This article is aimed at the analysis of the production process of a large-scale chemical enterprise producing 100,000 tons of chlor-alkali. The chemical plant has long used the tank position calculation method to measure the production of light and concentrated alkali, because the cross-section of the storage tank is large and the scales are divided The accuracy of the accuracy is poor, resulting in the accuracy of the calculated output is very unstable, but it cannot be calculated from time to time, and it is difficult to control the output of light and concentrated alkali. This problem has plagued the production department for a long time and has not been effectively resolved.When the company expanded its production capacity in 2000, in order to solve this problem and achieve the purpose of accurately measuring the output of light and concentrated alkali, two new electromagnetic flowmeters were installed.However, it was soon discovered that in the measurement process, occasional inaccurate measurements would occur.The company organizes technical personnel to investigate the reasons and draws conclusions. 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 both symptoms and root causes be solved. Fundamentally solve the problem of inaccurate instrument measurement.
For flow meters, it is difficult to verify and calibrate the products after they are put into production due to the limitation of use conditions, so careful installation and correct use are the prerequisites for accurate and reliable measurement.
This article is aimed at analyzing the measurement characteristics of the electromagnetic flowmeter instrument and the problems that arise in the production application, and finds out the causes of the problems. This method also has certain reference significance for other users.
Electromagnetic flowmeter Electromagnetic flowmeter
Electromagnetic flowmeter Electromagnetic flowmeter
1 Measuring principle of electromagnetic flowmeter
Electromagnetic flowmeters work on the basis 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, and the conductive fluid with an average flow velocity of v cuts the magnetic field line when it flows through the magnetic field area.Therefore, 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 velocity of the medium; D is the inner diameter of the instrument tube.
From the formula (1) we can see that B and D are constant, 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 volumetric flow rate formula for a fluid is:
Electromagnetic Flowmeter
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 correspondence functions, then U and Q are also proportional one-to-one correspondence functions, that is, the volume flow Q of the medium is also measured when the induced voltage U is measured.
2 Advantages of electromagnetic flowmeter
(1) There are no throttling or moving parts in the electromagnetic flowmeter tube, no pressure loss, and energy saving.Especially suitable for measuring liquid-solid two-phase media (such as suspension liquid).
(2) Only the lining and the electrode are in contact with the medium. As long as the two materials are selected properly, the purpose of anti-corrosion and anti-wear can be achieved. After long-term use, the accuracy of the instrument will not decrease. Compared with the oval gear flowmeter, its accuracy is 100%. Stability is much better.
(3) The measurement is only related to the average flow velocity of the medium, and has nothing to do with the change of the medium's flow state (laminar flow or turbulent flow), 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 of other conductive liquids or solid-liquid two-phase media without correction.
(4) The range ratio is wide, up to 1 100, and the range can be changed arbitrarily.
(5) No mechanical inertia, sensitive response, can measure instantaneous pulsating flow; and good linearity, can directly divide the scale.
(6) From the detection electrode to the front, only a 5D straight pipe section is required, which does not require too much installation space.However, other flowmeters generally require a straight pipe section of 10 D or more, and have strict requirements on the 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.But it is very important that the sensor must be completely filled with medium.The axis of the detection electrode should be in a horizontal position to prevent sticking and dirt, and to exclude losses caused by air bubble contact.
(2) There should be no strong interference electromagnetic field around the sensor.
(3) The installation of the sensor, its seal, and the grounding ring must be concentric with the pipe and cannot protrude inward.
(4) The measurement accuracy has a great relationship with the grounding effect of the sensor, and the electromagnetic flowmeter is very sensitive to the interference of the applied potential.Therefore its ground electrode must be an independent ground electrode.The purpose of grounding is to equipotentialize the conductive fluid flowing through the sensor with ground, not the pipe wall.
4 Selection, installation and use of electromagnetic flowmeter
4.1 Selection principle of electromagnetic flowmeter
(1) Considering the caliber and range, although the range of the electromagnetic flowmeter is set arbitrarily, the setting range is limited by the caliber.The setting of the range should consider that the normal flow exceeds half of the full range, so that the measurement accuracy is high.The flow rate is generally 2-4 m/s. If the medium is easy to wear the electrodes, a slightly lower flow rate can be selected; if the medium is easy to adhere, an appropriate higher flow rate can be selected.After comprehensive consideration, select the caliber of the meter according to the flow rate meter.
(2) Consider the pressure, temperature and corrosiveness of the medium, and choose different lining and electrode materials accordingly.
