Migration problem of differential pressure level gauge measurement in sealed containers

Ugh, it’s a bit like an exam. I’m just talking casually; please forgive any mistakes! Migration occurs when the lowest liquid level, the pressure tapping point, and the transmitter diaphragm box are not at the same horizontal level. A positive pressure difference means positive migration, and vice versa! ! !

The conditions that lead to migration are also related to the medium being measured and the density of the isolation fluid in the pressure conduit.

In the case of a double-flange transmitter, the silicone oil inside the transmitter can also cause measurement errors, and the purpose of migration is to eliminate these errors.

Simply put, when the measured liquid level is at the measurement zero point, if the differential pressure acting on the transmitter is not zero, then calibration is required. When the zero-point differential pressure is positive, positive migration is required ； When the zero-point differential pressure is negative, negative migration is required.

When using a conventional differential pressure transmitter to measure liquid level, if the steam measured on the negative pressure side tends to condense, it is common to install a condensate tank on that side in order to ensure accurate and stable measurements. The condensate tank is filled with liquid (the type of liquid depending on the medium in question). When there is no liquid level in the container, the pressure from the liquid in the condensate tank acts on the negative pressure side; therefore, negative migration is required to eliminate this pressure effect. If the differential pressure transmitter is installed at a higher position than the pressure sampling point in the positive pressure chamber, it also needs to be relocated; whether the relocation should be forward or backward can be calculated using statics principles.

For level measurement in closed containers, the indication value of a differential pressure level gauge is related not only to the liquid level height but also to the liquid density and the installation position of the differential pressure instrument. It is generally required that the pressure tapping point (zero liquid level) be at the same horizontal level as the inlet of the measuring instrument; otherwise, additional static pressure errors will occur. However, this requirement may not be met during actual installation. When using flanged differential pressure transmitters, changes in the density of the pressure transmission medium will generally cause additional static pressure errors, regardless of the height. In this case, correction can be made through calculation, or the pressure (differential pressure) transmitter can be zero-adjusted so that it outputs 0 at a liquid level of 0; this method is known as range shifting. Reasons for negative migration: Use of an isolation tank or flange-type pressure difference measurement. Reasons for positive migration: The transmitter is located below the liquid level reference plane

When using a differential pressure transmitter to measure the liquid level, if the positive and negative pressure chambers of the transmitter are at the same level as the pressure sampling point in the container, no offset adjustment is required. In practical applications, considering factors such as the installation location of the equipment and ease of maintenance, it is not always necessary for the measuring instruments to be at the same level as the pressure sampling points ； For example, when the medium to be measured is a highly corrosive or highly viscous liquid, it is not possible to feed this medium directly into the pressure measuring instrument; therefore, an isolation fluid tank must be installed, using the isolation fluid to transmit the pressure signal and thus prevent the measuring instrument from being corroded. At this point, it is necessary to consider the effect of the liquid columns of the medium and the isolation fluid on the readings of the pressure measuring instrument. There are mainly three installation methods for differential pressure transmitters used to measure liquid levels. In order to accurately indicate the level of the liquid, the differential pressure transmitter must undergo certain technical adjustments – namely calibration. Migration is divided into no migration, negative migration, and positive migration. The essence of positive and negative migration is to adjust the differential pressure transmitter in order to change the upper and lower limits of the range, while the size of the range remains unchanged. From the perspective of the negative pressure chamber, it can also be simply understood as positive migration – it’s as if an amount equal to ρgh is added in the negative pressure chamber – while positive migration corresponds to a reduction of ρgh in that chamber. Positive migration fault: To determine whether a differential pressure transmitter experiencing positive migration provides accurate measurements in actual use, one should first close the positive and negative pressure measurement chambers of the three-valve assembly associated with the differential pressure transmitter, and open the balance valve as well as the instrument vent plug; at this point, the instrument’s output should be below 4 mA. If the output is not less than 4 mA, it may be that there is some blockage in the positive pressure chamber leads or the three-valve assembly. Secondly, close the pressure sampling point in the positive pressure chamber and open the vent switch; at this point, the output should be 4mA. If the output is below 4mA, it may be due to a reduced migration amount or a low zero point ； If a sealing fluid has been poured in, it may be that the fluid wasn’t filled to capacity or leaked from somewhere else ； If the output is higher than 4mA, it indicates an increased migration amount or a high zero point. Negative migration fault: To determine whether a differential pressure transmitter affected by negative migration provides accurate measurements in actual use, first close the positive and negative pressure measurement chambers of the transmitter’s three-valve assembly, open the balance valve and the instrument vent plug; the instrument’s output should be 20mA. Next, close the pressure-taking points in the positive and negative pressure chambers and open the vent switch; at this point, the instrument’s output should be 4mA. If it is not 20mA or 4mA, check whether the wires leading to the positive and negative pressure chambers are blocked, whether there has been any change in the drift value, whether the zero point is accurate, and whether any isolation fluid has been lost.

It’s unclear why the original poster can rate others while answering their own questions. I’ll come and discuss it too. When a differential pressure transmitter is used for level measurement, whether positive or negative offset is required depends on its installation. It is generally not possible to ensure that the pressure taps on the positive and negative sides are horizontal; as a result, the liquid level pressure in these taps will be taken into account, which necessitates the use of positive or negative offset. If a balanced container is used, the positive-pressure side is generally connected to the gas phase, with the pressure tap filled with liquid; the negative-pressure side is connected to the liquid phase, followed by a full-scale negative migration. If the positive-pressure side is connected to the liquid phase and the negative-pressure side is connected to the gas phase, since the connection point of the pressure tap on the positive-pressure side is at a higher level than the measurement chamber, a certain column of liquid remains in the pressure tap, requiring positive migration.

Optional external ultrasonic level gauge for non-contact measurement. High precision

Here is the simplest way to calculate it; don’t keep getting stuck on some trivial concepts. 1. As a general recommendation, the installation location of the differential pressure transmitter should be at least 30 centimeters below the lower end. Although I have also tested double-flange capillaries, which can take accurate measurements even when moved back and forth between the upper and lower ends, it’s better to follow the guidelines outlined in the manual, including the installation specifications. 2. The simple way to calculate it is 0% (LRV); in this case, the pressure acting on the positive-pressure chamber minus the pressure in the negative-pressure chamber gives the differential pressure corresponding to LRV. 100% (URV); all that is needed is to add the pressure of the medium being tested (H*r), where H is the liquid level height and r is the specific gravity. 3. The so-called negative migration occurs when 0% is calculated, resulting in a negative value; conversely, it is positive migration. Positive and negative migration is caused by the difference in pressure sampling points corresponding to the positive and negative pressure chambers during installation. 4. As for whether to use a potting fluid for the pressure guide tube or a dry one (upper-pressure guide tube), it is necessary to have a good understanding of the manufacturing process; special care must be taken regarding volatile substances and vacuum conditions, as otherwise the liquid inside the upper-pressure guide tube may either escape or, if it was originally dry, liquid may enter it. The measurement has a large error. Of course, that’s why isolation tanks, double-flange capillaries, and the like were developed.