Solution to measurement error of electromagnetic flowmeter

This paper briefly introduces the basic principles of electromagnetic flowmeters, combined with reality, analyzes the causes of electromagnetic flowmeter measurement errors in detail, and gives Corresponding measures to reduce measurement errors.

Introduction Due to its many advantages, electromagnetic flowmeters have been widely used in many fields such as metallurgy, petrochemicals, chemical industry, electric power, environmental protection, wastewater and urban utilities, and play a huge role in flow measurement. . In practical applications, electromagnetic flowmeters have very strict usage requirements. If the equipment is improperly selected, installed or used, it will cause increased measurement errors and unstable indication values. Therefore, it is particularly important to study the causes of electromagnetic flowmeter measurement errors and corresponding solutions.

1 Basic Principle The basic principle of electromagnetic flowmeter is Faraday's law of electromagnetic induction, that is, when a conductor moves to cut magnetic lines of force in a magnetic field, an induced electromotive force is generated at both ends of the conductor. For electromagnetic flowmeters, when conductive liquid flows in a non-magnetic measuring tube perpendicular to the magnetic field, an induced electromotive force proportional to the flow rate will be generated in the direction perpendicular to the flow direction and magnetic field, and its magnitude is: E =kBDv (1)

2 Analysis of causes of measurement errors and countermeasures It can be seen from equation (3) that the changes in k, B and D, as well as the verticality of the direction of the magnetic field, the electrical connection and the fluid flow, will cause measurement errors, that is, electromagnetic There are many reasons for flow meter measurement errors, including instrument selection, installation environment, installation method, use and maintenance and many other factors.

2.1 It is a common phenomenon that the liquid to be measured contains bubbles. The formation of bubbles is due to external inhalation (such as deterioration of the sealing of the pump shaft, leakage of the negative pressure end pipe connection gasket, etc.) or leakage in the liquid. There are two ways for dissolved gas (air) to transform into free bubbles and precipitate. At this time, the measurement result is the flow rate including the bubble volume, which leads to measurement errors. At the same time, if the bubble diameter is greater than or equal to the electrical diameter, it can also cause the measurement value to be unstable and cause the measurement display value to fluctuate. For example, the fluctuation of a DN2200 electromagnetic flowmeter due to bubbles can reach 20% to 50%. Solution: 1) Change the installation position; 2) If the installation position is not easy to change, install a gas collector upstream of the flow meter to exhaust air regularly.

2.2 The liquid to be measured is not full. The non-full pipe is a situation where bubbles are contained. At this time, if the liquid level is higher than the electrical level, the front and rear straight pipe sections (*D and then 5D are preferred) are ideal. , the measurement is generally stable, but the measurement result includes the gas volume in the upper half of the tube, and the measurement error is large; if the liquid level is lower than the electrical level, the measurement loop is in an open circuit state, and the measurement results are seriously distorted. Solution: 1) Try to install the electromagnetic flowmeter on a vertical pipe with bottom-up flow; 2) In many cases, the electromagnetic flowmeter is installed horizontally. In this case, it should be installed at the low end of the pipe, and the electrical axis should be parallel to the horizon ( Otherwise, the low voltage is easily depositedCover); 3) The sensor should be installed downstream of the pump and upstream of the control valve to prevent negative pressure from occurring in the measuring pipe; 4) The sensor installation port should have a certain back pressure and should not be too close to the direct discharge port; 5) Now there are The new electromagnetic flowmeter can be used to measure free-flowing liquid flow on free surfaces such as non-full pipes, such as municipal engineering sewage, industrial wastewater discharge measurement, etc. 2.3 Drastic changes in the conductivity of the liquid to be measured. If the conductivity of the liquid to be measured changes drastically at high frequency, it will cause the displayed value to fluctuate significantly at high frequency, and may even prevent normal monitoring or the corresponding control system from working properly. This phenomenon is often seen in the chemical industry. The way to solve this problem is to inject chemicals from the downstream of the flow meter as much as possible. If it must be injected from upstream, a straight pipe section or reactor that is sufficient to complete the chemical reaction or ensure sufficient mixing of the materials must be installed upstream to ensure sufficient mixing distribution. Evenly. 2.4 The conductivity of the liquid to be measured is too low. The conductivity of the liquid to be measured decreases, which will increase the output impedance of the electricity (determined by the conductivity of the liquid to be measured and the size of the electricity), and the load effect caused by the input impedance of the converter will produce measurement errors. If the actual conductivity is lower than the lower limit (usually 5μS/cm), the instrument will not work properly and the indication will shake. Solution: 1) Choose other low conductivity electromagnetic flow meters that meet the requirements, such as capacitive electromagnetic flow meters; 2) Choose other principle flow meters, such as orifice plates, etc.

