From the working principle of JY-LDE electromagnetic flowmeter, it can be known that the fluid that can be used to measure the flow rate of electromagnetic flowmeter must be conductive. The electromagnetic flowmeter is used to measure the flow, because the non-conductive gas, steam, oil, acetone and other substances cannot be used to measure the flow.
JY-LDE electromagnetic flowmeter will cause inaccurate measurement due to various faults during operation. Generally, the faults generated by electromagnetic flowmeter during operation can be roughly divided into two categories. One is the failure caused by the failure of the flowmeter itself and the damage of components; the other is the failure caused by the change of external conditions, such as flow distortion, deposition and scaling caused by unreasonable installation.
1. There is a measurement failure due to the presence of air bubbles in the medium
The inhalation of gas from the outside of the medium or the transformation of dissolved gas in the medium into free bubbles are two ways to generate bubble-like gas in liquids. If there are large air bubbles in the medium, the whole electrode will be covered when the air bubbles pass through the electrode, which will make the flow signal input circuit open instantaneously, and the output signal will fluctuate. To determine the cause of this fluctuation, it can be done by disconnecting the loop current of the magnetic field. After disconnection, if the flowmeter still displays and is still in a state of fluctuation, it proves that the existence of air bubbles in the medium will cause the electromagnetic flowmeter to fluctuate. Using a pointer multimeter to measure the electrode resistance of the electromagnetic flowmeter, it will be found that the loop resistance of the electromagnetic flowmeter electrode is higher than the normal resistance value.
If the air enters the measured medium due to the installation position of the JY-LDE electromagnetic flowmeter, if the gas is stored due to the installation of the electromagnetic flowmeter at the high point of the piping system, or the flowmeter fluctuates due to the inhalation of air from the outside, it is necessary to replace the electromagnetic flowmeter. The installation position of the flowmeter is modified to be installed at the lowest point of the pipeline, or installed with a U-shaped pipe. However, in some cases, due to the large diameter of the electromagnetic flowmeter or the installation position is not easy to change, it can be solved by installing a gas collecting bag and an exhaust valve upstream of the flowmeter.
2. Electromagnetic flowmeter electrodes are corroded
Due to the contact with some highly corrosive media in the process of acetic acid production, when the electrode material of the electromagnetic flowmeter is not selected properly, the medium will corrode the electrode of the flowmeter, which will eventually lead to the failure of the sensor. As a result, the flow meter output fluctuates. Only when the electrode is corroded and the flowmeter fails, we can find that the electrode material is not corrosion-resistant, and the performance problem of this material itself cannot be identified before use. Therefore, the only way to solve this kind of failure is to replace the electrode with a new one. Therefore, the judgment and treatment of electrode corrosion faults are all methods of post-event maintenance treatment.
3. The medium is not full
Occasionally there is a phenomenon of non-full pipe in daily production. This phenomenon can be seen as a typical case of liquid containing air bubbles. When the electrode level is lower than the medium level, it is ideal to use straight pipe sections before and after the flowmeter, and the measurement data is relatively stable. However, the gas volume in the upper half of the pipe is also counted as the medium flow rate, so the measurement error is large in this case; when the electrode level is higher than the medium liquid level, the measurement loop of the JY-LDE electromagnetic flowmeter is in an open state, so The measured data is severely distorted. The following methods can be used to deal with the failure caused by the non-full pipe of this medium: try to install the electromagnetic flowmeter on the vertical pipeline that flows from the bottom to the top; in actual production, the electromagnetic flowmeter needs to be installed horizontally, in this case, it should be installed in the pipeline The lowest end of the electromagnetic flowmeter, and the electrode axis of the electromagnetic flowmeter is parallel to the horizon, (otherwise the sediment will cover the electrode in the low position); in order to avoid negative pressure in the measuring tube, the sensor of the flowmeter should be installed downstream of the pump, control Upstream of the valve; the mounting port of the flowmeter sensor should have some back pressure and should be away from the direct discharge port.
However, the most important thing is that the electromagnetic flowmeter should be installed to prevent the situation that the medium is not full.
4. The properties of the liquid to be tested cause measurement failures
If the conductivity of the measured medium decreases, the output impedance of the electrode will increase. At this time, the impedance of the converter input will cause a load effect, and the flowmeter will produce measurement errors. If this fault occurs in the electromagnetic flowmeter, only a low-conductivity electromagnetic flowmeter that meets the requirements, or a flowmeter with other principles such as an orifice flowmeter can be used.
5. Measurement failure caused by electrode scaling or electrode short circuit of JY-LDE electromagnetic flowmeter
When the liquid to be measured contains metal, the electrodes of the flowmeter are prone to short-circuit, and the measured value of the flowmeter is obviously small or tends to zero. This phenomenon does not often occur in daily production operations. When measuring high-viscosity medium, since the medium is easy to adhere and deposit on the tube wall, if the conductivity of the liquid to be measured is lower than the conductivity of the attached medium, the signal potential of the electrode will be shunted by the precipitation, so that it cannot work normally, and the phenomenon of electrode short circuit occurs. ; If the precipitated medium is a non-conductive layer, it will cause the open electrode flowmeter to not work properly. If the oxide rust layer is attached to the lining pipe wall, or the main component is metal sediment, its conductivity is greater than the liquid conductivity, and the actual flow value will be higher than the flow value measured by the flow meter; if the sediment is a scale layer such as calcium carbonate , the conductivity of the liquid to be measured is higher than that of the sediment, and the measured flow rate will be smaller than the actual flow rate.
