Pump Startup with Event Transients (Metric Units)

Pump Startup with Event Transients (English Units)

Summary

This example looks at three pump startup cases for a water transfer system. The objective is to determine the maximum pressures in the system and to assess whether the system cavitates or experiences sub-atmospheric pressure.

Topics Covered

• Starting one or more pumps

• Using steady-state and transient special condition features

• Using event transients

• Using Scenario Manager

• Creating Graph Sets

• Using animation features

Required Knowledge

This example assumes the user has already worked through the Beginner - Valve Closure example, or has a level of knowledge consistent with that topic. You can also watch the AFT Impulse Quick Start Video (Metric Units) on the AFT website, as it covers the majority of the topics discussed in the Valve Closure example.

Model File

This example uses the following file, which is installed in the Examples folder as part of the AFT Impulse installation:

• Metric - Pump Startup with Event Transient.imp

Step 1. Start AFT Impulse

From the Start Menu choose the AFT Impulse 10 folder and select AFT Impulse 10.

To ensure that your results are the same as those presented in this documentation, this example should be run using all default AFT Impulse settings, unless you are specifically instructed to do otherwise.

Step 2. Define the Fluid Properties Group

1. Open Analysis Setup from the toolbar or from the Analysis menu.

2. Open the Fluid panel then define the fluid:

   1. Fluid Library = AFT Standard

   2. Fluid = Water (liquid)

      1. After selecting, click Add to Model

   3. Temperature = 16 deg. C

Step 3. Define the Pipes and Junctions Group

At this point, the first two groups are completed in Analysis Setup. The next undefined group is the Pipes and Junctions group. To define this group, the model needs to be assembled with all pipes and junctions fully defined. Click OK to save and exit Analysis Setup then assemble the model as shown in the figure below.

The Add/Remove Segments button on the Toolbar can be used to create the bends in pipes on the Workspace. Note that these segments are only visual, and have no affect on the calculations.

The system is in place but now we need to enter the input data for the pipes and junctions. Double-click each pipe and junction and enter the following data in the properties window.

For Valve J9, this valve will use what is called an event transient. This means that the time zero on the Transient tab is with respect to some condition occurring in the system. If the condition is never reached, the valve transient is never initiated. Here we want the valve to open when the pressure at J7 is sufficient to cause the water to flow into the J10 reservoir. Flow will occur when the pressure at J7 reaches 3.10 barG (310 kPa(g)) or greater. The pressure at J7 is the same as that at the outlet of pipe P7, so we will use pipe P7 as the location of the event initiation.

