Online Inspection and Verification of Electromagnetic Flowmeters (Reposted)

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Mr. Cai Wuchang, a professor-level senior engineer with the Flow Meters Professional Association of the China Instrument and Control Industry Association, and an editorial board member of this journal. Keywords: Electromagnetic flowmeter, Flow calibration, Online inspection 1. Introduction Electromagnetic flowmeters are widely used in the process industry and utilities; as required, such instruments must operate under controlled conditions and be calibrated regularly. The methods for checking the flow value of flow meters usually include offline and on-site online inspection. Since both of these methods have their shortcomings, practical users have developed several indirect on-site inspection methods to verify or assess whether the flow measurement values of electromagnetic flowmeters have exceeded the range corresponding to the original calibration accuracy level, thereby providing a basis for deciding whether to continue using them or to conduct further inspections. This article provides a brief overview of recent developments in domestic and international fields, including inspection methods for on-site non-steady flow rate calibration, practical experience, development of specialized instruments, and the formulation of standards for on-site calibration. II. Current status of online inspection 　At present, there is a lack of comprehensive understanding regarding online inspection of electromagnetic flowmeters; only a few Japanese companies have published reports in recent years on the implementation of non-flow-based online inspection methods ； Additionally, the Shanghai area began exploring and developing “online inspection methods” a few years ago. 1. Chemical industry 　　Electromagnetic flowmeters are used in the chemical industry primarily for flow control. The fluids measured are mostly acidic or alkaline liquids and slurries, which are often corrosive and abrasive. In practical applications, failures and malfunctions of electromagnetic flowmeters are often caused by corrosion and leakage, reduced insulation, contaminated electrodes, or the accumulation of foreign substances. 　　The traditional regular maintenance procedure for electromagnetic flowmeters involves removing the flow sensor from the pipeline for cleaning and inspection, followed by flow calibration. To minimize damage to the lining caused by removing the flow sensor from the pipeline, it is first necessary to measure parameters such as insulation resistance on the pipeline in order to determine whether there are any abnormalities, and then decide whether to proceed with removing the pipeline for inspection or performing a calibration of the actual flow rate. The proportion of the three inspection methods used by Mitsubishi Chemical Co., Ltd. is as follows: (1) Online inspection alone accounts for 35% ； (2) 22% is allocated to inspection after removing the pipeline for cleaning the liquid contact area ； (3) Offline real-flow calibration accounts for 43%. 2. Water Supply Industry 　　(1) Tokyo Metropolitan Waterworks Bureau 　　The Tokyo Metropolitan Waterworks Bureau conducts a comprehensive inspection of electromagnetic flowmeters once a year. The inspection includes: visual inspection, testing of the converter’s characteristics, calibration of the measurement values, measurement of voltages in various parts, measurement of insulation resistance, and verification of the electrical circuit. When checking and adjusting instruments, zero drift makes it very important to adjust the zero point; however, \"on-line zero adjustment\" requires the flow of the medium being measured to stop, which is not easy to achieve. Therefore, on-site, the checks related to sensor operation can be omitted, and only the calibration of the converter is performed. Compare the results of this inspection with historical data to determine whether the instrument should be continued in use, repaired, or replaced. Whether to replace the sensor is determined based on the degree of degradation in the insulation resistance of the excitation coil being measured. 　　(2) Shanghai Water Supply Company and Raw Water Company Since the 1990s, the Shanghai Water Supply Company and Raw Water Company have begun to explore methods for conducting online inspections to detect any abnormalities. For pipelines with no possibility of being taken out of service, the flow sensors and converters are inspected separately; the converters are tested using analog signal generators and other standard instruments, achieving a high level of calibration accuracy (depending on the accuracy of the analog signal generators), using the same methods as those for offline inspections. Sensor inspection involves indirect methods such as testing the electrode contact resistance, checking the insulation resistance and copper resistance of the excitation coil including the excitation connection cables, as well as examining the excitation current output by the converter to verify the magnetic field strength. For pipelines with shutdown conditions, access can also be obtained through access holes pre-installed near the sensors, to inspect the condition of contamination/deposition on the electrodes and lining and to clean them. III. Contents of On-site Inspection The contents of the on-site online inspection of electromagnetic flowmeters are shown in Table 1. Except for the zero-point check, the flow sensor, converter, and connection cable are all tested separately. 