On-site instrument failure analysis (one question per week) – Question 10 (answers released)
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This post was last edited by Yiding on 2009-11-29 at 15:38. When an electromagnetic flowmeter in a certain device is in operation, if the signal becomes weaker or drops suddenly, what could be the possible reasons? How to handle it? (Note: The original title was an excerpt from the book; the answer is the standard one provided in the book.) Answer: When there is no working medium in the measurement conduit, the electrodes are actually insulated from each other. The main reason for the above situation is that the insulation between the electrodes deteriorates or there is a short circuit, preventing the signal from being sent out. The above situation should be considered from the following aspects: 1. There may be dirt deposited on the inner wall of the measurement catheter; the electrodes should be cleaned and wiped. 2. The lining of the measurement conduit may be damaged and should be replaced. 3. The signal socket may be corroded and should be replaced or cleaned.
Please explain the operating conditions of the electromagnetic flowmeter, as well as factors such as the surrounding environment; otherwise, any analysis conducted without this information can only be general in nature. Check whether the signal wire and ground wire are properly connected. 1. Determine whether there are bubbles flowing inside the sensor. 2. Check for any deposits inside the sensor’s liner. 3. Is the grounding proper? 4. Check whether the insulation between the excitation wire and ground has decreased. 5. Check whether the insulation between the signal wire and the shield wire has decreased. 6. Check for any external interference
1. When installing an electromagnetic flowmeter, a certain length of straight pipe section must be reserved; otherwise, bubbles will form in the pipeline, and when the electrodes come into contact with these bubbles, a zero input condition will occur； 2. External interference ； 3. Check grounding ；
The electrode is covered, the grounding is poor, and there is air mixing.
I think the most likely cause is poor grounding； Also, a tube not being fully filled with the medium is probably one of the reasons as well.
Yes, it is necessary to ensure certain straight sections before and after. For pipes with a larger diameter, a certain inclination is also required; if they are installed vertically, the inlet should be at a lower level and the outlet at a higher level. This is all to ensure that the pipe is filled completely during measurement, thereby guaranteeing accuracy.
First, from a process perspective, if the process flow rate indeed decreases, then there should be no problem with the equipment； If the process equipment is functioning properly, then it is necessary to analyze 1: how to determine whether the problem lies with the equipment on-site or with the equipment in the control room ； A standard signal is generated using a signal generator at the output terminal of the electromagnetic flowmeter on-site, to check whether the display in the control room is normal ； The purpose is to check if there are any issues with the line ； In particular, problems related to poor contact are easy to identify ； 2. Sensor analysis: 2.1 The sensor electrodes are gradually covered by sludge. 2.2 The medium does not fill the tube completely. 2.3 Poor grounding and interference issues
1. Caused by asymmetric flow 2. Caused by issues with fluid conductivity 3. Caused by the effects of deposits on the electrode lining 4. Caused by excitation problems 5. Caused by issues with sensor grounding ??
The signal from electromagnetic flowmeters is quite weak (only 2.5–8 mV at full scale), and poor grounding is likely to cause interference with this signal.
First, determine whether it is a problem with the on-site flow meter, or an issue with transmission and the DCS. If the issue is related to the flow meter, the possible causes are as follows: 1. The sensor’s probe is gradually covered by dirt, or there are hard particles in the fluid that get stuck in the probe. Solution: Flush the sensor (backwashing is preferred). 2. Due to the excessively high temperature of the medium, the sensor lining is gradually damaged; solution: replace the sensor. 3. There are bubbles in the medium, and the sensor is not operating in a full-battery condition. It is recommended to open the inspection valve on the pipeline for observation, or to check at the pipeline outlet. 4. There are welding machines in operation or other sources of electromagnetic interference around the flow meter; it is recommended that they stop those devices and give it a try.