FAQ - Frequently Asked Questions

How has AFT Fathom Been Verified?

There are several aspects to AFT Fathom verification.

First, AFT Fathom has been compared to a wide variety of published results. Models, comparisons and explanations for such published predictions are given in the Verification section of this site and the Verification folder installed with AFT Fathom.

Second, it can be verified that the AFT Fathom predictions agree with the fundamental equations for networks.

Finally, AFT Fathom predictions have been compared to test data and other analytical methods on numerous occasions and good agreement has been shown.

Can AFT Fathom Be Used to Model Gas Systems?

Yes, depending on the gas system (see Walters, 2000). For incompressible gas systems, AFT Fathom is perfectly appropriate. For compressible flow gas systems, a more sophisticated tool such as AFT Arrow should be considered.

Can AFT Fathom Model Two-Phase Flow?

No, AFT Fathom models single phase flow only.

How Does AFT Fathom Account for Heat Transfer in Pipe Systems?

Heat transfer effects show up in several places in AFT Fathom. First, for pipes users can input pipe and insulation data thermal conductivity data and thickness, as well as external convection coefficients. The thermal conductivity data can be constant or temperature dependent. These heat transfer calculations and the associated physical properties are determined for each pipe. Second, heat transfer can occur in Heat Exchanger junctions. Here the user assigns a heat load or chooses a heat transfer model for the heat exchanger. AFT Fathom performs the appropriate energy balance calculation across the heat exchanger. Third, heat transfer occurs in pumps due to inefficiencies.

Finally, at each branching location in the network an energy balance is performed so that energy is balanced from one pipe to the next.

Does AFT Fathom Perform an Energy Balance Across Each Junction in the Model?

Yes, every junction except for heat exchanger and pumps is assumed to be adiabatic.

Does AFT Fathom Model Transient System Behavior?

Yes, in the optional XTS add-on module.

Can I Model a Non-Newtonian Fluid in AFT Fathom?

Yes, AFT Fathom offers several non-Newtonian fluid models including Power Law and Bingham Plastic.

Can AFT Fathom Model Fire Sprinkler Systems?

Yes. The Spray Discharge junction is perfectly appropriate for fire sprinklers. The fire sprinkler K values can be entered in directly into the Spray Discharge. In addition, Spray Discharge junctions can be turned on or off with a single mouse click. Evaluating multiple fire location scenarios is easily done, and use of the Scenario Manager allows all cases to be kept in the same model file.

Can AFT Fathom Model Ducting System Like Those in HVAC, Ventilation and Dust Collection Systems?

Yes, AFT Fathom can model ducts. The rectangular geometry is modeled as a non-cylindrical pipe using a hydraulic diameter. Since ducting systems are usually incompressible, the solution methodology is appropriate.

Can I Keep All of My Design Cases Together in a Single File?

Yes, the Scenario Manager lets you create dependent design cases where changes are inherited by children.

What Is the Difference Between Using a Junction to Input Pressure Loss Data and the Fittings & Losses in Pipes?

A junction has several advantages. First, solutions are given at all junctions, so the user can check the results at the junction. In contrast, Fittings & Losses are lumped into the pipe and it is not possible to give results at the loss. Second, many junctions (such as valves) have the ability to specify a Restricted Flow Area for cavitation calculations. No such ability exists for Fittings & Losses; thus, cavitation is always ignored for Fittings & Losses. Third, when using a junction the location in the pipe system of the pressure loss is specified. In other words, the upstream and downstream pipe lengths are specified. In the case of Fittings & Losses, it assumed to act like a friction factor and be evenly distributed along the pipe. The Fittings & Losses approach has the advantage of being able to specify multiple losses quickly and easily, and not cluttering up the model Workspace with numerous junctions.

What Is the Difference Between a Reservoir Junction and an Assigned Pressure Junction?

There is an overlap in capability between the Reservoir junction and an Assigned Pressure junction and frequently they are interchangeable. Here are the differences. The Reservoir junction input pressure and temperature always corresponds to stagnation properties. In the Assigned Pressure junction, they can correspond to either static or stagnation properties. If static, only one pipe can be connected. If the stagnation option is used in the Assigned Pressure junction, it will behave identically to a Reservoir junction. Finally, the Reservoir junction allows pipes to connect at different elevations (i.e., depths), and also allows pipes to discharge above the liquid surface level.

