A. Place the pipes and junctions
At this point, the first four items are completed on the Checklist. The next Checklist item is to “Define Pipes and Junctions”. In the Workspace window, assemble the model as shown in Figure 3.1.
Figure 3.1 Layout of pipe system for Pump Selection with Flow Control Valves Example
B. Enter the pipe data
The system is in place, but now you need to enter the input data for the pipes and junctions. Double-click each pipe and enter the following data in the Specifications window (or use the Global Pipe Editing window):
All of the pipes are Steel, with standard roughness and the following data:
Pipe Length
C. Enter the junction data
J1 Reservoir
1. Name = Supply Tank
2. Tank Model = Infinite Reservoir (only visible if XTS module is enabled)
3. Liquid Surface Elevation = 5 ft 4. Surface pressure = 10 psig
5. Pipe Depth (on Pipe Depth and Loss Coefficients tab) = 5 ft J9 Reservoir
1. Name = Receiving Tank
2. Tank Model = Infinite Reservoir (only visible if XTS module is enabled)
3. Liquid Surface Elevation = 10 ft 4. Surface pressure = 30 psig
5. Pipe Depth (on Pipe Depth and Loss Coefficients tab) = 10 ft (for both pipes)
J3 Tee J4, J7 Control Valves 1. Elevation = 0 feet
2. Valve Type = Flow Control 3. Flow rate = 100 gpm
Using the Groups tool on the Edit menu, add the control valves to a group named Flow Control Valves.
J5, J8 Heat Exchangers 1. Elevation = 0 feet
2. Loss Model = Resistance Curve
3. Loss Curve Data = 10 psid @ 100 gal/min J2 Pump
1. Elevation = 0 feet
2. Pump Model = Fixed Head Rise 3. Fixed Head Rise = 20 feet
D. Check if the pipe and junction data is complete
Turn on Show Object Status from the View menu to verify that all the necessary data is entered. If so, the “Define Pipes and Junctions”checklist item will have a check mark. If not, the uncompleted pipes or junctions will have their number shown in red. If this happens, go back to the uncompleted pipes or junctions and enter the missing data. You can also open the List Undefined Objects window from the View menu to see what data is missing.
Step 4. Open the Goal Seek and Control Manager
The GSC data is entered in the Goal Seek and Control Manager window.
Open the Goal Seek and Control Manager from the View Menu. After opening the Goal Seek and Control Manager, the user specifies all of the system variables, as well as the desired goals. The Goal Seek and Control Manager is shown in Figure 3.2 below.
Figure 3.2 The Goal Seek and Control Manager is used to define GSC Variables and Goals.
Step 5. Add a variable
In the GSC module, variables are the parameters that AFT Fathom will modify in order to achieve the specified goals. In general there should be one applied variable for each applied goal.
Select the Variables tab on the Goal Seek and Control Manager window.
The Variables tab allows users to create and modify the system
variables. The object and junction type are selected, then the name and
number of the object to which the variable applies, and the object parameter that is to be varied are specified on the Variables tab.
For this example, you will be adding a variable for the Pump Fixed Head Rise. Select the “Add Variable” button, and input the following variable data:
1. Apply: Selected 2. Object Type: Junction 3. Junction Type: Pump
4. Junction Number and Name: J2 (Pump) 5. Variable Parameter: Head Rise
6. Link To: (None)
7. Lower Bound: Leave Blank 8. Upper Bound: Leave Blank
The Apply column allows users to specify which of the variables that have been defined will be used. This allows the flexibility of creating multiple variable cases, while only applying selected variables for any given run.
The Link To column allows users to apply the same variable to multiple objects. This allows users to force parameters for several objects to be varied identically.
Upper and lower bounds provide logical extremes during the goal search. For this case, leave the lower and the upper bounds blank.
After entering the data, the Variable tab should appear as shown in Figure 3.3.
Step 6. Add a goal
Goals are the parameters you want to achieve. The goals are achieved as AFT Fathom modifies the variables. Typically, there should be one applied goal for each applied variable.
Figure 3.3 GSC Variables are parameters that are changed by AFT Fathom to achieve the defined goals.
