With the continuous expansion of production scale and the development of production technology, there are increasingly higher requirements for the level of automation in the production process. Therefore, industrial instruments have also undergone a development process from non-existence to existence, from simplicity to complexity, and from single-function to multifunction. Initially, instruments could only measure and display temperature (such as glass thermometers), pressure (such as U-tube manometers), flow (such as glass rotameters), and liquid levels (such as glass tube level gauges) on-site and could only perform simple control. They have gradually developed towards remote centralized display and remote control. In addition to the increasingly complete detection elements and instruments for various parameters, the development of process control instruments has been rapid, experiencing leaps from pneumatic unit combination instruments, electric unit combination instruments, electronic comprehensive control devices to industrial computer control systems.
Industrial automation instruments come in a wide variety. Based on the process of obtaining, transmitting, reflecting, and processing information, industrial automation instruments can be divided into five categories: (1) detection instruments; (2) display instruments; (3) control instruments; (4) actuators; (5) centralized monitoring and control devices.
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Detection Instruments
During the production process, the temperature, pressure, flow, level, and other physical quantities of the medium at different parts of the equipment and pipelines change rapidly and are always in flux. Detection instruments are used to measure the values of the above physical quantities at every moment.
According to the different process parameters being measured, detection instruments can be classified as follows:
1. Temperature Instruments: Common temperature measurement instruments include glass thermometers, bimetallic thermometers, pressure (thermal bulb) thermometers, temperature switches, thermocouples, resistance temperature detectors, as well as radiation thermometers such as radiation high-temperature thermometers and optical high-temperature thermometers.
2. Pressure Instruments: Pressure measurement instruments are used to detect pressure, vacuum, and differential pressure. Based on their working principles, they can be divided into: elastic pressure gauges (which can be further divided into spring tube pressure gauges, diaphragm pressure gauges, capsule pressure gauges, pressure switches, etc.); sensor-type pressure gauges (such as resistive, capacitive, inductive, and Hall-effect pressure gauges); liquid column pressure gauges (such as U-tube, straight tube, and inclined tube pressure gauges); and piston pressure gauges, which are generally used for calibrating standard pressure gauges and have higher accuracy.
3. Flow Instruments: Flow measurement instruments are diverse. Currently, the most widely used are differential pressure flow transmitters combined with throttling devices. Common throttling devices include orifice plates, nozzles, and Venturi tubes. Other common flow instruments include water meters, rotameters, oval gear flow meters, target flow meters, electromagnetic flow meters, vortex flow meters, Annubar flow meters, and mass flow meters.
4. Level Instruments: Level instruments primarily measure the liquid level of a certain medium in towers, tanks, and containers or the interface of two liquids with different specific gravities as well as the level of solid materials. The most common level gauges are glass tube level gauges and glass plate level gauges. Others include differential pressure level gauges and buoyancy level gauges (such as float level gauges, level switches, float level gauges, buoy level gauges, steel tape level gauges, and tank level weighing instruments, etc.). For solid material level detection, there are resistive level gauges, capacitive level gauges, level switches, heavy-duty level detectors, tuning fork level gauges, ultrasonic level gauges, and radioactive level gauges.
5. Component Analyzers: Component analyzers are used to determine the composition of process media and measure the content of certain components (or some components up to all components). Based on their working principles, they can be divided into electrochemical analyzers (such as conductivity meters, industrial pH meters, zirconia analyzers, etc.), thermal analyzers (such as thermal conductivity analyzers, thermochemical analyzers, infrared analyzers), as well as magnetic conductivity analyzers, photoelectric colorimetric analyzers, mass spectrometers, industrial gas chromatographs, etc.
When installing online component analyzers, it is generally necessary to preprocess the samples to ensure that the state, temperature, pressure, flow, and other parameters of the samples meet the working conditions required by the analyzers. Therefore, a pipeline system consisting of filters, dust collectors, drying containers, coolers, rotameters, water seals, valves, and pipes is needed for general preprocessing of samples. For some special media (such as dirty flue gas samples, high-temperature gas samples, heavy oil sampling, corrosive component sampling, and environmental monitoring sampling), their sample preprocessing systems are even more sophisticated. These finished forms of sample preprocessing systems are called sample preprocessing devices.
Additionally, there are some physical property detection instruments such as moisture meters, hygrometers, density meters, concentration meters, turbidity meters, and viscometers that are often classified as component analyzers.
6. Mechanical Quantity Instruments: Common mechanical quantity instruments in industry include thickness gauges, thermal expansion detectors, tension detectors, deflection detectors, as well as devices for detecting shaft vibration, shaft displacement, and rotational speed used in rotating machinery (such as large steam turbine compressors) and weighing devices (such as electronic belt scales, belt deviation and slippage detection devices, weighing displays, and bagging devices, etc.).
