1. Assigned Pressure J1

   1. Name = High Pressure Steam Inlet

   2. Elevation = 0 feet

   3. Static Pressure = 2425 psia

   4. Temperature = 1060 deg. F

2. Area Change J2

   1. Type = Conical Transition

   2. Angle = 45 degrees

3. Branch J3,J4,J7 

   1. Elevation = 0 feet

4. Assigned Flowrate J5,J6

   1. J5 Name = Turbine 1

   2. J6 Name = Turbine 2

   3. Elevation = 0 feet

   4. Mass Flowrate = 504000 lbm/hr

      Note: Ensure that you change the unit in the Mass Flowrate from lbm/s to lbm/hr, as lbm/hr is not the default unit.

   5. Temperature =1060 deg. F

5. Dead End J8  

   1. Elevation = 0 feet

6. Bend J9

   1. Elevation = 0 feet

   2. Type = Standard Elbow (knee, threaded)

7. Bend J10, J12

   1. Elevation = 5 feet

   2. Type = Standard Elbow (knee, threaded)

8. Valve J11

   1. C_v = 3000

   2. X_t = 0.7

   3. Elevation = 5 feet

   4. In the optional tab, set the special condition to closed

   5. Click on the Transient Tab and enter the following data

9. Assigned Pressure J13

   1. Name = Steam Bypass Outlet

   2. Elevation = 0 feet

   3. Static Pressure = 435 psia

   4. Static Temperature = 660 deg. F

C. Check if the pipes and junctions are specified

• ØClick the List Undefined Objects button to see if the model is fully defined. If it is not, the undefined pipes and junctions window will show a list of incomplete items. If undefined objects are present, go back to the incomplete pipes or junctions and enter the missing data.

Step 3. Specify Analysis Setup

A. Specify Fluid panel

• ØOpen the Analysis Setup window from the Main Toolbar and open the Fluid panel. Under the "AFT Standard" fluid option, choose "Steam" from the list and click the "Add to Model" button. Click OK.

B. Specify the Pipe Sectioning and Output panel

• ØOpen the Pipe Sectioning and Output panel. Specify the Minimum Number of Sections Per Pipe as 1 and the Estimated Maximum Pipe Temperature During Transient to 1100 deg. F.

C. Specify the Simulation Mode/Duration panel

• ØOpen the Simulation Mode/Duration panel from the Transient Control group from the Analysis Setup window. Enter the Stop Time as 5 seconds. Click the OK button.

Step 4. Run the model

• ØSelect "Run Model" from the Main Toolbar. The model has an estimated run time of 5 minutes, but the run time is dependent on the speed of your PC.

Step 5. Graph the results

A. Graph the transient pressures at the turbines

• ØGo to the Graph Results window. The first graph we make will show the Static Pressure at the entrance to the steam turbines.

1. From the Graph Control tab on the Quick Access Panel, select the Transient Pipe tab

2. Select the pipe P5 outlet station

3. Ensure that time units are set to seconds

4. In the Parameters definition area, select "Pressure Static" and specify "psia" in the dropdown box under Units

5. Click Generate

This will generate the graph shown in Figure 6. The pressure variation is difficult to see using the default y-axis scale so we will adjust the scale using the following steps:

1. Right-click the y-axis

2. Uncheck the Auto Scale box

3. Set the Minimum to 2200 psia, the Maximum to 2600 psia, and Major Val to 100 (see Figure 7)

The graph should now match Figure 8. When the model contains multiple transient events, transient pipe graphs allow the user to see the cumulative impact of the events on system pressure. Note that the first peak contains an oscillation, which is attributable to the opening of the bypass valve.

B. Create a profile of the bypass line temperature and pressure

We also will animate the profile of the bypass line temperature and pressure in order to determine whether condensation occurs.

1. Click the "New Tab" button to create a new graph.

2. From the Graph Control tab on the Quick Access Panel, select the Profile tab (Figure 9)

3. Select pipes 6, 8, 9, and 10

4. Verify the Length unit is set to feet

5. Next to Animate Using, verify "Output" is selected, and verify "seconds" are the selected Animation Time Units

6. Add a second parameter in the parameters definition area, then specify one parameter to be "Pressure Static" and the other to be "Temperature Static"

7. Change the Pressure Static units to "psig" and the Temperature Static units to "deg. F"

8. Click the Generate button

9. Click the Play button to review the graph

Note: If you wish to adjust the speed of the transient in the profile, you can use the Speed Slider in the upper right-hand corner of the Graph Results window to speed up or slow down the animation.

Figure 10 is useful to determine whether or not condensation occurs in the system, as well as understanding the cause of condensation. Condensation can occur by a rise in pressure, a decrease in temperature, or a combination of both. Saturation line data was added for AFT Standard fluids in AFT xStream 2. In this model the temperatures stay well above the saturation line at all times, showing that the steam is not close to condensing.

It should be noted that the saturation data is limited to predicting the presence of saturation conditions as opposed to modeling the behavior of the multi-phase mixture.

C. Graph the mass flow rate and pressure drop through the bypass valve

Lastly, we shall create a graph to show the mass flow rate and the pressure drop through the bypass valve to gain insight into how the flow develops in the bypass line.

