Energy Plant Instrumentation and Controls 4th Edition PDF

Unit A-9 • Energy Plant Instrumentation and Controls Objective 1 Describe the concept and basic components of a control loop. Control Loops There are many ways to implement process control, using mechanical, pneumatic, electrical, analog, and digital techniques. The basic theory is similar for all...

Unit A-9 • Energy Plant Instrumentation and Controls Objective 1 Describe the concept and basic components of a control loop. Control Loops There are many ways to implement process control, using mechanical, pneumatic, electrical, analog, and digital techniques. The basic theory is similar for all methods. There are two basic types of controls: open loop and closed-loop. Open Loop Control Systems Open loop control systems simply estimate or predict the action that is necessary to accomplish a desired objective. In open loop control, no check is made to determine whether the corrective action has accomplished the desired objective. A washing machine is an example of an open loop system. The operator takes a measurement (how dirty the clothes are), compares this with a reference level (how clean the clothes need to be), and predicts how best to run the load (the washing machine cycle, the amount of soap, the water temperature, and how much bleach to use). The operator then starts the machine and hopes that the prediction will achieve the desired result. If all predictions are correct, the clothes will be perfectly clean. Thus, open loop control is capable of achieving the desired results. However, if any of the variables deviate from the conditions used to make the prediction, open loop will not give perfect control. Since the washing machine makes no final comparison of the actual and desired results, any error in the prediction will produce a difference between the desired and actual results. An open loop system is therefore defined as “one in which the output has no effect on the input.” This system cannot change the position of a control valve, because no data about the process conditions reaches the valve. So, the control device may act in a way that is totally unrelated to the desired process conditions. In the case of an open loop system, an operator must decide if error exists (the difference between the desired and actual conditions), and whether the error is increasing or decreasing. Then, the operator must manually adjust the position of the control valve. In other words, a human becomes a controller that observes the process conditions and manipulates a control valve manually in response to the observed conditions. The diagram in Figure 1 helps to illustrate open loop control. This process is for heating water using steam. Cold water enters from the left of the heat exchanger. Warm water exits at the right. The device that controls the heating process is a manual steam valve. To maintain the temperature of the water close to the desired value, the steam valve must be manually operated to throttle the steam. 1-4 4th Class Edition 3 • Part A Introduction to Energy Plant Controls and Instrumentation • Chapter 1 Figure 1 – Manual Control Water Heating System Warm Water Coil Cold Water Steam Valve Steam Condensate If variables such as water flow, steam flow, and steam quality are constant, the outlet temperature of the water can be maintained at the desired value by opening the steam valve the correct amount and leaving it there. However, if the temperature of the incoming cold water changes, or if the water flow changes, the outlet temperature will deviate from the desired value. In this case, the steam valve must be manually readjusted. To compensate for disturbances in the water heating process and maintain the desired water temperature, the operator must check the discharge water temperature frequently to see if there is any error (also called offset), and verify whether the error is increasing or decreasing. Then, the operator must adjust the opening of the steam valve to meet the energy requirements of the heat exchanger. This is a manual type of control system. All open loop systems are manual control systems, and need continuous attention. The desired value of the process variable is difficult to maintain when the operator is present, and impossible to maintain when the operator is absent. Closed Loop Control Systems Closed loop control systems are automatic control systems. They respond to changes in process conditions regardless of whether an operator is present. As well, depending on how critical the process is, closed loop systems can a) be tuned to keep the process conditions within narrow or wide parameters, b) be tuned to respond rapidly or slowly to process deviations, and c) be configured so that the process conditions always return to the desired condition. In a closed-loop control configuration, a measurement is made of the variable to be controlled; then it is compared to the desired condition (the set point). If error exists between the actual value and the desired value, the controller takes the necessary corrective action. On Track Closed loop systems get feedback about process conditions. Open loop systems do not. 4th Class Edition 3 • Part A 1-5 Unit A-9 • Energy Plant Instrumentation and Controls An example of a basic closed loop control system is shown in Figure 2. This diagram can be related to the process of heating water with steam. Consider the open loop water heating process described above, with the addition of a transmitter, a controller, and an automatic valve. Figure 2 – Closed Loop Control System The process box represents the heating of water, performed by the heat exchanger. The sensing element can be any one of a variety of temperature sensors. It responds to the temperature of the measured variable (the water leaving the heat exchanger). The temperature transmitter takes the value of the measured variable from the sensor and transmits it (the process variable) to the controller. The controller is like a human brain. It compares the process variable (the actual water temperature) to the water temperature