control valve sizing

CHEN 4570

Instrumentation & Process Control

Spring 2005

Control System Development with HYSYS1 This exercise provides a tutorial on the development of dynamic chemical process simulations using AspenTech’s HYSYS simulation software. The tutorial is based on implementing three conventional PID control loops on a single-stage flash drum unit for a multicomponent, nonideal feed stream. The steps in the development process are: 1) 2) 3) 4) 5) 6) 7)

steady-state design and simulation specifying dynamic characteristics of process equipment switching the simulation over to dynamics mode adding feedback controllers to the simulation adding stripchart displays carrying out dynamic tests and fitting transfer functions tuning the controllers

1. Steady-state Design and Simulation The process unit that is design, simulated and studied in this tutorial is a single-stage flash drum, as shown in the figure below.

Vapor Valve

Vapor Outlet

Flash Drum

Feed Feed Valve

Liquid Valve

Liquid Outlet

The feed specifications are Mass flow rate Pressure Temperature

5 kg/s 20 psia 80 °C

Composition acetone (mass fraction) 2-propanol water

0.294 0.484 0.222

The flash drum is to operate at atmospheric pressure, 14.7 psia. Pressure downstream of both vapor and liquid valves is 9.7 psia. With this information, you can create a steady-state simulation in HYSYS. Start by launching HYSYS and opening a new case, as shown below.

1

You may find it best to use this document on-line while you are completing the tutorial, opened in Acrobat Reader, rather than printing it out on a standard laser printer. In that way, you can see the colors.

CHEN 4570

Control System Development with HYSYS

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The Simulation Basis Manager window will appear, and you click on the Add button to create a fluid package for the component set.

Click the View button and the Component List View window will show.

Enter acetone into the Match field and click the Add Pure button. Do the same for 2-propanol and water.

Exit the Component List View by clicking on the tab of the Basis Manager window.

in the upper righthand corner. Click on the Fluid Pkgs

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Click on the Add button, as shown above. The Fluid Package window appears with the Setup tab selected.

By scrolling down the Property Package Selection list, find the NRTL item and click on it. The NRTL (non-random, two-liquid) activity model is appropriate for our aqueous mixture. Click on the Close ( ) button and return to the Simulation Basis Manager.

Click on the Enter Simulation Environment and, with the mouse, manually “stretch” the PFD window to fill the available HYSYS window space.

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Save the case as FlashDrumxyz (instead of xyz, use your initials). A .hsc extension (HYSYS Simulation Case) will be automatically added to the file. The case name should now appear at the top of the HYSYS window.

Click once on the blue arrow for Material Stream on the palette to the right.

Move the cursor to the left side of the PFD window, and you should see

. Click there and a light

blue arrow labeled 1 should appear. The light blue color indicates that the stream needs to be specified, so double-click on the arrow. A stream specification window, labeled 1, should appear.

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Start by clicking on the 1 to the right of Stream Name, and then enter Feed in the field that appears.

Once, you press the Enter key, Feed should now appear to the right of Stream Name and in the title bar of the window. Next, click to the right of Temperature, and type 80 in the entry field up top.

Notice that the default units are the desired units, °C, so press Enter. Next, click to the right of Pressure and type 20 in the entry field. The default units, kPa, are not correct for the 20 entry, so open the units list to the right and select psia.

Click on psia and the entry will be made, changing the display of the units to kPa and converting the numerical value to 137.9. In a similar fashion, enter the Mass Flow as 5, changing the entry field units to kg/s. This should leave you with the following window now.

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The rest of the quantities are still because the composition hasn’t been specified yet. So, click on the Composition item on the left,

and then on the Edit button. Make certain the Mass Fractions option button is selected and enter the feed specifications, [ 0.294 0.484 0.222 ], and confirm that the Total at the bottom is 1.0.

As shown, click OK, and the Feed window should show the compositions, along with a green OK banner at the bottom.

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Click on the Conditions item on the left, and the values should now be filled out.