Junction Properties

1. Reservoir J1

   1. Name = Supply Reservoir

   2. Tank Model = Infinite Reservoir

   3. Liquid Surface Elevation = 6 meters

   4. Liquid Surface Pressure = 0 barG (0 kPa (g))

   5. Pipe Depth = 6 meters

2. Pumps J2 & J4

   1. J2 Name = Transfer Pump #1

   2. J4 Name = Transfer Pump #2

   3. Inlet Elevation = 0 meters

   4. Pump Model tab

      1. Pump Model = Centrifugal (Rotodynamic)

      2. Performance Curve Used in Simulation = Standard Pump Curve

      3. Enter Curve Data =

Volumetric	Head
m3/hr	meters
0	45.7
227	42.7
454	36.6

4. Curve Fit Order = 2

5. Configuration Data tab

   1. End of Curve Flow Rate = 900 m3/hr

6. Optional tab

   1. Special Condition = Pump Off With Flow Through

7. Transient tab

   1. Transient = Speed vs. Time

   2. Transient Special Condition = None

   3. Initiation of Transient = Time

   4. Transient Data = Absolute Values

Time (seconds)	Pump Speed (Percent)
0	0
2	100
10	100

3. Valves J3 & J5

   1. J3 Name = Valve #1

   2. J6 Name = Valve #2

   3. Inlet Elevation = 0 meters

   4. Loss Model tab

      1. Valve Data Source = User Specified

      2. Loss Model = Cv

      3. Loss Source = Fixed Cv

      4. Cv = 1000

   5. Transient tab

      1. Transient Special Condition = None

      2. Initiation of Transient = Time

      3. Transient Data = Absolute Values

Time (seconds)	Cv
0	0
1	800
2	1000
10	1000

6. Optional tab

   1. Special Condition = Closed

4. Branch J6 - J8

   1. Elevation = 0 meters

5. Valve J9

   1. Name = Valve to Process #1 Tank

   2. Inlet Elevation = 0 meters

   3. Loss Model tab

      1. Valve Data Source = User Specified

      2. Loss Model = Cv

      3. Loss Source = Fixed Cv

      4. Cv = 500

   4. Optional tab

      1. Special Condition = Closed

   5. Transient tab

      1. Transient Special Condition = None

      2. Initiation of Transient = Single Event

      3. Event Type = Pressure Stagnation at Pipe

      4. Condition = Greater Than

      5. Value = 3.1 barG

      6. Pipe = 7 (Pipe) Outlet

      7. Transient Data = Absolute Values

Time (seconds)	Cv
0	0
2	400
3	500
10	500

6. Reservoir J10

   1. Name = Process #1 Tank

   2. Tank Model = Infinite Reservoir

   3. Liquid Surface Elevation = 30 meters

   4. Liquid Surface Pressure = 0 barG (0 kPa (g))

   5. Pipe Depth = 3 meters

7. Reservoir J11

   1. Name = Process #2 Tank

   2. Tank Model = Infinite Reservoir

   3. Liquid Surface Elevation = 3 meters

   4. Liquid Surface Pressure = 0 barG (0 kPa (g))

   5. Pipe Depth = 3 meters

ØTurn on the Show Object Status from the View menu to verify if all data is entered. If there are objects that are not defined, the uncompleted pipes or junctions will have their number shown in red on the workspace. If this happens, go back to the uncompleted pipes or junctions and enter the missing data. If all objects are defined, the Pipes and Junctions group in Analysis Setup will have a check mark.

Step 4. Define the Pipe Sectioning and Output Group

ØOpen Analysis Setup and open the Sectioning panel. When the panel is first opened it will automatically search for the best option for one to five sections in the controlling pipe. The results will be displayed in the table at the top. Select the row to use one section in the controlling pipe. 

Navigate to the Output Pipe Stations panel. To minimize run time and output file size, the default is set to Inlet and Outlet for all pipes. This will cause only the inlet and outlet station of each pipe to be saved to the output file. Data can be saved for all pipe stations or only selected stations. The selected stations are shown on the Stations In Output column (see Figure 4). With the list next to Change Selected Pipes To set to All Stations, click the All button, then click Change Selected Pipes To. This will save all pipe station data for all pipes, which will be useful later for animation purposes.

Step 5. Define the Transient Control Group

ØOpen the Simulation Mode/Duration panel in the Transient Control group. Enter 10 seconds for the Stop Time.

Go to the Transient Cavitation panel and clear the check box for Model Transient Cavitation (we are going to ignore this until it becomes clear we need to model cavitation).

Step 6. Create Child Scenarios

In this model we want to evaluate three pump startup cases:

1. Both pumps starting

2. One pump starts while the other pump stays off

3. One pump starts while the other is already running

We will create three scenarios to model the three cases. Open up the Scenario Manager on the Quick Access Panel. Create a child scenario by either right-clicking on the Base Scenario and then selecting Create Child, or by first selecting the Base Scenario on the Scenario Manager on the Quick Access Panel and then selecting the Create Child icon. Enter the name Two Pump Start in the Create Child Scenario window, and click OK. The new Two Pump Start scenario should now appear in the Scenario Manager on the Quick Access Panel below the Base Scenario. Select the Base Scenario and create another child and call it One Pump Start. Finally create a third child called One Pump Start with One Running. The children scenarios should now be displayed as shown in Figure 5.

