This article describes the instruments, technologies, and equipment used to develop and maintain process control loops. In addition, this article describes how process control equipment is represented in technical drawings of control loops.
The goal of this article is to be able to:
❑ Describe the basic function of and, where appropriate, the basic method of operation for the following control loop components:
• Primary element/sensor
• Transducer
• Converter
• Transmitter
• Signal
• Indicator
• Recorder
• Controller
• Correcting element/final control element
• Actuator
❑ List examples of each type of control loop component listed above.
❑ State the advantages of 4–20 mA current signals when compared with other types of signals.
❑ List at least three types of final control elements, and for each one:
• Provide a brief explanation of its method of operation;
• Describe its impact on the control loop;
• List common applications in which it is used.
❑ Given a piping and instrumentation drawing (P&ID), correctly label the:
• Instrument symbols (e.g., control valves, pumps, transmitters);
• Location symbols (e.g., local, panel-front);
• Signal type symbols (e.g., pneumatic, electrical).
❑ Accurately interpret instrument letter designations used on P&IDs.
Control Loop Equipment and Technology
Previously, we described the basic elements of control as measurement, comparison, and adjustment. In practice, there are instruments and strategies to accomplish each of these essential tasks. In some cases, a single process control instrument, such as a modern pressure transmitter, may perform more than one of the basic control functions. Other technologies have been developed so that communication can occur among the components that measure, compare, and adjust.
Primary Elements - Sensors
In all cases, some kind of instrument is measuring changes in the process and reporting a process variable measurement. Some of the greatest ingenuity in the process control field is apparent in sensing devices. Because sensing devices are the first element in the control loop to measure the process variable, they are also called primary elements. Examples of primary elements include:
❑ Pressure sensing diaphragms, strain gauges, capacitance cells;
❑ Resistance temperature detectors (RTDs);
❑ Thermocouples;
❑ Orifice plates;
❑ Pitot tubes;
❑ Venturi tubes;
❑ Magnetic flow tubes;
❑ Coriolis flow tubes;
❑ Radar emitters and receivers;
❑ Ultrasonic emitters and receivers;
❑ Annubar flow elements;
❑ Vortex sheddar.
Primary elements are devices that cause some change in their property with changes in process fluid conditions that can then be measured. For example, when a conductive fluid passes through the magnetic field in a magnetic flow tube, the fluid generates a voltage that is directly proportional to the velocity of the process fluid. The primary element (magnetic flow tube) outputs a voltage that can be measured and used to calculate the fluid’s flow rate. With an RTD, as the temperature of a process fluid surrounding the RTD rises or falls, the electrical resistance of the RTD increases or decreases a proportional amount. The resistance is measured, and from this measurement, temperature is determined.
Transducers and Converters
A transducer is a device that translates a mechanical signal into an electrical signal. For example, inside a capacitance pressure device, a transducer converts changes in pressure into a proportional change in capacitance.
A converter is a device that converts one type of signal into another type of signal. For example, a converter may convert current into voltage or an analog signal into a digital signal. In process control, a converter used to convert a 4–20 mA current signal into a 3–15 psig pneumatic signal (commonly used by valve actuators) is called a current-to-pressure converter.
Transmitters
A transmitter is a device that converts a reading from a sensor or transducer into a standard signal and transmits that signal to a monitor or controller. Transmitter types include:
❑ Pressure transmitters;
❑ Flow transmitters;
❑ Temperature transmitters;
❑ Level transmitters;
❑ Analytic (O2 [oxygen], CO [carbon monoxide], and pH) transmitters.
Signals
There are three kinds of signals that exist for the process industry to transmit the process variable measurement from the instrument to a centralized control system:
1. Pneumatic signal
2. Analog signal
3. Digital signal
Pneumatic Signals
Pneumatic signals are signals produced by changing the air pressure in a signal pipe in proportion to the measured change in a process variable. The common industry standard pneumatic signal range is 3–15 psig. The 3 corresponds to the lower range value (LRV) and the 15 corresponds to the upper range value (URV). Pneumatic signalling is still common. However, since the advent of electronic instruments in the 1960s, the lower costs involved in running electrical signal wire through a plant as opposed to running pressurized air tubes has made pneumatic signal technology less attractive.
