Migration of double-flange differential pressure transmitters

1. The migration amount of a double-flange differential pressure transmitter is the pressure generated by the silicone oil due to the height difference between the two flanges. Once the flange is installed, it can be moved; the root valve of the instrument is open (in operating condition). 3. Migration is only related to the height difference between the two flanges, not to their position

The amount of negative migration for a double-flange level transmitter is independent of the transmitter’s installation location; negative migration always occurs, and its value equals the product of the distance between the upper and lower flanges and the density of the liquid inside the capillary (make sure to use consistent units).

After all this time and so many posts, still don’t get it? It’s funny to see the OP looking so confused.

I think once the empty tank is installed, we can just move the range so that the output becomes 0

Use a remote controller to adjust the range; for example, if your gauge’s range is 0-5, and after installing an empty tank it shows -0.5 due to gravity, then use the remote controller to change it to -0.5-4.5

1. It has nothing to do with the installation location; it depends on the distance from the capillary on your flange surface to the gauge head

2. The vast majority of level transmitters need to be relocated, unless the installation height of the gauge head is the same as that of the positive pressure port; in the case of an empty tank, open the root valve and the drain valves of the three-valve or five-valve assembly before proceeding with the relocation

In industrial production, it is often necessary to measure and control the liquid level in certain equipment such as boxes, tanks, and vats. There are many instruments available for measuring liquid levels. For many special liquid media – those that are corrosive or contain crystalline particles, have high viscosity, or tend to solidify – dual-flange differential pressure transmitters can be used for liquid level measurement in order to avoid problems such as corrosion or blockage of the pressure transfer tubes. In this article, CHANGHUI Instruments shares the installation methods for double-flange differential pressure transmitters, as well as the calculation methods for the zero-point adjustment and range setting of such transmitters. It also outlines the calculation and verification techniques for double-flange differential pressure transmitters. As shown in Figure 1, the metal diaphragm, which is a sensitive component, is located within a closed system composed of an armored capillary and a measurement chamber; this system is filled with a sealing fluid, usually silicone oil, which serves as the pressure-transmitting medium. To ensure the durability of the capillaries, they are all protected by metal hoses on the outside. yunrun.com.cn/tech/1465.html 1. Installation and calculation of double-flange differential pressure transmitters in level measurement. Double-flange differential pressure transmitters can be installed at any height and location. However, it must not be higher than the horizontal level of the flange in the low-pressure chamber when used in vacuum environments (manufacturers of double-flange differential pressure transmitters designed for vacuum use usually take special measures, but they are still very prone to damage; for the reasons behind this, see the article titled \"Reasons Why Double-Flange Level Transmitters Are Not Suitable for Measuring Negative Pressure Levels\"). In level measurement, double-flange transmitters are commonly used in sealed containers; the flange in the low-pressure chamber serves to eliminate the effects of pressure (or vacuum) changes within the container, while the flange in the high-pressure chamber is used to measure the liquid level inside the container. If a double-flange differential pressure transmitter is used in an open vessel, the flange of the high-pressure chamber should be connected to the flange at the lower end of the vessel, while the flange of the low-pressure chamber should be exposed to the atmosphere and kept at the same height as the flange of the high-pressure chamber. ①The double-flange differential pressure transmitter is installed below the horizontal line centered on the lower flange of the open-container. The double-flange differential pressure transmitter is positioned below this horizontal line, with the flange of its low-pressure chamber being on the same horizontal level as the center of the lower flange of the open container, as shown in Figure 2: Figure 2 