[2025 Instrument Encyclopedia] Selection of Electrode Material for Electromagnetic Flow Meters

This post was last edited by zhoudingshengs on 2025-3-7 09:45. The replies below are not based on the text in the attached file and are for reference only. When selecting an electromagnetic flowmeter, the following seven key points should be considered: 1. Accuracy and functions: Choose an electromagnetic flowmeter with the appropriate accuracy level based on the measurement requirements, while also taking into account the desired functions such as two-way measurement and alarm capabilities. 2. Flow rate and diameter: Determine the range of fluid flow rates and select an appropriate sensor diameter to ensure measurement accuracy. 3. Fluid properties: Consider the fluid’s conductivity, corrosiveness, wear resistance, and cleanliness in order to select appropriate lining and electrode materials. 4. Pressure and temperature: Select an electromagnetic flowmeter with the appropriate pressure rating and temperature tolerance range, based on the operating pressure of the pipeline system and the highest operating temperature of the fluid. 5. Installation conditions: Consider the pipe dimensions, installation method, as well as grounding and shielding requirements, to ensure the safe installation and operation of the electromagnetic flowmeter. 6. Power Supply and Output: Determine the required power supply and type of output signal to meet the system’s requirements. 7. Compatibility and communication: Select an electromagnetic flowmeter that supports the required communication protocols to ensure compatibility with the automated control system.

There are eight requirements for the installation of electromagnetic flowmeters: 1. Installation location and direction: They can be installed vertically, horizontally, or at an angle, but it is necessary to ensure a full-bore condition; it is best for the fluid to flow through the instrument from bottom to top. 2. Avoid magnetic field interference: The installation location should be kept away from all sources of magnetism, such as large motors, large transformers, and welding machines. 3. Grounding requirements: The transmitter housing, shielded wires, measurement conduit, and the pipes at both ends must all be grounded, with separate grounding points provided. 4. Signal cable: Use the specified shielded cable; the signal wires and excitation wires should be laid separately and kept away from the power cables. 5. Power requirements: The transmitter and converter must use the same phase of power supply. 6. Length of straight pipe section: There should be a straight pipe section of not less than 5D on the upstream side, and one of not less than 3D on the downstream side. 7. Installation environment: Avoid installing in areas with large temperature fluctuations, corrosive gases, strong vibration sources, or direct sunlight. 8. Maintenance space: To facilitate installation, maintenance, and upkeep, there must be sufficient space around it for these purposes.

The nine methods for maintaining electromagnetic flowmeters include: 1. Resistance method: Checking the continuity of fuses, cables, and excitation coils, etc. 2. Current method: Measure the excitation current and output current. 3. Voltage method: Determine whether the power supply is correct and check the symmetry of the electrodes. 4. Waveform method: Measure the waveforms at key points to identify internal faults in the converter circuit board. 5. Substitution method: Utilize the interchangeability between converters and sensors to determine the location of the fault. 6. Signal tracking method: Use a PLC program to monitor the flow meter signal. 7. Observation method: Observe the display window to determine whether the converter is functioning properly. 8. Short-circuit method: Use a wire to short-circuit the signal terminal in order to identify faults in the converter, sensor, or transmission cable. 9. Multimeter measurement method: Measure the resistance of the sensor’s excitation coil and signal terminals to determine the sensor’s condition. These methods cover a comprehensive inspection from the power supply to the converter, sensors, and wiring system, to ensure the proper operation of the electromagnetic flowmeter.

　　Choosing the flow range for an electromagnetic flowmeter has a significant impact on its accuracy and service life. Under normal circumstances, the rotational speed corresponding to the flow rate during operation should not be too high. The operating conditions are divided into continuous operation and intermittent operation. Continuous work refers to working more than 8 hours per day, while intermittent work refers to working less than 8 hours per day. For continuous operation, the flow rate should be selected at a lower value within the instrument’s upper range; for intermittent operation, a higher value can be chosen. Under normal circumstances, for continuous operation, the actual flow rate is multiplied by 1.4 to obtain the upper limit of the flow rate range, while for intermittent operation, it is multiplied by 1.3. When the diameter of the instrument differs from that of the process pipeline, reducer pipes and straight pipes of the same diameter should be used for modification.

