Instrumentation Principals Introduction The purpose of instrumentation is to automate the measurement and control of process systems. Modern advances in instrumentation have facilitated automation of many processes that relied on manual control in the past. As a chemical operator, instrumentation enhances your eyes and ears to help you understand how a process or important process variable is operating. This example DCS screen monitors the mixing of two raw materials in a vessel and the pumping of the mixture to another location. A chemical operator will be able to use instrumentation to answer these kinds of questions about the process. How much material is flowing through this pipe? What temperature is the material in this pipe? How much material is in this vessel? How much pressure is in this pipe? In addition to basic process questions, learning about instrumentation will help you answer questions like: How does a particular instrument work? How are instruments communicating information? What malfunctions are common for a particular instrument? What kinds of safety systems are protecting a process? Automated Control Systems Instrumentation and automated control systems surround our modern lives. You use automated control systems, also called feedback control systems, when you adjust the thermostat to control the temperature in your home, or set the cruise control in a car. These automated control systems use feedback to determine a necessary action. We will examine a home thermostat in more detail to understand how its instrumentation works. The purpose of a common home thermostat is to maintain a comfortable air temperature. The thermostat serves two instrumentation functions: sensing and controlling. At a simplified level, the thermometer first senses the current temperature. The thermostat then takes action if the measurement strays too far from the desired temperature, which is called the set point. If a temperature adjustment is needed, the thermostat signals the heater or air conditioner to either heat or cool the air in the house. This is called feedback control because the action the controller will take depends on feedback from the system air temperature compared to the set point. The cruise control on a car is another example of a feedback control system. This system attempts to keep the car speed constant in spite of disturbances caused by changes in the slope of the road and variations in the wind and road conditions. The cruise control system senses the speed of the car and the current throttle position. A controlling computer compares the speed to the set point the driver has input. If an action is needed, the computer sends a signal to adjust the throttle valve to accelerate or decelerate the car speed. The magnitude of the signal the computer sends to the throttle valve is a complex calculation using the current speed and the sensed throttle position. You will learn more about these types of advanced control strategies later in this course. The thermostat and cruise control examples both use feedback to adjust their output. Feedback control is a “loop” of information where a group of instruments act together to control a single process variable. This continuous monitoring and adjustment cycle is also called closed-loop control. Open-loop control is different from closed loop control because it does not use feedback to determine the actions of the process. For example, let’s look at the thermostat control system again. If you programmed your home heater or air conditioner to turn on for five minutes every half hour regardless of the air temperature, you would be controlling an open-loop system. Open-loop control is not used as frequently as closed-loop control in chemical manufacturing. This is because closed-loop control is more useful for keeping a process close to a set point, which often is critical to safe and efficient operation. At its most basic level, a feedback control process will contain four elements: a sensor, a transmitter, a controller, and a final control element. A sensor measures, or senses, a process variable. That information is passed along by a transmitter to a controller. The controller compares a measured variable to a desired value and decides an appropriate output response. The final control element receives the signal from the controller and directly manipulates a variable, which causes the process to react. All feedback control systems will contain these four components. Process Variables Process variables can be classified by the way they are used in a control system. For instance, if you are driving a car, keeping the car in its proper lane on the road is a controlled variable. Controlled variables are the primary focus and are held at a target or set point. Manipulated variables are directly adjusted by the controller to correct a controlled variable. Acting as the controller in this example, the driver manipulates the steering wheel. The manipulated variable and the controlled variable are two different parameters. Indicated variables are not controlled or manipulated but are measured and monitored. For example, the driver may be monitoring the rear view mirror, but not using that information to control the cars position. Other variables that are not monitored, manipulated, or controlled can impact the process. These are uncontrolled or disturbance variables. For example, a rock in the road is an uncontrolled variable that disrupts the process. All of the instruments you will learn about in this course play a role in a feedback control loop. To help you better make sense of their uses, similarities, and differences, instruments can be categorized by their function in the control loop, their location, and the signal type they produce. Instrumentation Functions Part 1 of 3 Instruments can be categorized by the functions they provide in a control loop: sensing, indicating, transmitting, converting, recording, controlling, and manipulating. Instruments may fall into more than one category when they have been designed to provide multiple functions. For example, many sensing instruments include a transmitter in addition to a sensor. A thermometer is an example of a sensing instrument. Sensing instruments are the first instrument in a control loop. They will have some method of detecting the process variable and can be mechanical or electronic. Perhaps the most important function in the control loop is sensing the process variable. Without sufficiently accurate and precise sensing