ways to select the units to be used. For example, in the Output window you can change units 'on the fly' for a specific parameter by double clicking an output column header and selecting the desired unit from the Change Units window. To globally change the unit system, go to the Options menu and select Parameter and Units Preferences. Select the Unit Preferences tab, choose the Default Unit System then click the Apply Default Units button. This will both apply the default units for the selected unit system and...

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Setting Preferred Units

To choose only English or SI units, go to the Options menu, select Parameter & Unit Preferences, then the Unit Preferences tab. Choose the Default Unit System then click the Apply Default Units button. This will set the units to be used both on input and output windows.

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Modeling Component Resistance Curves

A variety of resistance modeling methods are available within AFT software, including constant or variable K values for a wide variety of components, K or Cv for valves. Also, for most components, you may directly model a resistance curve. This is especially handy when you pressure drop (or head loss) vs. flow data from the component manufacturer or test data. Whether you're modeling a valve, spray discharge, orifice or general component junction, you'll find this option on the Loss Model tab of the junction specification window listed as "Resistance Curve". Clicking the button will highlight an 'Enter Curve Data' button, which...

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Use Bookmarks to Accelerate Model Navigation

AFT software has a variety of features for editing and navigating around models. One of these is the ability to define and then easily jump to bookmarks. So, what are bookmarks? A bookmark is an identified location, a junction or pipe, within a model. To create a bookmark, select a junction or pipe by clicking it once. On the View menu, select Bookmarks\Set Bookmark. To jump to the bookmark, simply press F9 and the Workspace will pan to bring the junction or pipe into view and select it. Any number of bookmarks may be defined. When multiple bookmarks are set, pressing...

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