exercise split-range control - lab- · pdf fileex. 1-3 – split-range control procedure...

© Festo Didactic 86007-00 35

Learn the basics of split-range control through several examples of split-range control from the industry, including a typical flare setup.

The Discussion of this exercise covers the following points:

Divide and conquerDead band for split range control.

Using split-range to control the input and output of a reactor

Split-range control for a flare application

Divide and conquer

Split-range control is used when a single controller is employed to control two final-control elements (two valves for example). In such a system, the controller struggles to keep one controlled variable at the set point using two manipulated variables. Typically, split-range control is found in temperature control applications, but split-range control applications extend far beyond temperature control.

The concept of split-range control is easier to understand when illustrated using applications such as a temperature control. In such an application, the process needs to be heated or cooled depending on the product temperature. Figure 1-16 shows how the temperature transmitter, the controller, and the two control valves are connected for split-range control in a typical temperature control application.

Figure 1-16. Split-range connections for a temperature control application.

In the diagram above, the 0% to 100% range of the controller output is split in two between the two valves. If the controller output is between 0% and 50%, it is the cooling valve that operates. This valve is fully open when the controller output is 0% and fully closed when the controller output is 50%. If the controller output is

Split-range Control

Exercise 1-3

EXERCISE OBJECTIVE

DISCUSSION OUTLINE

DISCUSSION

4-20 mA

4-12 mA 12-20 mA

Cooling valve (fail open)

Heating valve (fail close)

Ex. 1-3 – Split-range Control Discussion

36 © Festo Didactic 86007-00

between 50% and 100%, it is the heating valve that is in operation. At 50%, the heating valve starts to open and it is fully open at 100% of the controller output.

In a split-range control installation, there are different ways to connect the valves so that they operate on two different ranges. In the example above, the current to pressure converters are used to split the controller output in two ranges. The first converter responds to a current from 4 mA to 12 mA, while the other operates in a range from 12 mA to 20 mA. When using this type of wiring, no special controller or configuration is required for split-range control.

Figure 1-17 shows a setup that performs the same split-range control but, in this case, the controller signal is a pneumatic signal. In such a setup, the two valves are mechanically different. The spring/diaphragm actuators of the two valves are selected so that their ranges of operation are different. The spring of the cooling valve is selected to allow the valve to open and close over a range of 20.7 kPa (3 psi) to 55.2 kPa (8 psi), while the heating valve closes and opens over a range of 55.2 kPa 8 psi to 89.6 kPa (13 psi).

Figure 1-17. Split-range control using pneumatic signals.

The opening and closing of the valves follow the same rules for both of the setups shown above. The graph of Figure 1-18 shows the relationship between the opening of the valves and the controller output.

Figure 1-18. Opening of the valves for split-range control.

Open

Closed

0 50 100 %

Cooling valve

Heating valve

20.7-89.6 kPa (3-13 psi)

20.7-55.2 kPa (3-8 psi)

55.2-89.6 kPa (8-13 psi)

Cooling valve (fail open)

Heating valve (fail close)

Valve opening

Controller output



Dead band for split range control

The examples above are some of the simplest setups for split-range control. Of course, a controller supporting split-range control may support more complex setups and correct some of the flaws inherent to split-range control. One problem that may arise with the setups detailed above is that the system may switch continuously between cooling and heating when the controller output is around 50%. To avoid continuous oscillation between the cooling and heating modes, a dead band is usually added between the two ranges. A dead band can be added, for example, by setting a range of 0% to 49% for the cooling valve and a range of 51% to 100% for the heating valve.

Figure 1-19. Opening of the valves for split-range control.

Using split-range to control the input and output of a reactor

Figure 1-20 shows another example of split-range control. In this example, chemical reactants come into a reactor via a first control valve that limits the input flow. Another control valve limits the output flow of the reactor. To ensure a high efficiency chemical reaction and a uniform product, the pressure inside the reactor must be kept above a given level. A pressure transmitter reads the pressure in the reactor and a controller, connected for split-range control, then changes its output to control the pressure inside the reactor. If the pressure is too low, the controller opens the input valve. If the pressure is still low, the controller closes the output valve. Figure 1-21 shows the opening of the control valves as a function of the controller output.