　　Generally, the choice depends mainly on its height. Currently, the accuracy levels for electromagnetic flowmeters are as follows: for liquids, the international standards are ±0.15%R, ±0.2%R, ±0.5%R, and ±1%R, while domestically produced models have accuracy levels of ±0.5%R and ±1%R. For gases, the international standards are ±0.5%R and ±1%R, whereas domestic models have accuracy levels of ±1%R and ±1.5%R. The above accuracy ranges correspond to a ratio of 6:1 or 10:1. In addition to being related to the quality of the product itself, it is also closely tied to the conditions of use.

　 Electromagnetic flowmeters require the fluid to be clean (or essentially clean), single-phase, and of low viscosity. Common fluids include: general fluids such as water, air, oxygen, high-pressure hydrogen, milk, coffee, etc.; petrochemicals such as gasoline, light oil, jet fuel, light diesel, naphtha, ethylene, polyethylene, styrene, liquefied gas, carbon dioxide, and natural gas; chemical solutions such as ammonia water, methanol, saline water, etc. ;Organic liquids: alcohol, ether, benzene, toluene, xylene, butadiene, carbon tetrachloride, methylamine, acrylonitrile, etc. Inorganic liquids: formaldehyde, ochre acid, sodium hydroxide, carbon disulfide, etc. For corrosive media, care must be taken in selecting the material; those with high impurities and abrasive media are not recommended.

　　When choosing an electromagnetic flowmeter for high-end applications, many economic factors need to be taken into consideration. The purchase cost of the instrument is only part of the total cost; other expenses such as auxiliary equipment (such as degasers, filters, etc.) or bypass branches including valves must also be taken into account. Regarding inspection costs, to maintain high standards, regular inspections are necessary, and even the installation of an online inspection system on-site incurs considerable costs; maintenance costs, as well as the replacement of wear-prone parts in electromagnetic flowmeters, are also required to ensure high performance.

　　Fluids containing impurities such as circulating cooling water, rivers, sewage, and fuel; areas where flow rates change rapidly, such as boiler feedwater systems and air hammer supply systems; when measuring liquids, when the pipeline pressure is low but the flow rate is high, the pressure on the downstream side of the instrument can approach the saturation vapor pressure, posing a risk of cavitation – in such cases, substances like liquid ammonia can flow freely from higher-level tanks, so the instrument should not be installed near discharge outlets; locations with severe electromagnetic interference, such as welders, motors, and contact relays; situations where the length of the straight sections upstream and downstream is insufficient, such as inside the engine rooms of ships; in boiler automatic feedwater systems, frequent starting and stopping of pumps can cause shocks to the impellers, leading to rapid damage of the sensors; careful consideration must be given when selecting instruments for use with corrosive or abrasive media.

Classified by excitation method: 1) DC excitation type – There are very few of these electromagnetic flowmeters, and they are used only for measuring the flow rate of liquid metals, such as mercury at room temperature, and liquid sodium and potassium at high temperatures. 2) The AC power-frequency excitation type, which was used in earlier electromagnetic flowmeters and relied on 50Hz power-frequency mains for excitation, has gradually been replaced by low-frequency rectangular excitation due to issues such as susceptibility to electromagnetic interference and zero-point drift. However, when measuring liquid-solid two-phase flows such as slurry and pulp, the low-frequency rectangular wave excitation method fails to overcome the spike noise generated by solids passing over the electrode surface, whereas meters with power-frequency AC excitation do not have this drawback; as a result, some electromagnetic flowmeters both at home and abroad still use AC excitation. 3) The low-frequency rectangular wave excitation type is used because it has low power consumption in the low-frequency rectangular wave excitation mode and does not cause instability at the zero point; it is currently the main excitation method for electromagnetic flowmeters. Its waveform has two types: binary \"positive–negative\" and ternary \"positive–zero–negative–zero\". In some electromagnetic flowmeters, the excitation frequency can be set by the user; generally, higher frequencies are used for small-diameter instruments, while lower frequencies are used for large-diameter instruments. 4) The waveform of the excitation current in the dual-frequency excitation type is a high-frequency rectangular wave superimposed on a low-frequency rectangular wave, primarily to overcome the slurry noise and motion noise associated with binary rectangular-wave excitation, thereby improving the stability and response characteristics of the instrument.