Causes and solutions of unstable zeroing of electromagnetic flowmeter
Check the unstable process of zero point electromagnetic flowmeter. First, make a preliminary inspection and judgment according to the process, and then carefully check and try to eliminate the problems one by one. The order criterion of the inspection items listed in the process is that it can be understood through observation or interrogation without major operation, that is, it is easy first and difficult later. According to the past on-site maintenance experience, the frequency is higher, and the order request required by the one with higher probability may be presented in the future. If the cause of the latter problems is confirmed by preliminary inspection, detailed inspection can also be carried out in advance.
The pipeline is not filled with liquid or the liquid contains bubbles. Objectively, it is thought that there is no activity in the flow sensor, but in practice, there is a small amount of activity. This kind of fault is mainly due to the poor tightness of the stop valve of the pipeline, and the small leakage detected by the electromagnetic flowmeter is misunderstood as zero change or unstable zero. Cases of incomplete sealing of valves due to long-term use of valves or liquid dirt are often encountered, especially for large valves. Another common reason is that the flow instrument has several branch pipes in addition to the main pipe, and the valves of these branch pipes are forgotten or neglected to be closed. Sometimes, it is difficult to confirm that there is no activity in the piping system on site. At this time, press to check whether there is leakage.
Change of power equipment status near the flow sensor (if the leakage current increases), the change of grounding potential will also cause the change of zero point of electromagnetic flowmeter. The influence of external interference such as pipeline stray current mainly depends on the good grounding maintenance of electromagnetic flowmeter. Generally, it is required that the grounding resistance is less than 10 and do not share grounding with other motors and electrical appliances. Sometimes the sub environmental conditions are good, and the electromagnetic flowmeter can work normally without grounding, but once it is good A good environment does not exist, and the instrument will show problems. It will be a lot of trouble to check again at that time.
The change or unevenness of liquid conductivity will change the zero point at rest and shake the output when it is active. Therefore, the flowmeter shall be located far away from the injection point or the downstream of the chemical section of the pipeline, and the flow sensor shall preferably be installed upstream of these places. If the liquid contains solid phase, or impurities accumulate on the inner wall of the flow metering pipe, or scale on the inner wall of the flow metering pipe, or the electrode is polluted by grease, etc., it may show zero change. Because the scale on the inner wall and the electrode pollution level cannot be completely the same and symmetrical, the equilibrium of the initial zero setting is destroyed. The active measures are to eliminate the dirt and accumulated scale; If the zero position changes little, you can also try to reset it.
Check the flow chart. The insulation drop of signal circuit will cause zero instability. The main reason for the insulation drop of the signal circuit is caused by the insulation drop at the electrode, but the insulation drop or damage of the signal cable and its wiring terminals cannot be removed. Sometimes, the on-site environment is very cruel. If the sealing of the instrument cover and wire connection is neglected, the moisture, acid mist or powder dust invades the instrument junction box or cable maintenance layer, resulting in the insulation drop. The insulation resistance inspection of signal circuit shall be stopped locally at the cable side and flow sensor respectively, and tested with a megger. Because the signal cable is easy to detect, it can be done first. The surface contact resistance of the electromagnetic flow metering electrode filled with liquid and the insulation resistance of the flow metering electrode behind the empty pipe.
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