Since the Base Scenario already has been setup with two pumps starting, we do not need to modify the Two Pump Start scenario.

Load the One Pump Start scenario by double-clicking the name in the Scenario Manager. Here we want to start only pump J2 and keep pump J4 off. Since pump J4 and valve J5 are off during the transient, if we delete their transient input data then they will stay turned off. Alternatively, we can specify that their transient data be ignored. The second option is what we will use here.

ØOpen the J4 Pump window and on the Transient tab in the Transient Special Condition area choose the Ignore Transient Data option and click OK. Do the same with the J6 Valve. This scenario is now completed.

ØLoad the Child Scenario One Pump Start with One Running.

Here we want pump J2 to be running at 100% speed during the steady-state and transient, and to start pump J4. Open the properties window for J2 and select the Optional tab. Set the Special Condition to None. This specifies that during the steady-state the pump will be on and operate on its curve. Since this pump will run at 100% speed during the transient, select the Transient Data tab and set the Transient Special Condition to Ignore Transient Data.

We also need to change the J3 Valve junction Special Condition to None, and the Transient Special Condition to Ignore Transient Data.

The third scenario is now complete.

Step 7. Run the First Scenario

ØLoad the Two Pump Start scenario in the Scenario Manager. Select Run Model. After completion click Graph Results at the bottom of the Solution Progress window.

Step 8. Examine the Output

First let's look at a pressure profile.

1. Select the Profile tab on the Quick Access Panel, as shown in Figure 6.

2. In the Pipes section click None.

3. Select pipes 1, 2, 3, 7 and 11.

4. Select Pressure Static as the Parameter to plot.

5. Change the units to barG.

6. Make sure the boxes for Mx and Mn are checked as these are the maximum and minimum values.

7. Click Generate.

Results (shown in Figure 7) indicate that the peak pressure occurs at the pump discharge, and that the minimum pressure is above atmospheric at all times.

To easily recreate this graph for the other scenarios, you can file this graph into the Graph List Manager at the top of the Quick Access Panel on the Graph Control tab. After the graph has been generated, click the Add Graph to List icon on the Graph Results Toolbar (or by simply right-clicking on the graph itself and choosing Add Graph to List). Give the graph a name, such as Pressure Profile Pump #1 to Process Tank #2, then click OK in order to file the graph into the default My Graphs folder within the Graph List Manager.

The profile through the other flow paths (there are four paths altogether) can also be plotted, and similar conclusions are obtained. Create Graph Sets for the other three paths.

Since the maximum pressure is at the pump discharge, it is of interest here to plot the pressure vs. time at the pump. Go to the Transient Jct tab on the Quick Access Panel. Add the two pumps to the Graph These Junctions section. Select the Graph Parameter as Pressure Static Outlet and set the units to barG. Click Generate. As would be expected, both pressure transients are very similar. Results are shown in Figure 8. Click the Add Graph to List button on the Graph Results Toolbar (or by right-clicking on the graph and choosing Add Graph to List) and type the name Pump Discharge Pressures to save this graph to the Graph List Manager as well.

The flow rates through the pumps are also of general interest, and can also be plotted as shown in Figure 9 by changing the parameter being graphed to Vol. Flow Rate Outlet. Add this graph to the Graph List Manager as well and name it Pump Discharge Flow Rates.

The flow rates into the two process tanks are of interest. On the Transient Pipe tab add the outlet of pipes P10 and P11. Select to plot the Volumetric Flowrate, click Generate, and then add this graph to the Graph List Manager with the name Process Tank Flow Rates, then click OK. Results are shown in Figure 10.

It should be noted that the graphs in Figure 8 and Figure 9 could also be generated by using the Transient Pipe tab to graph the static pressure and volumetric flow rate at the inlet of pipes P2 and P5.

Step 9. Animate the Results

ØLoad the pressure profile graph Pressure Profile Pump #1 to Process Tank #2 that was created in Step 8 from the Graph List Manager, and make the following updates, as shown in Figure 11.

