Application of magnetostrictive liquid level gauge in the measurement of carbon dioxide

1. Carbon disulfide metering difficulties

Carbon disulfide is the main raw material of viscose chemical fiber, in addition it is also widely used in medicine, pesticides, accelerators, additives, solvents, etc. It is an important chemical raw material. Therefore, the measurement of carbon disulfide is necessary both in the transaction and in the internal cost management of factory production. However, carbon disulfide is flammable and explosive. Among the many petrochemicals, it is one of the most dangerous chemicals. In order to ensure the safety of use, water seals are used for transportation and storage, because carbon disulfide has a density of 1.262 g/em under standard conditions, which is heavier than water. Therefore, water is added above the containers for transportation and storage. To avoid its volatilization and direct contact with the outside world. This is a guarantee of safety, but it adds to the difficulty of measurement. Since carbon disulfide is always accompanied by water, it cannot be simply measured with a flow meter. If metering with a flow meter, a device must be added before the flow meter to first determine whether water or carbon disulfide flows through, which complicates metering and greatly increases costs, so this method is not feasible; With water, weighing method is also not desirable. In view of this situation, volumetric methods are usually used.

With volumetric metering, the measuring instruments must do the following:

(1) It must be an interface measurement, not a simple level measurement;

(2) Must have high accuracy, at least better than 0.2%;

(3) In accordance with the explosion-proof specification, carbon disulfide should belong to the intrinsically safe, IIC grade T5 group. That is, the explosion-proof rating of the meter should be ialICT5 or more.

2. Current measurement methods and problems

At present, there are mainly the following types of carbon disulfide metering:

2.1 Manual Measurement

The copper rod is coated with lead oil and inserted into the container. Due to the chemical action of carbon disulfide and lead oil, the copper rod becomes black and the water does not react with the lead oil to maintain the original color. The copper rod is taken out of the container for a certain period of time and taken out. The length of the darkened portion is measured with a tape measure or a steel tape measure, which represents the height of the liquid level of carbon disulfide in the container. This method requires the use of lead oil for every measurement. The container must be opened and closed. It is time-consuming, laborious, and odorous, which impairs the operator's health. More important is the large measurement error. First of all, whether the copper rod is vertical in the container and whether the bottom of the container is horizontal is caused by the error; Secondly, the number of lead oil coating, the length of the copper rod insertion time, the stability of the interface and many other factors also cause measurement errors; The precision of the steel tape measure itself is not high, and the tension of the pull tape can also have an effect. Different people have different measurement results, and a large amount of accidental errors occur. Generally speaking, the absolute error of manual measurement is at least 5~10mm. For a large tank with a diameter of 2600mm and a length of 12912mm, the measurement deviation caused by the measurement is 262.5~416.5kg, and the value is about 682.5~1082. 9 yuan.

2.2 Magnetic flap level gauge

Using the difference in the density of water and carbon disulfide, the floating ball principle is used to measure the interface between the two, and the magnetic coupling is used to drive the flap to display the interface height, or trigger the reed relay to output the remote transmission electrical signal. Due to the mechanical action such as turning over and reed clutch, these instruments are prone to failure due to changes in time and environment, and their reliability is not high. General errors are within ±1%F·S, or even more, and they are difficult to measure.

2.3 RF Admittance Level Meter

The radio frequency admittance level meter is a new type of meter developed on the basis of the capacitance level meter. It measures the interface between the two based on the difference in the dielectric constant between water and carbon disulfide. This table is easy to install, reliable performance, the minimum accuracy of ± 0.5% F · S, as the general display and control is acceptable, but for high-precision measurement is not suitable.

Based on the above conditions, the measurement of carbon disulfide has always been a big problem. Finding high-precision and reliable measuring instruments has always been an urgent need for trading and production. Magnetostrictive level gauges are such an ideal meter.

3. Magnetostrictive liquid level meter principle and characteristics

The magnetostrictive level gauge is a new type of liquid level gauge that appeared only in the late 1980s and early 1990s. Because of its reliable performance and high precision, it caused a sensation in the measurement industry.

