According to the working principle of the electromagnetic flowmeter, the fluid that can select the electromagnetic flowmeter to measure the flow rate must be conductive. Strictly speaking, except for the high-temperature fluid, only any fluid with a conductivity greater than 20us/cm can use the corresponding electromagnetic flow. Measure the flow rate, because non-conductive gases, steam, oils, acetone and other substances cannot use electromagnetic flowmeters to measure flow.

Electromagnetic flowmeters may cause inaccurate measurement due to various faults during operation. In general, the electromagnetic flowmeters generated during operation may be classified into two types. One is the failure of the flowmeter itself, which is caused by damage to the components; the other is a failure caused by a change in the external conditions, such as an irrational installation that causes flow distortion, deposition, and scaling.
1. The presence of bubbles in the medium. Measurement failure Media inhaling gas from the outside or dissolving gas from the medium into free-form bubbles are two ways to produce bubble-like gas in the liquid. If there is a large air bubble in the medium, the entire electrode is covered when the air bubble passes through the electrode, and the flow signal input circuit is opened instantaneously, so that the output signal will fluctuate. Judging the cause of such fluctuations can be done by disconnecting the loop current of the magnetic field. If the flow meter still shows after the disconnection and is still in a fluctuating state, it is proved that the presence of air bubbles in the medium may cause the electromagnetic flow meter to fluctuate. Using an analog multimeter to measure the resistance of the electromagnetic flowmeter electrode, you will find that the loop resistance of the electromagnetic flowmeter electrode is higher than the normal resistance value.
If the electromagnetic flowmeter installation position causes air to enter the measured medium, if the electromagnetic flowmeter is installed at the high point of the piping system and the gas is stored, or the flowmeter fluctuation due to the external air intake, the electromagnetic flowmeter needs to be replaced. The location modification is installed at the lowest point of the pipeline or U-tube installation. However, in some cases, since the size of the electromagnetic flowmeter is large or the installation position is not easily changed, a gas-collection bag and an exhaust valve may be installed upstream of the flowmeter to solve this problem.
2. The electromagnetic flowmeter electrode is corroded. During the production of acetic acid, some highly corrosive media may be exposed. When the electrode material of the electromagnetic flowmeter is selected improperly, the medium will corrode the flowmeter electrode and eventually cause the sensor to fail. So there will be fluctuations in the flowmeter output. Only when the flowmeter malfunctions after the electrode has been corroded can we find that the electrode material is not resistant to corrosion, and the performance of the material itself cannot be discerned before it is used. Therefore, only replace the new electrode to solve this problem. Therefore, the electrode corrosion failure judgment processing belongs to the method of post-maintenance processing.
3. Occasionally there will be non-full tube phenomenon in the daily production of non-full tubes. This phenomenon can be seen as a typical case of bubbles in liquids. When the electrode level is lower than the liquid level of the medium, it is ideal to use a straight pipe section before and after the flowmeter, and the measurement data is relatively stable. However, the gas volume in the upper half of the tube is also counted as the medium flow, so the measurement error is large in this case; when the electrode level is higher than the medium level, the measurement loop of the electromagnetic flowmeter is in an open state, and the measured data Seriously distorted. Dealing with the failure of this medium's non-full pipe can have the following methods: Install an electromagnetic flowmeter on a vertical pipe that flows from bottom to top; in the actual production, the electromagnetic flowmeter needs to be installed horizontally. In this case, it should be installed on the pipe. The lowest end of the electromagnetic flowmeter and the axis of the electrode is parallel to the horizon (otherwise the sediment will cover the lower electrode); in order to avoid negative pressure in the measurement tube, the sensor of the flowmeter should be installed downstream of the pump and controlled. Upstream of the valve; the installation port of the flowmeter sensor should have a certain back pressure and should be kept away from the direct discharge port.
However, the most important thing is that when the electromagnetic flowmeter is installed, it is better to prohibit the occurrence of the medium non-full pipe.
4. The nature of the liquid to be measured leads to measurement failure. If the conductivity of the measured medium is reduced, the output impedance of the electrode will increase. At this time, the input impedance of the converter will cause a load effect, and the flowmeter will produce a measurement error. If the electromagnetic flowmeter appears this fault, then only select the low-conductivity electromagnetic flowmeter that meets the requirements, or use an orifice flowmeter or other principle flowmeter.
5. Measurement failure caused by electrode fouling or electrode short-circuiting of the flowmeter When the measured liquid contains metal, the flowmeter electrodes are prone to short-circuiting, and the flowmeter's measured value is significantly smaller or tends to zero. This phenomenon does not happen often in daily production operations. When measuring high-viscosity media, because the media is easy to adhere and deposit on the wall of the pipe, if the conductivity of the measured liquid is lower than the conductivity of the attached media, the signal potential of the electrode will be precipitated and shunted and will not work properly, resulting in an electrode short circuit phenomenon. If the deposited medium is a non-conductive layer, it will cause the electrode open flow meter to not work properly. If the oxidized iron rust layer adheres to the lining tube wall, or the main component is a metal precipitate, its conductivity is greater than the liquid conductivity, and the actual flow rate value is higher than the flow rate value measured by the flow meter; if the precipitate is a scale layer such as calcium carbonate The measured conductivity of the liquid is higher than the conductivity of the precipitate. As a result, the measured flow value will be less than the actual flow rate.
In order to prevent the sediment in the fluid from affecting the operation of the flowmeter, the electrodes of the flowmeter are selected from those that are not easily attached to the protruding tip or hemisphere, and can be replaced or cleaned. The optional electrode can manually scrape the dirt outside the sensor on a regular basis. Alternatively, the measuring circuit may be temporarily disconnected and a small amount of low-voltage and large current may be used to indirectly remove grease deposits between the electrodes. It can also be used to increase the liquid flow rate to clean the wall of the adhesive layer.
6. Asymmetric flow of the medium to be measured Under normal production conditions, the flow velocity of the fluid in the pipeline is axisymmetric and the magnetic field is uniform. The non-axisymmetric flow velocity distribution of the fluid in the actual pipeline often occurs. At this time, the fluid flow can be divided into a straight flow along the axis of the pipeline. The volumetric flow of the liquid to be measured is its integral to the cross section of the pipeline; the other is the vortex flow. . As the eddy currents affect the output of the sensor, the flowmeter produces errors. In order to eliminate the influence of vortex flow on the flowmeter sensor, the upstream of the flowmeter should have a straight section long enough to distribute the fluid flow concentrically; the inner diameter of the pipe near the flowmeter should be the same as the inner diameter of the flowmeter. The flow velocity is evenly distributed; otherwise flow regulators can be installed to partially compensate for the lack of upstream straight sections.
7. External Electromagnetic Interference There are stray currents, static electricity, electromagnetic waves, and magnetic fields in the production site. The flow signal of the electromagnetic flowmeter is very small, and it is very easy to be interfered by external electromagnetic interference, which affects the normal operation of the electromagnetic flowmeter. The so-called electric field disturbance refers to an abnormal fluctuation of the output signal after the potential balance in the flowmeter measuring tube is destroyed by noise.
In order to reduce the interference of the external magnetic field on the flowmeter, we must install the electromagnetic flowmeter sensor away from the strong magnetic field source. In addition, enhanced shielding measures are taken to prevent interference from strong electric fields. It is also possible to insulate the connection between the electromagnetic flow sensor and the pipe.
8, the flowmeter lining deformation caused by the measurement of fluctuations Flowmeters are generally fluorinated plastic lining, so that the lining of the flowmeter is very easy to deform, the phenomenon of metering failure. There are two main reasons for the deformation of the lining: thermal diffusion of the fluoroplastic lining into the steam, usually the lining material, thickness, temperature difference inside and outside, the type of fluid and steam, pipeline pressure and many other factors determine the degree of penetration; Depending on the process structure of the fluoroplastic lining material, PTFE is generally used as the fluoroplastic lining material. The non-adhesive force of the PTFE material is only combined with the wall of the pressure, so the negative pressure pipeline does not use this. Kinds of materials.
In order to prevent lining deformation, we generally take the following measures: increase the thickness of the insulation between the flange and the coil box, reduce the fluid temperature difference and reduce the thermal diffusion, so that the temperature difference between the inside and outside of the lining is improved to the maximum extent, so that the permeability can be reduced and the measurement can be slowed down. Condensation of steam in the pipe wall; in addition, the thickness of the PTFE lining is thickened or another type of lining is replaced.
9, other causes of failure
1) Lightning strikes. Electromagnetic flowmeters are susceptible to high voltages and currents in the line after being struck by lightning, damaging the flow agent.
2) Changes in environmental conditions. Once the environmental conditions of the flowmeter change and new interference sources occur during operation, the normal operation of the instrument will be disturbed and the output signal of the flowmeter will fluctuate.

