Thread Content
In enterprises, control valves are important actuator instruments in the process control of industrial automation, and they are widely used in industrial production process control. Process control instruments have evolved through various stages, from hydraulic and pneumatic instruments, electric instruments, electronic analog instruments, and digital intelligent instruments, to the most advanced computer-based distributed control systems. These days, everyone can notice that the instruments around us are gradually moving towards electrification; the original method of reading values from dials is slowly being replaced by direct displays on LED screens or centralized displays on electrical control panels. Process control instruments consist of three main hardware components: the measurement and transduction unit, the actuator, and the regulator. Among them, the measurement transduction unit is the transmitter; in some cases, the transmitter is essentially equivalent to a sensor. The actuator is something like the valves produced by valve manufacturers, while the regulator is the controller – it serves as the brain of the entire control system, such as a PID regulator. Shangnai would like to introduce here the most important component in digital instruments today: the transmitter. 1. Purpose of transmitters: In industrial production, after sensing elements detect parameters such as pressure, temperature, flow rate, and liquid level, transmitters are needed to convert the signals generated by these sensing elements into standard signal forms (such as 4–20 mA DC signals), which are then sent to display instruments or control devices for display, recording, or regulation. Therefore, a transmitter is not only a conversion medium but also a medium in itself. 2. Features of the transmitter: Since regulators are usually located in control rooms that are at a relatively considerable distance away, the transmitter functions as a combination of a signal generator and a driver for the transmission lines. It serves as the heart of the entire control system; it is through the transmitter that the control unit and the measurement unit are connected together, thus forming a complete control system. 3. Transmitter category? In some transmitters, the measuring and transmitting units are integrated into one unit (such as pressure transmitters); in others, there is only a transmitting function (such as temperature transmitters). Transmitters can be classified by their driving energy source into pneumatic transmitters and electric transmitters. Among them, differential pressure transmitters and temperature transmitters are the most commonly used in industrial production processes. 3.1 Common Types and Selection Principles of Differential Pressure Transmitters Differential pressure transmitters are used to convert measured parameters such as differential pressure, flow rate, and liquid level into a unified signal standard, and to transmit this standardized signal to indicating, recording instruments, or controllers, so that the measured parameters can be displayed on a screen. The main types include: force-balanced electric differential pressure transmitters and capacitive differential pressure transmitters. There are no standard specifications for differential pressure transmitters; each manufacturer has its own selection chart. The selection of differential pressure transmitters is primarily based on the property characteristics of the medium to be measured, with considerations for cost savings as well as ease of installation and maintenance. In cases where the medium under test is highly viscous, prone to crystallization, and highly corrosive, an isolated transmitter must be used. When selecting a differential pressure transmitter, it is necessary to take into account the erosion of the diaphragm box metal by the medium; therefore, the material of the diaphragm box must be chosen carefully. The materials used for diaphragm boxes in transmitters include ordinary stainless steel, 304 stainless steel, 316L stainless steel, and tantalum diaphragms. When selecting a differential pressure transmitter, the temperature of the medium to be measured must be taken into account. If the temperature is high, typically between 200°C and 400°C, a high-temperature model should be chosen; otherwise, the silicone oil will vaporize and expand, leading to inaccurate measurements. When selecting a differential pressure transmitter, the operating pressure rating of the equipment must be taken into consideration; the pressure rating of the transmitter must be suitable for the application in question. In terms of selecting the measurement range of the transmitter, most transmitters have a certain adjustable range; it is best to set the operating range within 1/4 to 3/4 of that range, as this ensures accuracy, which is particularly important for differential pressure transmitters. In practice, there are some application scenarios (such as level measurement) that require shifting the measurement range of the transmitter; the measurement range and the amount of shift are calculated based on the installation location on site, with shifts being either positive or negative. 