Diaphragm differential pressure transmitter in the use of two-level gas tank level gauge
0, Overview In our factory acetylene plant production, acetylene gas cabinet, cracking gas cabinet is an important part of the entire acetylene production process, the gas tank level measurement with high and low chain, is an important part of production automation control. The measurement of the gas tank level gauge is stable, and the stability of the system operation is a key measurement parameter. The problems with the steel belt level gauge: Transmitter differential pressure formula: With the influence of the liquid's own gravity, different pressures at different heights produce a pressure difference. The pressure difference is used to calculate the height of the liquid. This is the application of a differential pressure transmitter to measure the liquid level. As shown in Figure 4. Due to the nature of toxic, hazardous, flammable, explosive, and corrosive substances that may cause harm to human bodies in some liquid media of chemical plants, for safety reasons, flange separators are used to isolate the medium, and there is a capillary connection between the flange and the transmitter. The capillary is filled with silicone oil. Measure liquid level diaphragm differential pressure transmitter type: double flange diaphragm differential pressure transmitter, single flange diaphragm differential pressure transmitter and so on. According to the migration formula, due to the influence of the self-weight of the silicone oil, at the zero liquid level, the migration amount is also different if the following conditions exist: Transmitter differential pressure formula:
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1, the original liquid level measurement method Our factory acetylene device, the gas cabinet is a wet gas cabinet, set the sink, the use of water-tight forms, the cabinet consists of two towers, that bell (a tower), the middle section I (two Tower), the cabinets move up and down as the gas volume in the cabinet changes. Gas tank level measurement method The original steel belt level transmitter was used to measure the gas tank level. Figure 1. The steel belt type liquid level transmitter includes a steel belt mechanism, a displacement transmission mechanism, and a transmitter. The level gauge is installed under the gas cabinet, one end of the steel rope is fixed on the top of the gas cabinet in the steel belt mechanism, and the other end of the steel belt is connected with the liquid level gauge through the guide rail in the steel belt mechanism. With the rise and fall of the gas cabinet, the bell drives the steel belt, and the movement of the steel belt drives the displacement transmission mechanism in the level gauge. Among them, the transmission gear transmits the level change through the coupler to the potentiometer of the transmitter. The change in the resistance value caused by the change in the liquid level produces a 4-20 mADC output, from which the level of the gas cabinet is measured. 
(1) Liquid level gauge As the liquid level goes up and down, the steel belt repeatedly pulls the potentiometer shaft of the transmitter to rotate, resulting in wear and damage of the potentiometer. Frequent indicator fluctuates, causing the liquid level gauge to work normally for acetylene and cracking gas tanks. Hidden danger.
(2) The liquid steel pipe (steel pipe) of the liquid level gauge protrudes from the air, and the thunderstorm period in summer is easy to become a lightning strike point, which is easily struck by lightning.
(3) The transmission mechanism is often stuck.
(4) The influence of gas cabinet drift, wind blow, etc., the steel rope is easily derailed and stuck.
(5) The steel belt level gauge has a complex structure. The VAREC products used have long spare parts, are expensive, and have high costs.
In response to the above problems, measures have been taken to deal with them:
(1) The potentiometer is replaced, but after six months of use, the problem arises.
(2) Install a lightning rod at the top of the gas cabinet, and install a lightning protection ground wire in the transmitter case to achieve the lightning protection effect.
(3) Always send people to use, twist the reset device, and lubricate.
(4) After the steel rope was jammed, contact the big crane and it has been processed three times. The cost of the big crane is relatively high.
(5) Replacement of products, similar to domestic ones, but the first problem mentioned above occurs after six months of use.
Acetylene gas cabinets and pyrolysis gas cabinets are important parts of the production process of the acetylene plant. The level measurement of the gas tanks with high and low levels is an important part of production automation control. In view of the above reasons, we have modified the level gauge and used a single flange diaphragm differential pressure transmitter to measure it.
2, the current level measurement method
2.1 Differential Pressure Transmitter Working Principle The pressure from both sides directly acts on the diaphragms of the transmitter's sensors. It is conducted through the sealing fluid (usually silicone oil) in the diaphragm to the measuring element. The measuring element will measure The differential pressure signal is converted to the corresponding electrical signal and passed to the converter. After amplification, the signal is converted to a standard electrical signal (DC current of 4-20 mA DC). 
ΔP=P + -P -
According to the differential pressure formula, the differential pressure transmitter can work as long as there is a pressure difference.
2.2 Differential Pressure Transmitter Measurement The liquid level pressure is the force acting vertically on the unit area of ​​the object. It is called the pressure in physics and it is represented by P. The unit is Pascal and it is represented by Pa.
The reason for the liquid pressure is that the liquid is subject to gravity.