shows the double-flange differential pressure transmitter installed below the horizontal line of the lower flange of the open container (with the flange of the transmitter’s low-pressure chamber on the same horizontal level as the center of that lower flange). It is assumed that the density of the medium being measured is ρ1 (unit: kg/m3), and H is the height between the lowest and highest measurement levels (unit: m) ； h1 is the height between the lowest measured liquid level and the centerline of the lower flange of the container (unit: m) ； Then the range of the double-flange differential pressure transmitter is L=H×ρ1 ； The zero-point positive migration amount is A=h1×ρ1 ； At the lowest measured liquid level, the equivalent static pressure difference acting on the double-flange differential pressure transmitter is △P1=h1×ρ1=A ； At the highest measured liquid level, the equivalent static differential pressure acting on the double-flange differential pressure transmitter is △P2 = (H + h1)×ρ1 = H×ρ1 + h1×ρ1 = H×ρ1 + A ； The measurement range of the double-flange differential pressure transmitter is △P=△P1~△P2=h1×ρ1~H×ρ1+h1×ρ1=A~H×ρ1, and the output of this transmitter is 4-20mA. B. The double-flange differential pressure transmitter is installed below the horizontal line centered on the lower flange of the open-container, with the flange of the transmitter’s low-pressure chamber being below this horizontal line as well, as shown in Figure 3: Figure 3 shows the double-flange differential pressure transmitter installed below the horizontal line centered on the lower flange of the open container (the flange of the transmitter’s low-pressure chamber is below this horizontal line). Assuming that the density of the medium being measured is ρ1 (unit: kg/m3), and the density of the liquid filling the capillaries of the double-flange differential pressure transmitter is ρ2 (unit: kg/m3), H represents the height between the lowest and highest measurement levels (unit: m) ； h1 is the height between the lowest measured liquid level and the centerline of the lower flange of the container (unit: m) ； If h2 is the height between the flange of the low-pressure chamber and the flange at the lower end of the open container (in meters), then the range of the double-flange differential pressure transmitter is L = H × ρ1 ； The zero-point migration amount (positive migration) is A=h1×ρ1+h2×ρ2 ； At the lowest measured liquid level, the equivalent static pressure difference acting on the double-flange differential pressure transmitter is △P1=h1×ρ1+h2×ρ2=A ； At the maximum measured liquid level, the equivalent static differential pressure acting on the double-flange differential pressure transmitter is △P2 = (H + h1)×ρ1 + h2×ρ2 = H×ρ1 + A ； The measurement range of the double-flange differential pressure transmitter is △P=△P1~△P2=h1×ρ1+h2×ρ2~(H+h1)×ρ1+h2×ρ2=A~H×ρ1+A, and the output of this transmitter is 4-20mA. ②The double-flange differential pressure transmitter is installed above the horizontal line of the lower flange of the open-container. Specifically, it is placed above the center horizontal line of the lower flange of the open container, with the flange of the low-pressure chamber of the double-flange differential pressure transmitter being at a higher level than that center horizontal line, as shown in Figure 4. To avoid the occurrence of a dead zone during measurement, it is necessary to ensure that h1×ρ1 ≥ h2×ρ2; otherwise, the actual measurement range will be smaller than the transmitter’s specified range. Figure 4: The double-flange differential pressure transmitter is installed above the horizontal line of the lower flange of the open container (the flange of the transmitter’s low-pressure chamber is above this horizontal line). It is assumed that the density of the medium being measured is ρ1 (unit: kg/m3), while the density of the liquid filling the capillaries of the double-flange differential pressure transmitter is ρ2 (unit: kg/m3). H represents the height between the lowest and highest measurement levels (unit: m) ； h1 is the height between the lowest measured liquid level and the centerline of the lower flange of the container (unit: m) ； h2 is the height between the flange of the low-pressure chamber and the flange at the lower end of the open vessel (in meters); thus, the range of the double-flange differential pressure transmitter is L = H × ρ1 ； The zero-point migration amount is A = h1×ρ1 – h2×ρ2 ； At the lowest measured liquid level, the equivalent static pressure difference acting on the double-flange differential pressure transmitter is △P1 = A = h1×ρ1 – h2×ρ2 = A ； At