You can see additional estimated properties by clicking the Properties item. Close out the Feed window in the upper right corner. The stream arrow should now appear dark blue and with by clicking on the the title Feed.

Now, you will add the feed valve. Click on the valve symbol on the palette to the right. Move your cursor to the right of the Feed stream and click. You should see a red valve symbol there. Double-click on the VLV-100 valve symbol and you will get a window.

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Click in the Name field and enter directly there a name Feed Valve. Open the Inlet drop-down list. There should be only one item, Feed. Click on it.

In the Outlet field, type directly Drum Feed. Press Enter, and this stream will be created automatically by HYSYS.

The Feed Valve window is now “complaining” that it needs a pressure drop, Delta P. Eventually, this will be determined dynamically, but, for now, we can enter the steady state value, 5 psi. So, click on the Parameters item on the left and enter a value of 5 psi. You will have to adjust the entry units to do this.

The Feed Valve window shows about 35 kPa and the green OK at the bottom indicates that the window is . Later, we will have to come “happy and satisfied” now, so close out the window by clicking on the back to size the valve for dynamic simulation purposes.

.2

Your process flow diagram (PFD) should now look like

2

You should re-Save your case frequently by clicking on the

button.

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Now, you will add the major equipment component, the flash drum. Find the separator icon on the palette

and click on it. Place your mouse cursor to the right of the Drum Feed arrow on the PFD and click. The separator should appear in red.

Double-click on the separator to get its window, and immediately change its name to Flash Drum.

Click on the <> item under Inlets, and select Drum Feed from the drop-down list above.

In the Vapour Outlet field, type Vapor, and in the Liquid Outlet field, type Liquid.

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The Vapor and Liquid streams have been created automatically, and, for now, the Flash Drum window is “happy as a clam at high tide” as shown by the green banner with OK. We’ll have to come back to it later it closed for now. for dynamic specifications, but Your PFD should now look like

As you go along, you can “yank” components on the PFD around to neaten things up, if you’re one of those “drive between the lines” types.

Now, it’s a matter of installing the Vapor Valve and Liquid Valve, each with a 5 psi pressure drop and outputting to Vapor Outlet and Liquid Outlet streams. Based on your experience to this point, do that (not much guidance provided here) and your finished steady-state PFD should look like the figure below.

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To make sure you have everything correct and consistent, you can check the conditions and compositions of the to outlet streams3. These are shown below.

This completes the steady-state simulation development. Make sure you save your case.

2. Specifying Dynamic Characteristics of Equipment The steady-state material and energy balances for the flash drum process can be calculated without much of the process design being specified. For example, the actual size of the flash drum only has bearing on the dynamic behavior of the unit. But the steady-state, nominal operating conditions can be used as a basis for the rest of the design. Flash Drum Dimensional Specifications From the HYSYS steady-state case, the volumetric flow rate is 22.5 m3/h (99.1 gpm). The flash drum is sized for a 5-minute hold-up at this flow rate. This requires an operating liquid volume of 1.88 m3. This leads to the following vessel design. Diameter Operating Level Vessel height Operating Level

3

1.5 m 1.06 m 2.5 m 42%

If your results differ, did you enter mole fractions instead of mass fractions?

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These specifications are now entered into the HYSYS case4. Double-click on the flash drum unit and select the Rating tab along the bottom of the window. Enter the diameter as 1.5 m and the height as 2.5 m, then the volume will be computed automatically. This is shown below.

Now click on the Dynamics tab and enter a Liquid Volume Pct value of 42. This should look like

Close the window by clicking on the

4

.

It might be a good time to save your steady-state case, and then “Save As” to a new file name for your dynamics case.

