Regarding the issue of inaccurate measurements by differential pressure transmitters
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The LNG recryolzer is 15 meters tall and is equipped with three differential pressure level gauges, connected via pressure transfer pipes! The pressure guide tube lacks insulation! All three level gauges are showing incorrect readings now! 1. The first level gauge measures from 0 to 4.8 meters; it is installed midway between the high-pressure and low-pressure sides. 2. The second gauge measures from 5.7 to 14.2 meters; it is installed at a position higher than the low-pressure side, with the DCS setting the measurement range to 5.7–14.2 meters. 3. The third gauge measures from 0.8 to 14.2 meters; it is also installed at a position higher than the low-pressure side, with the DCS setting its measurement range to 0.8–14.2 meters. All three gauges are used together for measurement, and neither of the pressure conduits on the high-pressure or low-pressure sides has any insulation measures in place! None of them have been migrated! There is no isolation fluid in the pressure guide tube either! The current problem is that the LNG level inside the container is above 10 meters. However, the liquid level calculated based on the pressure readings from the differential pressure transmitter is 4.4 meters for the first reading, which is not at full scale; 12.1 meters for the second reading, and 12.4 meters for the third reading! Due to some minor configuration issues in the DCS, only calculations can be performed. With a density of 0.42 g/cubic centimeter and G=9.8, could you help analyze the reason for this? Urgent!
Test each transmitter individually. I’m not entirely sure about the installation method of the pressure guide tubes on your end, but first close the root valves of the positive and negative pressure chambers, open the flanges connected to the pressure guide tubes so that the positive and negative pressure chambers are exposed to the atmosphere, check whether the gauge reads 0, and then use the 475 handheld controller to conduct a circuit simulation test. This will help resolve most issues (such as jamming problems, configuration problems, or wiring problems).
If there’s a problem, first clarify the situation: 1. Is there an issue with the table itself (perform a single-check); 2. Are there any problems with the wiring and cables (check the wiring locations and verify the connections); 3. Are there any issues with the cards (have the configuration system check whether they correspond to the DCS I/O tables)
This post was last edited by Darija on 2020-5-25 at 12:22. Ordinary differential pressure transmitters come with on-site indicators to ensure that the readings match those on the DCS. The differential pressure itself is not prone to failure. It is generally required that the high-pressure side be a liquid, while the low-pressure side be a gas. Both cooling and heating should be crucial. As for the first liquid level gauge, there is liquid on both the top and bottom; due to the temperature of LNG, there is a difference in the density of the liquid inside the tower and that resulting from the differential pressure, which causes errors. It’s the same as in a fluidized bed reactor when measuring the bed height – adjustments need to be made for the density of the purge gas and the gas circulating within the bed.
What was measured on-site was LNG; the pressure guide tube had no insulation, and it was filled with gas phase. I’d like to confirm whether temperature and pressure have an impact on the density of LNG. Could increased pressure cause the density of the LNG liquid to decrease?
This post was last edited by Dariga on 2020-5-25 21:57. There is definitely an impact; you can conduct calculations and analyses using P = density × gh. It’s just a matter of quantity. Additionally, if there is air at the high-pressure end of the pressure guide tube, it affects the measurement; similarly, if there is liquid at the low-pressure end, it also affects the measurement. Measurement errors are also caused by the difference in density between the liquid at the high-pressure end of the pressure guide tube and the liquid inside the tower.
This post was last edited by HEJIYUER on 2020-6-4 23:31. For the pressure measurement of cryogenic liquids, it is necessary to ensure that the entire vertical section of the pressure conduit is in the gas phase; if there is liquid that has not been vaporized in this area, the measurements will be inaccurate. The low readings on the three gauges indicate that there is liquid inside the negative pipe. You can check whether the vertical section of the pressure guide tube (the outer one) is frozen; if it is, then there is liquid present. Ask about the process – whether the medium contains trace amounts of C3; if it enters the pressure guide tube, it cannot be vaporized by the ambient temperature. . . There is one method that is definitely effective: provide heat tracing and tie the pressure guide tubes together; the gas phase pressures in the two tubes being equal can cancel each other out. If there is no liquid phase in the pressure guide tube, then the measured differential pressure is generated by the hydrostatic head of the liquid level.
The pipes in the vertical sections are all at room temperature! There is no freezing in the liquid phase tube! Back then, I suspected that the deposition of heavy hydrocarbons was responsible for preventing the pipes from getting clogged, but that a pressure drop was the cause! The factory only has nitrogen gas, no high-temperature steam! I don’t know how to deal with it either! The pressure tap pipes are all welded and quite long; I’m not sure what to do with them
I believe you have already carried out all the routine checks on the pipelines/transmitters. It is most straightforward to use the first gauge; at this point, the differential pressure output should be at its full scale, with the differential pressure reading equal to 4800 mm x 0.42 = 2016 mm of water column. You say the current reading is 4.4 meters; therefore, the differential pressure reading on the gauge should be 2016 x 4.4/4.8 = 1848 mm of water column. What is the size of the pressure guide tube? What is the pressure of LNG?
The pressure guide tube is size 14, and the gas pressure inside the container is 0.78. The differential pressure value you calculated is correct! Around 18 Kpa