Application of electromagnetic flowmeter in chemical production

Flow rate is one of the important measurement parameters in chemical production. As far as measuring the flow rate of conductive liquids is concerned, whether the purpose is monitoring or metering, EMF can achieve stable, reliable, high-precision and easy-maintenance measurement. When selecting and using instruments, the author summarizes some key points based on practical applications for reference.

1 Working principle of EMF The

basic principle of EMF is Faraday's law of electromagnetic induction, that is, when a conductor cuts the magnetic force in a magnetic field, an induced electromotive force is generated at both ends. As shown in Figure 1, the conductive medium in the EMF measuring tube is equivalent to the conductive metal rod in the Faraday experiment, and the two electromagnetic coils at the upper and lower ends generate a constant magnetic field. When a conductive medium flows through, an induced potential is generated, which is measured by two electrodes inside the pipe and the induced voltage is derived. The measuring pipe uses a non-conductive lining (rubber, Teflon, etc.) to achieve electromagnetic isolation between the fluid and the measuring electrode. Assuming that the magnetic field strength B (T) generated by the electromagnetic induction coil is constant, the generated induced voltage Ue (V) is proportional to the average velocity V (m/s) of the fluid, the coefficient is k, and the inner diameter L (m) of the measuring tube is known, then the volume flow Qv (m3/s) can be calculated by the following formula:

EMF consists of two parts: flow sensor and transmitter. The sensor measuring tube is equipped with excitation coils above and below. When the excitation current is passed, a magnetic field is generated and passes through the measuring tube. A pair of electrodes are installed on the inner wall of the measuring tube in contact with the liquid, and the induced potential is drawn and sent to the transmitter. The excitation current is provided by the transmitter. 2

Characteristics of EMF

1) Simple structure. The measuring pipeline of EMF is a smooth straight pipe without flow-blocking disturbing parts or throttling parts, which is not prone to blockage, wear and other problems. Therefore, EMF is particularly suitable for liquid-solid two-phase fluids containing solid particles or fibers, such as pulp, mud, ore pulp, coal-water slurry and sewage.

Figure 1 Measurement principle

2) Since there is no throttling part, the instrument has almost no pressure loss and consumes only 10 to 20W of electrical power. Compared with the throttling differential pressure flowmeter, its energy-saving effect has become a major advantage of the development of EMF.

3) The volume flow obtained by EMF is actually not affected by the fluid density, viscosity, temperature, pressure, Reynolds number and the change of conductivity within a certain range.

4) The measurement range of EMF is very large, usually 20:1 to 50:1, and each range can be measured linearly from 2% to 100%. The measurement range of EMF can cover two velocity distribution states of turbulent and laminar flow, which is unmatched by other flowmeters.

5) The measurement principle is linear, with high measurement accuracy and fast response speed. It can measure pulsating flow and fast cumulative total, and can measure forward and reverse bidirectional flow. The straight pipe section before and after the sensor is low.

6) Good corrosion resistance, reliable use, easy maintenance and long service life.

However, EMF cannot measure liquids with very low conductivity, such as petroleum products and organic solvents; it cannot measure gases, steam, and liquids containing more and larger bubbles; it cannot measure liquids at higher temperatures due to the temperature limit of the lining material and the electrode insulation material. [page]

3 Key points for selecting EMF

In petrochemical design, there are many factors to consider when selecting a suitable flow meter. Specifically, when it comes to the characteristics of EMF, four factors must be considered, namely, instrument performance, flow characteristics, installation conditions, and environmental conditions.

3.1 Instrument performance

The functions of EMF on the market vary greatly. The simple ones only measure one-way flow and only output analog signals to drive the subsequent instruments; multi-functional instruments measure two-way flow, range switching, upper and lower limit flow alarms, empty pipe and power cut-off alarms, small signal removal, flow display and total calculation, automatic verification and fault self-diagnosis, communication with the host computer, and motion configuration. The serial digital communication function of some models of instruments can select a variety of communication interfaces and dedicated chips (ASIC) to connect to HART protocol systems, PROFTBUS, Modbus, FF fieldbus, etc.