Liquid pressure characteristics: (1) The liquid has pressure on the bottom and the wall of the vessel, (2) The pressure inside the liquid has pressure in all directions; (3) The pressure of the liquid increases with depth, at the same depth, the liquid flows in all directions. The pressure is equal; (4) The pressure of different liquids is also related to the density.
Liquid pressure calculation formula: P = Ïgh (Ï is the liquid density in kilograms per cubic meter; g is 9.8 N/kg; h is the depth, which refers to the vertical distance from the liquid free liquid to a point inside the liquid, in meters) .
According to the liquid pressure formula, the pressure of the liquid is related to the density and depth of the liquid, regardless of the volume and quality of the liquid. According to Pascal's law: the fluidity of a liquid, the pressure applied to any part of a closed liquid must be transmitted in all directions by the liquid in its original size. The formula for the pressure at a certain point in the liquid is P = P 0 +, where P 0 is the pressure in the container and Ï gh is the liquid pressure. For a known liquid level, Ï and g are constants and P0 does not change. The pressure (pressure) at this point is related to the height of the liquid, ie the pressure (pressure) increases with the depth of the liquid. As shown in Figure 3, there are three points A, B, and C in the liquid, points A and B are at the same depth, and point C is deeper than points A and B. The pressure relationship at the three points is: point A pressure PA = point B pressure PB, point C pressure PC> point A pressure PA. 
Transmitter differential pressure formula:
ΔP=P + -P -P + =Ïgh+P 0 ;P - =P 0
ΔP=P + -P - =Ïgh+P 0 -P 0 =Ïgh 
2.3 Single-flanged diaphragm differential pressure transmitter applications In open vessels, the measurement level is mostly measured with a single-flanged diaphragm differential pressure transmitter, Figure 5. Transmitter differential pressure formula:
ΔP=P + -P - ;P + =Ïgh+Ï Silicone oil gh 1 +P Atmospheric pressure; P - =P Atmospheric pressure ΔP=P + -P - =Ïgh+Ï Silicone oil gh 1 In the above formula, at level height h Zero level, differential pressure value ΔP is Ï silicone oil gh 1 , not zero, this is due to the influence of silicone oil liquid pressure in the capillary, which requires migration, in order to ensure that the actual liquid level is at zero level, the transmitter The output is also zero and the migration amount is Ï silicone gh 1 . Migration> 0, positive migration; migration <0, negative migration. 
(1) The flange installation height differs when setting the zero level.
(2) The height of the transmitter installation position has changed.
(3) The flange moves up and down with the mounting height of the device while the transmitter is stationary. To further understand the role of silicone oil pressure in the capillary, do one such experiment:
The single flange diaphragm differential pressure transmitter is fixed, and the negative pressure side is connected with the atmosphere. The positive pressure side is connected with the single flange diaphragm capillary. The circuit part is connected well, and the output of the transmitter is tested in the current loop with the ammeter. The experimenter took a single flange to move up and down, and the differential pressure transmitter output current also changed. This experiment shows that due to the gravity of the silicone oil liquid in the capillary, the liquid pressure is generated. Under a single flange diaphragm differential pressure transmitter, the differential pressure transmitter's differential pressure is determined by the pressure of the silicone fluid in the capillary when there is no external pressure applied. Produce and increase with height.
Using this experiment, our factory now uses a single flange diaphragm differential pressure transmitter to measure the level of the gas tank. Install as shown in Figure 6.
The single flange diaphragm differential pressure transmitter is fixed on the secondary maintenance platform, and the negative pressure side is open to atmosphere. The positive pressure side is connected by a single flange diaphragm capillary, and the flange is fixed on the top of the bell, and the pressure on the positive pressure side of the transmitter. That is, the pressure generated by the silicone oil in the capillary, the diaphragm moves up and down with the gas cabinet, and the input pressure of the differential pressure transmitter also changes accordingly, resulting in a 4-20 mADC output, thus measuring the gas tank liquid level. 
ΔP=P + -P -
P + =Ï Silicon oil gh+P Atmospheric pressure
P - = P Atmospheric pressure ΔP = P + -P - = (Ï Silicone oil gh + P atmospheric pressure) - P Atmospheric pressure = Ï Silicone oil gh Our factory Gas tank level measurement height is 18 meters, then the measuring range:
ΔP=Ï Silicone oil gh=0.93*1000*9.8*18=164.052kPa Silicon oil density Ï takes 0.93*1000kg/m3, g~9.8 N/kg When liquid level height h is zero liquid level, the effect of silicone oil self weight, differential pressure The value ΔP=Ï silicone oil gh 1 is not zero, which requires a migration to ensure that the transmitter output is zero. When the gas tank liquid level h is zero liquid level, the flange diaphragm is lower than the transmitter installation position, and the migration amount is <0, which requires negative migration. If the transmitter is fixed at the midpoint of the gas tank liquid level, h 1 = 9 meters, migration amount = Ï silicone oil gh 1 = 0.93*1000*9.8*(-9)=-82.026kPa, the configuration of the transmitter Measurement range: -82.026 ~ 82.026kPa.