instrumentation, controlling the process would be difficult or impossible. A pressure gauge dial is an example of an indicating instrument. These show the current state of the process through the use of a digital readout, dial, or pointing device. They can be stand alone instruments, but are often features of a sensor, transmitter, or controller. A pressure transmitter is an example of a transmitting instrument that sends a signal along the control loop to another instrument. A transmitter often provides both a sensing function and a transmitting function in the same physical instrument. A transducer is an example of a converting instrument. Sometimes the signal type generated by an instrument may not be understood by the next instrument in the control loop. In that case, a converting instrument is required to change the information signal from one form to another. For example, a transducer can convert a mechanical signal from a sensing instrument into an electronic signal needed by a controlling instrument. Converting instruments may not always be present in a control loop. A DCS system is an example of a recording instrument used to keep historical data on the process variable for a set period of time. These records can be useful in troubleshooting problems related to process instrumentation. A microprocessor-based electronic controller is an example of a controlling instrument. They use measurement information from sensors to determine the proper output. At a more detailed level, controlling instruments provide comparing, calculating, and correcting functions, which will be explained in more detail later in the course. A valve is an example of a manipulating instrument, which is also called a final control element. A signal sent from a controlling instrument arrives at the manipulating instrument, which initiates a change to a manipulated process variable. They are the final instrument in the control loop. Instrumentation Functions Part 2 of 3 Instruments can be categorized by their location depending on its distance from the process: local or remote. Local instruments are located near the process, and often are referred to as field instruments. For example, a sensor is categorized as a local instrument since it must be close to the process to detect the process variable. Remote instruments are located away from the immediate area of the process. These instruments would be receiving signals from the process location. Some instruments can be local or remote. If a controller is located in a control room it would be categorized as remote, but if the controller is near the process variable being manipulated it would be categorized as local. Instrumentation Functions Part 3 of 3 Instruments can be categorized by the type of signal they produce to communicate information along the control loop. An instrument can produce either an analog or digital signal. Analog signals change continuously mirroring the measured property. For example, second and minutes are displayed in a continuous manner on an analog clock. Digital signals are not continuous. Digital signals have a limited number of steps along its range. For example, a digital clock cannot communicate information smaller than the display is designed to show. This digital clock changes in one minute intervals. There are four basic types of signal transmission used for chemical manufacturing process instrumentation. Pneumatic Analog electronic Digital electronic, and Mechanical. Instruments powered by air or other gases are categorized as pneumatic instruments. Pneumatic signals are analog. The pneumatic pressure changes continuously in time mirroring the measured property to create a signal. While pneumatic instrumentation is an older technology, it is still used today, and desirable in certain situations. Pneumatically-actuated valves are a common final control instrument. The most common pneumatic signal works on a scale of 3 to 15 psi. Instruments powered by electricity are categorized as electronic. Electronic signals can either be analog or digital. Analog electrical signals work on a scale of 4 to 20 milliamps direct current standard. Electric analog is one of the most common signal types used in instrumentation. Digital signals are not continuous. The measured property is converted to a discrete signal of ones and zeros for signal transmission. A mechanical signal is a physical linkage transmitting a signal by physical motion. For example, on some gauges, the measurement signal is transmitted by mechanical motion. Control Loop Model This control loop model is a simplified diagram showing the flow of information along an instrumentation loop. To develop a detailed understanding of instrumentation and the ability to troubleshoot complex chemical manufacturing processes, you will need to use Piping and Instrumentation Diagrams. P&IDs display how instruments are interconnected and function together to control a process. P&IDs are designed to display an entire control loop in relation to the field instrumentation but may not accurately represent the actual physical location of an instrument. Instrumentation or equipment may be depicted close together on a diagram but are often spread out with a lot of distance between them. You will recall from previous courses that instruments are typically represented on piping and instrumentation diagrams by a combination of letters, numbers, and graphic symbols. Instruments are typically identified by an alphanumeric code, or tag number. The lettering and graphic symbol choices used for instrumentation are determined by a set of rules to help you understand the functions, location, control loop, and type of instrument. In this example, the letters FIC identifies that this instrument is a Flow Indicating Controller. The numbers 101 identifies the instrument’s loop identification number. The box around the instrument symbol indicates it is a shared control display. The line through the symbol indicates the instrument is in a remote location. Learning aid materials have been provided for you to supplement your understanding of the P&ID symbols that will be used in this course to depict instrumentation on P&IDs. The rest of this course is organized to introduce you to the types of instruments used to measure common process variables, how signals are transmitted between instruments, how controlling instruments make decisions to adjust processes, how final control elements make automated adjustments occur, the strategies used to control complex proce