Analog Signals
The most common standard electrical signal is the 4–20 mA current signal. With this signal, a transmitter sends a small current through a set of wires. The current signal is a kind of gauge in which 4 mA represents the lowest possible measurement, or zero, and 20 mA represents the highest possible measurement. For example, imagine a process that must be maintained at 100 °C. An RTD temperature sensor and transmitter are installed in the process vessel, and the transmitter is set to produce a 4 mA signal when the process temperature is at 95 °C and a 20 mA signal when the process temperature is at 105 °C. The transmitter will transmit a 12 mA signal when the temperature is at the 100 °C setpoint. As the sensor’s resistance property changes in response to changes in temperature, the transmitter outputs a 4–20 mA signal that is proportionate to the temperature changes. This signal can be converted to a temperature reading or an input to a control device, such as a burner fuel valve. Other common standard electrical signals include the 1–5 V (volts) signal and the pulse output.
Digital Signals
Digital signals are the most recent addition to process control signal technology. Digital signals are discrete levels or values that are combined in specific ways to represent process variables and also carry other information, such as diagnostic information. The methodology used to combine the digital signals is referred to as protocol.
Manufacturers may use either an open or a proprietary digital protocol. Open protocols are those that anyone who is developing a control device can use. Proprietary protocols are owned by specific companies and may be used only with their permission. Open digital protocols include the HART® (highway addressable remote transducer) protocol, FOUNDATION™ Fieldbus, Profibus, DeviceNet, and the Modbus® protocol.
Indicators
While most instruments are connected to a control system, operators sometimes need to check a measurement on the factory floor at the measurement point. An indicator makes this reading possible. An indicator is a human-readable device that displays information about the process. Indicators may be as simple as a pressure or temperature gauge or more complex, such as a digital read-out device. Some indicators simply display the measured variable, while others have control buttons that enable operators to change settings in the field.
Recorders
A recorder is a device that records the output of a measurement devices. Many process manufacturers are required by law to provide a process history to regulatory agencies, and manufacturers use recorders to help meet these regulatory requirements. In addition, manufacturers often use recorders to gather data for trend analyses. By recording the readings of critical measurement points and comparing those readings over time with the results of the process, the process can be improved. Different recorders display the data they collect differently. Some recorders list a set of readings and the times the readings were taken; others create a chart or graph of the readings. Recorders that create charts or graphs are called chart recorders.
Controllers
A controller is a device that receives data from a measurement instrument, compares that data to a programmed setpoint, and, if necessary, signals a control element to take corrective action. Local controllers are usually one of the three types: pneumatic, electronic or programmable. Contollers also commonly reside in a digital control system (Picture 1).
Picture 1: Controllers
Controllers may perform complex mathematical functions to compare a set of data to setpoint or they may perform simple addition or subtraction functions to make comparisons. Controllers always have an ability to receive input, to perform a mathematical function with the input, and to produce an output signal. Common examples of controllers include:
❑ Programmable logic controllers (PLCs)—PLCs are usually computers connected to a set of input/output (I/O) devices. The computers are programmed to respond to inputs by sending outputs to maintain all processes at setpoint.
❑ Distributed control systems (DCSs)—DCSs are controllers that, in addition to performing control functions, provide readings of the status of the process, maintain databases and advanced man-machine-interface.
Correcting Elements - Final Control Elements
The correcting or final control element is the part of the control system that acts to physically change the manipulated variable. In most cases, the final control element is a valve used to restrict or cut off fluid flow, but pump motors, louvers (typically used to regulate air flow), solenoids, and other devices can also be final control elements.
Final control elements are typically used to increase or decrease fluid flow. For example, a final control element may regulate the flow of fuel to a burner to control temperature, the flow of a catalyst into a reactor to control a chemical reaction, or the flow of air into a boiler to control boiler combustion. In any control loop, the speed with which a final control element reacts to correct a variable that is out of setpoint is very important. Many of the technological improvements in final control elements are related to improving their response time.
Actuators
An actuator is the part of a final control device that causes a physical change in the final control device when signalled to do so. The most common example of an actuator is a valve actuator, which opens or closes a valve in response to control signals from a controller. Actuators are often powered pneumatically, hydraulically, or electrically. Diaphragms, bellows, springs, gears, hydraulic pilot valves, pistons, or electric motors are often parts of an actuator system.
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