Accurate level measurement technology and level measurement instruments help to reduce material losses, use energy efficiently, increase productivity and reduce safety accidents. Accurate level measurement requires the correct selection of level instruments according to the requirements of industrial production. With the rapid development of the salt chemical industry, the level measurement of salt production units is particularly important. In this paper, we discuss how to select the correct level measurement instrument in the salt chemical industry, taking into account the classification of level measurement instruments and the needs of salt chemical production.

A, the classification of material measuring instruments

　　1、Ultrasonic level meter

　　The working principle of ultrasonic level meter is that the ultrasonic transducer sends out high-frequency pulse sound wave encountering the measured level surface is reflected, and the fold-back reflected echo is received by the transducer and converted into an electrical signal. It has the ability to improve the reliability of the instrument to solve the problem of uneven level measurement inaccuracy, and greatly strengthen the anti-interference ability, and can work stably in the vicinity of the substation transmission tower.

　　2、Radar level meter

　　Radar level meter belongs to the general-purpose radar level meter, which is based on the time travel principle of measuring instruments. It is suitable for non-contact continuous measurement of the level of liquid, slurry and granular materials, and is suitable for occasions with large changes in temperature and pressure, the presence of inert gas and volatilization.

　　3、Buoyancy level measuring instrument

　　It mainly includes constant buoyancy level meter and variable buoyancy level meter. It is characterized by low measurement accuracy, low maintenance workload, and can realize remote transmission and automatic adjustment.

　　(1) Constant buoyancy level meter. Constant buoyancy level meter is to rely on the float or float float in the liquid with the liquid level changes and rise and fall, it is characterized by simple structure, low price, suitable for a variety of storage tank measurement.

　　(2) variable buoyancy level meter. Variable buoyancy type is also called sink cylinder level meter, when the liquid level is different, the volume of the sink cylinder immersed in the liquid is different, so the buoyancy force is different and produces displacement, which is converted into angular displacement through mechanical transmission to measure the liquid level.

4、Differential pressure level measuring instrument

　　Mainly including pressure level meter, blowing method pressure level meter, differential pressure level (or interface) meter three forms.

　　(1) pressure level meter. The pressure level meter adopts the principle of static pressure measurement, which is stable, reliable and has a long service life. Easy to install, simple structure, economic and durable.

　　(2) blowing method pressure level meter. Blowing method pressure level meter types is inserted in the open container a blowing tube, air through the float flow meter and constant flow valve and blowing pipeline to be measured in the liquid, blowing flow is indicated by the float flow meter, the flow size set by the flow adjustment valve on the float flow meter, constant flow of gas from the lower port of the blowing tube inserted into the liquid escapes, bubbles and discharged into the atmosphere through the liquid. When there is a small amount of air bubble discharge at the lower end of the blowing tube, the pressure value indicated by the differential pressure transmitter can reflect the liquid level height. Mainly suitable for atmospheric pressure or open container, can measure corrosive media, but the measurement range is limited, the accuracy depends on the transmitter accuracy and the performance of the pressure stabilizer. All mechanical structure, good sealing, low failure rate and low maintenance

　　(3) Differential pressure level meter. Differential pressure level meter has two pressure ports for gas phase and liquid phase. The pressure at the gas phase pressure point is the gas phase pressure in the equipment; the pressure at the liquid phase pressure point in addition to the role of gas phase pressure, but also by the role of the liquid column static pressure, the difference between the liquid phase and gas phase pressure, is the static pressure generated by the liquid column. It is due to the density change directly affects the measurement results, and is suitable for the process of relatively stable density.

　　5、Direct liquid level measurement instrument

　　The main forms of glass tube level meter, glass plate level meter, is a connector type instrument, with a simple structure, inexpensive, intuitive, suitable for field use, not convenient for remote transmission and adjustment characteristics.

　　6、Electrical level measuring instrument

　　Mainly includes electric contact level meter magnetostrictive level meter, capacitance level meter.