Figure 1-20. Split-range control.

Open

Closed

0

49

100 %

Cooling valve

Heating valve

51

Dead band

Reactants Product

Valve opening

Controller output



Figure 1-21. Opening of the valves as a function of the controller output.

For this installation, the controller could also be configured so that the opening of the control valves overlap at 50% of the controller output, as shown in Figure 1-22. In this case, the input valve is fully closed and the output valve is fully open when the controller output is 0%, both valves are 50% open when the controller output is 50% and, finally, the input valve is fully open and the output valve is fully closed when the controller output is 100%.

Figure 1-22. Overlapping of the opening of the control valves.

Split-range control for a flare application

Gas flares are common in refinery and chemical plants. These devices are mainly used to burn the excess of flammable gas in a vessel and prevent an eventual explosion due to overpressure. When something goes wrong, the flare system is the insurance against disaster.

Many flare systems rely on split-range control for the evacuation of the gas. Figure 1-23 shows a typical split-range arrangement for a flare application.

Input valve

Output valve

Open

Closed

0 50 100 %

Output valve

Input valve

Open

Closed%

Valve opening

Valve opening

Controller output

Controller output

0% 50% 100%



Figure 1-23. Split-range control of a flare application.

In a flare application, a fluid or a gas is processed in a vessel. The chemical reaction or the physical condition to which the fluid is exposed allows obtaining the desired product plus an (undesired) excess of flammable gas. In the example above, it is a pressure transmitter that determines if the product is ready and if there is a gas excess.

Figure 1-25 shows the opening of the control valves as a function of the controller output for this flare application. If the pressure is low, the controller output is 0% and both valves are closed to allow pressure to build up in the vessel and the formation of the product. As the pressure increases, the control valve allowing the product to exit the vessel and go to the gas compressor opens. This valve continues to open until the controller output reaches 50%. At this point, the valve is fully open while the flare valve is still closed. If the pressure still increases, there is an excess of gas that must be evacuated. Thus, the controller increases its output and the flare valve starts to open to evacuate the gas and burn it. When the controller output reaches 100% both valves are fully open.

Figure 1-25. Opening of the control valves for a flare application.

Figure 1-24. Refinery flare.

Compressor valve

Flare valve

Open

Closed

Valve opening

Controller output

50% 0% 100%

Ex. 1-3 – Split-range Control Procedure Outline


The Procedure is divided into the following sections:

Setup and connections

Configuring the transmitter and the controller for split-range control

Controlling the flow loop

Setup and connections

1. Connect the equipment according to the piping and instrumentation diagram (P&ID) shown in Figure 1-26 and use Figure 1-27 to position the equipment correctly on the frame of the training system.

Table 1-8. Material to add to the basic setup for this exercise.

Name Model Identification

Differential-pressure transmitter (low-pressure range) 46921 LIT 1

Pneumatic control valve1 46950-B LCV 1-B

Color paperless recorder 46972 UR

Controller * LIC 1

1 The first control valve (LCV1-A) is included in the basic setup.

PROCEDURE OUTLINE

PROCEDURE

Ex. 1-3 – Split-range Control Procedure


Figure 1-26. P&ID – Split-range control loop.

Open to atmosphere



Figure 1-27. Setup – Split-range control loop.

a The paperless recorder (UR) is not displayed in the P&ID above. See Figure 1-28 for the suggested electrical connections.

2. Connect the control valves to the pneumatic unit.

3. Connect the pneumatic unit to a dry-air source with an output pressure of at least 700 kPa (100 psi).

4. Wire the emergency push-button so that you can cut power in case of emergency.

Air from the pneumatic unit (140 kPa (20 psi))



5. Do not power up the instrumentation workstation yet. You should not turn the electrical panel on before your instructor has validated your setup—that is not before step 10.

6. Connect the controller to the control valves and to the differential-pressure transmitter. You must also include the recorder in your connections. On channel 1 of the recorder, plot the signal from the transmitter, on channel 2, plot the signal from the first output of the controller (connected to LCV1-A), and on channel 3, plot the signal from second output of the controller (connected to LCV1-B). Be sure to use the analog input and outputs of your controller.

