Issues regarding pressure transmitter measurements!
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Our facility has a reboiler. A pressure transmitter is used for pressure measurement; the pressure signal is transmitted downward to the installation location via pressure conduits. The range of this pressure transmitter is 60 KPA. After normal calibration, the reading indicated on the transmitter is higher than the actual value. Does this mean that the height of the pressure conduits needs to be taken into account? It’s approximately 2 meters! Last edited by yame1996 on 2008-2-13 at 18:20.]
Your analysis should be accurate!
1. Perform zero drift adjustment on the pressure transmitter ； 2. Raise the installation position of the pressure transmitter.
Zero the pressure guide tube after filling it with liquid. When a 2-meter pressure conduit is filled with liquid, there is a pressure of about 20 KPa; if the zero point does not shift, the error will be quite large.
That’s indeed the case. In an installation like this, with the pressure-taking valve closed and no pressure measured inside the container, the positive pressure chamber of the pressure transmitter has already experienced a pressure equivalent to 2 meters of water, which is approximately 0.2 kg. When the pressure relief valve is opened, the pressure inside the container as measured by the transmitter is increased by this existing 0.2 kg of pressure; this combined value is what is displayed as the output from the pressure transmitter. Naturally, this value is higher than the actual pressure, by approximately 0.2 kg. Solution: 1. With the pressure tube filled with the medium, it is cooled to ambient temperature; at this point the pressure sampling valve is closed, so that the transmitter experiences only the pressure of those 2 meters of water column. Then perform positive migration, that is, directly set this position as the transmitter’s zero point. 2. It is also possible to accurately measure the height from the pressure tapping point to the transmitter, and then calculate the pressure of that liquid column based on density, followed by performing positive migration. 3. Reinstall the transmitter so that it is at the same level as the pressure tapping point. Of course, this is rather troublesome. I hope this helps you.
I guess this problem arises as fluid starts to accumulate in the pressure gauge tube over time; one can lead the pressure gauge tube upward for a while before letting it go downward, which facilitates the return of the accumulated fluid； Or, as mentioned above, fill it with liquid first and then migrate.
It’s a problem with the position of the pressure guide tube; online relocation is required when establishing zero point inside the tank
This is a problem that is frequently encountered in the installation and maintenance of instruments. Due to installation requirements, the transmitter is usually located below the pressure tapping point. The pressure measured by the poster is absolute pressure; in terms of gauge pressure, it represents negative pressure. Due to liquid accumulation in the pressure guide tube, it is necessary to take into account a zero-point adjustment upward, that is, adjusting the zero point downward by the pressure difference generated by 2 meters of liquid. However, the primary requirement is to fill the pressure guide tube with liquid in advance; if the liquid is prone to freezing, heating must also be provided.