How Does AFT Fathom Account for Pressure Losses at Tees and Wyes?

AFT Fathom uses sophisticated models to calculate losses at tees and wyes. The methods, which come from Idelchik, take into account losses that depend on the flow split, area change and angle of the branch pipe.

How Do I Size a Pump When There Are Not Any Flow Control Valves?

See Pump Sizing

How Do I Size a Pump When There Are Flow Control Valves?

See Pump Sizing

How Do I Model a Variable Speed Pump?

Enter the pump data as a polynomial in the Pump Properties Window and then set the pump speed directly. No entry is assumed to be 100% speed. If you enter something other than 100%, AFT Fathom uses the affinity laws to adjust the curve. If a control setpoint is entered, AFT Fathom will determine the speed necessary to meet that setpoint.

How Do I Find the Pump Speed to Obtain a Desired Flow or Pressure?

Enter the pump curve data in the Pump Properties Window and set the desired flow or pressure on the "Variable Speed" tab using the Controlled Pump option. AFT Fathom will calculate the pump speed required to produce the flow or pressure conditions specified and display the speed in the Pump Summary report.

How Do I Close a Valve, Pump or Pipe?

Select the pipe or junction you want to close and choose Special Condition from the Edit menu. By default, AFT Fathom will display a red "X" next to the pipe or junction label on the Workspace. It will also redraw your Workspace and show the closed sections of the model with dashed lines for the pipes and dashed outlines for each junction. Some special condition settings do not close the junction but have different purposes. For example, the normal condition for a relief valve is to be closed, so its special condition causes it to be open.

How Do I Model the Pressure Drop Across a Component When There Is Only One Data Point?

Use the Resistance Curve Loss Model.

How Do I Model a Relief Valve in a System?

You can model a relief valve using either a Relief Valve junction or a regular Valve junction. The Relief Valve junction is always closed when you run the model (unless you set its Special Condition), and AFT Fathom will run the Solver to determine if sufficient pressure exists to crack it. If so, it will run the Solver again with the relief valve open. If you know the condition you are modeling will crack the relief valve, it is more efficient to just use a regular valve junction. In this case, AFT Fathom assumes the valve is open from the start, and does not have to go through the extra step of solving the network to check for sufficient system pressure to crack it.

How Do I Show the Pipe or Junction Names on the Workspace?

Open the pipe or junction Properties Window, click the Optional tab, and check or clear the check boxes for showing the number or name. This will affect the current pipe or junction. You can set the default behavior in the Workspace Preferences window available in User Options.

How Do I Change the Display of Pipe or Junction Names on the Workspace After the Model Has Been Built?

You can use the Global Edit windows to change the current model settings for all pipes or junctions or only selected ones. See the Global Pipe Edit or Global Junction Edit window for more information.

How Do I Set My Preferred Engineering Units as Defaults?

From the Tools menu, select User Options. Then select Preferred Units under Unit System. Drop down the list of units for each Unit  Family to select the Preferred Unit. At this point, the preferred unit applies only to the current model. To make this the default for all models, click the Save as New User Defaults button at the bottom. The preferred units for a model can also be set in the Startup window.

How Do I Change the Input Data for Multiple Pipes or Junctions All at One Time?

The Global Pipe Edit and Global Junction Edit windows offer tremendous power and flexibility in changing all or parts of your model input all at once. See Global Pipe Edit and Global Junction Edit  topics for more information.

How Do I Create a Liquid Mixture?

You can create mixtures with the NIST REFPROP or Chempak libraries (Chempak is an additional add-on). The AFT Standard fluid library does not support mixing. To create a mixture, go to the Fluid panel in Analysis Setup and choose either the Chempak or NIST REFPROP library, and then click the “Create New Mixture and Add…” button. Here you can specify the mixture components and percentages.

How Do I Merge Two Models Together?

Use the Merge feature on the File menu to merge models together.

How Do I Automatically Run Multiple Scenarios or Models Right After Each Other?

Multiple scenarios or models can be run sequentially using the Batch Run feature.

How Do I Enable or Disable the Highlight Feature in the Pipe and Junction Properties Windows?

There are three ways to enable or disable the Highlight feature. The first is toggling the option on the Options menu. The second is pressing the F2 function key while in a Pipe and Junction Properties Window. The third is double-clicking in the blank space at the top of the Pipe and Junction Properties Window.

