Selection and daily maintenance of various instruments

1.1 Temperature meter
Compared with other kinds of instruments, the temperature meter is a relatively simple meter. Commonly used are bimetal field thermometers, thermal resistance thermometers, thermocouple thermometers, integrated thermal resistance (thermocouple) thermometers, etc. Because the accuracy of the on-site indication of the bimetal thermometer is not high, it is generally not taken seriously; in addition, the bimetal thermometer often has no remote transmission function, so the protection level is not as good as that of the remote transmission thermometer, often due to water seepage in the glass panel, the pointer and the transmission Institutional failure, poor linearity, and its actual use is less than ideal. If the process temperature is above 650 °C, thermocouples are often preferred; if the temperature measurement point is far from the control room, it is more suitable to use an integrated temperature transmitter. Because the thermocouple needs special compensation cable, the longer the length, the higher the cost. For example, in a thermal power plant, the boiler part is generally compact and the control room is more compact. The high temperature part uses more thermocouples, and the chemical equipment requirements and the control room must have a certain safety distance. Therefore, when the process temperature of the chemical plant is high, it is more suitable to use the thermocouple temperature change. Measuring the thermal resistance below the medium temperature is an economical and practical temperature meter, such as the three-wire PT100. No matter which kind of thermometer is used, it is better to use the sheath alone to be repaired and replaced, and the thermometer core should be shockproof to improve the working conditions of various flow rates.
1.2 Pressure gauge
Here, a pressure transmitter is taken as an example for a brief introduction. The pressure transmitters of domestic brands and imported brands actually have no big difference in accuracy; in terms of zero stability, domestic pressure transmitters are still inferior. Therefore, in high pressure work, domestic pressure transmitters can be used. For example, boiler steam pressure, sometimes working pressure is at least a few megapascals, even if there are several thousand kPa error, it will not affect the operation; but if the working pressure is low, in the finished tank area, for example, some finished tanks are often floating. In the top tank, the operating pressure is not very high, and even nitrogen protection is required. In this case, a more reliable pressure transmitter is required. As for the choice of flange diaphragm type or threaded connection, it depends on the characteristics of the process medium and working conditions. For example, in the case where the acid-base salt is corrosive, it is more suitable to use the flange corrosion-resistant diaphragm type; in the high temperature and high pressure working condition, it is more suitable to use the thread connection, and at the same time, take measures against the antifreeze and anticoagulation in winter to prevent freezing. Bad pressure gauge.
1.3 Flow meter
A wide variety of flow meters is also difficult to determine in the selection of instruments, but the big principle cannot be changed. For example, in the case of cross-border area measurement, it is more suitable to use a mass flow meter. It is also possible to select other flow meters, but it is inconvenient for future cost assessment. Because the mass flowmeter has high precision and good stability, it is difficult for other flowmeters to do. However, due to the high price of the mass flow meter, the physical properties of the medium should be fully considered in the selection. The mass flow meter is commonly used to measure gas and liquid. Domestic mass flowmeters have reliable performance when measuring liquid medium stable conditions. When measuring gas and process medium unstable liquids (such as propylene), compared with imported brand mass flowmeters, there is still a certain gap in signal processing. However, some gas-liquid two-phase conditions coexist is difficult to measure with any mass flow meter. For example, in the MTP process, the reactor feed is mostly gas, and the diameter of the pipeline is large. This condition cannot be solved by the mass flowmeter, and the accuracy of the orifice flowmeter is not high. In this case, the equilibrium flow can be used. The meter (flange-connected) transmitter is suitable for multi-parameters with temperature and pressure compensation. Typical EJA910X, 3051SMV, the performance is very reliable. As for the rotameter and electromagnetic flowmeter, they are commonly used in the project, and they are not introduced here.
1.4 Level measurement
Liquid level measurement According to the design specifications, pressure vessels tend to have at least two different measurement principle level gauges, while liquid level gauges with on-site indications are most common with magnetic flap level gauges; level gauges for remote transmission signals, The double flange differential pressure level gauge is the most common. These liquid level gauges are simple in structure, reliable in principle, and easy to maintain, but they are not necessarily used. Some conditions require special consideration. For example, in the production process, liquid separation is required. Commonly, oil-water separation is required, which requires an interface meter. According to different measurement principles, the interface meter has different forms such as magnetostriction and guided wave radar. As for which one to choose, it depends on the dielectric constant and separation of the material. The boundary gauge is more scientific with the top mounting method, and the side-mounted boundary gauge, the material density is heavy below, and the density is light above. If the side-mounted type cannot be operated at full level, the material can only enter the boundary gauge from the flange opening at the bottom, and the boundary gauge will not be able to measure the true boundary. This situation is often ignored.

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