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# Brief introduction of single and double flange type transmitter

Aug 29, 2019

In the process of industrial manufacturing, some of the measured tanks are easy to crystallize, highly viscous, highly corrosive, easy to solidity, and single and double flange differential pressure transmitter is often used in these occasions, such as: coking plant tanks, towers, kettles, tanks; liquid tanks for the production of evaporator units, desulfurization and denitrification plant level tank. Their main difference is the use of the open or sealed tanks. Double flanges are usually use closed tanks, and single flange type is both available.

1. The level measuring principle of single flange differential pressure transmitter

Single flange differential pressure transmitter do the level conversion by the density of the open tank.

The level measurement of the open tanks

When measure the level of the open tanks, the transmitter is mounted near the bottom of the container to measure the pressure corresponding to the height of the liquid above it.

The pressure of the tank level connect the high pressure side of the transmitter, and the low pressure side connect the air

If the minimum level of the range of measured liquid level change is above the transmitter installation position, the transmitter must perform a positive migration.

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Image 1-1 the example of the level measurement of the open tanks

Example

Assume:

X is the vertical distance between the lowest and highest measured liquid levels, X = 3175 mm.

Y is the vertical distance between the pressure tap of the pressure transmitter and the lowest level, y = 635mm.

ρ is the density of the liquid, ρ=1.

h is the maximum pressure head produced by the liquid column X and the unit is KPa.

e is the pressure head produced by the liquid column Y and the unit is KPa.

1mH2O=9.80665Pa (the same below)

Measurement range from e to e+h

And so:

h=X•ρ=3175×1=3175mmH2O=31.14KPa

e=y•ρ=635×1= 635mmH2O= 6.23KPa

The measuring range of the transmitter is 6.23KPa ~ 37.37KPa

In short, the actually measure the height of the liquid level:

Level height H=(P1-P0)/(ρ*g)+D/(ρ*g);

Note:

P0 is the current atmospheric pressure;

P1 is the pressure value measured on the high pressure side;

D is a zero point migration.

2. The level measuring principle of double flange differential pressure transmitter

Double flange differential pressure transmitter do the level conversion by the density of the closed tank.

Dry pressure connection

If the gas on the liquid surface does not condense, the connecting tube on the low pressure side of the transmitter will remain dry. This condition is called a dry pressure-pressure connection. The method for determining the transmitter's measuring range is the same as that for opening the container level.

Wet pressure connection

If condensation occurs on the gas above the liquid, the liquid gradually accumulates in the pressure tube on the low pressure side of the transmitter, causing measurement errors. In order to eliminate this error, a liquid is prefilled in the low pressure side pressure tube of the transmitter. This condition is called a wet pressure connection.

Let the low pressure side of the transmitter exist a indenter, so we must do the negative transfer (see image 1-2)

Image 1-2 the example of the level measurement of the closed tanks

Example of the wet pressure connection

Assume:

X is the vertical distance between the lowest and highest measured liquid levels, X = 2450 mm.

Y is the vertical distance between the pressure tap of the pressure transmitter and the lowest level, y = 635mm.

Z is the distance between the tip of the liquid-filled pressure guiding tube and the transmitter's reference line, Z=3800mm

ρ is the density of the liquid, ρ1=1.

ρ2 is the density of the full filled liquid in the low pressure side tube, ρ2=1

h is the maximum pressure head produced by the liquid column X and the unit is KPa.

e is the pressure head produced by the liquid column Y and the unit is KPa.

s is the pressure head produced by packing column Z and the unit is KPa.

Measurement range from (e-s) to (h+e-s), then

h=X·ρ1=2540×1 =2540mmH2O =24.9KPa

e=Y·ρ1=635×1=635mmH2O =6.23KPa

s=Z·ρ2=3800×1=3800mmH2O=37.27KPa

and so：e－s=6.23－37.27=－31.04KPa

h+e－s=24.91+6.23－37.27=－6.13KPa

In short, the actually measure the height of the liquid level:

Level height H=(P1-PX)/(ρ*g)+D/(ρ*g);

Note:

PX is the pressure value measured on the low pressure side;

P1 is the pressure value measured on the high pressure side;

D is a zero point migration.

3. Tip of the installation

3.1 The single flange type

1. For the open tank single-flange isolation diaphragm transmitter for liquid level measurement of the open liquid tank, the L side of the low pressure side interface should be open to the atmosphere.

2. For the sealed liquid tank, the pressure guiding pipe for guiding the pressure in the liquid tank should be piped to the L side of the low pressure side port. It specifies the tank reference pressure. In addition, the drain valve on the L side must be frequently unscrewed to drain the condensate in the L side chamber. Otherwise, there will be an error in the measurement of the liquid level.

3. The transmitter can be mounted to the flange as shown in Figure 1-3. The side flange of the tank is generally a movable flange. When it is fixed, it can be easily tapped to facilitate on-site installation.

Image 1-3 the installation example of the flange type liquid level transmitter

3.2 The double flange type

(1)When measuring the liquid level of the liquid tank, the minimum liquid level (zero point) should be set at 50 mm or more from the center of the high pressure side diaphragm seal. As shown in Image 1-4

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Image 1-4 the installation example of the liquid tank

2) Install the flange diaphragm on the high (H), low (L) pressure side of the tank as shown on the transmitter and sensor labels.

3) In order to reduce the influence of the ambient temperature difference, the high-pressure side capillaries may be used together, and the influence of wind and vibration may be prevented (the capillary of the extra-long portion shall be fixed together by rolling).

4) as far as possible during the installation work, do not apply the differential pressure of the sealing liquid to the diaphragm seal.

5) The transmitter body should be installed 600mm above the high pressure side remote transmission flange diaphragm seal installation part so that the differential pressure of the capillary seal fluid is added to the transmitter body as much as possible.

6) Of course, it cannot be installed at a position of 600 mm or more below the mounting part of the flange diaphragm seal part due to the setting conditions. Or, if the transmitter body can only be installed above the flange seal mounting part for objective reasons, the installation position must satisfy the following formula.

6.1 h: The height of the distal flange diaphragm seal mounting section from the transmitter body (mm);

• When h ≤ 0, the transmitter body should be installed above the flange diaphragm seal installation part h (mm).

• When h>0, the transmitter body should be mounted below the flange diaphragm seal mounting area (h (mm)). As shown in Figure 1-5:

6.2 P: pressure inside the tank (Pa abs);

6.3 P0: Lower pressure limit of transmitter body;

6.4 ambient temperature: -10 ~ 50 °C.

Image 1-5 the example of installation position in the tank