Refinery Relief System (English Units)

Refinery Relief System (Metric Units)

Summary

This example will demonstrate how to evaluate an emergency relief system for relief capacity, and mass and mole fraction of a discharge mixture for an environmental impact assessment.

Topics Covered

  • Evaluating Relief Valves

  • Using Scenarios

  • Using Global Junction Edit Feature

Required Knowledge

This example assumes the user has already worked through the Beginner - Air Heating System example, or has a level of knowledge consistent with that topic.  You can also watch the AFT Arrow Quick Start Video on the AFT website, as it covers the majority of the topics discussed in the Beginner: Air Heating System example.

Model File

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

Problem Statement

A new emergency relief system at an oil refinery is being considered, and the process calculations need to be evaluated.

The system provides relief to processes for methane, propane, and ethane. Each process is at 200 psig when the relief event occurs. 

The relief valve CdA is 15 inches2. Assume K=0 since this system will choke. The elbow is a standard elbow, and the tees should all be modeled as simple tees.  All of the pipes are STD (schedule 40) Steel - ANSI pipe, with standard roughness, and the flow can be assumed to be adiabatic.

The process engineer has evaluated the relief capacity at the minimum process temperature of 300 deg. F. Depending on the properties of the crude oil, all three processes can operate at up to 700 deg. F. The process engineer did not look at this case due to time constraints, but he thinks the higher temperatures will not affect the relief capacity. The process engineer also neglected any elevation changes to simplify the calculations, even though the relief point is 120 feet above the discharge header.

Determine the following:

  • The relief capacity (i.e. flow rate) of each process.

  • The mass and mole fraction of the discharge mixture for the environmental impact assessment.

  • Determine if neglecting the high temperature case and the elevation effects are acceptable.

Step 1. Start AFT Arrow

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

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

Step 2. Define the Steady Solution Control Group

There is an option available in Solution Control when using one of the marching methods that can sometimes result in an overall reduction in solution runtime. This option is to first solve the system using the Lumped Adiabatic method and then use these results as a starting point for the marching solution. Since the Lumped Adiabatic solution can typically be obtained much faster, this can provide an overall reduction in runtime for the marching method.

The Steady Solution Control Group is already defined using the default inputs. This means that no user input is required to run the model. However, the Lumped Adiabatic initialization will be used for this model.

To activate this option, do the following:

  1. Open Analysis Setup

  2. Navigate to the Solution Method panel in the Steady Solution Control group

  3. Click the box to turn on First Use Lumped Adiabatic Method To Obtain Initial Stating Point For Marching Solution

The Steady Solution Control Group is now defined for this model.

Step 3. 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 = NIST REFPROP

    2. Fluid = Methane

      1. After selecting, click Add to Model

    3. Repeat this to add Ethane and Propane

    4. Specific Heat Ratio Source = Library

Step 4. 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 on the workspace as shown in the figure below.

Figure 1: Model layout for Refinery Relief System example

The system is in place but now we need to enter the properties of the objects. Double-click each pipe and junction and enter the following properties.

Pipe Properties

  1. Pipe Model tab

    1. Pipe Material = Steel - ANSI

    2. Pipe Geometry = Cylindrical Pipe

    3. Size = Use table below

    4. Type = STD (schedule 40)

    5. Friction Model Data Set = Standard

    6. Lengths = Use table below

Pipe Size (inch) Length (feet)
1-3 3 50
4 3 25
5 4 25
6 6 150

Junction Properties

  1. All Tanks

    1. J1 Name = Methane Tank

    2. J2 Name = Ethane Tank

    3. J3 Name = Propane Tank

    4. Elevation = 0 feet

    5. J1 Fluid = Methane

    6. J2 Fluid = Ethane

    7. J3 Fluid = Propane

    8. Pressure = 200 psig

    9. Temperature = 300 deg. F

  2. J4 Bend

    1. Elevation = 0 feet

    2. Type = Standard Elbow (knee, threaded)

    3. Angle = 90 Degrees

  3. All Tees

    1. Elevation = 0 feet

    2. Loss Model = Simple

  4. J7 Valve

    1. Name = Relief Valve

    2. Elevation = 0 feet

    3. Valve Data Source = User Specified

    4. Loss Model = K Factor

    5. K Factor = 0

    6. Sonic CdA = 15 inches2

    7. Exit Valve (optional) = Checked

    8. Exit Pressure = 0 psig

    9. Exit Temperature = 70 deg. F

To show the names of the tanks and valve, a new Workspace Layer will be created.

  1. Create a new layer by selecting New Layer and then Blank Layer from the Workspace Layers tab of the Quick Access Panel.

  2. Name the layer Junction Labels.

  3. Edit the layer by selecting it and clicking the gear icon to bring up the Layer Settings window.

  4. Open the Show/Hide Labels panel and uncheck Force Shown Labels to Match Shown Objects.

  5. Toggle the visibility icon next to the tanks and valve to on. This will cause their labels to be shown.

  6. Navigate to the Junction Parameters panel and expand the Commonly Used Junction Parameters list.

  7. Double-click Junction Name to add it to the list on the right-hand side. This will add the Junction Name to the label of the junctions that were defined in the Show/Hide Labels panel.

