Reasons for large error in electromagnetic flow meter measurement

With the advantages of no pressure loss, high accuracy, and moderate price, electromagnetic flowmeters are widely favored by enterprises in petrochemical, chemical and other industries, and 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 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 influences causing electromagnetic flowmeter errors can be divided into three categories: improper selection, influence of the liquid to be measured, and interference.

Caused by incorrect selection

1: Liquid flow rate to be measured

The measurable flow rate range of the electromagnetic flowmeter is generally 0.5~10m/s, and the economical flow rate is The 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: Selection of electrode and lining materials

Electrode and lining materials are in direct contact with the liquid to be measured, and should be based on the characteristics of the liquid to be measured (such as corrosiveness, abrasiveness, etc.) and working conditions If the temperature selective electrode and lining materials are selected improperly, it will cause problems such as fast adhesion, corrosion, scaling, wear, and lining deformation, which will lead to measurement errors.

3: Excitation stability

The excitation methods of electromagnetic flowmeter include DC excitation, AC sine wave excitation and dual-frequency rectangular wave excitation. DC excitation is prone to electrode polarization and DC Interference problem, AC sinusoidal excitation can easily cause 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, which is an ideal method. 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.

IV. 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 selected, measurement errors will occur. In this case, a measurement error should be selected that will not cause the liquid-solid The sensor is installed at the phase-separated straight pipe section.

Influence 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, the displayed value will be larger. Fluctuation, if the problem is very serious, it will be difficult for the control system to operate normally; and when the conductivity of the liquid to be measured is too low, it will be difficult for the electrode to achieve normal output. If the conductivity of the liquid to be measured is below the lower limit during operation, then It is difficult for the electromagnetic flowmeter to function properly. In response to these situations, first of all, the type of electromagnetic flowmeter must be selected based on actual needs and relevant standards and requirements; secondly, a reactor or straight pipe section must be installed to ensure full mixing of materials and promote the smooth realization of chemical reactions; thirdly, Re-select the flow meter type.

2: The liquid to be tested has bubbles or a non-full tube

For bubbles, it mainly comes from the gas dissolved in the liquid developing into a free state.state 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 electrode diameter, an unstable state will occur during the measurement and display process, and fluctuations are unavoidable. In response to this situation, firstly, the gas collector can be installed on the electromagnetic flowmeter, and the exhaust operation can be performed according to the cycle; secondly, the installation position can be reasonably changed; thirdly, the vertical pipe can be installed on the electromagnetic flowmeter to ensure that the gas collector is installed on the electromagnetic flowmeter to ensure the flow from bottom to bottom. direction; fourth, when installing the sensor, avoid being too close to the discharge port; fifth, install the sensor at the position of the control valve, upstream of it, or downstream of the pump.

3: The conductivity of the liquid to be measured is too low

The decrease in conductivity of the liquid to be measured will increase the output impedance of the electrode, and cause a load effect caused by the input impedance of the converter, resulting in measurement errors. , if the actual conductivity is lower than the lower limit value, the instrument will not work properly and 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 asymmetrical

During the measurement, the liquid to be measured is asymmetrical, and there are two main flow combinations: one is a single vortex flow; the other The first is a straight flow along the axis of the pipeline, and the volume flow rate of the liquid is the integral of the pipeline 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 flowmeters are often used to measure non-clean fluids. The non-clean fluid contains some sediments and other substances, which contaminates the electrode surface of the electromagnetic flowmeter or the inside of the pipe, 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.