The electromagnetic sewage flow meter is buried underground with the pipeline. The sensor has an IP68 protection structure. The converter is installed in the meter box or indoors. The two are connected by cables. Due to changes in site conditions such as ground subsidence and humid mountain environment, the sensor and converter have not been thoroughly treated with moisture-proof (waterproof) treatment at the cable joint. If the joint is in a humid environment, such as a meter well, cable trench, etc., moisture will invade the cable joint, which may cause the following:
(1) The insulation of the excitation coil to the ground decreases, causing the measurement result to be smaller;
(2) The contact resistance at the connection of the excitation circuit cable increases, causing the converter's excitation circuit to be in a non-constant current operating area. The excitation current decreases, which also causes the measurement result to be smaller. If the contact resistance is unstable, the measurement value will fluctuate.
(3) The insulation between the signal line and the ground decreases, causing signal attenuation, which ultimately results in a smaller measurement result;
(4) The insulation performance of the signal line excitation line to the ground is reduced, making the measurement result much greater than the normal data. If this interference is unstable, the impact on the hot water flow meter will also change, resulting in fluctuations;
(5) The contact resistance at the signal cable connection increases, causing the measured value to decrease. If the contact resistance is unstable, the measured value cannot be stable and is prone to interference.
The above abnormal conditions occur in the signal circuit, so it is difficult to judge. Only by going deep into the site and eliminating other possible faults of the circulating water flow meter, can the cable be checked for problems. The on-site instrument maintenance personnel only need to compare the on-site excitation circuit resistance measurement value with the data provided by the manufacturer, test the grounding resistance, and gradually determine the problem. Then, they can take measures, such as replacing the entire cable or keeping the cable joints dry.
The sewage electromagnetic flow meter will basically not malfunction within more than one year of operation, but after one year, it will often work abnormally, which is manifested in the measurement value becoming larger or smaller, or constantly fluctuating. After on-site inspection, the possibility of the pipeline being not full or the medium containing gas has been ruled out. It is found that such problems may also be related to cable failure.
1. The sewage electromagnetic flowmeter has no flow output
1. Cause Analysis This type of failure is common during use. The reasons are generally: (1) The power supply of the instrument is abnormal; (2) The cable connection is abnormal; (3) The medium flow condition does not meet the installation requirements; (4) The sensor parts are damaged or the lining has an adhesion layer; (5) The converter components are damaged.
2. Solution
(1) Confirm that the power supply is connected and check whether the voltages output by the power circuit board are normal, or try to replace the entire power circuit board to determine whether it is good or bad.
(2) Check whether the cables are intact and connected correctly.
(3) Check the flow direction of the measured medium and whether the medium in the pipe is full. For sewage electromagnetic flowmeters that can measure in both directions, if the directions are inconsistent, the measurement can be performed, but the set display flow in both directions does not match, and must be corrected. If the workload of removing the sensor is large, you can also change the direction of the arrow on the sensor and reset the display instrument symbol. The main reason why the pipeline is not full of medium is that the sensor is installed in an improper position. Measures should be taken during installation to strictly follow the installation requirements to avoid the pipeline being not full of medium.
(4) Check whether the inner wall electrode of the transmitter is covered with a medium scarring layer. For measuring media that are prone to scarring, clean them regularly.
(5) If the fault is determined to be caused by damage to the converter components, simply replace the damaged components.
2. Output value fluctuation
1. Cause Analysis
Most of these faults are caused by the influence of the measuring medium or the external environment, such as strong magnetic fields and strong electromagnetic waves. Therefore, when installing, it is generally necessary to keep away from strong magnetic fields. The sewage electromagnetic flowmeter should meet the requirements of electromagnetic compatibility and work normally in the specified radiated electromagnetic field environment, and will not cause the instrument performance to decline or work abnormally in this environment. In addition, strong radio waves and stray currents on the pipeline may also become interference sources. The fault can be eliminated by itself after the external interference is eliminated. In order to ensure the accuracy of the measurement, this type of fault cannot be ignored. In some production environments, due to the large vibration of the measuring pipeline or liquid, the circuit board of the flowmeter will become loose, which may also cause fluctuations in the output value.