3.2 Nine key points to note when using differential pressure transmitters: 1) When measuring gas pressure, the pressure tap should be located at the top of the process pipe, and the transmitter should also be installed on the upper part of the process pipe, so that any accumulated liquid can easily be fed back into the process pipe. 2) The pressure guide tube should be installed in a location with minimal temperature fluctuations ； 3) Prevent the transmitter from coming into contact with corrosive or overheated media ； 4) Prevent residue from accumulating in the conduit ； 5) When measuring liquid pressure, the pressure tapping should be located on the side of the process pipeline to prevent the accumulation of sediment. 6) When measuring steam or other high-temperature media, a condenser such as a buffer tube (coil) must be used to prevent the operating temperature of the transmitter from exceeding its limits. 7) During freezing in winter, the transmitters installed outdoors must be equipped with anti-freezing measures to prevent the liquid in the pressure tapping from expanding due to freezing, which could cause damage to the sensor. 8) When measuring liquid pressure, the transmitter should be installed in a location where it is protected from liquid impacts (water hammer effect), to prevent the sensor from being damaged due to excessive pressure. 9) When making the connections, pass the cable through the waterproof connector (accessory) or flexible tube and tighten the sealing nut to prevent rainwater and other substances from seeping into the transmitter housing via the cable. 3.3 Fault Analysis of Differential Pressure Transmitters: During the measurement process, transmitters often experience various faults. Timely identification, analysis, and handling of these faults are crucial for ongoing production activities. Based on experience in daily maintenance, some judgment and analysis methods as well as analysis processes have been summarized. 1) Survey method. Review any sparking, smoking, unusual odors, power supply fluctuations, lightning strikes, moisture, improper operations, or incorrect maintenance that occurred prior to the malfunction. 2). Intuitive method. Observe external damage to the loop, leaks in pressure piping, loop overheating, status of power switches, etc. 3) Detection method. Circuit breakage detection: Isolate the section suspected to be faulty and check whether the fault disappears. If it does, the location of the fault can be determined; otherwise, further investigation can be carried out. For example, if an intelligent differential pressure transmitter cannot communicate via Hart over a distance, the power supply can be disconnected from the instrument itself, and an alternative power source can be used on-site to power the sensor in order to determine whether an electromagnetic signal of around 2 kHz is interfering with the communication. Short-circuit detection: To ensure safety, the relevant circuit sections are directly short-circuited. For example, if the output value of the differential transmitter is too low, the pressure guiding tube can be disconnected, and the differential pressure signal can be directly routed to both sides of the differential pressure sensor from outside the primary pressure tapping valve. By observing the transmitter’s output, it is possible to determine whether there are blockages or leaks in the pressure guiding pipeline. Replacement test: Replace the part suspected to be faulty to determine the location of the issue. For example: If there is suspicion that the transmitter’s circuit board has malfunctioned, a replacement board can be installed temporarily to determine the cause. Sectional testing: The measurement circuit is divided into several sections, such as the power supply, signal output, signal transduction, and signal detection. Testing is carried out on each section in turn, starting from the simplest parts and moving to more complex ones, as well as from the surface to the interior, in order to narrow down the scope and identify the location of the fault. 3.4 Temperature Transmitters: Temperature transmitters are used in conjunction with various thermocouples or thermal resistors; they enable the measured temperature to be converted linearly into a DC current signal of 0–10 mA or 4–20 mA, thereby facilitating display, recording, and coordination with control units. Depending on the type of temperature sensor used, temperature transmitters are classified into three types: DC millivolt transmitters, resistive temperature transmitters, and thermocouple temperature transmitters. Its advantage lies in the use of linear integrated circuits with low drift and high gain, which enhances the reliability, stability, and various technical performance of the instrument. At the same time, thanks to the use of a linearization circuit, a linear relationship is maintained between the transmitter’s output signal and the measured temperature signal. Moreover, safety spark-proof measures are implemented in the circuit, enabling it to be used for temperature measurement in hazardous locations.