  8. Close the Layer Settings window.

ØTurn on Show Object Status from the View menu to verify if all data is entered. If so, the Pipes and Junctions group in Analysis Setup 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.

Step 5. Run the Model

Click Run Model on the toolbar or from the Analysis menu. This will open the Solution Progress window. This window allows you to watch as the AFT Arrow solver converges on the answer. Once the solver has converged, view the results by clicking the Output button at the bottom of the Solution Progress window.

Step 6. Specify the Output Control

Open the Output Control window by selecting Output Control from the Toolbar or Tools menu.

With the Pipes button selected at the top, add Composition Mass Fraction and Composition Mole Fraction to the pipe output list by highlighting these parameters (holding CTRL will allow you to select both at the same time) and then clicking the purple Add arrow.

Step 7. Examine the Output

The Composition tab in the pipes output section displays the component mass flow rate through each of the pipes, as shown in Figure 2). 

The output for this model shows:

  • The total relief capacity is 37.628 lbm/sec

    • 7.152 lbm/sec for methane

    • 12.55 lbm/sec for ethane

    • 17.93 lbm/sec for propane

  • The mass fraction at discharge (observed from clicking on the Composition tab): 19.01% methane, 33.34% ethane, and 47.65% propane

  • The mole fraction at discharge from the Composition tab: 35.11% methane, 32.86% ethane, and 32.02% propane

Figure 2: Output window showing results for Refinery Relief System example

Step 8. Create a Child Scenario

In order to determine if the process engineer has made good assumptions by neglecting the higher temperature process and the elevation data, it will be necessary to change the model. 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

The Scenario Manager should currently appear as shown in Figure 3.

Figure 3: The Scenario Manager window on the Quick Access Panel is used to create and manage model scenarios

Create a child scenario by either right-clicking on the Base Scenario and then selecting Create Child. Enter the name High Inlet Temp in the Create Child Scenario window, as shown if Figure 4. Click OK. The new High Inlet Temp scenario should now appear in the Scenario Manager on the Quick Access Panel below the Base Scenario as shown in Figure 5.

Figure 4: Create Child Scenario window

Figure 5: Scenario Manager on the Quick Access Panel with New Child Scenario

Step 9. Modify the Higher Inlet Temp Scenario with Global Junction Editing

Double-click on the High Inlet Temp scenario in the Scenario Manager on the Quick Access Panel to make it the active scenario if it is not already.

To check the process engineer's assumption about the 700 deg. F process, the inlet temperature for all of the process tanks must be changed from 300 to 700 deg. F. These changes could be made by opening the Tank Properties window for each of the tanks and changing the inlet temperatures one at a time.

AFT Arrow has a feature that can be used to change the temperature for all of the tanks at the same time. This feature is called Global Junction Editing.

Open the Global Junction Edit window by selecting Global Junction Edit | Tanks from the Edit menu. Select all of the tanks in the Junction List by clicking All at the top (see Figure 6).

Figure 6: Global Junction Edit window

Click the Select Tank Data button. This will open a Tank Properties window. Enter a temperature of 700 deg. F and click OK to close the Tank Properties window. The Global Junction Edit window now contains a list of all the parameters that can be changed in the selected junctions. The data in the list is grouped together by the tab on the Properties window on which the data is entered. Any items that are selected in this list will be applied to all of the junctions selected in the Junction list.  Check the box next to Temperature in the list, which now shows a value of 700 deg. F  (see Figure 7).  Since this is the only item in the parameter list that is selected, it is the only parameter that will be changed in the selected tank junctions. Click Apply Selections at the bottom then click OK to close the Global Junction Edit window. The temperatures for all three of the tanks have now been updated to 700 deg. F. Open the Properties window for one of the tanks to verify that the change was made.

Figure 7: Global Junction Edit window with Temperature Parameter Selected

Step 10. Run the Model

Click Run Model on the toolbar or from the Analysis menu. This will open the Solution Progress window. This window allows you to watch as the AFT Arrow solver converges on the answer. Once the solver has converged, view the results by clicking the Output button at the bottom of the Solution Progress window.

Step 11. Examine the Output

See Figure 8 for the output from this scenario. The output for this model shows:

At 700 deg. F, the relief capacity is reduced to 29.767 lbm/sec, a 21% reduction, which is significant. The original assumption and calculation by the process engineer based on this assumption was not conservative, as he assumed.

Figure 8: Output window showing the results for High Inlet Temp Scenario

Step 12. Create and Run a Child Scenario for Elevation Effects

Create a child scenario to the High Inlet Temp scenario to examine the affect including the elevation information has on the model.

Change the elevation data for the J7 Valve to 120 feet. Rerun the model, and examine the results.

Adding the 120-foot elevation to the relief valve has a negligible effect on the system solution.