Ammonia Transfer with Cavitation (English Units)

Ammonia Transfer with Cavitation (Metric Units)

Summary

A ship carrying ammonia in a pressurized holding tank transfers the fluid to an onshore holding tank. The valve controlling flow to the holding tank must be closed as quickly as possible. Three different closing times are to be evaluated, and the resulting maximum pressures need to be determined.

Topics Covered

• Observing effects of varying valve closing time

• Evaluating systems with transient cavitation

• Using Scenario Manager

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 (English 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:

• US - Ammonia Transfer with Cavitation.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 = Ammonia (liquid)

      1. After selecting, click Add to Model

   3. Temperature = 75 deg. F

This calculates the fluid properties to use in the model. Notice the high vapor pressure of 141 psia. By changing the temperature to 70 or 80 deg. F, it can be seen that vapor pressure is very sensitive to temperature. In such a case, it would be worthwhile to find out if 75 deg. F is the highest expected temperature so that the analysis can be run with the highest potential vapor pressure.

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 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.

Pipe Properties

1. Pipe P1

   1. Pipe Material = Steel - ANSI

   2. Size = Use table below

   3. Type = STD (schedule 40)

   4. Friction Model Data Set = Standard

   5. Length = Use table below

Pipe	Size (inches)	Length (feet)
1	8	100
2	10	300
3	10	150

Junction Properties

1. Reservoir J1

   1. Name = Ship - Pressurized Tank

   2. Tank Model = Infinite Reservoir

   3. Liquid Surface Elevation = 40 feet

   4. Liquid Surface Pressure = 250 psig

   5. Pipe Depth = 40 feet

2. Area Change J2

   1. Inlet Elevation = 0 feet

   2. Type = Abrupt Transition (Cylindrical)

3. Valve J3

   1. Loss Model tab

   2. Inlet Elevation = 0 feet

   3. Loss Model tab

      1. Valve Data Source = User Specified

      2. Loss Model = Cv

      3. Loss Source = Fixed Cv

      4. Cv = 1000

   4. Transient tab

      1. Transient Special Conditions = None

      2. Initiation of Transient = Time

      3. Transient Data = Absolute Values

Time (seconds)	Cv
0	1000
0.5	0

4. Reservoir J4

   1. Name = Onshore Holding Tank

   2. Tank Model = Infinite Reservoir

   3. Liquid Surface Elevation = 20 feet

   4. Liquid Surface Pressure = 250 psig

   5. Pipe Depth = 5 feet

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 navigate to the Sectioning panel. When the Sectioning panel is first opened it will automatically search for the best (least number of sections within 10% variance) option for one to five sections in the controlling pipe. The results will be displayed in the table at the top. Make sure the second row designating two sections in controlling pipe is selected.

Step 5. Define the Transient Control Group

ØOpen the Simulation Mode/Duration panel.

1. Enter the Stop Time as 5 seconds.

2. Navigate to the Transient Cavitation Panel.

3. Make sure that Model Transient Cavitation is checked and that the selected model is Discrete Vapor Cavity Model (DVCM).

4. Click OK.

Step 6. Create Child Scenarios

In this model we want to see the impact of using different valve closure times. We will create three scenarios to do the comparison. This can be done easily using the Scenario Manager on the Quick Access Panel. Note that the Scenario Manager can also be accessed from the Tools menu.

The Scenario Manager is a powerful tool for managing variations of a model, referred to as scenarios.

The Scenario Manager allows you to:

• Create, name and organize scenarios

• Select the scenario to appear in the Workspace (the ‘current’ scenario)

• Delete, copy and rename scenarios

• Duplicate scenarios and save them as separate models

• Review the source of a scenario’s specifications

• Pass changes from a scenario to its variants

Check that the Scenario Manager is active on the Quick Access Panel. It should appear as shown in Figure 2.

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 0.5 Second Closure in the Create Child Scenario window, as shown in Figure 3. Click OK. The new 0.5 Second Closure 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 1 Second Closure. Finally create a third child called 2 Second Closure. See Figure 4.

Set up scenarios

Since the Base Scenario already has the 0.5 second closure time entered, we do not need to modify that child. Load the 1 Second Closure scenario in the list by double clicking the scenario name.

ØOpen the J3 Valve junction and on the Transient tab change the time of valve closure from 0.5 to 1, then click OK. Select the 2 Second Closure scenario in the list by double-clicking the scenario name in the Scenario Manager. Change the J3 Valve junction here to close at 2 seconds.

Step 7. Run the 0.5 Second Closure scenario

ØDouble-click on the 0.5 Second Closure scenario in the Scenario Manager on the Quick Access Panel to make it the active scenario. Alternatively, you can select this scenario and then click on the Load Scenario icon to load this scenario.

