【Help Request】The differential pressure transmitter gives inaccurate readings; urgent help needed!
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The LNG recondenser 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. These three level gauges work together for measurement purposes, and neither of the pressure conduits on the high-pressure or low-pressure sides has any insulation measures in place! None 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 12.6 meters for the third reading! Based on the trends, the difference between the second and third is relatively constant. Maintain it between 0.5 meters and 0.6 meters. The problems that need to be solved now are as follows: 1. Why, when the liquid level is clearly above 4.8 meters, is the measured value only 4.4 meters? ； 2. How to resolve the error issue between the second and third ones ； 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!
The density of the differential pressure tapping pipeline is different from that of the liquid inside the tower. Because LNG has a relatively low density, the deviation is that large.
This post was last edited by ZhiGE on 2020-5-25 at 13:18. Forced heating is applied to the pressure transfer tube to vaporize the LNG liquid column inside it, especially in the area near the pressure sampling port. You can use steam to purge the pressure guiding tube to see the effect; the first one should reach the full scale ； Also, check that there are no leaks in the pressure guide tubes and connectors.
The inside of the pressure guide tube contains gas that is essentially sealed, and the LNG inside the tower exerts force on this gas. There is only one collar throughout the second phase, and all checks show no air leaks.
Currently, the transmitter is installed above the gas phase pipe; a pressure guide tube is located behind the process root valve, and there is no heat tracing. When the high and low pressure sides are properly connected, the pressure gauge tube does not frost over and the temperature remains close to normal. However, when air is released from the pressure gauge tube, it starts to frost over rapidly from its base. Could it be an error on the transmitter? According to my gauge, it’s -100KPa to 100KPa.
1. I’ve sent you the reasons, methods, and drawings; can’t you give it a try to satisfy your curiosity? 2. If transmitter error is suspected, isolate the valve assembly and balance it to check the zero point; if that doesn’t work, perform an offline calibration. 3. The first liquid level corresponds to a pressure of P=4.8*9.8*0.42*10^3=19.76 kPa; this value should be within the range that the instrument can handle. 4. It is recommended that the differential pressure transmitter used for measuring the first liquid level be installed at the same level as the positive pressure side, with forced vaporization on the negative pressure side.
Floor 8 is the correct choice; the installation locations are extremely chaotic, and it’s difficult to make any changes (who knows whether there is liquid LNG inside the pressure transfer pipes?). It’s better to follow the suggestions in point 4 on Floor 8 by installing transmitters and ensuring that the negative-pressure pipes are vaporized
There is indeed a problem with the installation location； The plant site is an LNG receiving station; unlike chemical plants which have a variety of equipment, there is no steam at the site. It was purged once using 7 kilograms of nitrogen, and since the LNG has already been introduced into the equipment on site, it is no longer possible to make any changes. The second and third items have been processed; the results are pending observation ； The first one is still 4.4–4.5 meters; it’s not possible to reach 4.8 meters. The current approach is to lower the liquid level inside the container below 4.8 meters, in order to see whether there is a linear variation in the first case. Then try forced heating again. Since I have been working in the LNG industry, I have no experience with heat tracing. Does this only force vaporization on the negative pressure side?
But if there is LNG, or if the pressure guide tube becomes blocked, frost will form in that part of the pressure guide tube. The pressure guiding pipes on-site are all at room temperature. Ice forms only near the root of the pressure tap.