Problems with the use of double-flange level gauges
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1. The instrumentation team reported that the dual-flange type level gauge tends to give inaccurate readings whenever the container is shut down. I found this diagram online; does it mean that the liquid level should not be too low? If the container needs to be emptied, should the isolation valves for the high and low pressure measurements of the level gauge be closed in advance?2. During disassembly, how should it be done so as not to damage the diaphragm box? I think the high-pressure side should be closed first, followed by the low-pressure side; then the pressure from the high-pressure side should be released, after that the low-pressure side, and finally the flanges of the level gauge can be removed. I’m not sure if this is the correct approach. I hope the experts can give me some advice
There should be no problem; the pressure resistance of the diaphragms on both the high-pressure and low-pressure sides should be sufficient to handle the container’s pressure. Was the interior pressurized with air when it was emptied? Generally, it’s possible to reset the zero point when the system is put back into use, and there might be a slight deviation, but it won’t result in inaccurate readings.
Double flanges are more accurate; is there negative pressure during evacuation?
Since the issue of inaccurate readings occurs every time it is disabled, it is necessary to discuss whether there are any changes in operating conditions as part of the process analysis
There are many possible reasons for an inaccurate level gauge: foreign objects on the diaphragm (such as material crystals or scale), damaged capillaries, and so on. There is no requirement regarding the liquid level here: 4 milliamps correspond to zero, while 20 milliamps correspond to the maximum liquid level! Be careful with the membrane and capillaries when removing it.
Installing it this way is no problem; generally, it shouldn’t be installed too low either. Prevent deposits in the liquid from blocking or sticking to the transmitter diaphragm or blocking the installation stub. When it is shut down, the liquid level is very low; readings become inaccurate starting from the location of the short pipe on the high-pressure side, and the data even changes frequently. At this point, it should be indicated that operation can no longer be used as a reference. And perform a signal removal procedure. As for the issue of unilateral overvoltage, current transmitters have no problems with this, so there’s no need to worry about it
Is the installation location of the transmitter higher than the lower pipe outlet?
1. The level of the liquid does not affect the operation of the equipment; however, if the reading is inaccurate, it may be due to: 1) Checking whether the positive and negative sides of the level gauge are vented. If the reading still does not return to zero after venting, it indicates that there might be a blockage in those sides (in this case, it is necessary to check whether the vent valves are blocked) or that the diaphragm box of the transmitter is damaged. 2) If the transmitter is damaged, check the condition of the equipment. 2. Under normal conditions, it is not necessary to close the positive and negative pressure valves of the level gauge in advance when emptying the container; however, if the container needs to be evacuated or pressurized, the level gauge must be disconnected in advance. 3. The sequence for removing the level gauge is as you said; just take care to protect the diaphragm box during the disassembly process.
There shouldn’t be any inaccuracies; just check the zero point when it’s put into use. When disassembling, be careful not to pull the capillary and avoid damaging the diaphragm.
If feeding is done under vacuum, air can easily enter the capillary, leading to instability.
Generally, be careful not to create a negative pressure that could empty the chamber; gauge boxes are sensitive to this and cannot withstand too high negative pressures. If steam at too high a temperature is introduced, attention must also be paid to the tolerance temperature.