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Source: Boss Wu from the chemical industry. Boss Wu said that instrument maintenance personnel need to improve their ability to diagnose instrument failures and gain a better understanding of the principles, structure, and performance characteristics of instruments. However, there are over 60 different types of flow meters used in chemical production – how can instrument technicians maintain and ensure the proper operation of these instruments? Regarding the installation and troubleshooting of flow meters, this article covers one of the commonly used types of flow meters; feel free to leave comments for discussion! Characteristics and application scenarios of the four main types of flow meters: There are a wide variety of flow meters, but this article focuses on the installation and maintenance of electromagnetic flow meters! 01 Electromagnetic flow meter: An electromagnetic flow meter is a device for measuring conductive liquids, based on Faraday’s law of electromagnetic induction. An electromagnetic flowmeter consists of three components: a sensor, a converter, and cable connections, all of which are essential. If the cable connection is short, it is installed inside the flow meter housing. Electromagnetic flowmeters possess a range of excellent features that enable them to address issues that are difficult for other types of flowmeters to handle, such as the measurement of dirty or corrosive fluids. Advantages: The measurement channel is a section of smooth straight pipe that does not get clogged, making it suitable for measuring liquid-solid two-phase fluids containing solid particles, such as pulp, sludge, and sewage. It does not cause pressure losses resulting from flow measurement, thus offering good energy-saving effects. The measured volumetric flow rate is essentially unaffected by changes in fluid density, viscosity, temperature, pressure, or conductivity. It has a wide range of flow rates and a broad range of pipe diameters, and can be used with corrosive fluids. Disadvantages: It cannot measure liquids with very low conductivity, such as petroleum products; it cannot measure gases, vapors, or liquids containing large bubbles; it cannot be used at high temperatures. 02 Classification of electromagnetic flowmeters: The electromagnetic flowmeters commonly used in chemical plants are from joint-venture or imported brands such as E+H (Endress+Hauser), KROHNE, ABB, Yokogawa, Siemens, and so on. Domestic brands of electromagnetic flowmeters include Shanghai Guanghua, Chuanyi, Wilte, Xisen, Zhejiang Diyuan, and others. 03 Installation requirements for electromagnetic flowmeters – Selection of the installation location. To ensure the stable and reliable operation of electromagnetic flowmeters, the following considerations should be taken into account when selecting the installation site: 1. Try to avoid ferromagnetic objects as well as devices with strong electromagnetic fields (such as large motors and large transformers), so as to prevent the magnetic field from affecting the working magnetic field of the sensor and the flow signal. 2. It should be installed in a dry and well-ventilated area, away from direct sunlight and rain. The ambient temperature should range from -20 to +60°C, with a relative humidity of less than 85%. 3. There should be sufficient space around the flow meter to facilitate installation and maintenance. 04 Eleven Installation Tips 1 Ensure the measurement tube is fully filled: The electromagnetic flowmeter’s measurement tube must be in a fully filled state; it must not be partially filled. If the pipeline is not fully filled or there is a vent at the outlet, the sensor should be installed on a siphon tube. Avoid installing it upstream of pipes that lead downward; also avoid installing it at the highest point of the pipe, as this can lead to the accumulation of bubbles. The sensor can be installed both horizontally and vertically, but it is important to ensure that sediment and bubbles do not affect the measuring electrodes; it is best to keep the axis of the electrodes horizontal. When installed on a horizontal pipe, the two measuring electrodes should not be directly above and below the pipe. When installed vertically, the fluid should flow from bottom to top. The straight pipe section at the inlet should be greater than 5D, and the straight pipe section at the outlet should be greater than 2D. If there are specific installation requirements, the electromagnetic flowmeter should be installed in accordance with its instruction manual. The straight pipe section at the inlet should be ≥ 20 D, and the straight pipe section at the outlet should be ≥ 7 D (where D is the nominal diameter of the sensor). Installation in horizontal pipes, installation in vertical pipes. 3. Installation when the outlet is used for venting: When the outlet is in a venting mode, the sensor should not be installed at the point where the pipe vents; it should be installed at a lower location. When the sensor is installed below the pipeline, it must be ensured that the sensor is filled with liquid, with no empty space inside. 