Select the Goals tab on the Goal Seek and Control Manager window.
The Goals tab allows users to create and modify the system goals. The goal type, object type, and the goal parameter are selected. A criterion for determining if the goal has been met is then specified, along with a value and units for the goal parameter. The user then selects the object to which the goal applies, and, if applicable, the location on the object at which the goal applies (e.g., the inlet or outlet of a pipe object).
The Apply column allows users to specify which of the goals that have been defined will be used. This allows the flexibility of creating multiple goal cases, while only applying selected goals for any given run.
A Group Max/Min goal allows a single goal to be applied to a group of objects. Fathom applies a Group Max/Min goal by ensuring the final goal value is either greater than or equal to (a Min goal) or less then or equal to (a Max goal) the specified value. For this example, a Group Max/Min goal will be applied to ensure the minimum pressure drop across the flow control valves is at least 5.0 psid.
Create a new goal for the pressure drop across the flow control valves as defined below (if the option for a group goal is not visible you may have forgotten to create a group as discussed in Step 3c above):
1. Apply: Selected 2. Goal Type: Group
3. Object Type: Group Max/Min 4. Goal Parameter: Pressure Loss 5. Criteria: >=
6. Goal Value: 5 7. Goal Units: psid
8. Object ID: Flow Control Valves (this is the name of the group) 9. Object Location: NA
After entering the data, the Goals tab should appear as shown in Figure 3.4.
Figure 3.4 GSC Goals are the parameter values the user wants to achieve.
As variables and goals are added to a model, AFT Fathom will display symbols beside the pipes and junctions that have variables or goals applied to them. The default is a “V” for variables, and a “G” for goals.
The goal symbol is not displayed next to objects that are part of a group goal. This is illustrated in Figure 3.5. These symbols can be configured on the Workspace Preferences window.
“V” symbol for a GSC Variable.
“G” symbols for GSC Goals are not displayed for Group goals.
Figure 3.5 AFT Fathom displays symbols next to objects on the Workspace that have goals or variables defined.
Step 7. View GSC settings in Model Data
The Model Data window provides a summary of the GSC module variable and goal definitions. Once the variable and goal information has been added in the Goal Seek and Control Manager, the information is displayed on the Goal Seek and Control tab in the General section of the Model Data Window. Figure 3.6 shows the GSC variable and goal
Figure 3.6 The Goal Seek and Control parameters defined in the Goal Seek and Control Manager are displayed in the General section of the Model Data window.
Step 8. Enable goal seeking
After the GSC goals and variables have been defined, goal seeking must be enabled using the Analysis menu, as shown in Figure 3.7.
Step 9. Run the model
Select Run Model in the Analysis menu. This will open the Solution Progress window. This window allows you to watch as the AFT Fathom Solver converges on the answer.
Note: When using the GSC module there is an area displayed in Solution Progress that shows the specific progress of the GSC module.
As it makes progress, the Best (Lowest) value will decrease towards zero. The field in the far right displays how many complete hydraulic solutions have been run.
After completion, click the View Output button at the bottom of the Solution Progress window.
Figure 3.7 Select “Use” from the Goal Seek & Control menu item on the Analysis menu to instruct AFT Fathom to do goal seeking when it runs.
Step 10. Examine the results
The Output window contains all the data that was specified in the Output Control window. The results of the GSC analysis are shown in the General Output section.
The GSC Variables tab shows the final values for the variable
parameters, as shown in Figure 3.8. The GSC Goals tab shows the values achieved for the goals, as shown in Figure 3.9.
Figure 3.8 The final GSC Variable values are shown on the GSC Variables tab in the Output window General section.
Figure 3.9 The final GSC Goal values are shown on the GSC Goals tab in the Output window General section. The Actual and User values should be close if GSC was successful.
If not, a warning will appear.
Analysis summary
For this example, the minimum goal of 5 psid pressure drop across the flow control valves was achieved by applying a Group Max/Min goal.
The pump head requirement for the example system was determined to be 167.3 feet, as shown in Figure 3.8.