set point (the desired water temperature). If there is a difference between the process variable and the set point, the control output signal to the final control element (an automatic steam valve) changes in proportion to the difference. The change in the position of the final control element changes the energy input to the process, so that the water temperature can be maintained at the set point. Figure 2 shows a closed loop control system in general terms, so that its concepts can be applied to any process. For example, a process may be to maintain the water level in a tank. The sensing element may be a level sensor, like a float. The set point may be “keep the tank ½ full.” The final control element may be a valve that controls the flow of water into the tank, or a valve that controls how fast the tank drains. Automatic Process Control Principles Figure 3 shows the open loop water heating system described previously, but with an automatic control system applied to it. The temperature-sensing element and transmitter provide process condition feedback to the hot water controller. The hot water controller positions the final control element (automatic steam control valve) based on the error between the process variable and the temperature set point. The lines with small diagonal slashes are the signal lines between the transmitter and the controller and the controller and the final control element. These slashes show that these lines are pneumatic; therefore, this is a pneumatic control system. 1-6 4th Class Edition 3 • Part A Introduction to Energy Plant Controls and Instrumentation • Chapter 1 During normal operation, the control valve feeds the required amount of steam into the heat exchanger to keep the hot water temperature at the set point. In other words, the energy flowing into the heat exchanger equals the energy required to maintain the hot water temperature. In the case of a system disturbance (such as an increase or decrease in water flow through the heat exchanger), the hot water temperature changes. For the following discussion, consider what happens when water flow increases. When the water flow increases, the heat exchanger no longer provides sufficient energy to maintain the hot water temperature set point. For this reason, the hot water outlet temperature decreases. The purpose of the temperature sensor is to continuously detect the hot water temperature. Changes to temperature make a physical change to the condition of the sensor (such as its resistance, position, or voltage). The change in sensor condition is interpreted by the transmitter and converted to a signal (the process variable). The transmitter outputs the process variable as a signal that the temperature controller can interpret. As the hot water outlet temperature drops, the transmitted process variable signal decreases. The temperature controller receives the process variable from the transmitter, and a set point (the desired hot water temperature) from another source. The set point is typically entered manually by an operator, but in more sophisticated control systems, a set point may be the control output signal from another controller. The temperature controller compares the process variable with the set point, determines the error between the two, and produces a control output signal that is proportional to the error. In this case, the drop in hot water temperature causes the controller to output a higher signal. The control output signal goes from the temperature controller to the final control element. The final control element (the steam control valve) has a power source, and can position itself according to the control output signal. In this situation, the increase in the temperature controller output signal causes the steam control valve to open more. This allows more steam to enter the heat exchanger. When the hot water temperature increases, the opposite sequence of events occurs. The sensor will cause a proportional change in the transmitter output. This will cause the controller output to close the control valve, and reduce the heat energy input to the heat exchanger. Figure 3 – Automatic Control System 4th Class Edition 3 • Part A 1-7 Unit A-9 • Energy Plant Instrumentation and Controls The overall action of this automatic control system is illustrated in Figure 4. Trace the sequence of events, beginning from the change in water flow, to the change in position of the steam control element. Note how a change in the flow of the manipulated variable affects the sensed and transmitted value of the measured variable. Figure 4 – Automatic Hot Water Temperature Control System Flowchart 1-8 4th Class Edition 3 • Part A Introduction to Energy Plant Controls and Instrumentation • Chapter 1 Figure 5 shows the closed loop described in Figures 3 and 4 in general terms that can be applied to any automatic control system. Compare these three diagrams. Figure 5 – Automatic Hot Water Temperature Control System Flowchart As can be seen, there are significant differences between the open loop (manual) control system and the automatic (closed loop) control system. The open loop system has no feedback signal to reposition the control valve. An operator must notice changes in temperature on the thermometer, and manually adjust the valve. In the closed loop system, the controller is continuously informed of the value of the process variable. Corrective action can be made accurately and quickly to keep the process closer to the desired value. 