Case The electromagnetic flow sensor is insulated from the connecting pipe to eliminate the effect of large stray currents [2]

A water supply company in Zhejiang Province installed two DN900MT900 electromagnetic flowmeters, one of which was operating normally, and the other one had up to 50% FS fluctuations in a period of 1 to 2 hours. The user believes that the conditions of use of the two instruments are similar and that the fault is caused by the instrument. The environment surrounding the site was surveyed. Upstream and downstream of the flow sensor were two unlined short steel pipes with a length of 0.5 m that were well grounded, and then connected to steel pipes with cement lining. Grounding and other electrical connections are in line with requirements, while eliminating the possibility of pipe network flow artery.

The distance between the transducer and the sensor is about 10m. A three-hundred kilovolt three-phase transformer is installed nearby, about 2m and 8m away from the converter and the sensor.

There are two possibilities for analyzing the cause of a fault: (1) magnetic field interference from a high-power transformer; and (2) stray current disturbances on the pipeline. To prove whether it is the influence of the transformer magnetic field interference, due to the fact that the transformer needs to be turned off, it is arranged for the second-step inspection. First, it is checked whether the pipeline is stray current. Without excitation current, use an oscilloscope to measure the potential between the two poles. The value should be zero. However, the waveform distortion AC potential with a peak Vpp as high as 1V was actually measured. The initial judgment is that even if the ground is well grounded, the instrument will be affected by stray currents in the pipeline.

Take the electromagnetic flow sensor with two short steel pipes and pipe network electrical insulation, so that the flow sensor and liquid with the same potential. When the instrument is put into operation, the output display is stable and normal. It also excludes the influence of the magnetic field disturbance of the power transformer on the flow measurement. At the same time, the measured interference current is 60mA AC, and the current direction comes from the upstream of the flow sensor.

This measure is also applicable to pipelines with cathodic protection currents as a method to exclude the effects of current disturbances in the pipeline.

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