The pump head requirements for this system could also be determined without GSC by adjusting the pump head rise, and running multiple Fathom solutions until the minimum control valve pressure drop requirement was met. Alternatively, one could change the pump to a fixed flow pump, and allow Fathom to calculate the pump head requirement directly. However, this technique results in a reference pressure problem between the assigned flow pumps, and the flow control valves. To bypass this problem, one of the flow control valves must be changed to a constant pressure drop valve with a 5 psid pressure drop, until a pump head could be determined.
By using the GSC module to size the pump, the head requirements can be determined directly without user iterations or modifications to the flow control valves.
C H A P T E R 4
Modeling Extended Time Simulation
This chapter discusses how one uses the XTS module to model transient system behavior. Detailed information regarding XTS menus and functionality is given in this chapter. Chapter 6 provides a detailed hands-on XTS example.
What is the XTS module?
While steady-state modeling answers many design questions, some questions cannot be adequately answered without considering how systems behave over time. Questions such as how long it will take to fill a tank require a dynamic system model. The AFT Fathom XTS module answers such questions.
The XTS module allows you to model transient system behavior. Users can specify the time duration of the simulation, time step size, control system parameters, and how components such as pumps and valves operate over time. Operations such as valve position changes can occur during a specified time schedule, or can occur in response to events in the system thereby simulating control system actions.
Users can also specify tank volume on “finite” tanks, so that tank draining and filling can be simulated. Tanks can be open or pressurized, with the gas pressure automatically calculated as the liquid level changes.
The XTS module is a powerful tool which extends AFT Fathom’s powerful modeling capabilities into the dimension of time.
How does the XTS module work?
The XTS module can be described as using a lumped or quasi-steady approximation to time simulation. What it in fact does is represent the transient system behavior as a sequence of pseudo-steady-state solutions.
In between each pseudo-steady-state solution it adjusts transient parameters, performs mass balances on tanks, and changes component operations as specified by the user.
As a thought experiment, consider a system which you want to simulate for ten minutes in one minute increments. This would require eleven time step solutions (time zero and each minute up to ten). You could manually do this with standard AFT Fathom by running a steady-state model eleven times, and in between each run adjusting the input
parameters for the next run based on the results of the previous run. The XTS module automates this manual process.
Besides the automation benefits, the XTS module offers additional benefits such as transient output data management, consolidation and display, and graphing tools to review the transient results.
Using the XTS module
The user has the option of activating or not activating the XTS module when AFT Fathom first loads. After AFT Fathom is loaded, the XTS module can be activated or deactivated for use from the Options menu.
Whether or not XTS is activated impacts the Analysis menu, Checklist and Status Bar and multiple other AFT Fathom functions.
If the XTS module is active, the user can still run models in Steady Only mode by selecting this under Time Simulation on the Analysis menu.
Hence there are three possibilities for XTS.
1. XTS is not active
2. XTS is active and operated in Steady Only mode 3. XTS is active and operated in Transient mode
Table 4.1 lists the three possibilities and the impact on various AFT Fathom features.
Table 4.1 AFT Fathom feature accessibility based on XTS activation and Time Simulation mode
XTS Not Active XTS Active XTS Active
Feature No Mode Steady Only Mode Transient Mode
"Time Simulation" on Analysis
Menu Not Visible Visible Visible
"Transient Control" on
Analysis Menu Not Visible Visible but disabled Visible and enabled
"Transient Control" on
Checklist Not Visible Visible but disabled Visible and enabled
"Transient Control" on Status
Bar Not Visible Not Visible Visible
"Transient" tab on Junction Specifications Windows
Not Visible - data cannot be entered
Visible - data can be entered but will not be
used
Visible - data can be entered and will be
used Transient Pipe and Jct tabs in
Output Control Not Visible Visible Visible
"Transient" tab in Junction
Section of Model Data Not Visible Visible only if junction transient data exists
Visible only if junction transient data exists Time control slider and
buttons in Output window Not Visible Not Visible Visible Time control slider and
buttons in Visual Report Not Visible Not Visible Visible
"Transient" tabs on Output
Window Not Visible Not Visible Visible
"Transient" tab on Select
Graph Data Not Visible Not Visible Visible
Animation controls on Graph
Results Not Visible Not Visible Visible when selected
Enabling XTS transient mode
When the XTS module is active, two new menu items appear on the Analysis menu. At the top of the Analysis menu is the Time Simulation menu, from which the user can select “Steady Only” or “Transient”.
