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Introduction to common problems of electromagnetic flowmeter in coal chemical production

As the flowmeter widely used in the current process industry, electromagnetic flowmeters are classified into five categories according to their faults and causes in the application process. The following is a brief description of the causes, detection and discrimination methods of these common faults of electromagnetic flowmeters and the treatment of faults. .

1 The phenomenon of bubbles and non-full tubes in the liquid
The presence of bubbles in the liquid causes measurement inaccuracies or fluctuations in measured values (output fluctuations). The formation of bubble-like gas in the liquid has two ways of inhaling from the outside and converting the dissolved gas (air) into a free bubble in the liquid. If the liquid contains large bubbles, the entire electrode can be covered when the electrode is wiped, and the flow signal input circuit is instantaneously opened, causing the output signal to fluctuate. The phenomenon of non-full pipe can be seen as an extreme case of bubbles in a liquid. The liquid not filled pipe can be divided into two cases where the liquid level is higher than the measuring electrode level or lower than the horizontal plane. When the liquid level in the tube is higher than the horizontal level of the electrode, if the straight pipe section of the pipe system is ideal, the measurement of the electromagnetic flowmeter can be mostly stable, but the volume of the liquid metered by the flowmeter contains the volume of gas in the pipe, so this measurement exists. A lot of measurement error. When the liquid level in the tube is lower than the surface of the electrode, the electrode is exposed to the air at this time, and the measuring circuit is actually in an open state. The measured value and output of the electromagnetic flowmeter are in a random state, constantly fluctuating or full. This is also when the non-working state, that is, the pipeline has no liquid medium, the electromagnetic flowmeter indicates a random value, and the process personnel sometimes think that the flowmeter is faulty.
The simplest method of discriminating is to cut off the excitation loop current of the magnetic field when encountering fluctuations. If the meter still displays and is unstable at this time, it is mostly caused by the influence of bubbles or non-filled tubes. If the electrode resistance is measured with a multimeter at this time, the loop resistance of the electrode can be measured to be higher than normal; most of the phenomena causing this phenomenon are still in the installation, and the proper installation of the electromagnetic flow meter directly affects the measurement effect. At the same time, if the air contained in the measured medium is judged to be caused by the installation position, if the flowmeter is fluctuated due to the electromagnetic flowmeter being installed at the high point of the pipeline or the outside air is sucked, the replacement of the installation position is the most thorough. The solution is to install at the lowest point of the pipeline or with a U-tube. However, in many applications, the diameter is large or the installation position is not easy to change. It is recommended to install the air bag and the exhaust valve upstream of the flow meter. After the exhaust device is installed, the measurement returns to normal. Try to avoid non-full pipe when installing the flowmeter, as mentioned above at the lowest end of the pipeline or intentionally install the flowmeter on the U-pipe and vertical pipe. Typical failure: coal gasification 33 unit P-3301AB outlet flow, due to installation on the horizontal pipe of the pump outlet, the process water is not filled with electromagnetic flowmeter, causing large fluctuations in indication, that is, the measured value and output are in a random state. Continuous fluctuations or full scale; the process can not be referenced, the electromagnetic flowmeter is modified to the vertical pipeline or the appropriate horizontal pipeline, in order to completely solve the problem; there are 33FT-0002, 36FT-0001, 36FT-0002 Because of the improper installation position, the indication fluctuation is large.
2 electrode corrosion
After the elimination of the bubble factor, there is a situation in which the measured value is shaken due to corrosion of the electrode, and both ends up with the sensor failure.
Due to improper selection of the electrode material, the electrode is corroded by the liquid to be measured, which causes the flowmeter output to fluctuate. Once the electrode is corroded, the result is a solution to this, only the replacement of the new electrode.
3 electrode scaling and electrode short circuit
The determination of the short circuit of the electrode is relatively simple. If the measured medium contains a metal substance, the short circuit of the electrode is easier to diagnose, and the measured value is obviously small or tends to zero. But this phenomenon is rare in daily operations. Because electromagnetic flowmeters are often used in metering environments such as raw water and sewage, the incidence of electrode fouling is high. When the electrode is fouled, the signal is gradually reduced until the insulation makes the signal loop open, and the flow signal is isolated.
When the viscosity of the measured medium is high, it is easy to adhere and precipitate on the tube wall. If the attached medium is a conductive substance with higher conductivity than the liquid to be measured, the signal potential is shunted and cannot work, that is, the electrode is short-circuited, if it is non-conductive. The layer, which is what we call the electrode scaling, makes the electrode open and does not work. It is a relatively thorough method to remove the cleaning electromagnetic flowmeter in combination with the process conditions.
Typical case: coal gasification 14 unit, slag pool circulating water pump P-1402AB outlet to V-1402 slag pool water electromagnetic flowmeter 14FT-0003, because the slag pool water contains fine slag and dirt, plus horizontal installation, it is easier to cause lining The electrode is fouled, causing the flow meter to gradually decrease. The solution is to renovate the pipeline and increase the bypass to facilitate the delivery of the electromagnetic flowmeter during production.
4 lining deformation
Inaccurate measurement or damage to the electromagnetic flowmeter. Lining deformation, mostly occurs in the lining of fluoroplastics, fluoroplastic lining, especially the structure of polytetrafluoroethylene (PTFE) lining itself, because the PTFE and the pipe wall are only pressed and have no cohesive force, so it cannot For negative pressure piping. Polytetrafluoroethylene (PTFE) practical temperature can not be greater than 150 °, therefore, polytetrafluoroethylene (PTFE) lined electromagnetic flowmeter can not be used in high temperature applications, if the negative pressure is formed at high temperature, it will cause deformation of the liner.

Electromagnetic flowmeter after deformation of lining
Typical case: In the Shell coal chemical plant, the pump P-1601 hits the electromagnetic flowmeter 16FT-0012 of J-1602. During the production work, the indication gradually becomes smaller, the parking is disassembled and inspected, and the PTFE lining deformation is detected. Bubbling inside, as shown in Figure 3: After analysis, the temperature condition of the pipeline medium is above 150°. Because the electromagnetic flowmeter is PTFE lining, the use temperature should not exceed 150°, and the process stops the pump P-1601, and the discharge is not opened. The installation position of the electromagnetic flowmeter is high, and the temperature is cooled and condensed to form a negative pressure, which causes deformation of the lining.

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