(1) Single-flange diaphragm differential pressure transmitter to measure liquid level requirements:
1 The single flange diaphragm differential pressure transmitter needs to be customized. The Rosemount transmitter used in our factory has a maximum length of 15 meters. Since the gas cabinet uses two sections of tower sections and the total height reaches 20 meters, the transmitter only needs to be installed on the middle secondary maintenance platform.
In this way, it is possible to meet the length requirements when the gas cabinet reaches the highest and the lowest.
2 Inquire the detailed information provided by the transmitter manufacturer, silicone oil density must be accurate, otherwise affect the measurement, resulting in errors.
3 When setting the measuring range of the transmitter, it is necessary to fix the transmitter first and then communicate with the technician to confirm the middle height of the gas cabinet.
4 Transmitter should be installed in the position of the leeward, do not be blown by the wind, try to make the capillary stable, and make empty space, so that the capillary does not twist around with the surrounding frame.
5 Do not install the flange diaphragm in the upward direction to prevent additional force from being generated when the water is trapped and rain falls on the diaphragm, affecting the measurement results.
(2) Single flange diaphragm differential pressure transmitter calibration method:
1 Connect the negative pressure side of the instrument to the atmosphere and the positive pressure side fitting flange to the corresponding pressure. After turning on the power for 3 minutes, the transmitter is 4mA. If there is any deviation, adjust the zero point in the adjustment menu to make it output. It is 4mA.
2 The positive pressure side of the transmitter is equipped with a flange input range signal. The negative pressure side is equipped with a flange and the corresponding pressure. The transmitter output is 20mA. If there is any deviation, adjust the range in the adjustment menu to make the output 20mA.
3 Repeat steps (1) and (2) to meet the requirements.
4 The measurement range is divided into 5 points, according to 0%, 25%, 50%, 75%, 100%
Point by point input signal, the transmitter's output value should be within the allowable range, if the error, repeated adjustment of zero, range.
3. Comparison of Single Flange Diaphragm Differential Pressure Transmitter and Steel Band Liquid Level Meter Measurement, Advantages 3.1 Structure Different Single Flange Diaphragm Differential Pressure Transmitter has simple structure and few components: single flange diaphragm, capillary, differential pressure Transmitters, all sealed, no moving parts. The structure of the steel belt level gauge is complicated: steel belts, displacement transmission mechanisms, transmitters, and movable parts are more likely to jam.
3.2 Measuring principle Different differential pressure transmitters measure the pressure difference on the diaphragms of the two sides of the sensor. The measuring element converts the measured differential pressure signal to the corresponding electrical signal and transmits it to the converter. After amplification and other processing, it becomes standard. Electrical signal output. The steel belt type liquid level gauge drives the displacement transmission mechanism in the liquid level gauge through the movement of the steel belt, wherein the transmission gear changes the liquid level through the coupler to the potentiometer of the transmitter and is caused by the change of the liquid level. The change of the resistance value produces a standard electrical signal output. The long-term sliding of the potentiometer is easy to wear and damage. The resistance value has a dead zone, making the measured value fluctuate and unstable.
3.3 Calibration method Single-flanged diaphragm differential pressure transmitter calibration method is simple, convenient and easy to operate, only through the pressure on the membrane box, set the transmitter with 475 communicator (transmitter with HART protocol) Range. The steel belt level gauge needs to pull the steel rope on the bell and fit it up and down.
3.4 Labor cost Single flange diaphragm differential pressure transmitter requires only 2 people for maintenance and calibration. When the steel belt level meter is calibrated, it needs to cooperate up and down with 4 people.
3.5 Maintenance and Maintenance Costs Differential pressure transmitters have high versatility. If there is only a problem with the circuit board, just replace the header circuit board. The steel belt level gauge spare parts are strong.
3.6 Cost Cost Single flange diaphragm differential pressure transmitter spare parts meter is cheap as a whole, which is lower than the price of a circuit board of the original steel belt level meter.
Through one year of application, the single-flange diaphragm differential pressure transmitter measures the liquid level of the gas cabinet to meet the requirements of process production. The failure rate of the instrument and the daily maintenance amount are zero, eliminating the problems of the liquid level gauge of the steel belt. The stable operation of the gas cabinet provides the guarantee for the “long, full, excellent, and stable†operation of the entire installation.