Positive migration issue of level differential pressure transmitters

Are you double-flanged? ? What is being migrated is the distance between the two flanges! ! ! For single-flange types, it is the vertical distance from the pressure measurement point to the transmitter! ! ! The density is the density of the medium in the pressure guiding tube! ! !

If it is an open tank, dual flanges are not necessary. In the case of a closed tank, it is incorrect to connect the negative pressure chamber to the atmosphere for zero-point migration; the best approach is to perform calculations or keep the liquid level above the zero point during migration.

Reply to 1#: For the third equation, migration simply means finding a way to make the instrument display zero when the physical quantity being measured is at 0.

First of all, I need to tell you that neither of your two methods is correct; they are both based on conceptual errors. 1. The first method: before any feed is introduced, when there is no fluid in either the positive or negative pressure tubes, the instrument displays zero. If you adjust the positive pressure setting to its full range, the instrument will continue to show zero throughout the entire process; it only shows zero when the tank is completely filled (before any negative pressure is applied and fluid enters the tank). If the liquid level continues to rise, the negative pressure conduit will also gradually fill with the medium, and the gauge will slowly move towards negative values until it shows an over-limit value below zero. 2. The second method is even less desirable; since there is a negative pressure side, it means the container being measured is not open-ended, but rather a sealed container. You’re trying to connect the negative pressure to the atmosphere, and this can lead to serious accidents. First of all, if the medium being measured (whether in liquid or gas form) is toxic or harmful, connecting the negative pressure directly to the atmosphere means that people could be exposed to these toxins; and even if they aren’t exposed directly, the air will still get polluted. Second, if the tank becomes full, wouldn’t all the medium inside the container flow out from the negative pressure side? That would be terrifying! 3. There’s also the second method: if the container equipment is under pressure, then in the case of positive pressure measurement, there is not only the static pressure due to the liquid level but also the pressure inside the equipment container. Meanwhile, in the case of negative pressure, the pressure is released outside. Obviously, the error in this case is quite significant. For example, if the range of the transmitter is 40 kPa and the pressure in the container is 300 kPa, then the gauge will exceed its range as soon as it is put into use; it has to withstand a pressure of 300 kPa in positive-pressure conditions, while in negative-pressure conditions it is dealing with exhaust pressure, so the gauge will definitely exceed its range. It’s completely unusable. After talking about the risks, let’s discuss your conceptual issues. 1. You do not understand the migration issue – when to perform a positive migration and when to perform a negative one. Positive migration is generally used in open containers; it is necessary when the positive pressure sampling port is below the actual liquid level zero point, or when the transmitter is installed below the positive pressure sampling port. In such cases, positive migration is also required. What is removed here is the static pressure exerted by the medium in the container or pressure guide tube on the positive-pressure diaphragm at the zero liquid level. 2. Negative migration: Negative migration occurs in liquid level measurement of almost all sealed containers, unless the negative pressure in the gas phase prevents liquefaction and keeps the container from filling up, that is, there should be no liquid on the negative pressure side. If you cannot guarantee that there is no liquid on the negative pressure side, it is best to perform negative migration. During negative migration, both the positive and negative sides must be filled with an isolation fluid or medium in order to ensure accuracy; this is because what is being migrated occurs at a liquid level of zero, with the static pressure exerted by the liquid column on the negative pressure diaphragm playing a role in this process. Depending on your actual installation situation, it can be done as follows: 1. If it’s possible to make adjustments, try to simulate packet isolation by properly filling the system with positive and negative pressure at zero level. Or, before starting work, one applies effort to inject sealant into the positive and negative pressure areas; once it seems that those areas are filled, that is the process of zeroing. 2. Change to double flanges, but it’s more troublesome than using sealing packets; there are more parts that need to be modified, and flanges also have to be added. 3. Don’t make any changes; let the operator fill the tank completely. This fills the negative pressure area with the medium, setting the output at 100%. Then have the operator make a few adjustments back and forth; repeat this a few times, and it will be basically correct.

First, thank you to all the experienced users for their answers. I’m sorry, I didn’t make it clear – it’s a sealed container. It has double flanges, no capillary tube, and is equipped with pressure lead tubes. Lower flange: 0.5 meters, upper flange: 2 meters. The minimum liquid level is 0.9 meters, while the maximum is 1.9 meters. The transmitter is 1.5 meters below the lower flange. In this way, there will be no liquid on the negative pressure side. It would be very dangerous if the tank became full, so the process design ensures that it never gets full. So this should be positive transfer, right? When I say to vent the negative pressure side to the atmosphere, I mean to do so during migration. It still needs to be restored after migration. Lower flange: 0.5 meters, upper flange: 2 meters. The minimum liquid level is 0.9 meters, while the maximum is 1.9 meters. Assuming the medium density is 1.0, what is my migration amount then? What specific actions should I take on site? The books say that density times height is used to calculate the amount of migration. Do we calculate it this way too during actual on-site migrations? ? ? If isolation tanks are installed on the positive and negative pressure pipes, I fill the positive and negative isolation tanks with isolation fluid, and then power is supplied to the transmitter. Is the pressure value shown on the gauge the amount of migration at this point? Should this be negative transfer? It seems that the experienced workers didn’t do any kind of calculation when migrating. There’s no need to know the medium density either. I'm still a beginner. The experienced masters are all gone, and I feel a lot of pressure. . . . I hope the experts can continue to offer their guidance. Thank you very much

Reply to 5# newttang: Thank you for your answer. It is a sealed container. It has double flanges, no capillary tube, and is equipped with pressure lead tubes. Lower flange: 0.5 meters, upper flange: 2 meters. The minimum liquid level is 0.9 meters, while the maximum is 1.9 meters. The transmitter is 1.5 meters below the lower flange. In this way, there will be no liquid on the negative pressure side. It would be very dangerous if the tank became full, so the process design ensures that it never gets full. So this should be positive transfer, right? When I say to vent the negative pressure side to the atmosphere, I mean to do so during migration. It still needs to be restored after migration. Lower flange: 0.5 meters, upper flange: 2 meters. The minimum liquid level is 0.9 meters, while the maximum is 1.9 meters. Assuming the medium density is 1.0, what is my migration amount then? What specific actions should I take on site? The books say that density times height is used to calculate the amount of migration. Do we calculate it this way too during actual on-site migrations? ? ? If isolation tanks are installed on the positive and negative pressure pipes, I fill the positive and negative isolation tanks with isolation fluid, and then power is supplied to the transmitter. Is the pressure value shown on the gauge the amount of migration at this point? Should this be negative transfer? What is an isolated packet? I’ve never seen anyone use it. . I’m a beginner; I hope for lots of guidance. Thank you. . .

Reply to 4# dduumm: Exactly! I just can’t think of a solution, which is why I’m asking the experts for advice. . . Could you tell me the specific method?

Reply to 2# Rain on Moonlight: Is the distance between the two flanges in negative migration? Is it only when the medium under test enters the negative pressure side that the distance between the two flanges changes? Is that the case?

The HART375 handheld device is used to adjust the range of pressure transmitters, and it allows for positive migration

Why haven’t any experts come?