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Introduction to the correct selection and use of electromagnetic flowmeter

First, the working principle of electromagnetic flowmeter

The measurement principle of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. The measuring tube of the flow meter is a non-magnetic alloy short tube lined with an insulating material, and the two electrodes are fixed to the measuring tube through the tube wall along the tube diameter direction, and the electrode head is substantially flush with the inner surface of the lining. When the exciting coil is excited by the bidirectional square wave pulse, an operating magnetic field having a magnetic flux density of B is generated in a direction perpendicular to the axis of the measuring tube. At this time, if a fluid having a certain conductivity flows through the measuring tube, the cutting magnetic line induces the electromotive force E. The electromotive force E is proportional to the magnetic flux density B, the product of the inner diameter D of the measuring tube and the average flow velocity V, and the electromotive force E (flow rate signal) is detected by the electrode and sent to the converter through the cable. After the converter amplifies the flow signal, it can display the instantaneous flow and total amount, and can output signals such as pulse and analog current for flow control or adjustment.

The electromotive force formula is

E = 4BQ/(πD)

Where: E—inductive electromotive force; B—magnetic induction intensity; Q—flow rate; D—nominal diameter of the flowmeter.

Second, the structure, classification and selection of electromagnetic flowmeter

The electromagnetic flowmeter consists of two parts: an electromagnetic sensor and an electromagnetic flow converter.

According to the function of its converter, it is divided into two types: basic type and slurry type. The basic type is suitable for the detection of general process, and the sensor diameter is DN15~DN2400; the slurry type is suitable for the measurement of pulp, pulp and paste liquid, and the sensor diameter is DN15~DN600.

According to the basic type and the type of slurry and the composition of the sensor, it can be divided into an integrated flow meter (converter and sensor integrated) or a separate flow meter (converter and sensor are installed separately). The integrated flowmeter nominal diameter is limited to DNl5~DN400, and the split flowmeter includes all diameters. The integrated flow meter can be mounted directly on the pipe, and the separate flow meter can be mounted on a specific wall or instrument panel.

According to different installation places, electromagnetic flowmeters can be divided into ordinary electromagnetic flowmeters and explosion-proof electromagnetic flowmeters.

The flow, total output, display, alarm, parameter programming, communication and other functions of the various types of electromagnetic flowmeters are basically the same. A typical network connection is shown in Figure 2. The network connection is to realize communication between multiple electromagnetic flowmeters and a computer through a communication bus. The electromagnetic flowmeter converts the measured flow signal into a digital signal and sends it out. The MODBUS protocol can be used to connect multiple instruments on the network.

Third, the main performance and characteristics of electromagnetic flowmeter

1. Simple and reliable structure, no moving parts, no flow blocking parts, no pressure loss and fluid blocking, and low requirements for straight pipe sections.
2. No mechanical inertia, fast response, good stability, can be applied to automatic detection and program control systems.
3. It can be used for the measurement of corrosive fluids such as various strong acids, alkalis and salt solutions. It is not affected by fluid density, viscosity, temperature, pressure and conductivity with respect to turbines and vortex flowmeters.
4. The flow range is wide (30:1), and some manufacturers can even reach 1500:1, and the initial measurement flow rate is as small as about 0.2m/s.
5. The current electromagnetic flowmeter adopts intelligent design, which can display medium flow direction and empty tube signal, and has the function of removing small signal migration.
6. Remote control can be achieved through telephone networking.

Fourth, the electromagnetic flowmeter in the use of precautions

1. Requirements for measuring media:

(1) The fluid medium to be tested must have electrical conductivity;
(2) The fluid medium to be tested must be filled with pipes;
(3) The flow medium to be measured must be uniform.

2. Installation requirements:

(1) It should be ensured that there is at least 5D (pipe inner diameter) straight pipe section upstream of the sensor. If there is a fully open gate valve or regulating valve upstream, the length of the straight pipe section upstream of the flowmeter should be increased to 10D; the downstream pipe section of the flowmeter is generally straight. Should be greater than 3D;
(2) The magnetic field of the instrument installation site should be less than 400A/m;
(3) Covering facilities should be added to places exposed to sunlight, and damage caused by corrosive liquid dripping should be prevented;
(4) The most important thing in reliable measurement is the installation of electrodes. The product requirements of different manufacturers are different. For example, the E-mag type produced by Kaifeng Factory requires the electrode to be completely immersed in the liquid, while the YYD type produced by Yiwu is based on the inner diameter of the pipe. , tube wall, excessive flange length calculation. The specific installation method can be referred to the factory manual.

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