Is the electromagnetic flow measurement error large? The problem is probably here

by:Sure     2020-06-16
Electromagnetic flowmeter is widely favored by petrochemical, chemical and other industries for its advantages of no pressure loss, high precision and moderate price, and plays an important role in flow measurement. However, in practical application, due to improper operation, unreasonable equipment selection and unscientific installation, measurement errors are difficult to avoid, causing trouble to users. Therefore, the majority of instrument people should pay attention to various factors that cause electromagnetic flowmeter errors. In general, the main effects of electromagnetic flowmeter errors can be divided into three categories: improper selection, influence of liquid to be tested and interference. Xisen electromagnetic flowmeter. Jpg improper selection 1. The flow rate of the liquid to be measured, the flow rate range measured by the electromagnetic flowmeter is generally 0. 5 ~ 10 m/s, economic flow rate range is 1. 5 ~ 3 m/s. In actual use, the inner diameter of the measuring tube should be determined according to the flow to be measured and the measurable flow rate range of the electromagnetic flowmeter. Second, the electrode and lining material selection electrode and lining material directly contact with the liquid to be tested, should be based on the characteristics of the liquid to be tested (Such as corrosive, abrasive, etc) And the selection of electrodes and lining materials at the working temperature. If not selected properly, problems such as fast adhesion speed, corrosion, scaling, wear and tear, lining deformation and the like will be caused, thus generating measurement errors. 3. Excitation Stability. The excitation modes of electromagnetic flowmeter include DC excitation, AC sine wave excitation and dual-frequency rectangular wave excitation, etc. DC excitation is easy to cause electrode polarization and DC interference problems, AC sinusoidal excitation is easy to cause zero change, while dual-frequency rectangular wave excitation not only has excellent zero stability of low-frequency rectangular wave excitation, but also has strong suppression ability of high-frequency rectangular wave excitation to fluid noise, it is an ideal excitation mode. In actual application, the stability of power supply voltage and frequency should be ensured as much as possible to ensure constant magnetic field strength and reduce measurement errors caused by changes in magnetic field strength. Four, mixed phase fluid measurement, using electromagnetic flowmeter to measure liquid-solid mixed phase fluid (Such as silt-containing water) If the electromagnetic flowmeter calibrated by single-phase liquid is selected, measurement error will occur. At this time, the sensor should be installed at the straight pipe section that does not cause liquid-solid phase separation. Xisen FLDC split electromagnetic flowmeter. Jpg the influence of the liquid to be tested 1. The conductivity of the liquid to be tested changes drastically. When the conductivity of the liquid to be tested is large, it will cause a large fluctuation in the displayed value. If the problem is very serious, then it is difficult for the control system to realize normal operation; However, when the conductivity of the liquid to be tested is too low, it is difficult for the electrode to realize normal output. If the conductivity of the liquid to be tested is within the range below the lower limit during operation, then the electromagnetic flowmeter is difficult to function normally. In view of these situations, first of all, it is necessary to select the type of electromagnetic flowmeter based on actual needs and in combination with relevant standards and requirements; Secondly, install the reactor or straight pipe section to ensure the full mixing of materials and promote the smooth realization of chemical reactions; Again, re-select the type of flowmeter. Second, the liquid bubble or non-full tube to be tested for bubbles, mainly comes from the development of dissolved gas in the liquid into free bubbles and bubbles inhaled by the outside world. The flow rate containing a large amount of bubble volume will affect the accuracy of measurement. If the bubble diameter is too large or even exceeds the value of the electrode diameter, an unstable state will occur during the measurement and display process, and fluctuation cannot be avoided. In view of this situation, firstly, the gas collector can be installed on the electromagnetic flowmeter, and the exhaust operation can be carried out according to the cycle at the same time; Secondly, reasonably replace the installation position; Thirdly, the vertical pipeline is installed on the electromagnetic flowmeter to ensure the bottom-up direction; Fourth, when installing the sensor, avoid being too close to the discharge port; Fifth, install the sensor at the control valve position, at its upstream position, or downstream of the pump. 3. If the conductivity of the liquid to be tested is too low and the conductivity of the liquid to be tested decreases, the output impedance of the electrode will be increased, and the load effect will be caused by the input impedance of the converter, resulting in measurement error. If the actual conductivity is lower than the lower limit value, then the instrument cannot work normally and the indication value will fluctuate. Countermeasures: select other low conductivity electromagnetic flowmeters that meet the requirements, such as capacitive electromagnetic flowmeters; Use other principle flow meters, such as orifice plates, etc. Fourth, the measured liquid presents an asymmetric state. During the measurement, the liquid to be measured is asymmetric, and there are mainly two flow combinations: one is a single vortex flow; The other is a straight line flow along the pipeline axis, and the volume flow of liquid is an integral of the pipeline section. In view of the shortage of the upstream straight pipe section, the flow regulator can be used for adjustment; Secondly, ensure that the inner diameter of the pipeline and the inner diameter of the flowmeter have the same value within a reasonable range of upstream and downstream; Thirdly, leave enough straight pipe sections for the upstream. Five, there is a layer in the measuring tube, electromagnetic flowmeter is often used to measure non-clean fluid. The non-clean fluid contains some substances such as sediment, which pollutes the electrode surface of the electromagnetic flowmeter or the pipeline, resulting in error in measurement results. In view of this situation, first of all, clean the electromagnetic flowmeter regularly; Secondly, reasonably increase the flow rate and control it to 4 m/s; Thirdly, the lining of poly (tetrachloroethylene) and other materials is applied. FLDC internal shrinkage electromagnetic flowmeter. Jpg 1. The cable between the space electromagnetic interference converter and the sensor is relatively long. Under strong electromagnetic environment, it is easy to be interfered, thus causing non-linearity of the measured value of the instrument, which is difficult to display normally. In view of this situation, first of all, shielding measures can be introduced to introduce cables separately in grounded steel pipes and use standard shielded cables; Secondly, reasonably shorten the cable length; Again, keep a long distance from the strong magnetic field. Second, the problem of connecting cables. The essence of electromagnetic flow application is to connect the converter and sensor with the help of specific cables to form a complete system, therefore, the cross-sectional area of conductors, capacitors, cable sites, etc. will have adverse effects. In view of this situation, first of all, it is necessary to ensure that the cable model meets the requirements, realize effective connection at the end, and prevent the occurrence of intermediate joint phenomenon; Secondly, the control length range is usually as short as possible. Third, grounding problem, because the output signal of the sensor is very small, usually only a few millivolts, in order to improve the anti-interference capability, the zero potential of the sensor must be grounded independently and reliably, and the grounding point of the sensor output signal should be electrically connected with the measured fluid. The grounding resistance of the sensor should be less than 10Ω. When the pipeline connecting the sensor is coated with insulating layer or non-metallic pipeline is adopted, grounding rings should be installed on both sides of the sensor and reliable grounding should be carried out to make the fluid grounded, the fluid potential is the same as the ground potential. Four, electrode and excitation coil symmetry point installation point vibration electromagnetic flowmeter excitation coil and electrode need to ensure symmetry, once asymmetric, deviation in the production process will be caused, measurement results are difficult to ensure accuracy. In addition, the installation site should reach a higher anti-vibration standard, otherwise the accuracy of the measured values cannot be guaranteed, and even the abnormal operation of the instrument can be induced.
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