Selection and daily maintenance of various instruments

1.1 Temperature instrument

Compared with several other instruments, temperature instruments are relatively simple instruments, commonly used are bimetallic field thermometers, thermal resistance thermometers, thermocouple thermometers, integrated thermal resistance (thermocouple) thermometers, etc. Due to the low precision of bimetallic thermometers on site, they are generally not taken seriously; moreover, bimetallic thermometers often have no remote transmission function, so the protection level is not as good as the protection measures of remote transmission thermometers. Often due to water seepage in the glass panel, the pointer and transmission Mechanism failure, poor linearity, and its actual use effect is not ideal. If the process temperature is above 650 °C, it is often better to use a thermocouple; if the temperature measurement point is far away from the control room, it is more appropriate to use an integrated temperature transmitter. Because the thermocouple needs a special compensation cable, the longer the length, the higher the cost. For example, in a thermal power plant, the boiler part is generally compact with the control room, and the high temperature part uses more thermocouples, and chemical equipment requires a certain safety distance from the control room. , so when the process temperature of the chemical plant is high, it is more appropriate to use a thermocouple for temperature change. Measuring thermal resistance below medium temperature is an economical and practical temperature instrument, commonly used such as three-wire PT100. No matter which kind of thermometer it is, it is best to use a separate core-pulling type for the sheath to facilitate maintenance and replacement, and the core of the thermometer should be shock-proof to improve the adaptability to various working conditions with high flow rates.

1.2 Pressure gauge

Here, take the pressure transmitter as an example for a brief introduction. The pressure transmitters of domestic brands and imported brands do not have a big gap in accuracy; in terms of zero point stability, domestic pressure transmitters are still inferior. Therefore, domestic pressure transmitters can be used in high-pressure workplaces. For example, boiler steam water pressure, sometimes the working pressure is at least a few megapascals, even if there is an error of several thousand pascals, it will not affect the operation; but if the working pressure is low, take the finished product tank area as an example, often some finished product tanks are internal floating. For top tanks, the operating pressure is naturally not very high, and even nitrogen protection is required. In this case, a pressure transmitter with more reliable performance needs to be selected. 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 acid, alkali and salt corrosive working conditions, it is more appropriate to choose a flange corrosion-resistant diaphragm type; Bad pressure gauge.

1.3 Flow meter

There are many types of flowmeters, which is also difficult to determine in the selection of instruments, but the general principle cannot be changed. For example, in the case of cross-border area measurement, it is more appropriate to choose a mass flowmeter, and it is also possible to choose other flowmeters, but it will bring inconvenience to the cost assessment in the future. Because the mass flowmeter has high precision and good stability, it is difficult for other flowmeters to achieve. However, due to the high price of mass flowmeters, the physical characteristics of the medium should be fully considered in the selection process. Mass flowmeters are commonly used to measure gas and liquid. Domestic mass flowmeters have reliable performance when measuring liquid media in stable conditions. When measuring gases and liquids with unstable process media (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, which is difficult for any mass flowmeter to measure. For example, in the MTP process, the feed to the reactor is mostly gas, and the diameter of the pipe is large. This kind of working condition cannot be solved by the mass flowmeter. The accuracy of the orifice flowmeter is not high. In this case, the balance flowmeter can be used It is advisable for the meter (flange connection) transmitter to use multi-parameters with temperature and pressure compensation, typically EJA910X, 3051SMV, and the performance is very reliable. As for rotameters and electromagnetic flowmeters, they are commonly used in projects, so I won’t introduce them here.

1.4 Liquid level measurement

Liquid level measurement According to the design specifications, pressure vessels often have at least two liquid level gauges with different measurement principles, and the liquid level gauge with on-site indication is the most common magnetic flap level gauge; the liquid level gauge with remote transmission signals, The most common is the double flange differential pressure level gauge. These liquid level gauges are simple in structure, reliable in principle, and easy to maintain, but they are not necessarily used, and some working conditions require special consideration. For example, liquid separation is required in the production process, and oil-water separation is common, which requires an interface meter. According to the 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 of the material and the separation situation. It is more scientific to choose the top installation method for the interface level gauge. For the side-mounted interface level gauge, the dense material is on the bottom, and the light density is on the top. If the side-mounted type cannot operate at full liquid level, the material can only enter the interface level gauge from the flange port at the bottom, and the interface level gauge will not be able to measure the real interface level. This situation is often ignored.