The XTS Transient analysis mode is enabled by selecting Time Simulation -> Transient from the Analysis menu (Figure 4.1). This can be selected before or after a model is built. Pre-existing models built with standard AFT Fathom can be opened with XTS and transient data added.
Transient modeling can be turned off at any time by selecting Steady Only from the same menu. The Steady Only mode causes the XTS module to function like standard AFT Fathom. One difference is that users can still enter transient data and this data is retained in the model.
If the model is opened in standard AFT Fathom, this data will be lost.
Table 4.1 relates the differences between not using XTS and using it in Steady Only mode.
When the Transient analysis mode is selected, the Transient Control option on the Analysis menu is enabled. Transient Control is a required Checklist item when in Transient analysis mode. In Steady Only mode the Transient Control menu item is visible but disabled and thus not required.
Figure 4.1 Transient mode is enabled from the Analysis menu.
Transient control window
When the XTS module is in Transient mode, the Transient Control menu option is enabled on the Analysis menu. This becomes one of the
Checklist items, and hence the user must enter data in Transient Control.
The transient simulation is controlled by information the user enters in the Transient Control window (see Figure 4.2). This window is used to specify the duration of the transient simulation, the size of the time step, and the frequency of the data saved to the transient output file.
Figure 4.2 The Transient Control window is used to specify the transient run parameters.
Also available is Forward Difference and Central Difference options for Finite Tank Liquid Level Adjustments. If finite tank reservoirs are not
used in the model, then there is no difference between the two options.
The impact when one or more finite tanks do exist is discussed later in this chapter.
Entering junction transient data
Transient data may be entered in the Transient tab on the Specifications windows for junctions which can have transients defined. A typical Transient tab is shown in Figure 4.3 for a Valve junction. Table 4.2 lists the different types of junction transients that can be modeled.
In most cases, transient data can be entered as absolute values or as a percentage of steady-state. The distinction is made when choosing between Absolute Values and Relative to Steady-State Value (Figure 4.3).
Figure 4.3 Example Transient tab on Valve Specifications window.
The first data point always needs to match the steady-state value which is usually entered on the tab at the far left. If, for example in Figure 4.3, the steady-state data for Cv of 200 is changed to 250, the transient data
option is specified, the first data point would be 100%, and if the steady-state Cv is changed from 200 to 250 there is no need to change the transient data since the first data point is still 100%. But it is now relative to a different steady-state value.
Initiating transients
There are four ways to initiate transients. Time based transients are based on the absolute simulation time. Single event transients are initiated by one event such as a pressure, tank liquid level, or other parameter at a specified location. Dual Event Cyclic events initiate transients in a repeating cycle based on two event parameters. Dual Event Sequential events initiate two transients that do not repeat.
Table 4.2 Types of junction transients that can be modeled Junction Type Transient Parameters Modeled
Assigned Flow Flow rate Assigned Pressure Pressure
Branch Flowrate source or sink Control valve Setpoint
Pump Speed, flow rate, head rise, control setpoint Reservoir Liquid level, surface pressure
Spray Discharge Flow area Three-Way Valve Position
Valve K, Cv or open percentage
Once the type of transient initiation is selected, the transient data for the component must be entered in the table, as appropriate. See the “Time and Event Transients” chapter for more details.
Repeat transient
If the transient data is periodic, you can enter the data for one cycle of the period and then tick the check box for Repeat Transient. This will cause the one cycle of transient data to be repeated once it has reached the end. The repetition will continue until the end of the simulation.
If the transient data is periodic, you can enter the data for one cycle of the period and then tick the check box for Repeat Transient. This will cause the one cycle of transient data to be repeated once it has reached the end. The repetition will continue until the end of the simulation.