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Control Valve Sizing HYSYS helps you with the sizing of the control valves. Sizing is not required for the steady-state case, but must be done for dynamics with control. Start by double-clicking on the Feed Valve and click on the Rating tab. With the valve opening at 50% (default), the Cv radio button on (default), and linear radio button on (default), click the Size button. You should then have

Close out the Feed Valve window. Complete sizing of the other two valves. Just to check, the Cv values for the Liquid and Vapor Valves should be about 66 and 600 respectively. Control Valve Dynamics In flow control loops, often the control valve represents the slowest element of the loop. Once the control valve stem moves, the adjustment of flow and its measurement are relatively instantaneous. This is especially true with incompressible fluids. Most valves, except for big ones with some type of hydraulic motor drive, can be adequately modeled with a first order transfer function with a time constant of a few seconds. So, it is important to include valve dynamics in the simulation, especially for the feed flow control and pressure control loops here. It's possible to add dynamics to control valves in HYSYS, but you have to enable a special licensing option first. Select Integrator from the Simulation menu.

In the Integrator window, and select the Options tab. Click the Use HYSYS Fidelity box, and, while you're at it, click the Static Head Contributions box. The window should now look like

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One or two other windows related to the HYSYS Fidelity package may be displayed as you make these changes. In particular, the window below showing licensing options should be set up as shown.

The dynamic response of control valves is often available from their manufacturer. Here, you can use reasonable estimates. To set the dynamic response characteristics of the Feed Valve, double-click on the valve on the PFD, click on the Dynamics tab and the Actuator option on the left. As shown below, select the First Order option, and enter an Actuator Time Constant of 5 seconds and a Valve Stickiness Time Constant of 3 seconds. Then, you can close the window.

Make similar entries for the other two valves on the PFD.

3. Switching the Simulation Over to Dynamic Mode At this point, it would be appropriate to switch over to dynamic mode. As a first step, it is useful to change the color scheme of the PFD streams to one more appropriate to dynamic simulations. Click on the Color Scheme drop-down button on the right side of the title bar in the PFD window.

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Click on Dynamic P/F Specs as shown. You can see how the colors are assigned by clicking on the palette

and the Edit This Scheme button. Note the color codes.

and close the PFD Color Schemes box in the same Now, you can close this box by clicking on the way. You will see that the Feed stream to the process is now coded red. This means that both the pressure and flow of this stream is specified, and that HYSYS has to figure out the distribution of flows and pressures through the system starting with these specifications. Double-click on the Feed stream and on the Dynamics tab. You should see

Since we are going to use the Feed Valve to control the flow rate, you want to remove the Mass Flow spec active checkmark.

Close the Feed window, and the Feed stream arrow should now appear green5. Activate pressure specs on the Vapor Outlet and Liquid Outlet streams. These streams should then appear green. To switch over to dynamics mode, you need to click on the button on the HYSYS toolbar:

5

Just had to get rid of that red!

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You will then get a warning and need to confirm the switch.

Click on the Yes button. If all is well, you are in dynamics mode. There should be an “Integrator Reset” message in the window at the bottom and the Dynamics Model button should appear depressed.

The simulation is now in dynamics mode.

4. Adding Feedback Controllers to the Simulation There are three feedback controllers to be added: feed flow rate, flash drum liquid level, and flash drum pressure. Feed Flow Controller

Find the PID Click on the Control Ops item on the palette. controller and click on it. Move your cursor to a point above the Feed Valve on the PFD and click. You should then have

Now, it is necessary to configure the controller. First, notice what the mass flow rate is for the Drum Feed stream by double-clicking on it.

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Close the Drum Feed window and double-click on the PID controller icon. Change the Name field to Feed FC, as shown.

Click on the Select PV… button, and use the Object Browser to select the Drum Feed stream and the Mass Flow variable.

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Click OK. Now, click on the Parameters tab in the Feed FC window, and enter range values for the Process Variable (PV) as shown below.

Click back to the Connections tab and click on the Select OP… button. Then, as shown below, select the Feed Valve and click OK.

You will set up the control action and tuning parameters later. For now, close the Feed FC window. The Feed FC controller icon should now be “connected” to the PFD.