4th Class Edition 3 • Part A 1-9 Unit A-9 • Energy Plant Instrumentation and Controls Components and Definitions Process A process may be defined as an operation that uses energy to produce a change in a material, or an energy conversion. The process may take many forms, such as one of the following: a) Maintenance of water level in a boiler or tank b) Control of the flow rate of various liquids and gases c) Maintenance of pressure in a vessel Measured or Controlled Variable All processes have desired outcomes. The measured variable is the process condition to be controlled at a definite desired value. To control this condition, it must first be measured. Examples of measured variables include: • • • • • Pressure Level Flow Temperature Composition Process Variable The process variable is the current status of a measured variable under control. For example, a measure variable could be boiler drum pressure. The process variable (for example) could be 1710 kPa, which represents the current value of the drum pressure. On Track Commonly, measured variables are flow, level, pressure, and temperature. Each variable has a value, called the process variable. These process variables are the actual measurements of the flow, level, pressure, or temperature. For example: Example of a Measured Variable Example of a Process Variable Flow 100 l/min Pressure 1075 kPa Temperature 52°C Level 765 mm Set Point A set point is the desired value of a process variable. In Figure 3, the desired temperature of water from the heater could be 80°C. In the previous example, the desired boiler pressure could be 1700 kPa. Error Error, often referred to as offset or deviation, is the difference between the actual value of the measured variable and the set point. It is the margin by which an automatic controller misses the desired value. 1-10 4th Class Edition 3 • Part A Introduction to Energy Plant Controls and Instrumentation • Chapter 1 Sensing or Measuring Element A sensing element, also called detector or sensor, is a device that responds with a physical change (such as a change in position, length, electrical resistance, or generated voltage) when a measured variable changes in value. Sensors convert the measured condition (such as pressure, temperature, or flow) to a movement or signal that can be transmitted to a controller, recorder, or indicator. Disturbance A process disturbance is a change in a process that cannot be predicted. The disturbance may be a flow rate change, a variation in temperature of a fluid stream, or the variation of several factors which may change independently. Manipulated Variable In most cases, the manipulated variable is some form of flow that is adjusted to restore a process variable to a desired set point. In Figure 1, an operator must manually adjust the control valve to permit adequate steam flow to the heat exchanger, so the water will be as close as possible to the desired temperature. Steam flow, then, is the manipulated variable. Transmitter Sensing elements must be located where the process measurements are taken. In some control systems, the process controller is located near the sensing element, or the sensing element may be part of a local controller. However, in larger installations, the controller may be in a separate room, a considerable distance from the process. In this case, a transmitter is installed at the same location as the sensing element. The transmitter converts the sensor’s condition to a representative process variable signal, and transmits this signal to a controller via some transmission method. Indicators Indicators (such as thermometers, pressure gauges, and level indicators) are located in the field, close to their respective processes. Control room displays show the transmitted process values of the operators. Sometimes, the transmitted data may not be up-to-date, or it may be inaccurate. Field indicators show the field operators and maintenance personnel real-time process values. These values can be used to validate the conditions observed by control room operators. Recorders Recorders normally consist of one or more pen mechanisms that are positioned according to measured process conditions (pressure, temperature, level, or flow). The recorders make permanent records of process conditions for future reference. When a recorder is placed near the process at the point of measurement, the sensing element may be located directly in the recorder. If the recorder is in a remote location, the transmitter output signal is used to position the pen. A recorder pen continuously marks the value of a monitored variable on a chart moving at a constant speed. The time is printed on the chart. An operator can see the current value of the variable, and how it changed with time. An indicator and a recorder may also be built directly into a controller for simultaneous observation and control. Newer systems input digital data signals into computerized control systems. The values can then be charted, graphed, and printed in a variety ways. The historical data is stored in digital format for future reference. Controller The controller is a very important component of an automatic control system. It continuously compares the value of a process variable with a set point. It then responds to the error between the process variable and the set point by re-positioning the final control element. This changes the energy flow to the process by altering the flow of the manipulated variable. 4th Class Edition 3 • Part A 1-11 Unit A-9 • Energy Plant Instrumentation and Controls Refer to Figure 3. The controller positions the final control element (the steam control valve) according to the error measured between the set point and the temperature of the hot water (the process variable). The adjustment in the final control element changes the rate of flow of the manipulated variable (steam flow) to bring the process variable back to the set point. An operator, a computer, or another instrument may adjust the set point signal. Controllers are normally equipped with an auto-manual transfer station. This allows an operator to switch the process from automatic to manual or vice versa, with little delay and minimum process upset. Final Control Element A final control element is used to adjust the manipulated variable. Most final control elements are control valves. Dampers and variable speed motor drives are also common final control elements. The steam control valve in Figure 3 is the final control element in that controller loop. Boilers commonly use air dampers and variable speed drives to control combustion airflow, and control valves to control fuel flow. 1-12 4th Class Edition 3 • Part A