The Discussion of this exercise covers the following points: On-off control On-off controller with a dead band. Conductivity control

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1 Exercise 1-3 On-Off Conductivity Control (Optional) EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with on-off conductivity control. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: On-off control On-off controller with a dead band. Conductivity control DISCUSSION On-off control The oldest, least expensive, and simplest type of controller is the on-off or twoposition controller. This type of controller is a discontinuous controller because the control signal can take only certain discrete values, usually on and off ( and of the output signal). On-off controllers are most effective on large capacitance, slowly changing processes. They are commonly used in home heating and air conditioning systems. They can also be found in appliances such as refrigerators, freezers, and water heaters. Figure 1-41 shows the typical input-output relationship of an on-off controller in direct-action mode. When the controlled is below the set point, the output signal is at its minimum value (), this is the off state. When the controlled is above the set point, the output signal is at its maximum value (), this is the on state. The action of this controller is said to be direct because the controller output signal increases (passes from to ) as the controlled increases. Remember that for a controller in a process control loop, the manipulated, that is the controller output, is the input of the process. Similarly, the controlled, that is the transmitter input, is the output of the process. Manipulated Set point Off Controlled Figure Typical response of an on-off controller in direct-action mode. On Festo Didactic

2 Ex. 1-3 On-Off Conductivity Control (Optional) Discussion Figure 1-42 shows the typical input-output relationship of an on-off controller in reverse-action mode. The controller output signal is at its maximum value () when the controlled is below the set point and passes to its minimum value () when the controlled is above the set point. The action of this controller is said to be reverse because the controller output signal decreases (passes from to ) as the controlled increases. The on-off controller with reverse action is commonly used in home heating systems. When the temperature falls below the set point, the home heating system turns on. Set point On Manipulated Controlled Figure Typical response of an on-off controller in reverse-action mode. Figure 1-43 illustrates the response of an on-off controller in reverse-action mode. The controller output signal changes state every time the controlled crosses the set point. Thus, the value of the controlled oscillates continuously around the set point. The amplitude and frequency of the oscillations are directly related to the process capacitance. The amplitude is high and the frequency is low when the process capacitance is large and vice versa. This explains why on-off controllers are best suited for slow response processes which normally have large capacitances. Moreover, the oscillating nature of onoff control tends to wear on control valves and contactors, which makes this type of control less suitable for continuous industrial processes. Off 60 Festo Didactic

3 Ex. 1-3 On-Off Conductivity Control (Optional) Discussion Set point Time Manipulated Controlled Time Figure Typical response of an on-off controller in reverse-action mode. On-off controller with a dead band The oscillation of the controlled around the set point in process control systems using an on-off controller may result in potential problems when the oscillation frequency becomes too high. This may cause the equipment to wear out prematurely, especially the control element which is continuously switched on and off. One way of minimizing this drawback is to reduce the oscillation frequency by adding a dead band (sometimes called differential gap). The dead band is a zone around the set point where no control action is taken. The state of the controller changes only if the controlled is above (or below) the set point by at least half the value of the dead band. Figure 1-44 and Figure 1-45 show the typical input-output relationships of direct action and reverse action for on-off controllers with a dead band. The controller output signal changes state at two different values located on either side of the set point. The controlled must pass through the entire zone covered by the dead band before the controller output signal changes state. As a result, a different path is taken on the input-output relationship depending upon whether the controller output signal passes from the on state to the off state or from the off state to the on state. This phenomenon is referred to as hysteresis. Festo Didactic

4 Ex. 1-3 On-Off Conductivity Control (Optional) Discussion Manipulated Set point From to From to Dead band Controlled Figure Typical response of an on-off controller in direct-action mode with a dead band. Manipulated Set point From to Dead band Controlled From to Figure Typical response of an on-off controller in reverse-action mode with a dead band. On-off control is similar to proportional control with a very large controller gain. If the proportional band or a proportional controller is set to zero, the controller gain will be very large (infinite). Therefore, the proportional controller will act as an onoff controller. Figure 1-46 shows the response of an on-off controller set to reverse action when a dead band is present. Compared to the response of a controller without dead band, the oscillation frequency is smaller. This effect can be used to prevent cycling at an excessive rate in the system, therefore reducing the wear on control valves. However, the larger the dead time, the higher the amplitude of the oscillation, which means that the controlled will go farther from the set point. 62 Festo Didactic