The connecting lines are a bit complicated, so, next, you will flip the Feed FC icon horizontally. Rightclick on the icon and select Transform, then Mirror about Y, and click.

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Then, right-click on the Drum Feed stream arrow and select Transform, then Mirror about X and click.

Next, click on the Size Mode button on the PFD window toolbar.

And, right-click on the Feed FC icon again, but select Move/Size Label. Move the Feed FC label above the controller icon and “stretch” the label box so that it appears like

These are just cosmetic moves, but they neaten things up a bit. Repeat this process twice, adding controllers for Liquid Level and Vessel Pressure according to the specifications below:

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Name PV PV Min PV Max OP


Drum LC Flash Drum / Liquid Percent Level 0% 100% Liquid Valve

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Drum PC Flash Drum / Vessel Pressure 50 kPa 150 kPa Vapor Valve

Arrange the PFD so that it looks about like the figure below.

In order to operate the controllers, you will now insert controller “face plates” into the HYSYS window. To do this, double-click on the Feed FC icon and click the Face Plate button at the bottom. The inserted face plate should something look like

The Setpoint is shown by the red triangle. The current PV value is shown along with the current OP value. The controller mode is Manual and tuning parameters can be accessed via the Tuning button. Insert face plates for the other two controllers. Minimize the PFD by clicking on “slot” button in the upper right corner of its window.

For now, rearrange the 3 face plates so you can see them all.

the

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5. Adding Stripchart Displays In order to view the behavior of the dynamic simulation, and, in particular, the controllers, it is useful to add some stripcharts to the window. To do this, select Databook from the Tools menu.

In the Databook window, make sure the Variables tab is selected, and click on the Insert… button. You should see the following.

Using the Variable Navigator, you need to select the variables that will appear on your stripcharts. First, select Feed FC, for the feed flow controller, and PV, for its process variable, the Drum Feed volumetric flow rate. Then click the Add button as shown below.

Then, for the same Feed FC, “Add” the OP and SP variables. Add the same three variables (PV, OP and SP) for the other two controllers (Drum LC and Drum PC). These 9 variables will let you follow the progress of the controllers. Additionally, you may want to track what happens to the compositions in the liquid and vapor outlet streams. To do this, “Add” the following variables also:

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Object Liquid Outlet

Variable Comp Mole Frac

Vapor Outlet

Comp Mole Frac

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Specifics Acetone 2-Propanol Water Acetone 2-Propanol Water

Cancel or close the Variable Navigator window. If you “stretch” the Databook window now, it will look like

Now, click on the Strip Charts tab, click the Add button and change the Logger Name field to Feed FC.

Click on the boxes in the Active column for the three Feed FC variables (PV, OP, SP), and then click on the Strip Chart… button.

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A small graph window should appear, as shown above. Create 4 additional stripcharts with the titles: ¾ Drum LC ¾ Drum PC ¾ Liquid Comps ¾ Vapor Comps selecting the appropriate variables for each. Place each on the HYSYS window as above. Once you have done this, close the DataBook window, and you should see that last stripchart created in the middle of the HYSYS window. The rest of the stripcharts are hidden behind this one. Drag and resize the stripchart windows and reposition the controller faceplates so that your HYSYS window looks like the figure below. Note: you can close the Object palette by pressing the F4 key or clicking on the small in its upper right corner.

Your simulator is about ready for its first dynamic test, so it would be a good idea to save it now.

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6. Carrying Out Dynamic Tests and Fitting Transfer Functions Your dynamic simulation is set for steady-state conditions at the moment. That means, if we run a dynamic test for a period of time without changing any inputs, the process should stay at or near steady state. This is always a good first test, since it might reveal some serious problems with the simulation setup. To do this, select Integrator from the Simulation menu and click on the General tab. The Integrator window is used to control the numerical solution of the differential equations in the model developed by your PFD. For many simulation problems, it is necessary to adjust the fields in the Integrator window in order to get the simulation to run as fast as possible while preserving numerical accuracy. For our simple example, the default settings will do, but change the End Time to 30 min.