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Display Instruments
This type of instrument is used in conjunction with detection instruments to indicate or record the instantaneous values of the measured parameters, such as ratio meters and millivolt meters, dynamic coil indicating instruments, digital displays, and electronic potentiometers or electronic balance bridges (which can also be combined with electric or pneumatic regulators to form composite instruments) and instruments with flow accumulation functions.
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Control Instruments
Control instruments accept measurement signals from process detection instruments and transmitters for display, while also issuing adjustment signals to control the actions of actuators (actuating mechanisms and control valves), forming a closed-loop control system.
Control instruments can be divided into two main categories based on signal types: analog control instruments and digital control instruments.
1. Analog control instruments include base-type instruments, unit combination instruments (pneumatic, electric), and assembled instruments.
(1) Unit combination instruments are categorized based on their functions in the control system into different units. Each unit instrument exists independently and can be combined into different detection and adjustment systems as needed, making the system composition flexible and convenient. The signal transmission between units uses a unified standard signal (also known as an analog signal). Unit combination instruments were widely used in the early 1950s to the early 1970s and are truly functional distributed instruments, meaning a single instrument is used to accomplish a specific required function.
It should be noted that the transmitting unit instruments in unit combination instruments (except temperature transmitters) should be classified as detection instruments based on their function.
Unit combination instruments can further be divided into pneumatic unit combination instruments and electric unit combination instruments based on their power source:
Pneumatic Unit Combination Instruments: Pneumatic unit combination instruments have developed from original pneumatic instruments. These instruments use compressed air at 0.14 MPa as their power source, employing compressed air at 0.02-0.1 MPa as a unified signal. Because both their power source and signal transmission are compressed air, pneumatic unit instruments have a natural explosion-proof effect in refining and chemical production facilities. However, their disadvantage is that the signal transmission distance is generally within 150 meters; when the transmission distance is too long, signal transmission lags, affecting the sensitivity of display and adjustment. Pneumatic unit combination instruments include the following unit instruments:
a. Transmitting units (i.e., transmitters) include pressure transmitters, differential pressure transmitters, target flow transmitters, internal orifice flow transmitters, single (double) flange differential pressure (level) transmitters, internal (external) float level transmitters, and temperature transmitters, etc.
b. Display unit instruments such as color band indicators, bar indicators, multi-needle indicators, indicating recorders, accumulators, etc.
c. Adjustment unit instruments include indicating regulators, recording regulators, cascade regulators, proportional (integral, derivative) regulators, etc.
d. Calculation unit instruments such as adders, multipliers, ratio meters, etc.
e. Setting unit instruments such as constant value setters, time program setters, etc.
f. Auxiliary unit instruments such as pneumatic (Q-type) actuators, manual/automatic switching actuators, high (low) value selectors, relays, switches, limiters, proportioners, load distributors, and large flow filter pressure reducing valves, etc.
Electric Unit Combination Instruments: Electric unit combination instruments use DC power as their working energy. These instruments have undergone three development stages of basic electronic components: Type I (vacuum tube circuits), Type II (transistor circuits), and Type III (linear integrated circuits). Currently, Types I and II have been phased out. Type III is still widely used in refining and chemical production facilities. The electric unit combination instruments referred to here only refer to Type III. Electric Type III instruments use a DC 24V power supply, and the signal transmission between unit instruments installed in the control room uses DC 1-5V voltage signals, while the signal transmission between instruments in the control room and field-installed transmitters, control valves, and actuators is done using DC 4-20 mA current signals. To meet different explosion-proof requirements, the field-installed transmitters and the control room input/output units (safety retainers, safety barriers) have explosion-proof and intrinsically safe types. Additionally, due to the development needs of industrial computer control technology, intelligent unit instruments based on microprocessors have been developed in recent years, becoming a new force in electric unit instruments.
Electric unit combination instruments include the following units:
a. Transmitting units (i.e., transmitters) include pressure transmitters, differential pressure transmitters, target flow transmitters, internal orifice flow transmitters, single (double) flange differential pressure (level) transmitters, internal (external) float level transmitters, temperature (temperature difference) transmitters, intelligent pressure transmitters, and intelligent differential pressure transmitters, etc.
b. Display unit instruments such as single (double) needle indicators, color band indicators, single (double) needle alarm instruments, single (double) pen recorders, multi-point indicating recorders, proportional (square root) accumulators, etc.
c. Adjustment unit instruments include indicating regulators, SPC/DDC backup regulators, multi-channel valve position tracking regulators, special function regulators, integrators, and differentiators, etc.
d. Calculation unit instruments such as adders, multipliers, square rooters, etc.
e. Conversion unit instruments include current signal converters, pulse/voltage converters, frequency/current converters, impedance converters, function converters, electric/gas converters, gas/electric converters, etc.
f. Setting unit instruments include constant current setters, ratio setters, rate setters, alarm setters, parameter program setters, time program setters, etc.
g. Auxiliary unit instruments such as electric (D-type) actuators, DDC actuators, safety retainers, safety barriers, distributors, voltage boxes, signal selectors, isolators, reverse devices, lifters, signal dampers, signal inverters, signal limiters, and rate selectors, etc.
