Calculation of migration amount

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1． Migration of liquid level When using a differential pressure transmitter to measure the liquid level, if the positive and negative pressure chambers of the differential pressure transmitter are on the same level as the pressure tapping point of the container, there is no need to migrate. In practical applications, due to considerations such as equipment installation location and ease of maintenance, the measuring instrument may not always be on the same level as the pressure point. ； If the measured medium is a highly corrosive or heavy-viscosity liquid, the medium cannot be directly introduced into the pressure measuring instrument. An isolation liquid tank must be installed and the isolation liquid is used to transmit the pressure signal to prevent the instrument being measured from being corroded. At this time, it is necessary to consider the influence of the liquid column of the medium and isolation fluid on the pressure measuring instrument reading. There are three main installation methods for differential pressure transmitters to measure liquid level. In order to correctly indicate the height of the liquid level, the differential pressure transmitter must undergo some technical processing - that is, migration. Migration is divided into no migration, negative migration and positive migration. 1.1 No migration. Install the positive and negative pressure chambers of the differential pressure transmitter and the pressure tapping point of the container on the same horizontal plane, as shown in Figure 1. , Figure 1 Principle diagram of no migration Figure 2 Principle diagram of negative migration Suppose the pressure at point A is P-, the pressure at point B is P+, the density of the measured medium is ρ, and the gravity acceleration is g, then ΔP= P+- P-=ρgh+ P-- P-=ρgh ； If it is an open container, P- is the atmospheric pressure, ΔP=P+=ρgh. It can be seen that if the positive pressure chamber of the differential pressure transmitter is connected to the pressure point, and the negative pressure chamber is open to the atmosphere, the height of the liquid level can be known by measuring the gauge pressure at point B. When the liquid level changes from h=0 to h=hmax, the differential pressure measured by the differential pressure transmitter changes from ΔP=0 to ΔP=ρghmax, and the output changes from 4mA to 20mA. Assume that the instrument range required by the differential pressure transmitter to correspond to changes in liquid level is 30kPa. When the liquid level changes from empty to full, the measured differential pressure changes from 0 to 30kPa. The characteristic curve is shown in (a) in Figure 4. 1.2 Negative migration As shown in Figure 2, in order to prevent the liquid or gas in the closed container from entering the pressure chamber of the differential pressure transmitter, causing blockage or corrosion of the pressure pipeline, isolation liquid tanks are installed between the positive and negative pressure chambers of the differential pressure transmitter and the pressure point, and are filled with isolation liquid, whose density is ρ1. When H=0, P+=ρ1gh1 P-=ρ1g(H+h1) ΔP= P+- P-=-ρ1gH When H=Hmax, P+=ρ1gh1 +ρgH P-=ρ1g (H+h1) ΔP= P+- P-=ρgH-ρ1gH=(ρ-ρ1)gH When H=0, ΔP=-ρ1gH, there is a static pressure ρ1gH in the negative pressure chamber of the differential pressure transmitter, making the output of the differential pressure transmitter less than 4mA. When H=Hmax, ΔP=(ρ-ρ1)gHmax. Since ρ1»ρ in actual work, at the highest liquid level, the pressure of the negative pressure chamber is much greater than the pressure of the positive pressure chamber, so that the instrument output is still smaller than the instrument output corresponding to the actual liquid level. This destroys the normal relationship between the transmitter output and the liquid level. In order to make the instrument output correspond to the actual liquid level, it is necessary to eliminate the static pressure ρ1gH generated by the H liquid column in this section of the pressure line of the negative pressure chamber. To eliminate this static pressure, the differential pressure transmitter must be adjusted, that is, the differential pressure transmitter is subjected to negative migration. The static pressure ρ1gH is called the migration amount. When adjusting the differential pressure transmitter, the negative pressure chamber is connected to the input signal, and the positive pressure chamber is vented to the atmosphere. Assume that the measuring range of the instrument is 30kPa and the migration amount ρ1gH=30kPa. During adjustment, the negative pressure chamber is pressurized to 30kPa. Adjust the zero point knob of the differential pressure transmitter so that the output is 4mA. ； After that, the negative pressure chamber is not pressurized, adjust the range knob of the differential pressure transmitter until the output is 20mA, and press the three points in the middle to check the scale. The relationship between input and output is shown in Table 1). Table 1), when the liquid level rises from empty to full, the differential pressure of the transmitter changes from ΔP=-30kPa to ΔP=0kPa, and the output current value changes from 4mA to 20mA. Its characteristic curve is shown in (b) in Figure 4. 