3.1 magnetostrictive level gauge working principle

The magnetostrictive level gauge is composed of a measuring tube, a float and an electronic transmitter. The outer tube of the measuring tube is a stainless steel protective tube, and the inner part is a magnetostrictive wire. The float rises or falls along the measuring surface of the measuring tube. The float is equipped with a permanent magnet and forms a magnetic field around it; the electronic transmitter periodically issues a query (current) pulse Also known as the starting pulse, this pulse propagates down the magnetostrictive line, and in the downward motion, a magnetic field is also generated around it. The two magnetic fields encounter a momentary moment of force and act on the magnetostrictive line to form a return pulse. The pulse propagates upward at a fixed speed and is received by the electronic transmitter. By measuring the time interval between the start pulse and the return pulse, the height of the measured liquid level can be accurately determined.

3.2 The characteristics of the Weiwei Technology Magnetostrictive Level Gauge

(1) Floating ball measurement principle This table still uses the floating ball principle as the measurement basis, so it can not only measure the liquid level but also can measure the interface. In addition to the float, there are no other mechanical movable parts, so the performance is reliable, and the service life is long. The measuring range of the hard rod type is 6.0m, and the flexible cable type is up to 25m. At the same time it can also measure the temperature of the medium.

(2), measurement accuracy High precision is the outstanding advantage of the table, its accuracy can reach 0.01%F·S. In the existing level gauges, only the measurement errors of the float strip, radar, and optical fiber level meters can reach millimeters, but in practical applications, the reliability and accuracy are comparable to the magnetostrictive level gauges. There is still a difference of 42. For example, the accuracy of an optical fiber level gauge is only 0.03%F·S. Therefore, the magnetostrictive level gauge is the best choice for volume measurement.

(3) Easy installation and calibration The magnetostrictive liquid level gauge is convenient for installation, commissioning and calibration. After the installation site and the installation parts are determined, the corresponding positions of the liquid level (or interface) zero point and full scale on the protective tube can be accurately calculated. The power is debugged before installation and the floats are respectively placed at the zero point and full scale position. Zero and full-scale output are 4mA and 20mA, respectively, without having to adjust and calibrate through liquid level (or interface) lifting, which greatly saves manpower and material resources, and brings great convenience to users. The level gauge is generally adjusted to zero and full scale by the user before leaving the factory. As long as the power check meter is normal, it can be installed or even dispense with debugging and calibration.

(4) Explosion-proof magnetostrictive level gauges are slightly more difficult to achieve intrinsically safe explosion than other gauges. Not all types of magnetostrictive level gauges can meet the requirements of explosion-proof scenes, and special attention should be paid when selecting them. Quite ialICT5 explosion-proof grade in China, the European Union CE certification mark is EEXialICT5. The North American CM and CSA certification corresponding level should be Intrinsic Safety Class 1 Division 1 Group A, this explosion-proof rating, only a few models of magnetostrictive level gauge can be achieved.

4. Application and effect

Shandong Weifang Hailong Co., Ltd. is a large-scale chemical fiber manufacturer with a large amount of carbon disulfide. Originally used magnetic flip-plate level gauges, but more errors, errors, and most of them are measured manually. Manual measurement takes time and effort, and the deviation is large, and it cannot adapt to the needs of modern industrial production. Therefore, under the arrangement of the Hailong Company's Metrology Division, relevant experts conducted on-site surveys and designs. Magnetostrictive level gauges were installed on two horizontal tanks of diameter 2000mm and 2600mm in the second section of the short-filing plant for raw material metering. . . And equipped with 4-digit display instrument, the accuracy is better than 0.1%, the maximum error ≤ 2mm. Since it was put into operation, it has been operating well. Operators have eliminated tedious labor. The carbon disulfide storage capacity can be monitored at any time in the instrument room. The measurement and management departments believe that the measurement data are accurate and reliable, and they are very satisfied. The carbon disulfide supplier also considers this measurement to be relatively original. More scientific and fair, recognized. Therefore, it is a consensus for us to measure carbon disulfide with the Coriolis Scientific Magnetostrictive Level Gauge. The application is successful. Making full use of the magnetostrictive level gauge's function for carbon disulfide metering can be further increased by one step. For example, with two floats, one measures the carbon disulfide interface and the other measures the level of water, that is, the total liquid level, which can make production safer. The magnetostrictive level gauge also has a temperature measurement function that can measure the temperature of the carbon disulfide medium and correct the density of the carbon disulfide to make the measurement more accurate.