(2) Assembled Comprehensive Control Instruments
This is a new series in the development of process control instruments, also known as assembled comprehensive control devices. It adopts a component assembly structure, which can conveniently and flexibly form a process control system, with an internal system using a 0-10V DC voltage signal system. It can also receive various pneumatic and electric signals (including current, voltage, contacts, pulses, frequency, encoding, etc.) from field detection instruments and detection elements.
Assembled comprehensive control devices include the following instruments and components:
a. Input/output components: input conversion components, output conversion components, pulse conversion components, mV/V conversion components, P/E conversion components, power accumulation driver components, etc.
b. Signal processing components: signal buffering components, relay buffering components, signal generation components (ramp generation components, timing components, etc.), analog computing components (multiplication, division components, square root components, addition components, function components, limiting components, signal selection components, etc.), accumulation components, alarm components, logic components.
c. Adjustment components: PID components (proportional, integral, derivative components), dynamic compensation components, tracking components, multi-output interface components, sound and light control components.
d. Auxiliary components and other components: power distribution components, signal distribution components, switching components, setting components, relay components, monitoring components.
e. Display operation instruments: single (double) needle indicators, single (double) pen recorders, three (four) pen recorders, trend recorders, manual controllers, control display operators.
(3) Base-type Adjustment Instruments
In the process of industrial automation instruments evolving from on-site detection and display to centralized control, a type of instrument that integrates measurement, display, and adjustment functions has emerged, which we call base-type adjustment instruments or simply base instruments. For example, indicating record regulators with pneumatic regulators and some local regulators with single adjustment functions (such as temperature regulators, pressure regulators, differential pressure regulators, flow regulators). Base-type adjustment instruments can be further categorized into pneumatic and electric types based on their power source.
Self-acting regulators are also a type of local adjustment instrument, named so because they rely on the measured medium as their power source; hence they are also called direct-acting regulators. Additionally, because they are integrated with their control valves, self-acting regulators are also known as self-acting control valves. Common self-acting regulators include self-acting temperature regulators, self-acting pressure regulators, and self-acting flow regulators.
2. Digital Control Instruments
Digital control instruments include distributed control systems (DCS), programmable logic controllers (PLC), industrial control computers (IPC), safety control systems (FSC), etc.
In the 1960s, with the large-scale and complex nature of industrial production processes, the requirement for industrial automation control systems was to handle large amounts of data, perform advanced computational control, facilitate information communication, achieve centralized display operation and advanced control, and improve control accuracy. The simple use of conventional analog instruments could no longer meet these requirements, leading to the adoption of computer control systems, further enhancing the comprehensive control level of the production process. However, with the highly centralized control functions, the risk of accidents also became highly centralized. Once a computer control system fails, control, monitoring, and operation cannot proceed, greatly impacting production and potentially leading to major global accidents.
After the 1970s, with the advent of large-scale integrated circuits and the birth of microprocessors, new process control systems based on microprocessors and microcomputers were developed, such as the Distributed Control System (DCS). The distributed control system inherits the advantages of conventional analog instruments and computer control systems, maintaining centralized display operation and centralized management while decentralizing control, further enhancing the safety and reliability of control systems. This is because distributed control systems distribute microprocessors based on control functions or control areas. Each control station with a microprocessor can control several or dozens of loops, and several control stations can be combined to control the entire production process, thus achieving decentralized control and dispersing risks. On this basis, a large amount of information is transmitted to the central control room via data communication cables, with microprocessor-based CRT display operation stations concentrating on displaying or recording this information. At the same time, in conjunction with upper-level computers (process management computers and production management computers), centralized monitoring and management of the production process are implemented.
Distributed control systems can achieve continuous control, batch (intermittent) control, sequential control, data collection and processing, and advanced control, closely integrating operational management with the production process. The distributed control system also features self-diagnosis capabilities, checking the hardware and software of the system. Once a fault is detected, it emits sound and light alarms and displays the location of the fault.
Typically, distributed control systems consist of field control stations, CRT display operation stations, communication networks, and peripheral devices such as printers.
In subsequent developments, the control communication functions of distributed control systems have become increasingly refined and standardized. Based on their focus on control functions, programmable logic controllers (PLC) have been separated from distributed control systems (DCS), which primarily focus on loop control. The original purpose of PLCs was to replace traditional relay-based interlocking alarm systems, and their input/output signals are all switch signals. They execute functions like logic, sequencing, timing, counting, and calculations using software programming, suitable for more complex interlocks. The key feature of PLCs is their programmability; simply changing the program can alter the control scheme. Their reliability, flexibility, and operational speed, as well as the complexity of control schemes, cannot be compared to relay circuits.