4 Series and parallel installation: If several sensors need to be connected in series along the same pipeline in order, the distance between each sensor should be at least 2 times the length of one sensor. If two or more sensors are installed in parallel with each other, the distance between them must be greater than 1 m. 5. To ensure the stability of measurements, straight pipe sections should be installed in front of and behind the sensor when connecting elbows, valves, and pumps; their lengths are indicated in the figure below. If this cannot be achieved, a flow stabilizer should be used or the cross-sectional area of the measurement point reduced. To avoid negative pressure, the sensor should not be installed at the pump’s inlet, but rather at its outlet. 6 Installation of a bypass pipe: The pipeline in which the flow meter is installed should be equipped with a valve that can shut off the fluid flowing within it, to facilitate routine repairs and maintenance. When the process does not allow flow interruptions. A bypass pipe can be added to the pipeline where the flow meter is installed. 7 Install the flow meter sensor in the calibration direction: When there is a clear indication of flow rate, it should be installed in the calibration direction. 8 inlet ports; for flow meters connected to cables and conduit tubes. The cables and conduit tubes should have a downward bend with the opening at the lowest point, to prevent water from entering and causing short circuits. 9 When installing a welded flange flow meter, the flange should be welded first and then the flow meter installed; it is not possible to weld the flange directly with the flow meter sensor in place, as this could cause the lining to be damaged by welding slag. 10 Distance between converter and sensor: The signal cable between the converter and the sensor must not be longer than 50 meters, and it must be enclosed in a galvanized tube. 11 Grounding measures for sensors: The flow signal generated by sensors is very small, amounting to only a few millivolts at full scale; therefore, the sensors must be properly grounded. The grounding requirements for electromagnetic flowmeters involve two aspects: From the perspective of the working principle of electromagnetic flowmeters and the circuit of the flow-induced signal current, the grounding terminals of the sensor and converter must be at the same potential as the medium being measured. Grounding, using the earth as a zero potential point, helps reduce external interference. Under normal circumstances, process pipes are metal pipes that are grounded by default, so this requirement is easily met. However, in situations with significant interference from external electromagnetic fields, an additional grounding system should be installed for the electromagnetic flowmeter. The grounding wire should be made of multi-strand copper wire with a cross-sectional area of more than 5 mm2. The grounding wire of the sensor must not be connected to the common grounding wire of the motor or other equipment, in order to avoid the effects of leakage current. The ground resistance should be less than 10Ω. The sensor is installed on the metal pipe (which has no insulating coating on its inner wall) and grounded as shown in the figure below. The sensor is installed on plastic pipes or pipes with insulating linings. Grounding rings, grounding flanges, or short tubes equipped with grounding electrodes should be installed at both ends of the sensor, as shown in the diagram below for grounding purposes. It ensures that the medium flowing inside the pipe is short-circuited to the ground, resulting in a zero potential; otherwise, the electromagnetic flowmeter cannot function properly. 05 Faults of Electromagnetic Flow Meters and Troubleshooting Methods Acceptance of Electromagnetic Flow Meters During the acceptance phase, the sensor, converter, and connection cables are generally inspected. For sensors, measurement can be carried out based on air traffic conditions: electrode insulation performance ； Excitation coil resistance and insulation performance ； Check the quality of the lining material ； For converter testing, an analog signal generator should be used; it is imperative not to check the converter’s output after connecting it to the sensor, as the output at that time will be random, with the displayed information also being arbitrary. For the inspection of connection cables, it is mainly checked whether the cable model and length meet the requirements. The initial assembly and calibration of electromagnetic flowmeters should focus on the following analyses and checks: whether the operating conditions of the instrument meet the requirements (humidity, pressure, flow rate, protection rating) ； Is the instrument selection correct? ； Check whether the instrument is installed properly (grounding, straight sections before and after it, wiring); there are five improper installation methods to avoid ； Gas gathering point – bubble accumulation should be avoided. Partially filled pipe – ensure the pipe is fully filled during measurement. Siphonage prevention – siphonage phenomenon must not occur. Pilot valve – the control valve should be installed downstream of the flow meter. Backward pump – a pump should be installed upstream to eliminate negative pressure. External interference from