Electromagnetic flowmeter to reduce the impact of elbow installation on metering

As a precision flow measurement instrument, electromagnetic flowmeters have relatively strict requirements during installation, such as the impact of external electromagnetic interference, grounding requirements, and requirements for front-end and rear-end straight pipes. , however, sometimes due to on-site conditions, the installation cannot reach the ideal state. Therefore, the instrument installer needs to take corresponding measures to adjust and optimize the on-site installation situation, so that the operation of the instrument can ultimately meet the design requirements. Various situations will also be encountered during the installation and use of electromagnetic flowmeters, which may cause large errors in measurement or even make it impossible. This requires instrument installation and operating personnel to be able to perform inspections and repairs in accordance with the standard use and maintenance specifications of instrument equipment. Having been engaged in electromagnetic production and service for many years, the company's technical staff have rich experience in the installation and maintenance of electromagnetic flowmeters, and have relatively complete solutions for various situations that arise. This article explains this and I hope it will be helpful to you.

Check and take measures if the flow measurement value does not match the actual value 4.1 Cause of failure 1) The converter setting value is incorrect. 2) The sensor is installed in an improper position, the tube is not full or there are bubbles in the liquid. 3) The signal cable is not properly handled or the cable insulation is degraded during use. 4) The flow conditions upstream of the sensor do not meet the requirements. 5) The resistance between the sensor electrodes changes or the electrode insulation decreases. 4.2 Troubleshooting and taking measures 1) Review the converter settings and check the zero and full scale values. First check whether the numbers of the matching sensors and converters are correct. Most contemporary sewage flowmeters have the calibrated instrument constants marked on the sensor (or the "Instruction Manual" attached with the meter) after real-flow calibration at the manufacturer, and are set in the matching converter. Therefore, before commissioning a newly installed instrument, you must first review the instrument constants, whether the sensor number and the converter number match. Because this type of mismatch occurs from time to time, it is necessary to review the setting values ​​such as caliber, range and measurement unit. 2) Check the condition of the pipeline fluid and whether it contains bubbles. This type of failure is mainly caused by poor pipeline network engineering design or imperfect related equipment. Please refer to the content that the pipeline is not filled with liquid or the liquid contains a few bubbles. 3) Check the signal cable system; check whether the connecting cable is properly matched? Is the connection correct? Is the insulation reduced? Usually people check the cause of the failure of the sewage flow meter to measure flow inconsistency, and often ignore the cable system connecting the sensor and the converter. Often encounter In the following cases: a) Cut off the entire attached cable and reconnect it. After a period of use, moisture is absorbed into the connection and the insulation decreases; b) The end of the signal line is not properly treated, and the inner shielding layer, outer shielding layer and signal core are in contact with each other. There is a short circuit, or a short circuit with the shell; c) The cable of the specified model (or attached) is not used; d) The cable length exceeds the upper limit restricted by the liquid conductivity; e) The liquid conductivity is low and the sensor and converter are far apart Far away, 4) Investigate the flow conditions upstream of the sensor. The flow conditions upstream of the sensor are often limited by the installation space and deviate from the specified requirements. For example, close to theThe flow-disturbing blocking parts and the lack of a straight pipe section of sufficient length will introduce factors that affect the accuracy of the measurement. Especially if a regulating valve or a gate valve that is not fully open is installed upstream of the proximity sensor, a satisfactory solution is to change the installation position of the sensor; if the length of the upstream straight pipe section is insufficient, adjust the installation position. 5) Detect the contact resistance and electrode insulation between the electrode and the liquid; the contact resistance value between the electrode and the liquid mainly depends on the contact area and liquid conductivity. When filled with domestic and industrial water with a conductivity of 150 × 10 - 6 s /cm, the contact resistance between the electrode and the liquid is about 15 kΩ. The electrode insulation resistance should be greater than 100 MΩ.

2. Check and take measures for no flow signal output 1.1 Cause of failure 1) Power supply failure. 2) The connecting cable (excitation circuit, signal circuit) is faulty. 3) Failure in liquid flow conditions. 4) Sensor parts are damaged and faulty. 5) The converter components are damaged. 1.2 Troubleshooting and taking measures 1) Check for power supply failure. First, check the main power supply and excitation current fuses. If a new fuse that meets the specified current value is connected and then energized and it blows, the fault must be found. Check whether the output voltage of each channel of the power circuit board is normal, or replace the entire power circuit board. 2) Check for faults in the connecting cable system. Check separately whether the cables connecting the excitation system and the signal system are connected and whether the connections are correct. 3) Check the direction of liquid flow and the filling of the liquid in the tube. The direction of liquid flow must be consistent with the direction of the arrow on the sensor housing. For sewage flow meters that can measure forward and reverse, if the directions are inconsistent, they can still measure, but the set display flow direction does not match the forward and reverse directions and must be corrected. If the pipeline is not filled with liquid, it is mainly due to improper pipeline network engineering design or improper sensor installation location. If the sensor is installed at positions a and e and dotted pipe discharge position b, it should be modified to positions c and d, see the figure.