2. Solution
(1) Confirm whether the fluid is pulsating due to the operation of the process personnel. At this time, the flow meter only reflects the flow condition truthfully. The fault can be eliminated by itself after the pulsation ends.
(2) Electromagnetic interference caused by external current, etc. Check whether there are large electrical appliances or welding machines working in the instrument operating environment, and make sure that the instrument is grounded and the operating environment is good.
(3) After the sewage electromagnetic flowmeter is properly grounded, it can avoid the influence of most stray currents in the pipeline.
(4) When the pipe is not full of the measured liquid or the liquid contains bubbles, both are caused by process reasons. At this time, you can ask the process personnel to confirm that the output value can return to normal after the liquid is full or the bubbles are calmed.
(5) The transmitter circuit board is a plug-in structure. Due to the large vibration of the on-site measurement pipeline or liquid, the power board of the flow meter often becomes loose. If it is loose, the flow meter transmitter can be disassembled and the circuit board can be re-fixed.
3. The sewage electromagnetic flow meter value does not match the actual value
1. Cause Analysis
(1) Is the transmitter circuit board intact? (2) When the liquid flow rate is too low, the measured liquid contains tiny bubbles, which rise and gradually gather above the pipe, causing the liquid flow area to change. When there is a lot of gas, interference signals will be generated, affecting the measurement accuracy. 3) The signal cable is not connected well or the insulation performance of the cable deteriorates during use, causing inaccurate measurements. (4) The parameter setting value of the converter is inaccurate.
2. Solution
(1) Check whether the transmitter circuit board is intact. If the junction box is flooded or corroded by the corrosive liquid being measured, it may cause the electrical performance to deteriorate or be damaged. In this case, the circuit board should be replaced.
(2) Ensure that the flow rate of the measured liquid in the pipeline is above the minimum flow limit value so that the transmitter can work normally.
(3) Check whether the signal cable connection and the insulation performance of the cable are intact. If the signal cable is loose, reconnect it. If the insulation of the cable does not meet the insulation requirements, replace it with a new cable.
(4) Reset the converter setting value and calibrate the zero point and full scale value of the converter.
4. The output signal of the sewage electromagnetic flowmeter exceeds the full range
1. Cause Analysis
The main reasons for this type of failure are: (1) the signal cable wiring is incorrect or the cable connection is disconnected; (2) the converter parameters are incorrectly set; (3) the converter and sensor models are not compatible.
2. Solution
(1) Check whether the signal circuit is connected normally. If the signal circuit is disconnected, the output signal will exceed the full value. At this time, the signal cable needs to be reconnected correctly. At the same time, check whether the insulation performance of the cable is intact. If it does not meet the requirements, replace it with a new cable.
(2) Check in detail whether the parameter settings, zero point and full scale of the sewage electromagnetic flowmeter converter meet the requirements.
(3) If the converter and sensor models are found to be incompatible, you need to contact the manufacturer for replacement.
5. Zero point is unstable
1. Cause Analysis
(1) The pipe is not filled with liquid or the liquid contains bubbles.
(2) Subjectively, people believe that there is no flow of liquid in the tube pump, but in fact there is a small flow.
(3) Reasons related to the liquid (such as poor uniformity of liquid conductivity, electrode contamination, etc.).
(4) Water enters the terminal box or the excitation coil becomes damp, causing the insulation of the excitation coil circuit to the ground to decrease.
2. Solution
(1) If the pipeline is not filled with liquid or there are bubbles in the liquid, it is due to process reasons. At this time, you should ask the process personnel to confirm. After the process is normal, the output value can return to normal.
(2) There is a small amount of flow in the pipe, which is not a malfunction of the sewage electromagnetic flowmeter.
(3) If 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, the zero point may change. In this case, it must be cleaned. If the zero point change is not large, you can also try to re-zero it.
(4) Due to the influence of environmental conditions, water, dust, oil, etc. may enter the terminal box. Therefore, it is necessary to check whether the insulation of the electrode part is degraded or damaged. If it does not meet the insulation requirements, it must be cleaned.