Electromagnetic flowmeter principle of operation

The working principle and selection principle of electromagnetic flowmeter

The electromagnetic flowmeter is a flowmeter that measures flow based on Faraday's law of electromagnetic induction. The advantage of electromagnetic flowmeter is that the pressure loss is extremely small and the measurable flow range is large. The ratio of maximum flow rate to minimum flow rate is generally more than 20:1. It is suitable for a wide range of industrial pipe diameters, up to 3m. The output signal is linear with the measured flow rate. The accuracy is high and the conductivity can be measured ≥1μs/cm. The fluid flow of acid, alkali, salt solution, water, sewage, corrosive liquid, mud and slurry. But it cannot measure the flow of gas, steam and pure water.

1. Measurement Principle

When a conductor moves to cut magnetic field lines in a magnetic field, an induced electric potential will be generated in the conductor. The size of the induced electric potential is related to the effective length of the conductor in the magnetic field. It is proportional to the speed of the conductor moving perpendicular to the direction of the magnetic field. In the same way, when the conductive fluid flows in the vertical direction in the magnetic field and cuts the magnetic induction force lines, it will also generate an induced electric potential on the electrodes on both sides of the pipe. The direction of the induced electric potential is determined by the right-hand rule, and the magnitude of the induced electric potential is determined by the following formula:

Ex=BDv Formula (1)

Where Ex—induced electric potential, V;

B—magnetic induction intensity, T

D—pipe inner diameter, m

v—average flow velocity of liquid, m/s

However The volume flow rate qv is equal to the product of the fluid flow rate v and the cross-sectional area of ​​the pipe (πD2)/4. Substituting equation (1) into this equation, we get:

Qv=(πD/4B)* Ex equation (2)

It can be seen from the above formula that when the pipe diameter D is fixed and the magnetic induction intensity B is kept constant, the measured volume flow rate and the induced potential have a linear relationship. If an electrode is inserted on both sides of the pipe, the induced potential Ex can be introduced. By measuring the magnitude of this potential, the volume flow rate can be obtained.

2. The structure of electromagnetic flowmeter

The structure of electromagnetic flowmeter is mainly composed of magnetic circuit system, measuring conduit, electrode, shell, lining and converter.

Magnetic circuit system: Its function is to generate a uniform DC or AC magnetic field. The DC magnetic circuit is implemented with *magnets. Its advantage is that the structure is relatively simple and is less disturbed by the AC magnetic field. However, it easily polarizes the electrolyte liquid passing through the measurement catheter, causing the positive electrode to be surrounded by negative ions and the negative electrode to be surrounded by positive ions. Surrounding, that is, the polarization phenomenon of the electrodes, causes the internal resistance of the two electrodes to increase, thus seriously affecting the normal operation of the instrument. When the diameter of the pipe is large, the magnetic field magnet is also large, bulky and uneconomical. Therefore, electromagnetic flowmeters generally use alternating magnetic fields and are excited by 50HZ power frequency power supply.

Measuring catheter: Its function is to allow the conductive liquid to be measured to pass through. In order to cause the magnetic flux to be shunted or short-circuited when the magnetic lines of force pass through the measuring conduit, the measuring conduit must be made of materials that are non-magnetic, have low electrical conductivity, low thermal conductivity and have a certain mechanical strength. Non-magnetic stainless steel, fiberglass, etc. can be used.High-strength plastic, aluminum, etc.

Electrode: Its function is to elicit an induced potential signal that is proportional to the measured value. The electrodes are generally made of non-magnetic stainless steel and are required to be flush with the lining so that fluid can pass through unimpeded. Its installation position should be in the vertical direction of the pipe to prevent sediment from accumulating on it and affecting the measurement accuracy.

Shell: Made of ferromagnetic material, it is the outer cover of the excitation coil of the distribution system and isolates the interference of the external magnetic field.

Lining: There is a complete electrically insulating lining on the inside of the measuring conduit and on the flange sealing surface. It directly contacts the measured liquid, and its function is to increase the corrosion resistance of the measuring tube and prevent the induced potential from being short-circuited by the metal measuring tube wall. Lining materials are mostly corrosion-resistant, high-temperature-resistant, wear-resistant polytetrafluoroethylene plastics, ceramics, etc.

Converter: The induced potential signal generated by the liquid flow is very weak and is greatly affected by various interference factors. The function of the converter is to amplify and convert the induced potential signal into a unified standard signal and suppress it. main interference signal. Its task is to amplify and convert the induced potential signal Ex detected by the electrode into a unified standard DC signal.

3. Selection and installation of electromagnetic flowmeter

Reasonable selection and correct installation of electromagnetic flowmeter are extremely important to improve the measurement accuracy of the flowmeter and extend the life of the instrument. The selection principles are as follows:

1. The fluid to be measured must be a conductive liquid. It cannot measure gas, steam, petroleum products, glycerin, alcohol and other substances, nor can it measure pure water.

2. Selection of caliber and range. The diameter of the flowmeter is smaller than the inner diameter of the pipe. The full-scale scale of the flowmeter is selected based on the principle of not being less than the expected maximum flow value. The common flow rate exceeds 50% of the full-scale range, so that higher measurement accuracy can be obtained. The common flow rate is 2-4m/s which is most suitable.

3. Pressure selection: The operating pressure must be lower than the rated working pressure of the electromagnetic flowmeter, generally not exceeding 16×105Pa.

4. Temperature selection. The temperature of the measured medium cannot exceed the allowable operating temperature of the lining material, which is generally ≤200°C.

IV. Precautions for the installation of electromagnetic flowmeter:

1. Installation location. The electromagnetic flowmeter can be installed vertically or horizontally, but it is recommended to install it vertically and the measured fluid flows from bottom to top. It can also be installed horizontally, but the two electrodes must be on the same level. When installing horizontally, ensure that the measuring tube is filled with liquid at all times.

2. The signal of the electromagnetic flowmeter is relatively weak, only a few millivolts at full scale, and only a few microvolts when the flow rate is very small. A slight external interference will affect the measurement accuracy. Therefore, the flowmeter's shell, shielded wire, and measuring conduit must be grounded. It is required to set up a separate grounding point and never connect it to the motor or upper and lower pipes.

3. The flowmeter should be installed away from all magnetic sources (such as high-power motors, transformers, etc.).

4. The electromagnetic flowmeter is a velocity flowmeter. When the streamline promulgation does not meet the set conditions, measurement errors will occur. Therefore, there must be a straight pipe section of about 10D in front of the electromagnetic flowmeter to eliminate the influence of various local resistances on the symmetry of streamline distribution.