2.5 Space electromagnetic wave interference If the cable between the sensor and the converter is long and there is strong electromagnetic interference around it, the cable may introduce interference signals, forming common mode interference, causing display distortion, nonlinearity or large amplitude. Shake. Solution: 1) Try to stay away from strong magnetic fields (such as near large motors, large transformers and power cables); 2) Try to shorten the length of the cable; 3) Use shielding measures, including using qualified shielded cables and passing the cable alone in a grounded steel pipe (cannot be threaded in the same pipe as the power cord). 2.6 Sensor grounding 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 reliably grounded, and the sensor output signal grounding point should be electrically connected to the fluid being measured. The grounding resistance of the sensor should usually be less than 10Ψ. When the pipe connecting the sensor is coated with an insulation layer or a non-metallic pipe is used, grounding rings should be installed on both sides of the sensor and reliably grounded so that the fluid is grounded and the fluid potential is the same as the ground potential. 2.7 There is an attached layer in the measuring tube. Electromagnetic flowmeters are often used to measure the flow of non-clean fluids that are prone to adhesion, precipitation, scaling, etc. The electrical surface and the inner wall of the pipeline are often contaminated. If the conductivity of the adhesion layer is similar to the conductivity of the fluid, there will be no principle error, and the value indicated by the meter will still be normal, but the actual flow area of ​​the fluid will be reduced; if the adhesion layer is a material with high conductivity, it will cause the sensor to The electrical resistance becomes very small, or even short-circuited, and the output shows a negative deviation, or even cannot work normally; if the adhesion layer is an insulating material, the electrical impedance increases or even opens the circuit, causing the errorincrease, and may not even work properly. Solution: 1) Try to use linings such as glass or polytetrachlorethylene that are difficult to adhere to and precipitate; 2) Clean regularly, using mechanical or chemical methods; 3) The flow rate should not be less than 2m/s, and the hao should be increased to 3 to 4m/s. s or above, this can achieve the purpose of automatically cleaning the pipeline to a certain extent and prevent adhesion and precipitation.

2.8 Under normal circumstances, the asymmetric flow of the liquid to be measured requires that the flow velocity of the fluid in the pipeline is axially symmetrically distributed, the magnetic field is uniform, and E is proportional to v. In fact, non-axisymmetric flow velocity distribution often occurs. At this time, any flow direction can be divided into a combination of two flows: one is a straight flow along the axis of the pipeline, and its integral over the cross-section of the pipeline is the volume flow rate of the liquid to be measured. ;The other is pure vortex flow, whose integral over the cross-section of the pipe is zero. If the vortex flow affects the output, it will produce an error. Solution: 1) There is enough straight pipe section upstream (more than 5D, depending on the specific situation) to ensure that the flow rate is distributed in concentric circles; 2) The inner diameter of the flow meter should be the same as the inner diameter of the pipe within a certain range upstream and downstream, otherwise it will cause The flow velocity distribution is uneven; 3) If the upstream straight pipe section is insufficient, a flow regulator can be installed, which can only partially compensate.

2.9 Problems with connecting cables The electromagnetic flowmeter connects the sensor and converter into a system by a certain cable. The cable length, insulation condition, number of shielding layers, distributed capacitance and conductor cross-sectional area will all affect It will have an impact on the measurement results, and in severe cases, the flow meter may not work properly. Solution: 1) The shorter the cable, the better, and its length should be within the allowable range. The maximum length is determined by the conductivity of the liquid to be measured, the number of shielding layers, distributed capacitance and conductor cross-sectional area; 2) Intermediate joints should be avoided , the ends should be properly handled and connected; 3) Try to use the specified type of cable. 2.10 Problems in selecting electrical and lining materials. Electrical and lining materials are in direct contact with the liquid to be measured. Electrical and lining materials should be selected according to the properties of the liquid to be measured (such as corrosiveness, abrasiveness, etc.) and working temperature. If the selection is inappropriate, the It will cause problems such as fast adhesion, corrosion, scaling, wear, and lining deformation, which will lead to measurement errors. Therefore, great attention should be paid to equipment selection. 2.11 Excitation stability issues The excitation methods of electromagnetic flowmeters include DC excitation, AC sine wave excitation and dual-frequency rectangular wave excitation. DC excitation is prone to electrochemical and DC interference problems, AC sine excitation is prone to zero point changes, and dual-frequency Rectangular wave excitation has both the excellent zero-point stability of low-frequency rectangular wave excitation and the strong suppression ability of high-frequency rectangular wave excitation on fluid noise. It is an ideal excitation method. In practical applications, the power supply voltage and frequency should be kept as stable as possible to ensure constant magnetic field intensity and reduce measurement errors caused by changes in magnetic field intensity.

2.12 Problems with the flow rate of the liquid to be measured. The measurable flow rate range of the electromagnetic flowmeter is generally 0.5 ~ 10m/s, and the economic flow rate range is 1.5 ~3m/s. In actual use, the inner diameter of the measuring tube must be determined based on the flow rate to be measured and the measurable flow rate range of the electromagnetic flowmeter. 2.13 Mixed-phase fluid flow measurement issues When using an electromagnetic flowmeter to measure the flow rate of a liquid-solid mixed-phase fluid (such as water containing sediment), if an electromagnetic flowmeter calibrated by a single-phase liquid is used, measurement errors will occur. The sensor should be installed in a straight pipe section that will not cause liquid-solid phase separation. 2.14 Symmetry problem of electrical and excitation coils During the manufacturing process, the electrical and excitation coils of the electromagnetic flowmeter must be strictly symmetrical, otherwise there will be asymmetric deviations, which will have a certain impact on the measurement results and cause measurement errors. 2.15 Vibration problem at the installation point Electromagnetic flowmeter has strict requirements on the vibration of the installation site, especially the integrated electromagnetic flowmeter, which must be installed in a place with small vibration, otherwise a certain measurement error will occur, and in severe cases, the instrument cannot work properly. 3 Conclusion There are many factors that affect the measurement accuracy of electromagnetic flowmeters, but as long as we have a deep understanding of the working principle of electromagnetic flowmeters and carefully analyze all aspects of the electromagnetic flowmeter measurement system, it is not difficult to find reasonable ways to reduce measurement errors. , thereby ensuring the stable operation and accurate measurement of the electromagnetic flowmeter.