Then minimize the Integrator window by clicking on its “slot” button in the upper right. Start the simulation by clicking on the Integrator Active button on the toolbar.

After a few seconds, flatline traces should appear across your stripcharts and the status window at the bottom should show

If flatline traces do not appear, that is, if the traces are shifting significantly with time, you've done something incorrectly. It’s probably worthwhile at this point to do some cosmetic work on your stripchart displays. Drag the small red triangle to the left to cover all of the dark blue stripe.

Before:

After:

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Right-click on the chart and choose Select Curve and Feed FC – PV, as shown below.

Right-click on the chart and click on AutoScale All Axes. Right-click the chart again and choose Graph Control… The Strip Chart Configuration – FeedFC window should appear with the General selected. Double-click on the black Background color rectangle you should see the Color palette window.

on tab and

Click on the white rectangle, as shown, and click the OK button. Close the Strip Chart Configuration window. Your FeedFC stripchart should now look something like

Make similar cosmetic changes to the DrumLC and DrumPC stripcharts. Do so also to the two composition stripcharts, but don’t worry about selecting a particular variable to display. Your HYSYS window should then look like

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Return to the DataBook window by selecting Tools then DataBook from the HYSYS menu. Select the FeedFC stripchart entry and change the Sample Interval from 20 seconds to 1 minute, as shown below.

The Sample Interval should then show as

.

Make this change for the other 4 stripcharts too. This will now be considered a base case for starting dynamic test simulations, so save the case with the name FlashDrumxyzBase.hsc where xyz are your initials, as before. The first dynamic test is run with the Feed FC loop. The dynamics here a relatively fast, so, temporarily, it will be advantageous save data at a shorter time interval. Go back to the DataBook, select the Strip Charts tab, and the Feed FC stripchart again. Change the Sample Int. to 1 second for now. Click on the

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Setup... button and change the Logger Size to 2000. This will allow enough data to be stored during your test.

Close out the small Logger Set-up window and the Databook window too. For the first dynamic test, change the FeedFC controller output from 50% to 60%. Place your mouse cursor on the OP field in the FeedFC box and double-click.

Type 60 into the field, as shown above, and press Enter.

Since the controller is in manual, you have direct control over the signal going to the valve, OP.

, and set the End Time to 60 min. From here, if you Restore the Integrator window, wish, you can just click the Continue button to restart the simulation and then click the Minimize button for the Integrator window to get it out of the way.

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Minimize button

When the Integrator has stopped at 60 min, “stretch” the stripchart plots again by dragging the red triangle all the way to the left on each stripchart.

Three of the stripcharts, at this point, should look something like

The FeedFC chart on the left shows the step change in the OP signal to the Feed Valve and a very rapid response in flow rate in the Drum Feed stream. There are also noticeable effects on the Drum pressure, as shown in the middle stripchart, and the integrating nature of the flash drum shows in the DrumLC stripchart on the right. Next, you will need to export and save the data from your test. To do this, access the Databook again from the Tools menu. Select the FeedFC stripchart and click on the Historical… button.

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Click the Save to .CSV File button

and name the file FeedFCxyzStepTest. Pick a location to save the file, like the Desktop and click the Save button.

Now, you can close out the Historical Data window and the Databook window.

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Launch Excel and Open the FeedFCxyzStepTest.csv file. If you neaten up the column widths a bit, the spreadsheet should look something like

To prepare this file for Control Station, delete the first 11 rows with the result

You can delete the rows of data up to three or four values of 50 before the change to 60. Also, column D is the SP variable and is not needed, so delete it. The result is

You can also re-origin the time column to zero. To do this, insert a column between A and B [click on B, right-click, and Insert], and enter the formula =A1-$A$1 in the empty cell B1. Then, double-click on the fill handle to copy that formula down.

