Measurement cases of electromagnetic flow meter for light concentrated alkali

This article is an analysis of the production process of a large chemical enterprise that produces 100,000 tons of chlor-alkali. This chemical factory has long used the slot calculation method to measure the output of light and concentrated alkali because the cross-section of the storage tank is large. , the scale graduation accuracy is poor, resulting in the accuracy of the calculated output being very unstable, but it cannot be calculated at all times, and it is difficult to control the output of light and concentrated alkali. This problem has troubled the production department for a long time, and has not been effectively solved. solve. When the company was expanding production capacity in 2000, in order to solve this problem and achieve the purpose of measuring the output of light and concentrated alkali, two new electromagnetic flow meters were installed. However, it was quickly discovered that during the measurement process, measurement inaccuracies occasionally occurred. The company organized technical personnel to explore the reasons and came to the conclusion that many problems are not only caused by the instrument itself, but also many external working conditions are also important reasons for inaccurate measurement. Only in this way can we solve both the symptoms and the root causes. Fundamentally solve the problem of inaccurate meter measurements. For flow meters, it is difficult to verify and calibrate the product due to limitations of usage conditions after it is put into production.

Therefore, careful installation and correct use are prerequisites to ensure accurate and reliable measurement. This article analyzes the measurement characteristics of electromagnetic flowmeter instruments and problems that arise in production applications to find out the causes of problems. This method also has certain reference significance for other users. 1. Measuring principle of electromagnetic flowmeter. Electromagnetic flowmeter works based on the principle of Faraday’s law of electromagnetic induction. The measured medium should be a conductive liquid. In an interval perpendicular to the flow direction of the medium, there is a stable magnetic field with a magnetic induction intensity B generated by the transmitter. When the conductive fluid with an average flow rate v flows through the magnetic field area, it moves to cut the magnetic lines of force. As a result, an induced voltage U is generated on the two detection electrodes parallel to the cross-section of the pipeline and perpendicular to the direction of the magnetic field lines. According to the law of electromagnetic induction: U= k B v D (1) In formula (1): U is the induced voltage; k is the instrument constant; B is the magnetic induction intensity; v is the average flow rate of the medium; D is the inner diameter of the instrument tube. It can be seen from formula (1) that B and D are unchanged, and k is a constant. Therefore, U is only proportional to v. As long as the magnitude of the induced voltage U is measured, the average flow velocity v of the medium can be measured. The volume flow formula of fluid is: where: Q is the volume flow rate of the medium. According to formula (2), it can be seen that Q and v are proportional one-to-one corresponding functions, then U and Q are also proportional to the one-to-one corresponding functions, that is, the volume flow rate Q of the medium is measured when the induced voltage U is measured. 2 Advantages of electromagnetic flowmeter (1) There are no throttling or movable parts in the electromagnetic flowmeter tube, no pressure loss, and energy saving. Especially suitable for measuring liquid-solid two-phase media (such as suspended liquid). (2) Only the lining and the electrode are in contact with the medium. As long as the two materials are selected, the purpose of anti-corrosion and anti-wear can be achieved. The accuracy of the meter will not be reduced after long-term use. Compared with the oval gear flowmeter, its accuracy is better. The stability is much better. (3) The measurement is only related to the average flow velocity of the medium, but not to the flow state of the medium (laminar flow or turbulent flow), has nothing to do with changes in temperature, viscosity, density, pressure and conductivity (within a certain range). Therefore, after the electromagnetic flowmeter is calibrated with water, it can be used to measure the flow rate of other conductive liquids or solid-liquid two-phase media without correction. (4) The range ratio is wide, up to 1100, and the range can be changed arbitrarily. (5) It has no mechanical inertia, is sensitive in response, and can measure instantaneous pulsating flow; it has good linearity and can be directly divided into equal parts. (6) From the detection electrode forward, only a 5 D straight pipe section is required, and no large installation space is required.