1. Zero-point check of the entire unit (Item 1) This check requires that the measurement tube of the flow sensor be filled with liquid and free of flow, but this is not possible in many field conditions. When the conditions are not met, it is necessary to give up zero-point inspection and adjustment of the entire unit, and instead perform only separate zero-point inspection and adjustment on the converter. It only makes sense after the sensor has been inspected, and provided that the insulation resistance of both the sensor excitation circuit and the signal circuit is normal (including the cables); otherwise, the entire device cannot operate properly. Typically, the zero point of the converter is negative and its value is quite small. If the absolute value exceeds 5% of the full scale, it is necessary to conduct an inspection first; adjustments should be made after the cause has been identified. Under normal circumstances, the difference between the zero point of the entire electromagnetic flowmeter and the zero point of the converter alone is less than 1%. In many cases where the zero value is greater than 5%, it is caused by the user failing to perform proper zeroing when the pipeline valves are not properly closed. 2. Connection cable inspection (Item 2) This inspection method is the same as the offline inspection; it checks the conductivity of each core of the signal lines and excitation lines as well as their insulation resistance, and verifies whether the grounding of each shielding layer is proper. 3. Converter inspection (items 3, 4) This inspection method is the same as the offline inspection. On-site or in the control room, general-purpose instruments along with analog signalers matching the flow meter model are used to replace the sensors and provide flow signals for zeroing and calibration. Calibration includes zero-point check and adjustment, setpoint check, excitation current measurement, current/frequency output check, etc. Some inspection items need to be compared with the previously recorded inspection values (or factory values) to determine whether there are any changes and whether those changes exceed the allowable range. 4. Flow sensor inspection (items 5–11): By checking the excitation coil and the excitation current measured by the converter, it is possible to indirectly assess whether the magnetic field strength has changed ； Measure the electrode contact resistance to assess the degree of contamination on the electrode surface and the condition of the coating adhering to the lining ； Check the insulation resistance of various components to determine the degree of component degradation and assess whether interference may result therefrom. If it can be taken out of service, the thickness of the electrode and lining adhesion layers can be observed and measured, allowing an estimation of the changes in flow rate resulting from variations in flow area before and after cleaning the adhesion layers. 　　(1) Measuring the copper resistance of the excitation coil: Use a high-precision digital multimeter or a Wheatstone bridge to measure the coil resistance; if necessary, apply a temperature coefficient correction before comparing it with the values recorded in the instrument’s database. Verify that the coil conducts properly and there is no inter-turn short circuit. 　　(2) Check the insulation resistance of the excitation coil. When the excitation coil and its connection terminals become damp, the insulation of the excitation circuit with respect to ground decreases, which can lead to the excitation signal being introduced into the flow signal transmission circuit. This results in a larger voltage division across the insulation resistance and signal resistance, thereby generating a significant common-mode interference signal. When this interference signal exceeds the suppression capability of the converter’s preamplifier, it causes the converter’s zero point to drift. When the drop in insulation resistance is not too severe, this phenomenon is not easily detectable while the instrument is in operation. Except for the IP68 version without terminal boxes, in practice due to negligence, the terminal boxes fail to remain sealed and become exposed to moisture; when the insulation resistance of the terminals drops below 5–6 MΩ, faults are likely to occur. Dry the terminals, and the fault is usually resolved. 　　(3) Check the electrode contact resistance The electrode contact resistance of the flow sensor should be measured immediately after the newly installed instrument has been calibrated, and the value should be recorded. In the future, measure once after each maintenance; analyzing and comparing these data helps determine the cause of instrument failures. 　　The contact resistance value between the electrode and the liquid depends on the conductivity of the liquid being measured at the contact surface. There are significant differences in the resistance values measured across different media. The electrode contact resistance can be measured using an analog multimeter by determining the resistance between each electrode terminal and ground when the measurement tube is filled with liquid. Experience shows that the difference between the measured contact resistance values of the two electrodes should be less than 10%–20%; otherwise, it indicates a fault. 　　If the measured electrode contact resistance does not match the original value, there may be three possible explanations: (a) the insulating coating on the two electrodes is not evenly distributed, or the insulation resistance of the signal circuit for one of the electrodes has decreased ； (b) The resistance value increases because the electrode surface is covered by an insulating layer ； (c) The resistance value decreases due to the adhesion of a conductive deposition layer on the surface of the lining near the electrode, or a decrease in the insulation of the electrode assembly (such as insulating sleeves). Sometimes, although the above phenomena occur, no fault is generated; this can serve as a precursor to a potential fault, and relevant measures should be taken in advance. 　　