　　(1) electric contact level meter. Electric contact level meter is designed according to the different resistivity of water and steam, the electrode of the measuring cylinder in the water to the cylinder impedance is small in the steam to the cylinder impedance. With the change of water level, the number of electrodes in the water to produce changes. With the following three features: with flashing, sound alarm function; with 4 ~ 20mA or 0 ~ 10mA analog signal output, can be connected to DCS system, with protection interlock output function; with self-power supply function can continue to work 4h after power failure.

　　(2) Magnetostrictive level meter. Magnetostrictive level meter is generated by the electronic part of the transmitter, the strain pulse returns along the waveguide and is received by the electronic unit. By accurately measuring the time interval between the interrogating pulse and the returning pulse, the high precision and high repeatability of the level value can be obtained, which has the characteristics of high precision.

　　(3) Capacitance type level meter. Capacitance level meter uses the change of measuring capacitance to measure the level of liquid, with high sensitivity.

　　7、Radioactive level meter

　　Radioactive level meter is based on the "ray absorption principle". It is mainly used in high temperature and high pressure, corrosive, toxic, smoky and other complex and harsh working conditions where the general instrumentation cannot be used or difficult to use.

Second, the selection of level measuring instruments in the salt chemical industry

　　1、The distribution of level measuring instruments

　　(1) Top-mounted magnetic flap level gauge. Mainly used in fine chemical plants, bromine plants, chemical plants, installed in the tank without crystallization, the instrument interface above the tank or into the underground material liquid tank.

　　(2) Side-mounted magnetic flap level meter. Mainly used in fine chemical plants, bromine plants, chemical plants, installed on the tank without crystallization.

　　(3) Radar level meter. Mainly used in fine chemical plants with agitation on the material liquid storage tank, chemical plants on the fish seed salt crystallizer.

　　(4) Float level transmitter. Mainly used in liquid bromine storage tank and dilute acid liquid tank in fine chemical factory, liquid bromine storage tank in bromine factory and chemical factory.

　　(5) Single flange liquid level meter. Mainly used in fine chemical plant refined salt plant, chemical plant, installed on the material liquid tank, outdoor brine tank.

　　(6) Input level meter. Mainly used in fine chemical plants at the wastewater pool.

　　(7) Double flange level meter. It is mainly used in the vacuum liquid tank of fine chemical plant, the evaporation tank of refined salt plant, the evaporation tank of potassium chloride in chemical plant, etc.

　　(8) Magnetostrictive level meter. Mainly used in the condensate balance barrel of refined salt plant.

　　2, level measurement instrument selection experience

　　(1) medium for crystallization, precipitation, tank bottom with stirring device of the liquid tank, preferred radar level meter or ultrasonic level meter. Now nearly 30 radar level meter has been used, the material according to the measurement medium is divided into the choice of 316L or 304 lined with PTFE, non-contact installation, a high degree of accuracy.

　　(2) the medium is no crystallization, no precipitation of the liquid tank (such as brine tanks, dilute acid tanks, clear water tanks, balance barrels, etc.) preferred magnetic flap level meter, can realize the site and remote display of two places, according to the tank level meter installation is divided into side and top mounted two, according to the material is divided into 304, 316, PP, 304 lined with PTFE several. When the measurement medium is brine, dilute acid and other corrosive liquids, PP (engineering plastic) material or 304 lined with tetrafluorine is selected, when the measurement medium is non-corrosive liquid, 304 material can be selected.

　　(3) Liquid bromine is easy to volatilize and has very strong poisonous and corrosive properties. In the liquid bromine storage tank, the float level transmitter is selected, and the float level transmitter is all lined with PTFE, and the connecting flange is also all lined with PTFE, so that the corrosion of liquid bromine to the level meter can be cut off better.

　　(4) The medium in the material liquid tank is liquid with strong hard foam above, can choose differential pressure level transmitter. In addition, differential pressure level transmitter is also installed on the evaporation tank (such as 120,000 Ua vacuum salt project, 150,000 t/a vacuum salt project), the saturated brine evaporates in the evaporation tank, when it reaches about 25 ° Be, salt can be precipitated, then the salt particles will be blocked in the differential pressure transmitter diaphragm of the liquid phase, for this reason, the liquid phase on the convex flange can be selected.