Differential pressure level transmitters are mainly used for level measurement in closed pressurized vessels. As the pressure of the gas phase in the plate influences the pressure of PB, the differential pressure value between the gas and the liquid can only be measured with a differential pressure meter, and we can know to know the liquid level is high or low. From the measurement principle, it is clear that any instrument capable of measuring differential pressure can be used to measure fluid levels in closed vessels.
1. Zero migration
When using differential pressure level transmitters to measure liquid levels, there is generally a zero migration problem due to different installation positions. Therefore, we can be divided into three kinds of situations to discuss: no migration, positive migration, and negative migration.
We need to connect the positive and negative pressure chamber of the differential pressure transmitter to the lower and upper-pressure points pl and p2 of the container, respectively, if the density of the measured liquid is ρ, the differential pressure acting on the positive and negative pressure chamber of the differential pressure transmitter is
∆p= p 1- p 2=Hρ g (4—10)
When the liquid level changes from H=0 to high-level H=Hmax, Δp changes from zero to considerable differential pressure Δpmax, and the transmitter corresponds to an output of 4 to 20 mA. Assume that the transmitter range Δp corresponding to the level change requirement is 5000 Pa, called no migration.
2. Positive migration
We measure in practice, the installation position of the transmitter is sometimes lower than the lower part of the vessel to take the pressure position, the measured medium is also less dense, non-corrosive, non-crystalline, and the gas phase part does not condense, the transmitter installation height is lower than the distance of the lower limit of measurement h. This time the relationship between the liquid level height H and the differential pressure Δp is
△p=p1一p2=Hρg+hρg
When H=0, △p= hρg >0 and a constant term, acting on the transmitter to make its output greater than 4mA; when H=Hmax, the large differential pressure △p=Hmaxρg+hρg, making the output of the transmitter greater than 20mA. This can be achieved by adjusting the zero migration spring of the transmitter so that the output of the transmitter is 4mA when H=0 and △ρ=hρg, and the range of the transmitter is still Hmaxρg; when H=Hmax, large differential pressure △p=Hmaxρg+hρg, the output of the transmitter is 20mA, thus achieving a normal correspondence between the output of the transmitter and the liquid level.
3. Negative migration
We understand that some media can have a corrosive effect on the instrument, or the gas phase section can produce condensation, making the condensate in the conduit change over time. In these cases, we install separate isolation or condensation tanks between the positive and negative pressure chambers and the pressure take-off point. As a result, there is also an additional static pressure acting on the transmitter from the negative side of the draft tube, making the differential pressure not equal to zero at the measured level H=O. For the sake of discussion, only one installation case is discussed, as shown in Figure 4-9(c). Transmitter installation height and the lower part of the container to take the pressure position at the same size, but due to the gas-phase medium is easy to condense and condensate height with time, then the negative pressure conduit can be filled with the measured liquid in advance, then the relationship between the level height H and the differential pressure △ p will be
△p= Hρg-hρg
When H=0, ∆p-hpg < 0, acting on the transmitter will make its output less than 4mA; when H=Hmax, the large differential pressure ∆p=Hmaxρg-hpg, making the output of the transmitter less than 20mA. This can be achieved by adjusting the zero migration spring of the transmitter, so that when H=O, ∆p= -hρg < 0, its output is 4mA and the range of the transmitter is still Hmax; Pg; when H = Hmax; large differential pressure △p = Hmax; Hmaxρg – hρg, the output of the transmitter is 20mA, thus achieving a regular correspondence between the output of the transmitter and the liquid level.
As can be seen from the above, the essence of positive and negative migration is to change the zero point of the differential pressure transmitter by migrating the spring so that the measured level is zero, the output of the transmitter is the starting value (4mA), so-called the zero-point migration. It only changes the upper and lower limits of the measuring range of the transmitter, while the size of the range will not change.
4. The working principle of differential pressure transmitters
We need to note that not all differential pressure transmitters come with a migration effect. We will have positive or negative migration problems in the actual measurement due to the different installation heights of the transmitter. Therefore, when choosing a differential pressure level transmitter, it should be stated in the specifications of the differential pressure transmitter whether there is a positive or negative migration device, and the size of the migration should be stated.
4.1 Corrosive, crystalline, or highly viscous media
When we measure the level of corrosive or crystalline particles, as well as dense, easy to solidify, and other media, we can use flange type differential pressure to solve the problem of corrosion or blockage of the pilot line transmitter. The flange of the transmitter is directly connected to the flange on the vessel, as a sensitive element of the measuring head 1 (metal membrane box) connected to the measuring chamber of the transmitter by capillary tube 2, in the membrane box, capillary tube, and measuring room consisting of a closed system filled with silicone oil, as a pressure transfer medium, to play a role in making the transmitter and the measured medium isolated. The working principle of the transmitter itself is the same as that of a general differential pressure transmitter.
The capillary tube diameter is small (generally 0.7 ~ 1.8mm inner diameter), outside the set of metal snakeskin tube for protection, it has a flexible, single capillary tube length typically in 5 ~ 11m can be selected, the installation is more convenient. Flange type differential pressure transmitter has a single flange, double flange, inserted or flat flange, and other structural forms can be selected according to the different conditions of the measured medium.
4.2 Fluidised powder and solid granular media
In petrochemical production, we often encounter measurements of fluidized powder catalysts in fluidized bed heights in reactors. Because fluidized powder or pellet catalysts have the properties of a typical fluid, we can treat them as fluids when we measure their bed height or charge. The principle of measurement also turns the problem of measuring bed height into a situation of measuring differential pressure. However, when carrying out these measurements, the pressure measurement points and lead lines can easily be blocked due to solid powders or particles. Therefore, a back-blowing system must be used, i.e., a differential pressure transmitter using the blowing method.
5. Conclusion
A differential pressure transmitter is a sensitive element (multi-use membrane box) divided into two chambers. It is introduced into the two chambers of pressure, the sensor in the joint action of the two sides of the pressure to produce displacement (or the trend of removal), the amount of this displacement, and the difference in pressure between the two chambers (differential pressure) is proportional to this displacement into a standard signal output that can reflect the size of the differential pressure.