How Do I Automatically Save My Results After the Model Is Run?

Open the Output Control window from the Analysis menu, change to the Format & Action tab, and choose to select the Transfer Results to Initial When Done, Transfer Valve States When Done, and Save Model When Results are Transferred options.

How Do I Quickly Find a Particular Pipe or Junction on the Workspace?

Use the Find feature to quickly find a pipe or junction.

How Do I Change the Reference Positive Flow Direction for One or More Pipes?

Select the pipe or pipes and choose the Reverse Direction feature on the Arrange menu.

How Do I Access Pipe and Junction Properties Windows from the Model Data Window?

In the Model Data tabular displays, double click on a row to automatically launch that pipe/junction's Properties Window.

How Do I Display Only Selected Pipes or Junctions in the Output?

Open the Output Control window from the Tools menu, change to the Show Selected Pipes/Jcts folder tab, and select the pipes and or junctions you want to display.

How Do I Quickly Change the Units for an Output Parameter?

Whereas the Output Control window allows you specify units for all parameters, when in the Output window viewing results you can quickly change the units for parameters in the tabular displays by double-clicking the column header.

How Do I Model Elevation Changes in My Pipe System?

Set the elevations in the elevation fields for each junction. Pipes are assumed to be straight between junctions. If you need to model a system other than a stationary earth-based system, the gravitational acceleration can be changed on the Environmental Properties panel of Analysis Setup.

How Do I Show Data for Only Selected Pipes or Junctions in Visual Report?

Open the Visual Report Control window, change to the Show Selected Pipes/Jcts folder tab, and use the provided features to specify which pipes and junctions should display data and which should not.

How Do I Make the Pipe Drawing Tool Stay Active so That I Can Continue to Draw Pipes Without Having to Click It Each Time?

If you hold down the CTRL key when completing the pipe drawing (just before releasing the mouse button), the Pipe Drawing tool remains active, and you can draw a series of pipes without returning to the Toolbox each time. If you double-click the Pipe Drawing tool it remains active until you click it again a single time. This allows you to draw a series of pipes without returning to the Toolbox each time.

How Do I Add Graphical Segments to a Pipe so It Is Not Constrained to a Single Straight Line?

Use the Add/Remove Segments tool found on the Arrange Menu.

How Do I Set Up Custom Libraries on My Local PC?

See Libraries.

How Do I Set Up Custom Libraries on Our Local or Wide Area Network?

See Network Libraries Overview.

How Do I Change the Icon for a Junction in AFT Fathom?

Open the Junction Properties Window, click the Optional tab, and then click the Change Icon button.

Can I Customize the Junction Icons in AFT Fathom?

No, AFT Fathom icons are in a resource file that cannot be edited by the user.

What Is the Limit to the Size of Model I Can Create with AFT Fathom?

There are no theoretical limits to model size, but there are a few practical limits. First, AFT Fathom accepts pipe and junction ID numbers up to 30,000. This limits the model size to 30,000 pipes and 30,000 junctions. Before you reach that limit, however, you will likely encounter a limitation of your available RAM to hold all of the solver parameters. To determine how much RAM you need, add up the number of branches and tees in the model. Take the square of this number. Then multiply it by 32 to get the amount of RAM that must be available. For example, with 1,000 branches and tees, the square is 1 million, and after multiplying by 32 you need 32 million bytes of RAM (i.e., 32 MB).

Can AFT Fathom Model Reacting Flows?

No, AFT Fathom models only non-reacting flows.

Can I Install AFT Fathom Onto a Local or Wide Area Network?

Yes, AFT Fathom can run off the network or local PC. When installed on a network, the number of concurrent users is limited to the number of licenses purchased. See the Information on Software Installation topic.

Where Can I Find Reference Information About Pipe Materials?

See Pipe Material References

What Kind of Transients Can Fathom XTS Model?

A wide range, including liquid levels, valve open/close, pump speed changes, relief valve opening and more. Just as important, Fathom XTS can model time transients, event transients and inherent transients. Time transients are changes that are specified to occur based on the model time. Event transients, on the other hand, are triggered by an event that occurs in the system. Many kinds of events can be used, such as pressure, flow, velocity and liquid level. Inherent transients are changes that occur by the very nature of a system component. One example would be the liquid level change that occurs in a finite reservoir (or tank), while another would be a check valve opening and closing as a result of flow reversals.