In order to prevent the sediment in the fluid from affecting the operation of the flowmeter, the electrode of the flowmeter is selected to be a pointed or hemispherical shape that is not easy to adhere to, and can be replaced or cleaned. Optional electrodes can be used to manually scrape off the grime outside the sensor on a regular basis. Alternatively, the measurement circuit can be temporarily disconnected, and a short-time low-voltage high current can be passed between the electrodes to incinerate and remove the grease deposits. The method of increasing the liquid flow rate can also be used to clean the adhesion layer of the pipe wall.
6. Asymmetric flow of the medium to be measured
Under normal production conditions, the flow velocity of the fluid in the pipeline is axisymmetrically distributed, and the magnetic field is uniform. However, the non-axisymmetric flow velocity distribution of the fluid in the actual pipeline often occurs. At this time, the fluid flow direction can be divided into a straight flow along the axis of the pipeline, and the volume flow of the liquid to be measured is the integral of the cross section of the pipeline; the other is the vortex flow. . Since the appearance of the vortex flow affects the output of the sensor, the JY-LDE electromagnetic flowmeter will produce errors. In order to eliminate the influence of the vortex flow on the flowmeter sensor; the upstream of the flowmeter should have a long enough straight pipe section so that the flow rate of the fluid can be distributed in concentric circles; the inner diameter of the pipe near the flowmeter should be the same as the inner diameter of the flowmeter, so that the The flow velocity distribution is uniform; otherwise, a flow regulator can be installed to partially compensate for the shortage of the upstream straight pipe section.
7. External electromagnetic interference
In the production site, there are interference sources such as pipeline stray current, static electricity, electromagnetic waves and magnetic fields. 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 interference refers to the abnormal fluctuation of the output signal after the potential balance in the measuring tube of the flowmeter is destroyed by noise.
In order to reduce the interference of the external magnetic field to the flowmeter, we need to 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. The connection between the electromagnetic flow sensor and the pipeline can also be insulated.
8. Flowmeter lining deformation causes measurement fluctuations
The lining of JY-LDE electromagnetic flowmeter is generally made of fluoroplastic, so the lining of the flowmeter is very prone to deformation and measurement failure. There are two main reasons for the deformation of the lining: one is that the fluoroplastic lining penetrates into the steam to cause thermal diffusion. Usually, the lining material, thickness, temperature difference between inside and outside, the type of fluid and steam, pipeline pressure and many other factors determine the degree of penetration; Second, it depends on the process structure of the fluoroplastic lining material. Generally, polytetrafluoroethylene is used as the fluoroplastic lining material. The polytetrafluoroethylene material has no adhesive force and is only combined with the pipe wall by pressing, so the negative pressure pipeline does not use this material.
In order to prevent the deformation of the lining, we generally take the following measures: increase the thickness of the insulation between the flange and the coil box, reduce the temperature difference of the fluid and reduce the thermal diffusion, so that the temperature difference between the inside and outside of the lining can be improved to the greatest extent, which can reduce the permeability and slow down Measure the condensation of steam in the pipe wall; in addition, increase the thickness of the Teflon lining or replace it with another form of lining.
9. Failure caused by other reasons
a. Lightning strike. After the electromagnetic flowmeter is struck by lightning, it is easy to induce high voltage and current in the line, which damages the flow agent.
b. Changes in environmental conditions. Once the working environment conditions of the flowmeter change and new interference sources appear during operation, the normal operation of the meter will be disturbed, and the output signal of the flowmeter will fluctuate.
Case Electromagnetic flow sensor is insulated from connecting pipes to eliminate the influence of large stray currents
A water company in Henan Province installed two DN900 JYLDEDN900 electromagnetic flowmeters, one runs normally, and the other fluctuates up to 50% FS within a 1-2h cycle. The user believes that the operating conditions of the two instruments are similar, and the fault is caused by the instrument. When investigating the surrounding environment of the site, the upstream and downstream immediately following the flow sensor are two 0.5m long unlined short steel pipes with good grounding, which are then connected to the cement-lined steel pipes. Electrical connections such as grounding meet the requirements, and at the same time, the possibility of pulsation of the pipeline network is excluded.
The converter is about 10m away from the sensor. There is a three-phase transformer with hundreds of kVA installed nearby, about 2m and 8m from the converter and sensor, respectively.
There are two possible reasons for analyzing the failure:
1. Magnetic field interference generated by high-power transformers;
2. Stray current interference on the pipeline. To prove whether it is the influence of the magnetic field interference of the transformer, because the shutdown of the transformer involves a wide range, it is arranged as the second step of inspection. First, check whether it is the interference of the stray current of the pipeline. Measure the potential between the two poles with an oscilloscope without adding excitation current, and its value should be zero. However, the waveform distortion AC potential with peak Vpp as high as 1V was actually measured. Preliminary judgment even if it is well grounded, the instrument will still be affected by the interference of pipeline stray current.
The JY-LDE electromagnetic flowmeter sensor and two short steel pipes are electrically insulated from the pipe network, so that the flow sensor and the liquid are at the same potential. When the instrument is put into operation, the output display is stable and normal, and the influence of the magnetic field interference of the power transformer on the flow measurement is also excluded. 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 current, as a method to try to eliminate the influence of pipeline current interference.