4.2 The specific selection and installation of our company
Since the measured medium is light alkali and concentrated alkali with a temperature lower than 100°C, the corrosion to stainless steel electrodes is very weak, but it has a corrosive effect on various rubber linings, so we choose polytetrafluoroethylene lining.Considering the expansion of production in the future, we choose the diameter as 150mm.The specification and model of the electromagnetic flowmeter selected by our company is: K300-(150)11431021, the diameter is 150 mm, standard type, molybdenum dititanium electrode, polytetrafluoroethylene lining, maximum working pressure 1.6 MPa, no Grounding ring, output 4 ~ 20 mA, accuracy class 1.0.Installed on a horizontal pipeline, there is a long enough straight pipe section before and after, the grounding electrode is independent, and the grounding is good.
There are 2 pumps used to transport light alkali, both of which have a measuring range of 100 m3/h and a head of 50 m, and use a single pump to transport light alkali intermittently.The range of the flowmeter is set to 160 m3/h.From the flow rate table, the flow rate when the diameter is 150mm and the average flow velocity v=1 m/s is 63.617 m3/h.
Electromagnetic Flowmeter
m/s, slightly lower.If the flow meter range is changed to 250 m3/h, and the normal flow rate is continuously delivered at 200 m3/h when the two pumps are turned on at the same time, the effect may be better.However, due to production constraints, it can only be delivered intermittently.
4.3 Take the light alkali flowmeter as an example to illustrate the use of the flowmeter
When the flowmeter was first put into use, compared with the calculated number of the slot position, the difference between the metered output per shift and the estimated output was only 1 t, and the error was small.In the next few months, a large deviation was found.Table 1 is a comparison between metered output (folded *) and estimated output (folded *) for the first half of the year.
Electromagnetic Flowmeter
It can be seen from Table 1 that in January when the flow meter was installed, the difference between the meter and the calculation is not large, and the difference is only 101.1 t for the whole month.Here, the estimated number is taken as the standard. In fact, the error of the estimated number is relatively large. It is hard 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 searched for the cause from the installation and parameter setting of the flowmeter, and found no problem.So we found the original records for careful analysis, and finally found out the reason for the deviation.It turned out that in February, due to the expansion of caustic soda production, a new 200 m3/h light alkali pump was installed, which was sorted as 1# pump.From the shift record of the electrolysis section in February:
From 8 to 16 o'clock on February 1: 3# pump is changed to 2#;
From 8 to 16:00 on February 4: 2# pump changed to 1#;
From 8 to 16:00 on February 7: 1# pump changed to 2#;
From 8 to 16:00 on February 9: 2# pump changed to 3#;
The 1# pump is 200 m3/h, and the 2# and 3# pumps are 100 m3/h.
Find out from the operation log of the evaporation section:
February 3: The metered light alkali production was 876 m3 throughout the day;
February 4: The metered light alkali production was 2 298 m3 throughout the day;
February 5: The metered light alkali production was 3 305 m3 throughout the day;
February 6: The metered light alkali production was 3 028 m3 throughout the day;
February 7: The metered light alkali production was 1 695 m3 throughout the day;
February 8: The metered light alkali production was 962 m3 throughout the day.
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, which was nearly 4 times the normal amount.On February 7th, the flow rate returned to normal on the 8th after the day shift switched to 2# pump, that is, the 100 m3/h pump.This shows that the over-range of the meter is the cause of the large error, and the meter can no longer be used at this time.Since the craftsman did not understand the instrument, the instrument staff was not notified in time to change the measuring range of the instrument when the large pump was added.It was not until mid-March that the instrument personnel found out the reason, and changed the range to 320 m3/h by doubling method, which met the requirements of the large pump, while for the small pump, the single-pump liquid volume was only 30% of the full range. , It also caused the meter measurement to be low when the small pump was turned on, thus causing a negative deviation from April to June.Until July, the 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 follow-up analysis of metered output and estimated output in the second half of the year, we believe that relatively good results have been achieved.The deviation between the two numbers per month is within 100 t, which is generally due to the high output of the meter.The analysis reason may be that the flowmeter is installed on the horizontal pipeline, but because 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, there may be dissatisfaction with the pipe, resulting in high metering.With the further expansion of production, when the pump can be continuously opened to transport alkali, the measurement accuracy will be higher.
5 Conclusion
From the above analysis, it can be seen that even the electromagnetic flowmeter with high accuracy must be used correctly.The inaccuracy of the flowmeter is not necessarily the cause of the meter itself, but has a lot to do with the use process, or even a decisive factor.When selecting, using, and maintaining instruments, instrument maintenance personnel should not only consider the instrument itself, but must fully consider whether the performance of the process and equipment meets the requirements for normal use of the instrument.Only in this way can each instrument play its due function and provide accurate and reliable measurement data for production and operation.