The development of PLCs has been rapid, enhancing analog control capabilities, computational functions, and even including CRT dynamic graphic displays, database management, and file generation. DCS has also absorbed the technological characteristics of PLCs, strengthening batch processing and sequential control functions. The overlap of functions between these two systems has blurred their distinctions. With the continued development of distributed control systems, especially in terms of miniaturization, intelligent field transmitters, standardization of field buses, and communication networks, as well as the integration of monitoring computers and PCs into DCS systems, the system software has been further refined, making distributed control systems more adaptable to various process control needs and achieving better technical and economic benefits.
Fieldbus (FCS) is a digital, serial, multi-point communication bidirectional transmission data bus installed between the automatic control devices in the production field and the control room. Its basic idea is to connect the control stations of DCS and PLC in the control room, intelligent regulators, etc., not one by one through their respective input/output (I/O) channels to the field instruments (such as transmitters, control valves, switches), but rather to connect them through their respective serial interfaces to the fieldbus H2 high-speed channel. Then, through the H2/H1 bridge, they are connected to the H1 fieldbus to achieve communication between H1 and H2 field instruments, monitoring and detecting the production process.
Since the fieldbus is the communication network interconnecting the lowest level of field devices (field equipment and field instruments), integrating field control and field communication functions, the nodes of the fieldbus communication network are intelligent transmitters (including temperature, pressure, flow, level, process analyzers, etc.) and intelligent actuators.
Industrial computers are classified based on their control and management functions into basic automation control devices and management computers. The basic automation devices are the first level of multi-pole control, including distributed control systems (DCS), programmable logic controllers (PLC), direct digital control devices (DDC), and fieldbus control systems (FCS). Process management computers are upper-level computers for basic automation devices and belong to the second level of multi-pole control; production management computers are suitable for the third to fifth levels of multi-pole control.
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Actuators
Actuators, also known as control valves, consist of two parts: the actuator and the valve. Based on the power source of the actuating mechanism, there are four main categories of control valves: pneumatic control valves, electric control valves, hydraulic control valves, and hybrid control valves. Pneumatic control valves are further divided into membrane control valves, piston control valves, and long-stroke control valves based on their actuating mechanism forms.
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Centralized Monitoring and Control Devices
Centralized monitoring devices utilize detection elements or detectors to concentrate and display the measured variable or alarm point signals; centralized control devices control a series of measured variable signals to actuators according to set programs. Centralized monitoring and control devices include various data acquisition devices,巡回检测装置, signal alarm devices, safety detection devices, industrial television, remote devices, and sequential control devices, etc. Centralized monitoring and control devices are generally divided into the following categories:
1. Safety monitoring devices include combustible gas detection alarm devices, toxic gas detection alarm devices, flame monitors, automatic ignition devices, combustion safety protection devices, oil leak detection devices, and high resistance detection devices, etc.
2. Industrial television consists of cameras and their auxiliary equipment (lighting, purging, cooling devices, and electric turntables), displays, and auxiliary devices (controllers, distributors, compensators, and switches), etc.
3. Remote devices accept input variable signals, process information, display alarms, and output control signals to the control end.
4. Signal alarm devices include flashing signal alarms, intelligent flashing alarm devices, relay line alarm systems, and various types of signal alarm devices.
5. Sequential control devices include relay interlocking protection systems, logic monitoring devices, sequential control devices, and intelligent sequential controllers, etc.
6. Data acquisition and巡回检测报警 devices include data acquisition devices and巡回检测 alarm instruments, etc.
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Self-control Other Equipment
This type of equipment mainly includes various types of (channel-type, cabinet-type, frame-type, screen-type) instrument panels, instrument boxes, operation consoles, insulation (protection) boxes, and power supply boxes, etc.
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Self-control Materials
Self-control materials refer to the materials required for instrument installation, which are diverse, such as pressure transmission pipes (seamless steel pipes, stainless steel pipes, high-pressure pipes), gas supply pipes (galvanized steel pipes, brass pipes), pneumatic signal pipes (copper pipes, copper cable, nylon pipes, connection boxes), electrical conduit materials (welded steel pipes, galvanized steel pipes), valves, flanges, and pipe fittings in various types of piping, electrical equipment materials for self-control (cables, wires, junction boxes, electrical devices, and components), instrument cable trays, and metal materials such as angle steel and channel steel used to make instrument equipment brackets and supports, heating insulation materials, and anti-corrosion paint materials, etc.
Source: Global Chemical Equipment Network
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