In industries such as petrochemicals, chemicals, electricity, metallurgy, municipal engineering, and pharmaceuticals, the technical difficulty and complexity of flow measurement are quite high. Research on flow measurement plays an important role in improving product quality, reducing operating costs for enterprises, and solving energy conservation and environmental protection problems. Electromagnetic flow meters have the advantages of no resistance and no pressure loss, which can fully reduce the resistance inside pipelines and meet the requirements of energy conservation and consumption reduction. Therefore, electromagnetic flow meters have been widely valued in the industry.

Such a user-friendly electromagnetic flowmeter often encounters problems in measurement. Once problems arise, it can be quite troublesome. Today, let's introduce the reasons for the data deviation of electromagnetic flowmeters?

In general, the main factors causing errors in electromagnetic flow meters can be divided into three categories: the influence of the measured liquid, improper selection, and interference.

The impact of the test liquid

1. The liquid to be tested contains bubbles

This is a common phenomenon, caused by both external inhalation and internal liquid dissolution. However, electromagnetic flow meters cannot distinguish between liquid and bubbles, so measuring them together will result in errors.

2. Non full tube of liquid to be tested

It can be said that a non full tube contains bubbles, which means that the liquid inside the tube is not full and there are a large number of bubbles at the top. If the liquid has not yet passed through the electrode, the measurement results will be greatly discounted. This is undoubtedly a mistake in engineering design.

3. The conductivity of the liquid to be tested undergoes drastic changes

When the conductivity of the liquid to be tested is high, it will cause significant fluctuations in the displayed values. If the problem is severe, it will be difficult for the control system to operate normally; When the conductivity of the liquid to be tested is too low, it is difficult for the electrode to achieve normal output. If the conductivity of the liquid to be tested is below the lower limit during operation, the electromagnetic flowmeter will have difficulty functioning properly.

In response to these situations, firstly, it is necessary to base on actual needs and combine relevant standards and requirements to select the type of electromagnetic flowmeter; Secondly, install reactors or straight pipe sections to ensure sufficient mixing of materials and promote the smooth implementation of chemical reactions; Again, re select the type of flowmeter.

4. Short circuit effect of conductive deposition layer

On the electrolytic cutting process test device in the tool workshop of a diesel engine factory, a DN80mm instrument is used to measure and control the flow rate of saturated salt electrolyte to obtain cutting efficiency. At first, the instrument operated normally, but after 2 months of intermittent use, I felt that the flow display value was decreasing until the flow signal approached zero.

Due to the gradual deposition of conductive substances, there is a short circuit phenomenon in the flow signal. This type of fault usually does not occur during the debugging period, but rather after running for a period of time before it becomes apparent.

5. Liquid conductivity exceeds the allowable measurement range

A chemical (smelting) plant in Shanghai used more than 20 Hastelloy B-electrode electromagnetic flow meters to measure high concentration hydrochloric acid solutions, but experienced unstable shaking of the output signal. On site inspection confirmed that the instrument is normal and ruled out other interference causes that may cause output shaking.

However, the Hastelloy B electrode instrument performed well in measuring hydrochloric acid in multiple other locations. Due to the fact that the output impedance of the electrode is determined by the conductivity of the measured liquid and the size of the electrode, when the conductivity is below the lower limit, the instrument cannot function properly and the displayed value may shake.

6. Space electromagnetic wave interference

Generally speaking, 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 significant shaking.

Improper selection

1. Flow rate of the liquid to be tested

The flow velocity range that can be measured by an 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 based on the size of the flow rate to be measured and the flow rate range that can be measured by the electromagnetic flowmeter.

2. Selection of electrode and lining materials

The electrode and lining materials are in direct contact with the liquid to be tested. The electrode and lining materials should be selected based on the characteristics of the liquid to be tested (such as corrosiveness, abrasiveness, etc.) and the working temperature. Improper selection can cause problems such as fast adhesion, corrosion, scaling, wear, and deformation of the lining, resulting in measurement errors.

3. Excitation stability

The excitation methods of electromagnetic flow meters include DC excitation, AC sine wave excitation, and dual frequency rectangular wave excitation. DC excitation is prone to electrode polarization and DC interference problems, while AC sine wave excitation is prone to zero point changes. 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, making it an ideal excitation method.

In practical applications, it is necessary to ensure the stability of the power supply voltage and frequency as much as possible to ensure a constant magnetic field strength and reduce measurement errors caused by changes in magnetic field strength.

4. Measurement of mixed phase fluids

When measuring the flow rate of liquid-solid mixed phase fluids (such as water containing sediment) using an electromagnetic flowmeter, if a single-phase liquid calibrated electromagnetic flowmeter is selected, measurement errors will occur. In this case, a sensor should be installed at a straight pipe section that will not cause liquid-solid phase separation.

Interference impact

1. Space electromagnetic interference

The cable between the converter and the sensor is relatively long, and in a strong electromagnetic environment, it is easily affected by interference, which can cause nonlinear measurement values in the instrument and make it difficult to display normally. In response to this situation, firstly, shielding measures can be introduced by separately introducing cables into the grounding steel pipe and using shielded cables that meet the standards; Secondly, reasonably shorten the cable length; Again, maintain a relatively long distance from the strong magnetic field.

2. Connection cable issue

The essence of electromagnetic flow applications is to use specific cables to connect converters and sensors, forming a complete system. Therefore, the cross-sectional area of conductors, capacitance, cable sites, etc. can all have adverse effects. In response to this situation, firstly, it is necessary to 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

Due to the small output signal of the sensor, usually only a few millivolts, in order to improve anti-interference ability, the zero potential of the sensor must be reliably grounded separately, and the grounding point of the sensor output signal should be electrically connected to the measured fluid. The grounding resistance of the sensor should be less than 10 Ω. When the pipeline connecting the sensor is coated with insulation layer or non-metallic pipeline is used, grounding rings should be installed on both sides of the sensor and reliably grounded to ensure that the fluid is grounded and the fluid potential is the same as the ground potential.

4. Symmetrical installation point vibration of electrodes and excitation coils

The excitation coil and electrode of the electromagnetic flowmeter need to be symmetrical. Once asymmetrical, deviations can occur during the production process, making it difficult to ensure accurate measurement results. In addition, the installation location 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.