The double-flange level gauge is inaccurate
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The remote transmission value of a double-flange level gauge changes as the pressure at the top of the tank it is installed in changes. Why? Thank you
Firstly, a double-flange level gauge that is free from faults will not experience any changes in its reading due to changes in tank pressure; it is necessary to determine whether the liquid level itself is fluctuating. If there is an on-site indication device such as a magnetic flip gauge, it can be used to check whether there are fluctuations as well. If there is no fluctuation, check again whether there is grounding in the instrument circuit cables, whether the shielding is properly grounded, and whether the DCS channels are functioning properly.
Regardless of how the pressure inside the tank changes, the pressure applied to the positive-pressure side and the negative-pressure side is the same. Secondly, the liquid level of a differential pressure level gauge is determined by three main factors ； Pressure difference, medium density, installation location (pressure tapping point).
I think it might be the control of the pressure regulating valve that is used to regulate the liquid level. You can check the DCS logic.
It generally won’t have an impact; check whether there are any faults with the double flanges
Check whether the density of the liquid changes when the pressure is high
Reply to 6# slayerstar: It would be better to have an on-site gauge for comparison, so that the actual liquid level can be seen; only then can it be determined whether the problem lies with the differential pressure transmitter or if there is a change in the liquid level itself. Moreover, the differential pressure transmitter does not change with the pressure in the tank – it measures the pressure difference between the upper and lower flanges
I encountered similar problems when debugging equipment in the past. At that time, the gas pressure was 150 Bar, and we also had a magnetic flap level gauge; there was a significant difference in the levels indicated by these devices. The separator we used was of the type that rotates at high speed, with gas entering from the top of the container and exiting from the bottom – as a result, the differential pressure readings were inaccurate, and I’m not sure why
In this situation, the gas phase pressure changes rapidly, causing the pressure measured at the upper flange to fluctuate. The lower flange fails to detect these rapid changes in gas phase pressure in time, which leads to fluctuations in the liquid level. Rosemount now offers a new type of dual-flange level gauge, in which a cable connects the upper flange to the transmitter; this allows for timely detection of changes in gas phase pressure, resulting in a much faster response compared to systems that use silicone oil for pressure transmission
Do your DCS systems use safety barriers? Is the shielding grounded? I have encountered this problem before: the pressure and level signals are on the same circuit board, and the wiring terminals of the level gauge are grounded, which causes interference to all the points on that circuit board. (My personal opinion is for reference only)