Application of electromagnetic flow meter in urban water supply industry

Project Overview

In recent years, with the continuous improvement of the automation technology level of the domestic water supply industry and the requirements for trade settlement and measurement, electromagnetic flowmeters have been more and more commonly used and promoted, especially in the Shanghai water supply industry, SP-LDE electromagnetic flowmeters. The design application has been widely recognized.

2 Basic Principles

According to Faraday’s law of electromagnetic induction, that is, when a conductive liquid cuts magnetic lines of force in a magnetic field and moves, an induced electric potential is generated in the conductor (see Figure 1), and the magnitude of the induced electric potential It is directly proportional to factors such as the magnetic induction intensity and the intensity of the fluid's cutting magnetic field lines.

Figure 1 Schematic diagram of induced electric potential generation

The electric potential is given by the following formula:

E=K×B×V×D

In the formula: E——induced voltage;

K——instrument constant;

B——magnetic induction intensity;

V——average flow velocity; < /p>

D——Measure the inner diameter of the tube.

When flow fluid, the fluid flows through a magnetic field perpendicular to the flow direction. The flow of conductive liquid induces a voltage proportional to the average flow rate or unit volume flow rate. Therefore, as long as the fluid being measured has a minimum The conductivity, its induced voltage signal is detected through two electrodes in direct contact with the liquid, and transmitted to the amplifier through a cable, and then converted into a unified output signal.

3 Main technical features and usage and installation requirements

3.1 Main technical features of electromagnetic flowmeter

① Since the measurement results are related to the pressure, temperature and conductivity of the liquid It has nothing to do with other physical parameters, so the measurement accuracy is high and the work is reliable.

② There are no flow obstructions in the measuring pipe, so there is no additional pressure loss, that is, there is no additional head loss for tap water measurement.

③ Only the pipes and electrodes are in contact with the liquid being measured. Therefore, as long as the electrode materials are properly selected, the corrosion resistance and wear resistance requirements can be achieved. It has the characteristics of long service life and low maintenance requirements.

④ It can measure the flow rate of fluid in forward and reverse directions.

⑤ The output signal mode is more flexible. There are pulse, current, frequency and other methods.

3.2 Requirements for the use and installation of electromagnetic flowmeters

① The upstream pipeline of the electromagnetic flowmeter has a straight pipe section with a length of at least 5 times the pipe diameter. There should be a straight pipe section downstream of at least three times the pipe diameter. If conditions permit, relax the length of the straight pipe in front of the flow meter to more than 10 times the pipe diameter.

② During the installation of the electromagnetic flowmeter, the sensor grounding ring must be reliably grounded, and the grounding resistance must not be greater than 1 OΩ.

③ When choosing the use environment, pay attention to avoid external electromagnetic interference, especially interference from power frequency electromagnetic fields.

④ For valves installed outside the leading distance in the upstream pipeline of the flow meter, especially butterfly valves or right-angle elbows that seriously affect the flow characteristics, it is best to install the valve in the flow meter. A rectifier is installed in a straight pipe 5 to 10 times the diameter of the upstream pipe to stabilize the water flow pattern and improve measurement accuracy.

4 The installation, configuration and use of electromagnetic flow meters in the water supply industry

The principle flow diagram of the water treatment process currently commonly used in water companies in various regions in Shanghai is shown in Figure 2.

Figure 2 Water process treatment process

Based on electromagnetic flowmeterThe measurement principle and measurement characteristics have now become a widely used product in the water supply industry. Since the first electromagnetic flowmeter was used in 1989.