The effect of high static pressure on level measurement

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This post was last edited by ウ① dot ゞ on 2017-11-18 at 18:46. There is a real case; after analyzing it, we obtained some data that we would like to share with everyone, in the hope that it will be helpful to all of you. Site conditions: The double-flange level transmitter is used to measure the liquid level under static pressure (8.0 MPa–14 MPa). During the nitrogen-based static pressure sealing test prior to startup, as the static pressure inside the reactor increased from 0 to 9.19 MPa, the liquid level indication rose from 0% to around 13%. Inspection of the vent valves on the positive and negative pressure sides showed that no liquid was discharged from the reactor. After analysis, the cause was identified: under high static pressure, the calculations performed by differential pressure level gauges must take into account the change in density of the gas in the upper layer due to compression. When the pressure is high enough, the gas in the upper layer becomes denser as a result of compression; therefore, it is necessary to determine the interface position by considering both layers of medium in order to accurately determine the liquid level. The high static pressure in the on-site sealed tank affects the zero point of the transmitter after negative migration at normal pressure; therefore, negative migration should be performed once the static pressure has stabilized. The high static pressure in the enclosed tank at the site affects liquid level measurement; therefore, the gas layer above needs to be used as the medium on the upper side of the interface, and a calculated interface measurement method must be employed for migration purposes. Actual data calculation: Flange spacing of 2m, range of 15.25 kPa, density of 777.8 KG/M3. During the pressure testing, after the tank was moved to atmospheric pressure, the DCS indicated 0%; when the pressure was increased to 9.19 MPa, the instrument showed 14%. Based on the scale range, the zero-point deviation was approximately: 15.25 * 14% = 2.135 kPa. Subsequently, using other software, it was found that the density of nitrogen at 101.325 kPa-G and 25 degrees Celsius is 1.2504 KG/M3. The gas density of nitrogen at a pressure of 9.19 MPa and 25 degrees Celsius is 107.048 KG/M3. The pressure of the gas after compression to a height of 2 meters is: 107.048 * 9.8 * 2 = 2.098 kPa. Given that the transmitter’s range is 15.25, the deviation is approximately 13.7%. The calculation did not take into account the effect of temperature on the compressibility factor, nor did it consider the impact of gas pressure at standard conditions; as a result, the calculated value differs slightly from the actual conditions on site, but it is generally in line with them. Previously, some forum users also raised similar questions. Today, I would like to share this real case along with the results of the analysis and calculations, in the hope that it will be helpful to everyone.