1. Click the "New Tab"button to add a new graph tab

2. From the Graph Control tab on the Quick Access Panel, select the Transient Jct tab

3. In the Select Junctions section, select J11 and click the ">" arrow

4. Add a second parameter in the Select Parameters section. Specify the parameters as "Mass Flow Rate Inlet" and "Pressure Drop Stagnation Total".

5. Set the units for Mass Flow Rate Inlet to "lbm/hr" and Pressure Drop Stagnation Total to "psid"

6. Click Generate

As shown in Figure 12, the flow rate increases and the pressure drop decreases as the valve opens over time. While the observed trends for mass flow rate and pressure drop are typical for a valve that is opening, it is still worthwhile to make Transient Jct graphs when multiple transient events occur in order to fully understand the interactions between them.

Note: Figure 12 has modified axes titles and its legend removed for clarity.

Step 6. Results of Other Libraries and Discussion

A. Review the results of other libraries

In addition to the AFT Standard fluid library, the model built in this example was run using the Chempak, NIST REFPROP, and ASME Steam libraries. Similar results to the AFT Standard Fluid run were obtained for both the bypass valve and the turbine pressure vs time graph, so these results are not shown below. Nevertheless, it is sensible to run your system using different fluid property models as they have been known to impact results. The temperature profile for the bypass line showed more significant differences, as is shown below comparing the results using NIST REFPROP to the AFT Standard fluid library. ASME Steam and Chempak gave nearly identical results to NIST, and therefore are not shown.

Note that the minimum temperature reached at J11 is 56 degrees Fahrenheit higher in the AFT Standard fluid library run compared to the NIST REFPROP run.

Note: The axes of the below graphs were modified for clarity.

B. Advice for preliminary runs

Obtaining final results from AFT xStream will require multiple iterations. To effectively work through those iterations, we recommend the iterative process begins with the following steps:

Step 1: Run the steady-state model

It is recommended that the first runs are completed using a simplified version of your model in the AFT Arrow™ Steady-State Solver with each fluid property model before considering transient conditions. This will allow for larger issues in the model to be caught before having to commit to Transient runs, which are more time consuming. This also can inform the types of variables that need to be considered during refined runs (i.e. if the AFT Arrow™ steady state runs are close to the saturation temperature, condensation is a phenomenon that should be monitored in subsequent runs).

Step 2: Complete a transient run using an AFT Standard fluid with minimal sectioning

You should then run your simplified model in the Transient Solver using an AFT Standard fluid with minimal sectioning. This is not always possible, such as when mixtures are present, but this step should always be attempted if allowable. This will allow for larger issues to be caught before committing to extended run times. Only after you are satisfied with the model should you attempt increasing the number of sections.

C. Selection of fluid property model for final runs

A number of factors must be taken into consideration while selecting the fluid property model for an AFT xStream model. If there are no mixtures, and the compound is present in the AFT Standard library, it is recommended that the AFT Standard library be the first library used. AFT Standard has the shortest run times of the four libraries and is an excellent choice for early runs focused on troubleshooting the model. However, if there is a need to model mixtures, and simplifying the system is not possible, initial runs should be conducted with NIST REFPROP or Chempak.

NIST REFPROP and Chempak both have greater accuracy than AFT Standard and can represent mixtures. However, both have slower run times than AFT Standard. Chempak runs faster than NIST REFPROP with comparable accuracy. However, Chempak is an optional add-on while the NIST REFPROP library comes standard with AFT xStream.

Lastly, ASME Steam provides a well-established, trustworthy source of steam data. ASME Steam has the longest run times of the four, and it is recommended that a high processing power computer paired with ample time be used to run your analysis if this library is selected. If either of those elements is unavailable, it is recommended that one of the three other libraries be used, with the added note that NIST REFPROP is the most similar in accuracy to ASME steam. Table 1 summarizes this example's run time with the four different fluid property models. The run times will be affected by the specifications of your computer as well as the memory available.

Table 1 shows data for runs conducted on the computers at AFT as of publication. Additionally, Table 2 provides a library compatibility chart to assist with fluid property model selection.

It should be noted that if a tremendous amount of accuracy is needed, appropriate library selection will not be sufficient to give a high accuracy model. The single parameter that will have the greatest effect on accuracy is the Minimum Number of Sections per Pipe in the Sectioning panel. Ultimately, appropriate sectioning remains the single most important factor in creating a high accuracy model and should be weighed just as heavily as the fluid library when setting the parameters your transient gas system uses.

* Evaluating the complexity of a system is most easily done in early runs of the model. It is suggested that all early runs should be run with a version of AFT Standard if they can, and the run time recorded. This run time should inform your decision on which fluid library is needed to run the more thorough runs when high accuracy results are needed. As seen from Table 1, the run times can vary wildly between the 4 libraries, so make sure you evaluate your time constraints and accuracy needs when weighing this decision. Factors that can impact the run time include the ratio in the length of the smallest pipe to the largest pipe, presence of mixtures, usage of Junctions containing resistance curves, and the presence of sonic choking. See the Reducing Run Time section for more information.

Conclusions

The mass flow rate through the bypass line was modeled and the cumulative effect of multiple transient events was observed.