7. Figure 1-28 shows how to connect the different devices together.

Figure 1-28. Connecting the instruments together for split-range control.

8. Before proceeding further, complete the following checklist to make sure you have set up the system properly. The points on this checklist are crucial elements to the proper completion of this exercise. This checklist is not exhaustive, so be sure to follow the instructions in the Familiarization with the Training System manual as well.

Ch2Ch1

24 V

In1 Out2Out1 Ch3



f

The hand valves are in the positions shown in the P&ID.

The control valves are fully open.

The pneumatic connections are correct.

Be sure the pneumatic panel is set so that the pressure at the inlet of each

control valve does not exceed 240 kPa (35 psi).

The controller is properly connected to the differential-pressure transmitter

and to the control valves.

The paperless recorder is connected correctly to plot the appropriate

signals on channel 1, channel 2, and channel 3.

9. Ask your instructor to check and approve your setup.

10. Power up the electrical unit, this starts all electrical devices as well as the pneumatic unit. Activate the control valve of the pneumatic unit to power the devices requiring compressed air.

11. With the controller in manual mode, set the output of the controller to 0%. The control valves should be fully open. If they are not, revise the electrical and pneumatic connections and be sure the calibration of each I/P converter is appropriate.

12. Test your system for leaks. Use the drive to make the pump run at low speed to produce a small flow rate. Gradually increase the flow rate, up to 50% of the maximum flow rate that the pumping unit can deliver (i.e., set the drive speed to 30 Hz). Repair any leaks and stop the pump.

Configuring the transmitter and the controller for split-range control

13. Bleed the impulse line of the transmitter and configure it for level measurement. Adjust the zero of the differential-pressure transmitter.

Set the parameters of the transmitter so that a 4 mA signal is sent if the column is empty and a 20 mA signal for a level of 80 cm (about 31 in).

14. Configure your controller for split-range control so that the opening of the control valves as a function of the controller output corresponds to what is shown in Figure 1-29. The setpoint of the controller is the level transmitter output. Refer to the manual of your controller for details on how to make the connections for split-range control.



Figure 1-29. Opening of the control valves as a function of the controller output.

Controlling the flow loop

15. Set your controller so that both ranges (i.e., 0%-50% and 50-100%) have the same PID tuning parameters. Test your configuration with the controller in proportional mode by setting the controller gain to .

16. Start the pump with the drive speed set to 30 Hz. Watch the evolution of the controlled variable and of the opening of the control valves as a function of time. You can stop the pump and the recording once both control valves are fully closed for the first time.

17. Transfer the data from the paperless recorder to a computer. Plot a graph of your results and compare it to the graph of Figure 1-29 to verify that your controller is properly configured for split-range control.

18. Let the column drain and, again, start the pump with the drive speed set to 30 Hz. Let the system run for approximately two minutes. Record and transfer the data to a computer. Plot a graph of your results.

19. According to your graph, does the system seem to reach equilibrium quickly or at all? Why?

20. Controllers supporting split-range control allow setting different tuning parameters for the 0% to 50% range and the 50% to 100% range. This implies that both ranges must be tuned independently. Tune both ranges of your controller using the tuning method of your choice. Test your tuning and verify that your system reaches steady state quickly.

LCV1-A

LCV1-B

Controller output

0% 50% 100%

Closed

Open

Valve opening

Ex. 1-3 – Split-range Control Conclusion


21. Transfer the recorded data to a computer. Plot a graph of your results.

22. Stop the system, turn off the power, and store the equipment.

In this exercise, you have learned various types of split-range control installations. You tested a setup in which the opening and closing of the control valves as a function of the controller output is similar to a flare application.

1. What is split-range control?

2. In a split-range installation, why is it important to carefully select the spring/diaphragm actuators of the valves?

3. What is the purpose of a dead band in a split-range installation?

4. Give three examples of split-range control applications.

5. What is the purpose of a flare application?

CONCLUSION

REVIEW QUESTIONS

exercise split-range control - lab- · pdf fileex. 1-3 – split-range control procedure...

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