connected cables also needs to be taken into account, as well as the possibility of faults in the instrument itself ； After power is applied, the zero point must be adjusted while the electromagnetic flow meter is filled with liquid and at rest. Inspection tasks after the electromagnetic flow meter starts operating include performing zero-point checks on a regular basis when flow is stopped, in order to check for issues such as wear and tear, corrosion, leakage, and scaling of the sensor electrodes. Especially for precipitated or easily contaminated electrodes, non-cleaning liquids containing solids ； Decrease in exciter coil insulation ； Decrease in converter insulation ； Converter circuit failure ； The connection cable is damaged, short-circuited, or damp ； New changes in the operating conditions of the instruments cannot be ruled out. 06 Q&A Common Faults During the Operation of Electromagnetic Flow Meters and Their Solutions 1: No signal output for the measured flow rate? Cause analysis: Abnormal power supply to the instrument ； The cable connection is abnormal ； The liquid flow conditions do not meet the installation requirements ； Damage to sensor components or an adhesive layer on the inner wall being measured ； The converter component is damaged. Solution: Confirm that power is connected, check whether the voltages output by the power circuit board are normal, or try replacing the entire power circuit board to determine if it is faulty. Check whether the cable is in good condition and the connection is correct. Check the direction of liquid flow and whether the tube is filled with liquid. For electromagnetic flowmeters that can measure in both forward and reverse directions, although measurement is possible when the direction is different, the displayed flow rate will indicate the wrong direction; this must be corrected. If removing the sensor is too time-consuming, it is also possible to change the direction of the arrow on the sensor and reset the symbols on the display instrument. The fact that the pipe is not filled with liquid is also caused by an improper installation location of the sensor; measures should be taken during installation to prevent the pipe from not being fully filled with liquid. Check whether the electrodes on the inner wall of the transducer are covered with a layer of liquid scum; for measurement liquids that tend to form scum, cleaning should be carried out regularly. If it is determined that the fault is caused by a damaged converter component, simply replace the damaged component. 2 Fluctuations in output value? Cause analysis: Such failures are mostly caused by the measuring medium or external environmental factors; once the external interferences are removed, the fault disappears on its own. To ensure the accuracy of measurements, such faults cannot be ignored either. In some environments, excessive vibration of the measurement pipeline or liquid can cause the circuit board of the meter to become loose, which in turn can lead to fluctuations in the output values. Solution: Determine whether it is due to process operations that cause pulsation in the fluid; in such cases, the flow meter simply reflects the actual flow conditions, and the fault will resolve on its own once the pulsation stops. Electromagnetic interference caused by external stray currents and the like. Check whether there are any large electrical appliances or welders in operation in the environment where the instrument is located; ensure that the instrument is properly grounded and that its operating environment is suitable. When the pipeline is not filled with liquid or contains bubbles, both situations are caused by process-related factors. At this point, it is possible to ask the process engineer to confirm that once the tube is filled with liquid or the bubbles subside, the output value will return to normal. The transmitter’s circuit board has a plug-in design, and due to the significant vibrations in the pipelines or liquids at the installation site, this often leads to the loosening of the power supply board of the flow meter. If it becomes loose, the flow meter can be disassembled and the circuit board reattached. 3 Is a signal output when there is no flow after power is applied? Cause analysis: Poor input shielding or grounding, resulting in electromagnetic interference ； The instrument is located near high-voltage equipment or sources of high-frequency pulse interference ； The pipeline experiences strong vibrations ； The converter’s agility is too high ； Solution: Improve shielding and grounding to eliminate electromagnetic interference ； After a long period without interference sources present, isolation measures are taken to improve power supply filtering ； Adopt damping measures and enhance signal filtering to reduce the sensitivity of the amplifier ； Reduce agility and increase the trigger level. 4 Is the measurement error large? Cause analysis: Excessively high pressure inside the pipe; incorrect selection of operating pressure ； Seal damaged ； The sensor has been eroded ； Solution: Adjust the high voltage and change the installation location ； Choose a higher-grade engineering pressure sensor ； Replace the seal ； Take anti-corrosion and protective measures.