5 Ex. 1-3 On-Off Conductivity Control (Optional) Discussion Set point Controlled Dead band Time Manipulated Time Figure Typical response of an on-off controller in reverse-action mode and with a dead band. Some devices can only achieve on-off control. However, PID controllers can be set to act like an on-off controller by using a large value for the parameter called the controller gain,. Conductivity control Conductivity measurement and control applications are numerous. From waste water treatment to fruit peeling applications, conductivity control is essential and has many faces. A conductivity limitation application, where the conductivity of the process water is kept under a given limit to prevent damages or optimize the system operation, is an example of applied conductivity control. Systems keeping conductivity low are common in steam boiler installations where conductivity is especially important. Specifically, keeping the amount of dissolved solids low in a boiler reduces the risk of scale formation, carryover, corrosion, and embrittlement. Desalination plants are another example where conductivity control is important to ensure that sea water has been desalinated correctly. More extreme examples of systems where conductivity control is important include semiconductor fabrication plants and pharmaceutical industry, both requiring ultrapure water for their respective manufacturing processes. On systems where the conductivity must be kept below a given limit, it is sometime expensive and complex to use a PID control system to keep the conductivity at a precise set point. For this reason, most conductivity control applications have the process water pass through several filters or use simple on-off control to keep conductivity below a given value. Festo Didactic

6 Ex. 1-3 On-Off Conductivity Control (Optional) Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Set up and connections Configuring the controller Conductivity control Curve analysis PROCEDURE Set up and connections 1. Use the piping and instrumentation diagram (P&ID) shown in Figure 1-47 to connect the equipment. If your training system is a Series 3532, use Figure 1-48 and Figure 1-49 to position the equipment correctly on the training system frame. If your training system is a Series 3531, use Figure 1-50 to position the equipment correctly on the training system frame. Use the basic setup presented in the Familiarization with the Training System manual. Table 1-6 lists the equipment you must add to the basic setup in order to set up your system for this exercise. Table 1-6. Equipment required for this exercise. Name Part number Identification Controller * AIC Table salt Volumetric flask Scopulla Graduated cylinder Precision scale Latex gloves Pipette Safety glasses Conductivity transmitter AIT Metering pump Paperless recorder UR Chemical tank Festo Didactic

7 Ex. 1-3 On-Off Conductivity Control (Optional) Procedure Filter Figure P&ID. Festo Didactic

8 Ex. 1-3 On-Off Conductivity Control (Optional) Procedure Figure Front setup (series 3532). 66 Festo Didactic

9 Ex. 1-3 On-Off Conductivity Control (Optional) Procedure Figure Back setup (series 3532). Festo Didactic

10 Ex. 1-3 On-Off Conductivity Control (Optional) Procedure Figure Setup (series 3531). 2. Wire the emergency push-button so that you can cut power in case of an emergency. The Familiarization with the Training System manual covers the security issues related to the use of electricity with the system as well as the wiring of the emergency push-button. 68 Festo Didactic

11 Ex. 1-3 On-Off Conductivity Control (Optional) Procedure 3. Wire the conductivity transmitter and the controller to the paperless recorder as shown in Figure Analog input In1 Out1 Ch1 Ch2 24 V Analog output Figure 1-51 Connecting the equipment to the recorder. 4. Wire one of the controller relay to a 24 V source and to the solenoid valve. Figure 1-52 shows a typical connection setup where the solenoid valve is connected to a 24 V source through a normally closed relay. a Connections may differ on your controller depending on its configuration. Refer to your controller manual for details. Normally closed relay 24 V Figure 1-52 Connecting the solenoid valve to the controller. 5. Do not power up the instrumentation workstation yet. Do not turn the electrical panel on before your instructor has validated your setup that is not before step Fill a chemical tank with a solution of sodium chloride (i.e., table salt) containing 5 grams of salt per liter. Carefully follow the procedure in the Measurement manual to prepare this solution. 7. 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 for the proper completion of this exercise. This checklist is not Festo Didactic

