How to Correctly Select Process Measurement and Control Instruments in Chemical Production

This article outlines
     Chemical production is an important part of a country's industrialization process, involving all aspects of national production. Many sectors such as heavy industry and light industry cannot do without the support of the chemical industry. At the same time, chemical production itself is also a part of industrial production, and it plays a significant role in industrial production and people's lives. The production process of chemical enterprises requires the use of various detection and control instruments. The detection instruments of chemical enterprises are divided into three categories according to the energy form: pneumatic, electric, and hydraulic. Typical representative products of pneumatic are gas turbine flowmeters, representative products of hydraulic are electromagnetic flowmeters , and electric ones are thermocouples. Generally speaking, the valve body is universal and can be matched with pneumatic actuators or other actuators such as electric. The selection of chemical instruments is an important task in the construction and daily management of chemical enterprises. Whether the appropriate instrument type and brand can be selected is related to production and safety, and the company's costs and benefits. The working principle of chemical instruments and the status of related production enterprises should be understood in detail to ensure accurate selection. This article will explain the simple principles and selection precautions of related instruments based on practical cases encountered in the production of chemical enterprises.
 


1. Selection of pressure gauges
　　The pressure gauges purchased by a certain chemical technology limited liability company in Shanxi Province through our company include local pressure gauges and remote pressure transmitters. The local pressure gauges mainly include diaphragm pressure gauges and ordinary non-diaphragm pressure gauges. Both pressure gauges have shockproof types. Pressure gauges are roughly divided into four categories according to their conversion principles: liquid column pressure gauges, elastic pressure gauges, electrical pressure gauges, and piston pressure gauges. The selection of pressure gauges is first based on the requirements of the process for pressure detection and the properties of the measured medium. The type, model, range and accuracy of the instrument are determined based on the on-site environmental conditions, and whether it has remote transmission, alarm and other functions. The local pressure gauge selected by our company belongs to the elastic pressure gauge. According to the process medium, ordinary water and gas medium working conditions, ordinary local domestic pressure gauges are used. For the medium of chlorosilane, imported diaphragm pressure gauges are used. The remote pressure transmitter uses Rosemount capacitive pressure and differential pressure transmitters. According to the different process media, it is divided into two types: capillary and ordinary pressure transmitters. The transmitter with capillary is mainly used for pressure and differential pressure measurement of corrosive media such as chlorosilane.
 
2. Selection of temperature instruments
　　According to the working principle, temperature instruments can be divided into expansion thermometers, pressure thermometers, thermocouples, thermal resistors and radiation pyrometers. Bimetallic thermometers belong to expansion thermometers and local instruments. Thermal resistors, thermocouples, and integrated temperature transmitters all require remote signal transmission, and the signals are sent to the control room via cables for display or control. Thermal resistors are based on the principle that the resistance of metals changes with temperature, and use semiconductor materials, generally copper and platinum. The corresponding relationship between temperature and resistance can be found according to the graduation number. Most of our company's thermal resistors are armored platinum thermal resistors pt100, which are 2-wire systems. Thermocouples use different metals to weld together to form a potential difference. The welding point is called the hot end and the other end is called the cold end. According to the different constituent metals, there are different thermocouples such as S, R, B, K, E, T, and J. Our company uses K-type thermocouples, which use special compensation wires and are 2-wire systems. Our company's thermocouples are mainly used in synthesis furnaces. Their characteristics are long lengths, up to 10m. The integrated temperature transmitter converts the thermal resistor or thermocouple signal into a standard 4-20MA signal. It is easy to use and is mainly used to measure the gas temperature in air compressor stations.
 
3. Selection of flow detection instruments
　　The flow meters used by the company include: rotor flowmeter, vortex flowmeter, mass flowmeter, average velocity tube flowmeter, integrated orifice flow transmitter, etc. Flow detection instruments can be divided into velocity type, volumetric type and mass flowmeter. Among them, rotor, electromagnetic, vortex, average velocity tube and orifice flowmeters are velocity flowmeters. The mass flowmeters used by our company are thermal and Coriolis. The average velocity tube flowmeter and the integrated orifice flow transmitter are differential pressure flowmeters, which are composed of a throttling device, a pressure pipe, a differential pressure gauge and a display instrument. Its throttling devices are average velocity tube and orifice plate. The electromagnetic flowmeter uses the principle of electromagnetic induction and can be used to measure the volume flow of conductive liquids such as acids, alkalis and salts. The rotor flowmeter is a flow measurement instrument based on the throttling principle when the fluid flows. The rotor flowmeter requires the fluid to flow from bottom to top, and can be divided into two types: local indication and remote transmission. The vortex flowmeter uses the principle of fluid oscillation for measurement. A non-streamlined column is inserted into the fluid and measured using the Karman vortex column. It can measure the flow of gas, liquid and steam in the pipeline, but it cannot be used in working conditions with severe vibration. It is characterized by high accuracy, wide range, no mechanical wear and easy maintenance. The mass flowmeter is different from the above volume flowmeters. It can directly measure the mass of the medium without conversion and correction. The Coriolis and thermal mass flowmeters are based on the fact that when the liquid flows in the vibrating tube, it will generate a Coriolis force proportional to the mass flow rate, that is, using a U-shaped tube, the fluid will produce a certain distortion when passing through. Using this phenomenon, the phase difference is converted into a corresponding level signal through an electromagnetic detector, and finally converted into a 4-20Ma analog signal proportional to the mass. In the actual selection, it is based on the medium of the measured fluid. For example, when measuring a trace amount of chlorosilane, a vortex flowmeter was used at the beginning, but because the flow rate was too small, only a mass flowmeter could be used in the end, such as an electromagnetic flowmeter, which must measure conductive media. The vortex flowmeter cannot be used to measure vibration conditions, but is mostly used for gas measurement, and temperature and pressure compensation must be performed according to temperature and pressure.
 