1. For Animate Using, select Output. This is where changing the output to store output data for all stations in Step 4 comes into play.

2. Uncheck the boxes for Mx and Mn so that the maximum and minimum values will NOT be displayed.

To use the Time Animation feature in conjunction with the Use Output File option, which we are using here, all pipe stations need to be saved in Transient Control (Step 5). In addition, it is frequently best to save all time step data as well (from the Save Output to File area in the Transient Control window). We selected both of these. This animation option allows you to start, pause, record, restart animation and also allows you to slowly move forward and backward in the animation by changing one time step at a time.

Another animation option in Figure 11 is Solver. This option actually re-runs the Transient Solver to generate the data for animation, and thus does not need to read it from the output file. Therefore, it does not require all of the data to be saved in Transient Control. This will reduce the run time of the initial simulation run, but will not have the advantage of being able to pause the animation and backup time step by time step to go back to the time where significant spikes start to appear.

ØClick Generate. Additional animation control features appear on the Graph Results window (Figure 12). Press the Play button and watch the pressure waves move.

There is also animation available in the Workspace via Layers, which can be useful to see how the flow paths interact with each other.

ØGo to the Workspace window and select Workspace Layers in the Quick Access Panel. Select New Layer, indicated by the green plus sign in the Layers section. Select Color Map/Animations and give the New Layer a name such as Static Pressure Animation. Open up the Layer Settings with the gear icon.

Set the Display Mode at the top of the window to Animate, then choose the settings as follows. (Figure 13)

1. Color Map/Animation Layer Settings

   1. Layer Type = Animation

   2. Data Type = Absolute Values

   3. Smooth colors between pipe sections = Checked

   4. Parameter = Pressure Static

   5. Units = barG

Click Close. The Animation should now be visible on the Workspace. Note that the animation toolbar again appears, similar to when using Graph Animation. It is recommended to save data for All Stations when creating a Workspace Layer Animation, but it is not required to do so. The Pipes to Animate section shows what data is being used to create the animation.

ØPress the play button to watch the pressure transients in the system.

Step 10. Run the Other Scenarios and Graph the Results

Using Scenario manager load the other two scenarios and run them.

In the One Pump Start With One Running scenario, it should be noted that there is a warning given for the pump. This is indicated by the red message in the status bar. The warning can be viewed in the top third section of the Output window on the Warnings tab, and states that there was reverse flow at pump J4. This warning message indicates that backwards flow occurred at the pump, and insufficient information was available to perform the head calculations. It is recommended to adjust the model to better account for this reverse flow.

Depending on the system, there are several actions that may be appropriate. One change may be to adjust model inputs such as the discharge valve opening profile to prevent reverse flow to the pump. This is only acceptable if the changes are physically accurate for the system. Otherwise, it will be necessary to choose an appropriate four quadrant data set for the pump to better predict the head at reverse flow conditions. See the Pump Trip With Backflow - Four Quadrant Modeling example for more information on specifying the pump using four quadrant data.

Note: Whenever you see caution, warning, or critical warning messages in general, you can click on the message itself, then hit the F1 button on the keyboard to pull up the help system in order to read a description of what the message means.

Results

Create graphs similar to Figure 6 through Figure 9 for each of the scenarios by double-clicking on the graph names in the Graph List Manager. This will reveal that the maximum pressure for the One Pump Start scenario occurs at the pump discharge location, similar to the first scenario, but in the One Pump Start With One Running scenario the maximum pressure occurs in pipe P11. Also the pressure drops below atmospheric, but does not reach the vapor pressure of water.

The numerical maximum pressure can be found in the Output window of each scenario and is summarized in Table 1. For these cases the maximum pressures do not significantly differ from each other.

Table 1: Summary of maximum static pressure for the three cases

Case	Max Pressure (barG)
Two Pumps Start	5.0156
One Pump Start	4.9306
One Pump Start with One Running	5.0640