When using a pointer multimeter to measure the electrode contact resistance, the following points should be noted: (a) The resistance value should be taken as the maximum deflection of the pointer at the moment the measuring probe makes contact with the terminal, and the measured value should be based on this first reading. If the values measured due to polarization are inconsistent upon remeasurement. 　　(b) When measuring the resistance of the two electrodes, the polarity of the probes at the grounded terminal must be the same; that is, use the same probe from the multimeter to connect to one electrode, while keeping the other probe grounded at all times. 　　(c) To compare the two measurement values before and after regular inspections, the same type of multimeter must be used, along with the same range; the voltage range setting of 1.5V batteries is commonly used, such as the “×1kΩ” setting. 　　Using a multimeter to measure the electrode contact resistance only provides an approximate value. Accurate measurement must be performed using a digital bridge or an AC bridge (such as the Kohlrausch bridge, etc.). 　　(d) Measurement of the polarization voltage between the electrode and the liquid. Measuring this voltage helps determine whether the electrode is contaminated or covered, which could lead to issues such as unstable zero points or fluctuating outputs. 　　Use a digital multimeter set to the 2V DC range to measure the polarization voltage between each electrode and ground; electromagnetic flowmeters can be measured with power connected or without it. The magnitude of the polarization voltage depends on the electrode potential of the electrode material and the properties of the liquid; the measured value can range from a few millivolts to several hundred millivolts. If metal powder adheres to the electrode during operation, it will change the polarization potential ； The alternating attachment/detachment of metal powder to the electrodes also causes unstable output. Table 2 shows the natural potentials of several metals in seawater. 　　(e) Check the insulation of the signal circuit and the insulation between the excitation circuit and the signal circuit. The purpose of this check is also to determine whether interference has arisen due to reduced insulation. When checking the signal circuit, the signal wire must be temporarily disconnected from the electrode. There are many reasons for a decrease in insulation, such as moisture entering due to unsealed junction boxes, and inadequate moisture protection when cutting and reconnecting the cables of IP68-rated sensors. 　　(f) Check the insulation resistance of the electrodes and the condition of the lining. For small-diameter instruments, this inspection can only be carried out by removing the pipeline; for large-diameter instruments, the accumulated fluid (water) must first be drained, after which access to the inside of the pipeline is possible through an access hole. Dry the inner surface of the lining, and use a 500VDC megohmmeter to measure the insulation resistance between the two electrodes and ground. If there is an adhesive layer on the lining that must be removed, the future cleaning interval should be determined based on the thickness of the layer. If the conductivity of the adhesion layer is the same as that of the liquid and it is not thick, the additional error due to changes in area can be ignored ； If the attached conductivity is lower than that of the liquid, a positive additional error will be generated ； Conversely, it generates a negative additional error. 　　It is generally required that the insulation resistance of the electrodes be greater than 100 MΩ. Most cases of decreased insulation are caused by the electrodes, bushings, etc. becoming wet due to exposure to water from the outside environment; sometimes using a hot air blower to remove the moisture can restore normal performance. If the insulation is damaged (for example, due to corrosive fluid penetrating through the seals), the sensor must be replaced and sent back to the manufacturer for repair. IV. Development Trends in Online Inspection 1. Formulation of Standard Documents for Online Inspection 　With the cooperation of electromagnetic flowmeter manufacturers, the water supply companies and raw water suppliers in Shanghai have accumulated experience in inspecting more than 300 large-diameter electromagnetic flowmeters. Between 1997 and 1998, they drafted the \"Methods for Online Calibration of Large-Diameter Electromagnetic Flowmeters,\" which were then tested on a pilot basis by enterprises under the Shanghai Public Utilities Administration. Currently, the Shanghai Water Affairs Bureau is formulating a local industry standard, the \"Specifications for On-line Calibration of Electromagnetic Flow Meters\", based on this document. 2. Development of specialized instruments for online testing Starting from the mid-1990s, world-renowned manufacturers of electromagnetic flowmeters began to develop specialized testing instruments one after another. For example, ABB Kent was the first to develop the CalMaster inspector for water utilities between 1996 and 1997 ； In November 2002, Krohne introduced the MagCheck instrument (an electromagnetic flowmeter testing and inspection device) to the water industry market. The application areas for such specialized instruments have now expanded from the water industry to process industries such as the chemical industry. 　　Specialized testing instruments need to be used in conjunction with portable PCs; at the site or in the control room, one end of the specialized instrument is connected to the electromagnetic flow converter, while the other end is connected to the PC, thereby enabling testing and inspection.