12 Ex. 1-3 On-Off Conductivity Control (Optional) Procedure exhaustive. Be sure to follow the instructions in the Familiarization with the Training System manual as well. f All unused male adapters on the column are capped and the flange is properly tightened. The hand valves are in the positions shown in the P&ID. The chemical tank is filled with the appropriate solution and is carefully labeled. You are wearing the appropriate PPE. The vent tube is properly installed. The controller is properly connected to the conductivity transmitter and the solenoid valve. The paperless recorder is set up and configured to record the output of the conductivity transmitter and the output of the controller The solenoid valve is wired to one of the controller relay, ready for the controller to be configured so that if the conductivity is above the set point the relay opens (i.e., no power is supplied and the valve closes). 8. Ask your instructor to check and approve your setup. 9. Power up the electrical unit, this starts all electrical devices. Configuring the controller 10. The setup described above is designed so that there is always a small quantity of water passing through the filter no matter the conductivity value. When the solenoid valve is open (i.e., it is energized), the majority of the flow is not filtered. When the solenoid valve is closed, the water is forced through the filter and maximum deionization occurs. The controller is used to open or close the solenoid valve depending on the conductivity of the process water. Since the solenoid valve can be energized only via a 24 V source, the 4-20 ma output of your controller cannot be used for this purpose. The configuration of your controller to achieve on-off control of the solenoid valve depends on the type of controller you use. Nevertheless, on most controllers, the on-off control setting is accomplished using one of the available relays. Usually, the relay can be activated as a function of the controller output. This allows the use of the full on-off control mode of the device with all its capabilities, such as adding a dead band. Other controllers are not as convenient; they allow only minimal on-off control via a relay (for example, through comparison of the input with the set point). In any cases, refer to your controller manual for details on how to configure it for on-off control via a relay. 70 Festo Didactic

13 Ex. 1-3 On-Off Conductivity Control (Optional) Procedure 11. Configure your controller so that the solenoid valve closes if the conductivity of the process water is above 500 S/cm. Conductivity control 12. Before taking a conductivity measurement, you must perform an air calibration of the conductivity sensor as described in the Familiarization with the Training System manual. 13. Configure the conductivity transmitter so that when the conductivity is null the transmitter produces a 4 ma signal and when the conductivity is 1000 S/cm the transmitter produces a 20 ma signal. 14. Test your system for leaks. Use the drive to make the pump run at low speed in order to produce a small flow rate. Gradually increase the flow rate, up to 5 of the maximum flow rate that the pumping unit can deliver (i.e., set the drive speed to 30 Hz). Repair all leaks. 15. Start the pump and set the drive speed to 30 Hz. 16. Fill the column up to 20 cm (8 in) of water, then close HV1 and open HV6 to put the process workstation into recirculation mode. 17. Once the process workstation in recirculation mode, wait about 1 minute to allow water to mix properly and the expel air form the filters and tubing. 18. Put the controller in automatic mode. 19. If the conductivity is smaller than 500 S/cm, use the metering pump connected to the chemical tank containing the sodium chloride solution to inject salt into the process water in order to increase the conductivity close to 500 S/cm. 20. Once the conductivity is close to 500 S/cm, start the metering pump and inject saline solution into the process water with the metering pump delivery rate set to 5 of its maximum delivery rate. 21. On the conductivity transmitter, watch the conductivity of the process water increase up to 500 S/cm. 22. When the water conductivity reaches 500 S/cm, carefully observe the solenoid valve and tubing to ensure that the valve closes when the conductivity is above the 500 S/cm set point. Festo Didactic

14 Ex. 1-3 On-Off Conductivity Control (Optional) Conclusion 23. If your controller reacts correctly to the change in conductivity, let your process run for about 15 minutes. 24. On the paperless recorder, watch how the conductivity of the process water changes when the process water is filtered. Make sure the process responds correctly to the on-off control. If not, check the configuration of your controller and make sure that the ac drive is set to 30 Hz and that the metering pump delivery rate is set to 5. a Increasing the ac drive frequency increases the filtering when the solenoid valve is open. Similarly, decreasing the ac drive frequency reduces the filtering when the solenoid valve is open. 25. If the on-off control is satisfactory, follow the procedure in the Familiarization with the Training System manual to transfer the data from the paperless recorder to a computer. 26. Stop the system, turn off the power, and store the equipment. Do not forget to rinse the ph probe and store it into a storage solution as described in the Familiarization with the Training System manual. 27. Stop the system, turn off the power, and store the equipment. Curve analysis 28. Plot the data using spreadsheet software. CONCLUSION You are now familiar with on-off conductivity control. You should also be able to configure a controller and use one of its relay for on-off control. REVIEW QUESTIONS 1. In which type of process it the on-off control most effective? 2. What shape is the process response curve likely to have when the process is controlled by an on-off controller without a dead band? 72 Festo Didactic

15 Ex. 1-3 On-Off Conductivity Control (Optional) Review Questions 3. Describe hysteresis for a process using on-off control with a dead band. 4. Define dead band. 5. Name two advantages of on-off control. Festo Didactic