the maximum measured liquid level, the equivalent static differential pressure acting on the double-flange differential pressure transmitter is △P2 = (H + h1)×ρ1 – h2×ρ2 = H×ρ1 + A ； The measurement range of the double-flange differential pressure transmitter is △P=△P1~△P2=h1×ρ1-h2×ρ2~(H+h1)×ρ1-h2×ρ2=A~H×ρ1+A, corresponding to an output of 4-20mA for this type of transmitter. B. The double-flange differential pressure transmitter should be installed above the horizontal line centered on the lower flange of the open-container, with the flange of the transmitter’s low-pressure chamber being on the same horizontal line as the center of the lower flange of the open container, as shown in Figure 5: Figure 5 shows the double-flange differential pressure transmitter installed above the horizontal line of the lower flange of the open container (with the flange of the transmitter’s low-pressure chamber and the center of the lower flange of the open container on the same horizontal line). Assuming the density of the medium being measured is ρ1 (unit: kg/m3), and the density of the liquid filling the capillary of the double-flange differential pressure transmitter is ρ2 (unit: kg/m3), H represents the height between the lowest and highest measurement levels (unit: m) ； h1 is the height between the lowest measured liquid level and the centerline of the lower flange of the container (unit: m) ； If h2 is the height between the flange of the low-pressure chamber and the flange at the lower end of the open container (in meters), then the range of the double-flange differential pressure transmitter is L = H × ρ1 ； Zero-point migration amount (positive migration) A = h1 × ρ1 ； The measurement range of the double-flange differential pressure transmitter is △P=△P1~△P2=h1×ρ1~(H+h1)×ρ1=A~H×ρ1+A, and the output of this transmitter is 4-20mA. ③The double-flange differential pressure transmitter is installed below the horizontal line of the lower flange of the sealed container, as shown in Figure 6. The high-pressure chamber flange is connected to the lower flange of the sealed container, while the low-pressure chamber flange is connected to the upper flange of the sealed container. Figure 6: The double-flange differential pressure transmitter is installed below the horizontal line of the lower flange of the sealed container. It is assumed that the density of the medium being measured is ρ1 (unit: kg/m3), while the density of the liquid filling the capillaries of the double-flange differential pressure transmitter is ρ2 (unit: kg/m3). H represents the height between the lowest and highest measurement levels (unit: m) ； h1 is the height between the lowest measured liquid level and the centerline of the lower flange of the container (unit: m) ； h2 is the height between the flange of the low-pressure chamber and the flange at the lower end of the open container (in meters); h3 is the height between the centers of the upper and lower flanges of the closed container. Thus, the range of the double-flange differential pressure transmitter is L = H × ρ1 ； The zero-point migration amount is A = h1×ρ1 – h3×ρ2 ； At the lowest measured liquid level, the equivalent static pressure difference acting on the double-flange differential pressure transmitter is △P1 = h1×ρ1 – h3×ρ2 = A ； At the maximum measured liquid level, the equivalent static differential pressure acting on the double-flange differential pressure transmitter is △P2 = H×ρ1 + h1×ρ1 – h3×ρ2 = H×ρ1 + A ； The measurement range of the double-flange differential pressure transmitter is △P=△P1~△P2=h1×ρ1-h3×ρ2~H×ρ1+h1×ρ1-h3×ρ2=A~H×ρ1+A, and the output of this transmitter is 4-20mA. ④The double-flange differential pressure transmitter is installed above the horizontal line of the lower flange of the sealed container, as shown in Figure 7. The high-pressure chamber flange is connected to the upper flange of the sealed container, while the low-pressure chamber flange is connected to the lower flange of the sealed container. To avoid a dead zone in the measurement process of double-flange differential pressure transmitters, it is necessary to ensure that h1×ρ1≥h2×ρ2. Figure 7: The double-flange differential pressure transmitter is installed above the horizontal line of the lower flange of the sealed container. It is assumed that the density of the medium being measured is ρ1 (unit: kg/m3), while the density of the liquid filling the capillaries of the double-flange differential pressure transmitter is ρ2 (unit: kg/m3). H represents the height between the lowest and highest measurement levels (unit: m) ； h1 is the height between the lowest measured liquid level