Electromagnetic flowmeters are widely used in petrochemical, chemical, sewage treatment, water conservancy, farmland irrigation, beer, milk, drinking water measurement and other industries because of their advantages of no pressure loss, high precision, and moderate price. They play an important role in flow measurement. . However, in practical applications, due to improper operation, unreasonable equipment selection, and unscientific installation, measurement errors are difficult to avoid, causing confusion to users. Therefore, instrument manufacturers 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: 1. Improper selection, 2. Influenced by the medium and external interference.
1. The flow velocity of the liquid to be measured: the flow velocity range that can be measured by the electromagnetic flowmeter is generally 0.5~10m/s, and the economic flow velocity range is 1.5~3m/s. In actual use, the inner diameter of the measuring tube should be determined according to the measured flow rate and the range of flow velocity that can be measured by the electromagnetic flowmeter.
2. Selection of electrode and lining material: The electrode and lining material are in direct contact with the liquid to be tested. The electrode and lining material should be selected according to the characteristics of the liquid to be tested (such as corrosiveness, abrasiveness, etc.) and the working temperature. Causes problems such as fast adhesion, corrosion, scaling, wear, and lining deformation, resulting in measurement errors.
3. Excitation mode of excitation stability electromagnetic flowmeter: there are DC excitation, AC sine wave excitation and dual-frequency rectangular wave excitation, etc. DC excitation is prone to electrode polarization and DC interference problems. The frequency square wave excitation has both the excellent zero point stability of the low frequency square wave excitation and the strong suppression ability of the high frequency square wave excitation to the fluid noise. It is an ideal excitation method. In practical application, the stability of the power supply voltage and frequency should be ensured as much as possible to ensure the constant magnetic field strength and reduce the measurement error caused by the change of the magnetic field strength.
4. Mixed fluid measurement: When using an electromagnetic flowmeter to measure the flow of a liquid-solid mixture fluid (such as water containing sediment), if an electromagnetic flowmeter calibrated by a single-phase liquid is used, measurement errors will occur. Install the flowmeter sensor in the straight pipe section that will cause liquid-solid phase separation.
Measured liquid effects:
1. The conductivity of the tested liquid changes drastically: when the conductivity of the tested liquid is large, it will cause a large fluctuation of the displayed value. If the problem is very serious, it is difficult for the control system to achieve normal operation; When it is low, it is difficult for the electrode to achieve normal output. If the conductivity of the liquid to be measured is below the lower limit during operation, the electromagnetic flowmeter cannot be detected normally. In view of these situations, first of all, we should consider the actual needs and combine the relevant standards and requirements to select the type of electromagnetic flowmeter; secondly, install the reactor or straight pipe section to ensure the full mixing of materials and promote the smooth realization of the chemical reaction; again, Redo the flowmeter type selection.
2. Measured liquid bubbles or non-full tube: For the bubbles, the main sources of the bubbles are the bubbles in which the dissolved gas in the liquid develops into a free state and the bubbles inhaled by the outside world. Flow rates that contain a large volume of bubbles can affect the accuracy of the 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 fluctuations 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; secondly, the installation position can be replaced reasonably; thirdly, the vertical pipe can be installed on the electromagnetic flowmeter to ensure Bottom-up orientation; fourth, when installing the sensor, avoid getting too close to the discharge port; fifth, install the sensor at the control valve location, upstream of it, or downstream of the pump.
3. The conductivity of the tested liquid is too low: the decrease of the conductivity of the tested liquid will increase the output impedance of the electrode, and the load effect caused by the input impedance of the converter will cause measurement errors. If the actual conductivity is lower than the lower limit, the instrument will If it does not work properly, the displayed value will fluctuate. Solution: choose other low-conductivity electromagnetic flowmeters that meet the requirements, such as capacitive electromagnetic flowmeters; or choose other principle flowmeters, such as orifice plates.
4. The measured liquid is asymmetrical: During the measurement, the measured liquid is asymmetrical, and there are mainly two flow combinations: one is a single vortex flow; the other is a straight flow along the axis of the pipeline, the volume flow of the liquid is the integral of the pipe section. In view of the insufficient upstream straight pipe section, the flow regulator can be used to adjust; secondly, ensure that the inner diameter of the pipe and the inner diameter of the flowmeter within a reasonable range of upstream and downstream have the same value; thirdly, leave enough straight pipe section for the upstream.
5. There is a layer in the measuring tube: Electromagnetic flowmeters are often used to measure non-clean fluids. The non-clean fluid contains some sediment and other substances, which pollutes the electrode surface or the pipeline of the electromagnetic flowmeter, resulting in errors in the measurement results. In response to this situation, first, the electromagnetic flowmeter should be cleaned regularly; secondly, the flow rate should be reasonably increased to control it at 4m/s and the uppermost state; thirdly, the lining of materials such as polytetrachloroethylene should be used.
1. Space electromagnetic interference converter and sensor: The cable is long, and in a strong electromagnetic environment, it is easy to be interfered, resulting in nonlinear measurement of the instrument, which is difficult to display normally. In response to this situation, firstly, introduce shielding measures, and the cable can be introduced separately in the grounded steel pipe, and use the shielded cable that meets the standard; secondly, the length of the cable should be shortened reasonably; thirdly, keep a long distance from the strong magnetic field.
2. Problems with connecting cables The essence of the electromagnetic flowmeter application is to use a specific cable to connect the converter and the sensor to form a complete system. Therefore, the cross-sectional area of the conductor, the capacitance, and the cable site will have adverse effects. In view of this situation, first of all, it is necessary to ensure that the cable type meets the requirements, to realize the effective connection of the ends, and to prevent the phenomenon of intermediate joints; secondly, to control the length range, usually the shorter the better
3. Grounding problem Because the output signal of the sensor is very small, usually only a few millivolts, in order to improve the anti-interference ability, 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 to the fluid to be measured. The grounding resistance of the sensor should be less than 10Ω. When the pipe connecting the sensor is coated with an insulating layer or a non-metallic pipe is used, a grounding ring should be installed on both sides of the sensor (the electromagnetic flowmeter manufactured now has 3 electrodes: positive electrode, negative electrode, grounding electrode), and ground it reliably so that the fluid is grounded, and the fluid potential is the same as the ground potential.
4. Electrode and excitation coil symmetrical point mounting point vibration
The excitation coil and electrodes of the electromagnetic flowmeter need to be symmetrical. Once they are asymmetrical, deviations will occur in the production process, and it is difficult to ensure accurate measurement results. In addition, the installation site needs to meet a high anti-vibration standard, otherwise the accuracy of the measured value cannot be guaranteed, and even the instrument may not work properly.
The flowmeter is connected to the ground wire, and the ground point of the sensor output signal should be electrically connected to the fluid to be measured. The grounding resistance of the sensor should be less than 10Ω. When the pipeline connecting the sensor is coated with an insulating layer or a non-metallic pipeline is used, a grounding ring should be installed on both sides of the sensor and be grounded reliably so that the fluid is grounded and the fluid potential is the same as the ground potential.