4. Selection of physical (liquid) level detection instruments
　　The related instruments used by Shanxi Lu'an High Purity Silicon Technology Co., Ltd. include magnetostrictive level gauges, radar level gauges, differential pressure transmitters, radio frequency admittance level gauges, magnetic flap level gauges, etc. Magnetostrictive level gauges are mainly used to measure the liquid level of chlorosilane storage tanks. Radar level gauges are equipped with waveguides and are mainly used to measure the level of silicon powder storage tanks for solid and liquid-solid mixed working conditions. Differential pressure transmitters mainly measure the liquid level of non-corrosive media. Instruments for measuring physical (liquid) levels can be divided into direct reading, differential pressure, buoyancy, electromagnetic, nuclear radiation, acoustic wave, optical and other types according to their working principles. Magnetostrictive level gauges are buoyancy type and cannot be used for the measurement of solids and non-contact liquids. In the selection of the above instruments, our company mainly considered the safety level, measurement accuracy, medium, etc. of the instruments. Despite this, in actual use, some instruments still have to be replaced.
 
5. Selection of pneumatic actuators
　　Pneumatic actuators use pneumatic actuators, which have simple structure, large output force, and fire and explosion prevention functions. Pneumatic actuators receive air pressure signals from positioners and solenoid valves, and convert them into corresponding push rod displacements according to the size of the air pressure to drive the mechanism to move. Pneumatic actuators are divided into three types: diaphragm type, piston type, and long stroke. The valve body is divided into straight-through single-seat valve, double-seat valve, angle valve, butterfly valve, three-way valve, etc. According to the adjustment form, it can be divided into cut-off, adjustment, and adjustment cut-off types. Our company uses many types of valves, including ordinary spherical regulating valves, spherical regulating valves with glass tubes, soft-sealed ball valves, hard-sealed ball valves, butterfly valves, disc valves, slide valves, eccentric rotary valves, etc. Due to the special working conditions of polysilicon, mainly involving some media containing silicon powder, which will cause serious abrasion to the valve, so valves such as disc valves and slide valves are used in large quantities. Disc valves are a kind of fast-opening and fast-closing valves that realize switching by sliding discs. Its design is specifically for materials with wear and erosion characteristics, and it can work normally for a long time under harsh working conditions. It has a hard sealing surface and a self-grinding effect, which continuously improves its sealing performance during the application on wear and erosion materials, and its service life is several times longer than that of ordinary ball valves. The slide valve is a special valve used in solid material conveying systems to open and close pipelines. The completely sealed structure can be applied to the bottom of silos, hoppers or related equipment. The blade-type gate design applies force to the valve seat by the inclined surface when closing, and has good sealing performance; the material can be selected from ceramics and metals, etc. Most polysilicon companies use the German Siruisi ceramic slide valve. For the alternating working conditions required by some chlorosilanes, that is, the medium temperature is either cold or hot, and the temperature difference is large. Among the world-renowned brands of ball valves, only a few can meet the requirements, which should be paid attention to. In other media without silicon powder, the first thing to consider is whether to use the regulating or cutting function. For corrosive media such as chlorosilane, hydrogen chloride, etc., the regulating valve should be selected with bellows seal, and the cut-off valve should be selected with hard seal valve to ensure that the valve ball and valve seat surface are hardened by rocket spraying.
 
VI. Conclusion
　　The selection method of common chemical instruments should be based on the technical characteristics of each manufacturer and different working conditions, and the products of companies with specific adaptation and research and development should be selected. According to the required medium and working conditions, the type, material, parameters and electrical interface of the instrument should be selected to ensure the normal operation of the enterprise.