and the centerline of the lower flange of the container (unit: m) ； h2 is the height between the flange of the low-pressure chamber and the flange at the lower end of the open container (in meters); h3 is the height between the centers of the upper and lower flanges of the closed container. Thus, the range of the double-flange differential pressure transmitter is L = H × ρ1 ； The zero-point migration amount is A = (h1×ρ1 – h2×ρ2) – (h3 – h2)×ρ2 = h1×ρ1 – h3×ρ2 ； At the lowest measured liquid level, the equivalent static pressure difference acting on the double-flange differential pressure transmitter is △P1=(h1×ρ1-h2×ρ2)-(h3-h2)×ρ2=A ； At the maximum measured liquid level, the equivalent static differential pressure acting on the double-flange differential pressure transmitter is △P2=-(h3-h2)×ρ2=H×ρ1+A ； The measurement range of the double-flange differential pressure transmitter is △P=△P1~△P2=A~H×ρ1+A, corresponding to an output of 4-20mA for this type of transmitter. ⑤The double-flange differential pressure transmitter is installed above the horizontal level of the upper flange of the sealed container, as shown in Figure 8. The flange of the high-pressure chamber is connected to the lower flange of the sealed container, while the flange of the low-pressure chamber is connected to the upper flange of the sealed container; however, the transmitter itself is installed above the horizontal level of the upper flange of the sealed container. Figure 8: The double-flange differential pressure transmitter is installed above the horizontal line of the upper flange of a sealed container. Assuming the density of the medium being measured is ρ1 (unit: kg/m3), and the density of the liquid filling the capillaries of the double-flange differential pressure transmitter is ρ2 (unit: kg/m3), H represents the height between the lowest and highest measurement levels (unit: m) ； h1 is the height between the lowest measured liquid level and the centerline of the lower flange of the container (unit: m) ； h2 is the height between the flange of the low-pressure chamber and the flange at the lower end of the open container (in meters); h3 is the height between the centers of the upper and lower flanges of the sealed container; h4 is the height from the centerlines of the positive and negative pressure chambers of the transmitter to the centerline of the upper flange of the container; h5 is the height from the centerlines of the positive and negative pressure chambers of the transmitter to the centerline of the lower flange of the container. Thus, the range of the double-flange differential pressure transmitter is L = H × ρ1 ； The zero-point migration amount is A=h1×ρ1-(h5-h4)×ρ2 ； At the lowest measured liquid level, the equivalent static pressure difference acting on the double-flange differential pressure transmitter is △P1 = h1×ρ1 – (h5–h4)×ρ2 = A ； At the maximum measured liquid level, the equivalent static differential pressure acting on the double-flange differential pressure transmitter is △P2 = (h1 + H)×ρ1 – (h5 – h4)×ρ2 = H×ρ1 + A ； The measurement range of the double-flange differential pressure transmitter is △P=△P1~△P2=A~H×ρ1+A, corresponding to an output of 4-20mA for this type of transmitter. For the above scenarios, if the installation positions of the high and low pressure chamber flanges of the transmitter are swapped, the same analytical calculations can be carried out; only the zero-point shift changes from a positive shift to a negative shift. 2. The “calculation and verification method” for double-flange differential pressure transmitters: When using a double-flange differential pressure transmitter to measure liquid levels, since the density of the liquid inside the transmitter’s armored capillary and the density of the liquid being measured are known, it is possible to easily determine the transmitter’s range (L), zero-point shift (A), as well as the equivalent static pressure differences (△P1 and △P2) acting on the transmitter at the lowest and highest measurement levels. This is achieved by determining the installation parameters of the transmitter (h1, h2, h3, h4, h5). Once these values are known, it is straightforward to carry out the calculations. Subsequently, the flanges of the transmitter’s positive and negative pressure chambers are placed at the same height for verification; the verification range is ΔP = ΔP1 to ΔP2, corresponding to an output of 4-20 mA from the transmitter. In this way, by installing the calibrated double-flange differential pressure transmitter on the container as required, it is possible to accurately measure the liquid level inside the container.