4) Check the integrity of the sensor. Mainly check the integrity of each terminal and excitation coil. Failures of the excitation coil and its system often include: a) coil disconnection; b) insulation degradation of the coil or its terminals. The frequency of insulation degradation among type II faults is relatively high. Coil disconnection and insulation degradation can be easily checked with a multimeter and megger. 5) Check for converter failure. The inspection method of sewage flow meter converter is often to use circuit board spare parts replacement method to try to troubleshoot.

3. Causes, inspections and measures of output sloshing failure 2.1 Causes of failure 1) The flow itself is fluctuating or pulsating, which is not essentially a fault of the sewage flow meter. It only reflects the flow conditions truthfully. 2) The pipe is not filled with liquid or the liquid contains bubbles. 3) Electrical and magnetic interference such as external stray current. 4. 2． 2. Troubleshooting and taking measures 1) Fluctuations in the flow itself. If the flow itself fluctuates, the instrument output fluctuation will truly reflect the fluctuation situation. Inspection methods can be asked to operators and process technicians at the site of use or if there are any fluctuations.source. Check whether there are flow-blocking parts in the upstream pipe of the sewage flow meter to generate vortex. In pipelines with pulsating flow sources, to slow down their impact on flow meter measurements, the flow sensor is usually placed away from the pulsation source and the pipe flow resistance is used to attenuate the pulsations; or an air chamber called a passive filter is installed at an appropriate position in the pipeline. Buffer, absorb pulsation. 2) The pipeline is not filled with liquid or the liquid contains bubbles. This type of failure is mainly caused by poor engineering design of the pipeline network, which causes the measuring tube of the sensor to not be filled with liquid or the sensor is improperly installed. There is no back pressure or insufficient back pressure downstream of the sensor. If it is installed in position e, the liquid flows through a short section of pipe downstream and is discharged into the atmosphere. If valve 2 is fully open, the sensor measuring tube may not be filled with liquid. Sometimes if the flow rate of the fluid is large enough to fill the meter, the meter will run normally. If the flow rate decreases, the meter may not be filled enough and cause the meter to malfunction. If the liquid contains gas, the output signal will fluctuate even more. The liquid contains tiny bubbles, which will gradually accumulate at high points or dead ends during the flow process, covering the electrodes and causing the output to shake. 3) External electromagnetic interference. Sewage flow meters are susceptible to external interference due to their small flow signals. The main sources of interference include pipeline stray currents, static electricity, electromagnetic waves and magnetic fields. Stray current in pipelines is mainly protected by good grounding of the sewage flow meter. Usually the grounding resistance should be less than 10 Ω. Do not share the grounding with other motors and electrical appliances.

IV. Inspection and measures for zero point instability 3.1 Cause of failure 1) The pipeline is not filled with liquid or the liquid contains bubbles. 2) Subjectively thinking that there is no flow of liquid in the pipeline system but in fact there is a slight flow; in fact, it is not a fault of the sewage flow meter, but a misunderstanding that truthfully reflects the flow conditions. 3) The sensor is not grounded perfectly and is subject to external interference such as stray current. 4) Reasons related to liquid (such as liquid conductivity uniformity, electrode pollution, etc.). 5) The insulation of the signal circuit decreases. 3.2 Troubleshooting and measures to be taken 1) The pipeline is not filled with liquid or the liquid contains bubbles. Please refer to the content that the pipe is not filled with liquid or the liquid contains bubbles. 2) There is trace flow in the pipeline. This type of fault is mainly caused by the poor sealing of the stop valve of the pipeline, and the small leakage detected by the sewage flow meter is misunderstood as zero point change or zero point instability. 3) Imperfect grounding is affected by external interference and changes in ground potential. The influence of external interference such as stray current in pipelines mainly depends on the good grounding protection of the sewage flow meter. The grounding resistance is usually required to be less than 10 Ω, and the grounding should not be shared with other motors and appliances. 4) Check the physical properties of the liquid. Changes or unevenness in the conductivity of the liquid will cause the zero point to change when stationary and cause the output to shake when flowing. If the liquid contains impurities, or impurities are deposited on the inner wall of the measuring tube, or scale is formed on the inner wall of the measuring tube, or the electrode is contaminated with grease or other contamination, etc., the zero point may change. The measure is to remove dirt and scale deposits; if the zero position changes greatly, you can also try to reset it to zero. 5) Check the signal line insulation. Decreased insulation of the signal loop will cause zero point instability. The main reason for the insulation degradation of the signal loop is the degradation of the insulation of the electrode parts, but it cannot be ruled out that the insulation of the signal circuit is degraded.The insulation of the cable and its terminals is reduced or damaged. Because sometimes the on-site environment is very harsh, and the instrument cover and wire connections are not sealed carefully, moisture, acid mist or powdery dust invades the instrument junction box or cable protective layer, causing the insulation to degrade. The insulation resistance of the signal loop is checked on the cable side and the flow sensor side respectively, and tested with a megohmmeter. Carry out in two steps. a) Fill the liquid measuring electrode surface with the liquid contact resistance flow sensor signal line. Use a multimeter to measure the resistance between each electrode and the ground point. The resistance value of the two electrodes to the ground should be between 10! and 20!. b) Empty tube measurement electrodes are insulated and vented to the measuring tube. Wipe the inner surface with a dry cloth. After it is dry, use an H500 VDC megohmmeter to measure the resistance value between each electrode and the ground. The resistance value must be above 100M Ω.