1.3 Positive migration In actual measurement, the installation position of the transmitter is often not on the same horizontal plane as the lowest liquid level, as shown in Figure 3. The container is an open container, and the position of the differential pressure transmitter is h distance lower than the lowest liquid level, ΔP=P =ρgH+ρgh. When H=0, ΔP=ρgh, there is a static pressure in the positive pressure chamber of the differential pressure transmitter, making its output greater than 4mA. When H=Hmax, ΔP=ρgH+ρgh, and the transmitter output is much greater than 20mA. Therefore, the static pressure of ρgh must also be eliminated. This is positive migration. Figure 3 Principle diagram of positive migration Figure 4 Relationship between measurement range, measuring range and migration amount When adjusting, the positive pressure chamber is connected to the input signal, and the negative pressure chamber is vented to the atmosphere. Assume that the instrument range is still 30kPa, and the migration amount ρgh=30kPa. The relationship between input and output is shown in Table 2). Table 2), its characteristic curve is shown in (c) in Figure 4. If the differential pressure transmitter selected on site is intelligent and can communicate with the HART handheld communicator, it can be adjusted directly with the handheld communicator. 1.4 The relationship between measurement range, measurement range and migration amount. The measurement range of a differential pressure transmitter is equal to the sum of the measurement range and migration amount, that is, measurement range = measurement range + migration amount. As shown in Figure 4, the measuring range a is 30kPa, there is no migration, and the measuring range is equal to the measuring range 30kPa. ； bThe measuring range is 30kPa, the migration amount is -30kPa, and the measuring range is -30~0kPa ； c The measuring range is 30kPa, the migration amount is 30kPa, and the measuring range is 30~60kPa. Figure 4: The relationship between measurement range, measuring range and migration amount. It can be seen from this that the input and output characteristic curves of positive and negative migration are the characteristic curves without migration amount that are translated along the abscissa representing the input amount. Positive migration moves in the positive direction, negative migration moves in the negative direction, and the distance moved is the amount of migration. To sum up, the essence of positive and negative migration is to change the upper and lower limits of the measuring range by adjusting the differential pressure transmitter, while the size of the measuring range remains unchanged. If viewed from the negative pressure chamber, it can also be simply understood as positive migration, which is like increasing the migration amount of ρgh in the negative pressure room, and positive migration is like reducing the migration amount of ρgh in the negative pressure room. 2. Use the migration principle to improve the liquid level measurement method. From the above analysis, we can understand the principle of positive and negative migration of the differential pressure transmitter to measure the liquid level. In this way, in practical applications, the liquid level measurement method can be improved accordingly according to the process conditions of the production device, the usage conditions of the instrument and the surrounding environment. 3 Fault Analysis of Differential Pressure Transmitter with Migration 3.1 Positive Migration Fault To determine whether the measurement of the differential pressure transmitter with positive migration is accurate during on-site use, first, the positive and negative pressure measurement chambers of the three-valve group of the differential pressure transmitter should be closed, and the balance valve and instrument vent plug should be opened. At this time, the instrument output should be lower than 4mA. If the output is not less than 4mA, the positive pressure chamber lead or the three-valve manifold may be blocked. Secondly, close the pressure point of the positive pressure chamber and open the vent switch. At this time, the output should be 4mA. If the output is lower than 4mA, it may be that the migration amount is small or the zero position is low. ； If there is isolation fluid filled, it may be that the isolation fluid is not filled fully or is leaking from the side. ； If the output is higher than 4mA, it means that the migration amount has become larger or the zero position is higher. 3.2 Negative migration fault To determine whether the measurement of a negative-migration differential pressure transmitter is accurate during on-site use, first close the positive and negative pressure measurement chambers of the three-valve group of the differential pressure transmitter, open the balance valve and the instrument vent plug, and the instrument output should be 20mA. Secondly, close the pressure points of the positive and negative pressure chambers and open the vent switch. At this time, the instrument output should be 4mA. If it is not 20mA or 4mA, check whether the leads of the positive and negative pressure chambers are blocked, whether the migration amount has changed, whether the zero position is accurate, whether the isolation fluid is lost, etc.