The caliber range of EMF is wider than other types of flow meters. For example, E+H provides three types of sensors: W, P, and H, which are used in the water and sewage industry (W type), the chemical and food industry (P type), and the food and pharmaceutical industry (H type). The caliber range is from 2 mm to 2 meters. The transmitter can provide multiple options of 10, 50, 23, and 53. 10 is an economical type developed for the water industry, 50 is a common type, 53 is an enhanced type, and 23 is a two-wire system. The accuracy can be selected from 0.5% and 0.2%. It can measure forward and reverse bidirectional flow, as well as pulsating flow.

For media that are prone to adhesion, deposition, scaling, etc., the flow rate requirement should not be less than 2m/s, and it is best to increase it to 3-4m/s or more to play a role in self-cleaning and preventing adhesion and deposition. For highly abrasive fluids such as slurry, the flow rate should be less than 2-3m/s to reduce the wear of the lining and electrodes. The selection of lining, electrode and grounding ring materials is also critical. You can refer to the relevant "material manuals" suitable for different media, such as the "Manual of Commonly Used Materials and Devices for Automatic Control".

3.2 Fluid Properties

The premise of using EMF is that the measured liquid must be conductive and cannot be lower than the threshold (i.e., the lower limit value). If the conductivity is lower than the threshold, it will cause measurement errors until it cannot be used. If it exceeds the threshold, it can be measured even if it changes, and the indication error does not change much. The threshold of the general EMF is between 10-4 and (5×10-6) S/cm, depending on the model. When using it, it also depends on the length of the flow signal line between the sensor and the converter and its distributed capacitance. The manufacturer's instruction manual usually specifies the signal line length corresponding to the conductivity.

The conductivity of industrial water and its aqueous solution is greater than 10-4S/cm, and the conductivity of acid, alkali, and salt solution is between 10-4 and 10-1S/cm. There is no problem in using it. Low-degree distilled water is 10-5S/cm, which is also no problem. From the data, it is found that some pure liquids or aqueous solutions have low conductivity and are considered unusable. However, in actual work, there are examples where they can be used because they contain impurities. Such impurities are beneficial to increasing conductivity. For aqueous solutions, the conductivity in the data is measured in the laboratory using pure water. The aqueous solution actually used may be mixed with industrial water. The conductivity will be higher than the one found, which is also beneficial for flow measurement.

3.3 Installation conditions

are classified according to the assembly method of the EMF's own converter and sensor. There are two types: separate type and integrated type. For example, in sewage treatment plants, large-caliber flowmeter sensors are mostly installed underground, so it is recommended to choose IP68 (dust-proof and submersible level). If installed on the ground, it is recommended to choose IP67 (dust-proof and splash-proof level). Small-caliber flowmeters are mostly integrated. Usually, EMF has the following requirements for the installation site:

a. When measuring mixed phase fluid, choose a place that will not cause phase separation; when measuring two-component liquid, avoid installing it downstream where the mixture is not yet uniform; when measuring chemical reaction pipelines, install it downstream of the reaction section that is fully completed; try to meet the requirement that the front and rear straight pipe sections are not less than 5D and 2D respectively;

b. Avoid negative pressure in the measuring tube as much as possible;
c. Choose a place with small vibration, especially for integrated instruments;
d. Avoid large motors, large transformers, etc. nearby to avoid electromagnetic field interference;
e. A place where it is easy to achieve separate grounding of the sensor;
f. Avoid high concentrations of corrosive gases in the surrounding environment as much as possible;
g. Avoid direct sunlight as much as possible.

3.4 Environmental conditions

General instruments used in explosive hazardous environments should be selected according to the provisions of standard GB3836.1 "General Requirements for Electrical Equipment in Explosive Environments", and instruments that are adapted to the atmosphere of the use environment, the classification and grouping of explosive mixtures, the type of equipment, and other safety rules or standards should be selected. In addition, external magnetic fields, electrical interference, ambient temperature and humidity are also key points to consider in EMF selection.

4 Conclusion

EMF is easy to use and requires little maintenance, mainly cleaning and replacement of electrodes. There are also electrode-free EMFs that require almost no maintenance. Of course, it is also very necessary to perform regular online calibration and inspection of EMF. As long as you have a comprehensive and in-depth understanding of the performance and characteristics of EMF and have strategies to cope with various working conditions, EMF will definitely become a powerful assistant in chemical measurement. (end)