ØClick Run Model from the toolbar. This will open the Solution Progress window, which allows you to watch as the Impulse Solver converges on the answer. This model runs very quickly. Now view the results by clicking the Output button at the bottom of the Solution Progress window.

Step 8. Examine the Output

ØGo to the Graph Results window by clicking the Graph Results tab. First create a pressure profile graph as follows:

1. Select the Profile tab in the Graph Control section of the Quick Access Panel.

2. In the Pipes section click All.

3. Make sure Pressure Static is selected in the Parameter section.

4. Change the units to psig.

5. Make sure the boxes for Mx and Mn are checked to display the maximum and minimum values.

6. Click Generate.

Results (shown in Figure 5) indicate that the peak pressure occurs at 400 feet along the transfer line. This corresponds to the valve inlet. Notice that the minimum pressure is flat along almost the entire length of pipe. This value is the vapor pressure, which indicates that the liquid cavitates through most of the length of the pipe.

ØSave this graph to the Graph List by selecting Add to Graph List from the toolbar, and give the graph an appropriate name, such as Static Pressure Profile. This will allow the graph to be quickly reloaded later in the other scenarios.

ØNavigate to the Transient Pipe tab on the Quick Access Panel. Add the outlet station of pipe 2 to the stations to be graphed. Make sure the selected graph parameter is Pressure Static with units of psig. Click Generate. This shows the transient pressures at the valve inlet over time (see Figure 6). Here you can see that the low pressure reaches the vapor pressure repeatedly. Save this graph to the Graph List as well.

From Figure 5 and Figure 6 you can see that the maximum pressure is about 570 psig. There are several ways to see the actual numerical value. Here is one way:

1. Change to the Output window by selecting the Output tab.

2. In the Pipes section the Transient Max/Min tab should be selected.

3. By default the maximum static pressure is not displayed, but the maximum stagnation pressure is. Typically these values are fairly close. Looking at the Maximum Stagnation Pressure for pipe 2 shows that it is about 570.2 psig (see Figure 7). If you want to see the maximum static pressure, you can add it to the Output window using the Output Control window.

Step 9. Run the Second and Third Scenarios

ØUse Scenario Manager to load and run the 1 and 2 second closure cases. Use the techniques in Step 8 to find the maximum pressure for these cases. A tabulation of the results is shown in Table 1. If the results for the 2 second case are graphed, one can see that there is no cavitation. The results corresponding to Figure 6 appear much different.

To see the difference in results with and without cavitation, you can turn the transient cavitation model off in the Transient Control window.

Step 10. Compare DVCM Output with DGCM

As a verification of the cavitation results, you can change the Transient Cavitation model in the Transient Control window to use the Discrete Gas Cavity Model. Do this for the 0.5 Second Closure scenario as follows:

1. Create a child of the 0.5 Second Closure scenario and name it DGCM

2. Open Analysis Setup and go to the Transient Cavitation panel in the Transient Control group

3. Under Model Transient Cavitation change the Model to Discrete Gas Cavity Model

4. Click OK

ØRun the model, then go to the Graph Results window.

Load the Static Pressure Profile graph by double-clicking the name in the Graph List Manager at the top of the Quick Access Panel. This will load the Pressure Profile with the results for the current scenario. In order to compare the DGCM results to the results using DVCM we can select both scenarios to be graphed using the Multi-scenario graphing feature.

ØIn the Graph Parameters Section of the Quick Access panel click the Multi-Scenario button to select the scenarios to graph. Check the box next to the 0.5 Second Closure scenario to include it in the graph, and ensure that the DGCM scenario is still selected, then click OK. Click Generate to update the graph.

Figure 8 shows the multi-scenario graph. Notice that the maximum pressure at the valve is nearly identical for both cavitation models, though there is some deviation in the maximum pressures upstream of the valve.

Repeat the process above to generate a multi-scenario graph of the Valve Inlet Pressure over time that was shown in Figure 6 above. The resulting multi-scenario plot is shown below in Figure 9. Notice that the models produce very similar results for the first pressure spike after cavitation begins at around 1.2 seconds, after which the results begin to diverge. This provides high confidence in the validity of the first pressure spike, as two separate calculation methods produced the same conclusion.

A similar analysis can be performed with the 1 Second Closure scenario, which results in slightly more deviation in the results. Performing this comparison with the 2 Second Closure scenario would produce no differences, as the minimum pressure never reaches vapor pressure in that scenario.

Summary

Table 2 provides a summary of the maximum pressure in each of the scenarios using each of the cavitation models. Lengthening the closure time reduced the maximum pressure with both cavitation models.

Table 2: Max Stagnation pressures with different closure rates

Closure Time (seconds)	Max Pressure DVCM (psia)	Max Pressure DGCM (psia)
0.5	630.9	640.1
1	562.3	551.0
2	382.5	382.5