Other flowmeters generally require a straight pipe section of 10 D or more, which requires strict installation space, otherwise the measurement accuracy cannot be guaranteed. 3 Installation of electromagnetic flowmeter (1) Theoretically, the sensor can be installed anywhere in the running pipeline. However, it is required that the sensor must be completely filled with media, which is very important. The axis of the detection electrode should be in a horizontal position to prevent dirt sticking and eliminate losses caused by bubble contact. (2) There should be no strong interfering electromagnetic fields around the sensor. (3) The sensor, its seal, and the grounding ring must be installed concentrically with the pipe and cannot protrude inward. (4) The measurement accuracy has a great relationship with the grounding effect of the sensor. The electromagnetic flowmeter is very sensitive to the interference of external electric potential. Therefore, its ground electrode must be an independent ground electrode. The purpose of grounding is to allow the conductive fluid flowing through the sensor to be at the same potential as the ground, not to ground the pipe wall. 4 Selection, installation and use of electromagnetic flowmeter 4.1 Principles of selection of electromagnetic flowmeter (1) Consider the caliber and range. Although the range of the electromagnetic flowmeter can be set arbitrarily, its setting range is limited by the caliber. . The setting of the measuring range should consider that the normal flow rate exceeds half of the full scale, so that the measurement accuracy is high. The flow rate is generally selected from 2 to 4 m/s. If the medium is easy to wear the electrode, a slightly lower flow rate can be selected; if the medium is easy to adhere, an appropriately higher flow rate can be selected. After comprehensive consideration, select the caliber of the instrument according to the flow rate table. (2) Consider the pressure, temperature and corrosiveness of the medium, and select different lining and electrode materials accordingly. 4.2 Our company’s specific selection and installation. Since the measured medium is light alkali and concentrated alkali with a temperature below 100, the corrosion on stainless steel electrodes is very weak, but it has a corrosive effect on various rubber linings, so we choose polytetrafluoroethylene Vinyl lining. Considering future expansion of production, we chose the caliber to be 150 mm. The specific specifications and models of electromagnetic flowmeters selected by our company are: K, caliber 150 mm, standard type, molybdenum titanium electrode, PTFE lining, high working pressure 1.6 MPa, no grounding ring, output 4~20 mA , accuracy level 1.0. Installed on horizontal pipes, there are sufficiently long straight pipe sections in front and behind, the grounding pole is independent and the grounding is good. There are two pumps used to transport light alkali, with a range of 100 m3/h and a lift of 50 m. A single pump is used to transport light alkali in an intermittent manner. The measuring range of the flow meter is set to 160 m3/h. From the flow rate table, the flow rate when the outlet diameter is 150mm and the average flow velocity v= 1 m/s is 63.617 m3/h. m/s, slightly lower. If the flow meterThe measuring range is changed to 250 m3/h. When the two pumps are turned on at the same time, the normal flow is continuously transported at 200 m3/h, and the effect may be better. However, due to production limitations, it can only be transported intermittently. 4.3 Take the light alkali flowmeter as an example to illustrate the use of the flowmeter. When the flowmeter is first put into use, compared with the calculated number of slots, the difference between the metered output and the estimated output per shift is only 1 t, and the error is small. However, in the following months, larger deviations were discovered. Table 1 is a comparison of the calculated production (discounted at 100%) and the estimated production (discounted at 100%) for half a year. As can be seen from Table 1, in January when the installation of flow meters began, the difference between the meter meter and the calculated one was only 101.1 t for the whole month. Here, the estimated number is used as the standard. In fact, the error in the estimated number is relatively large. It is difficult to say what caused the difference of 101 t. However, the difference in February was astonishingly large, reaching 1 133.7 t, and it became a negative deviation again from April to June. We first looked for the cause from the installation and parameter setting of the flow meter, and found no problem. So we found the original records and conducted careful analysis, and finally found the cause of the deviation. It turns out that in February, due to the expansion of caustic soda production, a new 200 m3/h light alkali pump was installed, ranked as pump #1. From the shift handover record of the electrolysis section in February, we found: February 1, 8-16:00: 3# pump was changed to 2#; February 4, 8-16:00: 2# pump was changed to 1#; February 7 From 8:00 to 16:00 on February 9: 1# pump is changed to 2#; from 8:00 to 16:00 on February 9: 2# pump is changed to 3#; while 1# pump is 200 m3/h, 2# and 3# pumps are 100 m3 /h. From the operation log of the evaporation section, it was found that: February 3: All-day meter light alkali output was 876 m3; February 4: All-day meter meter light alkali output was 2,298 m3; February 5: All-day meter meter light alkali output was 2,298 m3 The output of light alkali is 3 305 m3; February 6: The output of light alkali is 3 028 m3 throughout the day; February 7: The output of light alkali is 1 695 m3 throughout the day; February 8: The output of light alkali is 1 695 m3 throughout the day Yield 962 m3. Comparing the two records, it can be seen that the 200 m3/h large pump was put into operation from February 4 to 7, and the light alkali flow rate was very large, nearly 4 times the normal amount. On February 7, the day shift switched to pump #2, a 100 m3/h pump, and the flow rate returned to normal on the 8th. This shows that the over-range of the instrument is the cause of the large error, and the instrument can no longer be used at this time. Because the process personnel did not understand the instrument well, they did not promptly notify the instrument personnel to change the instrument range when adding a large pump. It was not until mid-March that the instrument personnel found out the cause and used the doubling method to change the measuring range to 320 m3/h, which met the requirements of the large pump. However, for the small pump, the liquid volume of a single pump was only 30% of the full scale. , and also caused the meter measurement to be low when the small pump was turned on, thus causing a negative deviation from April to June. It was not until July that the measuring range was changed to 250 m3/h, and it was stipulated that only one pump should be turned on when the large pump was turned on, and two pumps should be turned on at the same time when the small pump was turned on. Through the tracking and analysis of the metered production and estimated production in the second half of the year, we believe that we have achieved relatively good results. The difference between the two figures per month is 1Within 00 t, the meter output is generally high. The reason may be that the flow meter is installed on a horizontal pipeline, but since the pipeline is hundreds of meters long, the installation location is not necessarily the lower point of the pipeline. When the pump is started and stopped, the pipe may not be full, resulting in high measurement. With the further expansion of output, when the pump can be continuously turned on to deliver alkali, the metering accuracy will be higher. 5 Conclusion From the above analysis, it can be seen that even the most accurate electromagnetic flowmeter must be used correctly. The inaccuracy of the flow meter is not necessarily the cause of the meter itself, but has a lot to do with the use process, and even is the determining factor. When selecting, using, and maintaining instruments, instrument maintenance personnel should not only consider the instrument itself, but should comprehensively consider whether the performance of processes, equipment, etc. meets the requirements for normal use of the instrument. Only in this way can each instrument exert its due function and provide accurate and reliable measurement data for production and operation.