Electromagnetic flowmeter measurement error problems and cause analysis

Case introduction

Analysis of electromagnetic flowmeter measurement error problems and causes:

With the advantages of no pressure loss, high precision, and moderate price, electromagnetic flowmeters are widely favored by enterprises in petrochemical, chemical and other industries and play an important role in flow measurement. plays an important role in. However, in practical applications, 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 professionals should pay attention to various factors that cause errors in electromagnetic flowmeters. In general, the main influencing factors causing electromagnetic flowmeter errors can be divided into three categories: improper selection, influence of the liquid to be measured, and interference. Improper selection 1. The measurable flow rate of the liquid flow rate to be measured by the electromagnetic flowmeter is generally 0.5 to 10m/s, and the economic flow rate range is 1.5 to 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. Selection of 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. Improper selection will cause adhesion. Problems such as high speed, corrosion, scaling, wear, and lining deformation will lead to measurement errors. 3. The excitation methods of electromagnetic stable electromagnetic flowmeter 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 causing zero point changes, while dual-frequency rectangular wave excitation is both It has excellent zero-point stability of low-frequency rectangular wave excitation and 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. 4 Mixed-phase fluid measurement 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. In this case, you should choose not to Install sensors in straight pipe sections that may cause liquid-solid phase separation. Effect of the liquid to be measured 1. The conductivity of the liquid to be measured changes drastically. When the conductivity of the liquid to be measured is large, it will cause large fluctuations in the displayed value. If the problem is very serious, it will be difficult for the control system to operate normally; while the conductivity of the liquid to be measured is When the conductivity is too low, it is difficult for the electromagnetic flowmeter to achieve normal output. If the conductivity of the liquid to be measured is below the lower limit during operation, it will be difficult for the electromagnetic flowmeter to function normally. In response to these situations, firstly, the electromagnetic flowmeter type should be selected based on actual needs and relevant standards and requirements; secondly, a reactor or straight pipe section should be installed to ensure full mixing of materials and promote the smooth realization of chemical reactions; thirdly, Re-select the flow meter type. 2. Bubbles in the liquid to be measured or a non-full tube. Bubbles mainly come from the gas dissolved in the liquid developing into free bubbles and bubbles inhaled from the outside. Flows containing large bubble volumes can affect the accuracy of the measurement. If the bubble diameter is too large, or even exceeds the value of the electrical diameter, an unstable state will occur during the measurement and display process, and fluctuations are unavoidable. In view of this situation, first, the gas collection can beInstall the sensor on the electromagnetic flowmeter, and perform exhaust operations according to the cycle; secondly, change the installation location reasonably; thirdly, install the vertical pipe on the electromagnetic flowmeter to ensure the bottom-up direction; fourthly, when installing the sensor, Avoid being too close to the discharge port; fifth, install the sensor at the control valve, upstream of it, or downstream of the pump. 3. The conductivity of the liquid to be measured is too low. The conductivity of the liquid to be measured is reduced, which will increase the output impedance of the electricity, and cause the load effect caused by the input impedance of the converter, resulting in measurement errors. If the actual conductivity is lower than the lower limit, the instrument will not work properly. work, the indication value will fluctuate. Countermeasures: Choose other low-conductivity electromagnetic flowmeters that meet the requirements, such as capacitive electromagnetic flowmeters; choose other principle flowmeters, such as orifice plates, etc. 4. The measured liquid is in an asymmetric state. During measurement, the liquid to be measured is asymmetric. There are two main flow combinations: one is a single vortex flow; the other is a straight flow along the axis of the pipeline. The volume flow rate of the liquid is integral of the cross section. If the upstream straight pipe section is insufficient, a flow regulator can be used to adjust it; secondly, ensure that the inner diameter of the pipe and the inner diameter of the flow meter have the same value within a reasonable range of the upstream and downstream; thirdly, leave enough straight pipe section for the upstream. 5. There is a layer in the measuring tube.

Electromagnetic flowmeter is often used to measure non-clean fluids. The non-clean fluid contains some sediments and other substances, which contaminates the electrical surface of the electromagnetic flowmeter or the inside of the pipeline, causing errors in measurement results. In response to this situation, firstly, clean the electromagnetic flowmeter regularly; secondly, reasonably increase the flow rate and control it at 4m/s; thirdly, apply a lining of polytetrachlorethylene and other materials. Interference impact 1. The cable between the space electromagnetic interference converter and the sensor is long. In a strong electromagnetic environment, it is easy to be interfered, which causes non-linear conditions in the measured values ​​of the instrument, making it difficult to display normally. In response to this situation, first, introduce shielding measures, such as single cable introduction in a grounded steel pipe, and use qualified shielded cables; secondly, shorten the length of the cable reasonably; thirdly, keep a long distance away from strong magnetic fields. 2. Connection cable problem The essence of electromagnetic flow application is to use certain cables to connect the converter and the sensor to form a complete system. Therefore, the cross-sectional area of ​​the conductor, capacitance, cable site, etc. will have adverse effects. First, ensure that the cable model meets the requirements, achieve effective connection at the end, and prevent the occurrence of intermediate joints; secondly, control the length range, usually the shorter the better. 3. Grounding issues: The output signal of the sensor is very small, usually only a few millivolts, in order to To improve the anti-interference ability, the zero potential of the sensor must be grounded reliably, and the sensor output signal grounding point should be electrically connected to the fluid being measured. The grounding resistance of the sensor should be less than 10Ω. 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. 4. Electrical and electromagnetic coil symmetry points. The excitation coil and electric power of the vibration electromagnetic flowmeter need to be symmetrical. Once asymmetrical, deviations will occur during the production process, and it is difficult to ensure accurate measurement results.In addition, the installation site needs to meet high anti-vibration standards, otherwise the accuracy of the measured values ​​cannot be guaranteed, and even abnormal operation of the instrument may be induced.