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Then, select the filled cells in column B, initiate a Copy (Ctrl-c), and Paste Special from the Edit menu. Click on the Values radio button and then OK, as shown below.

Then, you can delete column A, and you will be left with

When dealing with data files from other tests, the exact time window required to capture the dynamic change may be different. Creating a graph in Excel may help you decide what time window to include and then which rows to eliminate.

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62

22500

22000 60 21500 58 21000

56

20500

20000

54

19500 52 19000 50 18500

48

18000 0

200

400

600

800

1000

1200

1400

1600

1800

2000

Now, you need to save the file in the .TXT form. To do this, first execute Save As (F12 key) and save the spreadsheet as an Excel workbook file (.XLS). Then execute Save As again and save the file as a .TXT file. Close out Excel, and launch Control Station. Click on Design Tools. In the Design Tools window, click on the Open File button.

Browse and open your .TXT file. The Label Data Columns … window should open, and the Process and Manipulated labels may be reversed. Fix this then click OK.

Next, click on the Plot button to see a plot of your test. It should look something like

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The form of the output suggests a 2nd-order-plus-lead transfer function with dead time. Close out the Print Data window by clicking on the X. Back in the Design Tools window, click on the Select Model button and make the settings shown below.

Click the Done button and then the Start Fitting button. Eventually, you should see a fit plot like

Close the Print Data window and the model parameter results are shown back in the Design Tools window6.

6

Your results may differ somewhat because your test data will not be identical.

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Control Station also states that it cannot compute controller tuning parameters for this type of model.7 But, with a little more knowledge of stability analysis and controller design, based on the model parameters, suggested tuning parameters for a PI controller are Kc = 0.1

τI = 30 s = 0.5 min

Close out Control Station. Return to the HYSYS window now.

7. Tuning the Controllers So, it is possible to test the process by making step changes in the controller outputs. Then you can save data files of the tests, trim them up with Excel, and fit transfer functions with Control Station. Frequently, Control Station will provide controller tuning parameters too. That is something that most industrial users of HYSYS do not know how to do. Starting with the base case conditions (all controller outputs at 50% and the process at steady state8), you should now run step tests for the other two controllers and develop tuning parameters for PI controllers. Once you have done that, you’re ready to proceed with the next step. • • • Click on the Tuning button on FeedFC faceplate. Enter the values above for Kc and Ti (use the minutes value).

Note that the default Action is Reverse. This is correct here but would change to Direct for the other two controllers in the PFD. Close the tuning window. Change the mode on the FeedFC faceplate from Man to Auto. 7

Note: for the Drum PC controller in the HYSYS model, an FOPDT model should work well, and then Control Station will give you tuning parameters. An SOPDT/Integrator will be needed for the DrumLC controller and IMC tuning parameters might be used (Table 12.1 in the text). 8 You can run the simulation back to base steady-state conditions, or you can re-open your base case.

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Change the Setpoint to 18000 kg/h by double-clicking in the PV field.

Run the simulation for about 30 min more to observe the performance of the FeedFC controller. The FeedFC stripchart should look something like

The controller appears to perform well. You can leave it in Auto. At this point, you should implement your tuning settings in the DrumLC and DrumPC controllers, one at a time, and confirm that they perform well. Occasionally, it is necessary to make a “field modification” of your initial tunings to improve performance. When you have all three controllers performing well, run a test where you make a 10% (up or down) change in the FeedFC Setpoint and observe how well (or poorly?) your DrumLC and DrumPC maintain the conditions in the Flash Drum. Your results might look something like those on the next page. You can document your stripcharts by right-clicking on them and clicking on Print Plot. You can document your HYSYS case via Reports on the Tools menu. This gives you a good example, although for a simple process, of how dynamic simulation and control system development can take place in an environment like HYSYS. Other simulation products, such as Aspen+ and SimSci ProVision, have similar features available.

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The axis scales on several of the above stripcharts have been adjusted to exaggerate the dynamic changes.

control valve sizing

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