Abstract: For the double flange differential pressure transmitter in the open container, closed container liquid level measurement, different installation positions and verification methods were described and analyzed.
The double flange differential pressure transmitter is a type of thermal measuring instrument that is very common in industrial production. It mainly acts on measuring physical parameters such as pressure, liquid level and flow rate of liquid medium. At present, in the industrial automation production equipment, the application range of the double flange differential pressure transmitter is more extensive, but as a general-purpose measuring instrument, the operation failure often occurs, and if there is a problem in production, it needs to be carried out in time. Failure to solve, if not handled quickly, will certainly affect the operator's parameter control to a certain extent, production can not be carried out normally, and some serious failures may even endanger personal safety.
The differential pressure transmitter is a transmitter that measures the difference in pressure across the transmitter and outputs a standard signal (eg, 4 mA to 20 mA, 0 V to 5 V). Differential pressure transmitters differ from general pressure transmitters in that they all have two pressure ports. Differential pressure transmitters are generally divided into positive pressure and negative pressure terminals. Under normal circumstances, differential pressure transmitters are positive pressure. The pressure at the end should be greater than the pressure in the negative pressure section to measure. Differential pressure transmitters are used to measure the level, density and pressure of liquids, gases and vapors and then convert them into a current signal output of 4 mA to 20 mA DC. The JT-3051DP can also communicate with each other via the BRAIN Communicator or the CENTUM CS/μXL or HART 275 Communicator for setting and monitoring.
The pressure and differential pressure transmitters are used as the detection and transformation part of the process control system to convert the process parameters such as differential pressure (pressure), flow rate and liquid level of liquid, gas or steam into a unified standard signal (eg DC 4 mA ~ 20 mA). Current), as an input signal to the display instrument, arithmetic unit and regulator, for continuous detection and automatic control of the production process. The double flange differential pressure transmitter consists of a measuring part and a conversion amplifying circuit. The measuring part of the double flange differential pressure transmitter often adopts a differential capacitor structure, and the central movable plate and the fixed plates on both sides constitute two planar capacitors H C and L C . The movable plate and the fixed plates on both sides form two pressure sensing chambers, and the medium pressure acts on the central movable plate through the filling liquid in the two chambers. Generally, an ideal liquid such as silicone oil is used as the filling liquid, and the medium to be measured is mostly a gas or a liquid. The function of the isolating diaphragm not only transmits pressure, but also avoids damage to the capacitor plates.
Measuring conversion circuit differential capacitance structure
When the positive and negative pressures (differential pressure) are applied to the isolating diaphragm on both sides of the bellows by the positive and negative pressure guiding ports, the liquid silicone liquid is transferred to the center measuring diaphragm through the chamber, and the central pressure sensitive diaphragm is displaced. The spacing between the moving plate and the left and right plates is not opposite, forming a differential capacitor. If the edge electric field is not considered, the differential capacitor can be regarded as a plate capacitor. The relative change value of the differential capacitor is proportional to the measured pressure, and is independent of the dielectric constant of the fill liquid, which eliminates the error caused by the change of the dielectric constant.
A differential pressure transmitter is a transmitter that measures the difference in pressure across the transmitter. The measured result is the pressure difference. Different from the general pressure transmitter, the differential pressure transmitter has two pressure interfaces. The differential pressure transmitter is generally divided into a positive pressure terminal and a negative pressure terminal. Under normal circumstances, The pressure at the positive pressure end of the differential pressure transmitter should be greater than the pressure in the negative pressure section to measure. The pressure difference between the positive and negative sides of the medium is measured and converted into a standard current signal (4 mA to 20 mA) that can react to the pressure difference.
3. 1 Transmitter installed in open container level measurement
The high pressure chamber flange is connected to the lower end flange of the vessel, and the low pressure chamber flange is mounted in the atmosphere at the same level as the flange of the high pressure chamber.
Connection of the high pressure chamber flange to the lower end flange of the container
Then the transmitter range is: L = H·p 1
The zero migration is: A = h 1 · p 1
At the lowest level of measurement, the equivalent static pressure difference acting on the transmitter is: P 1 = h 1 · p 1 = A
At the highest level of measurement, the equivalent static pressure difference acting on the transmitter is: P 2 = (H + h 1 )·p 1 = H + p 1 + A
Where: H———the height between the lowest measured liquid level and the highest measured liquid level; h 1 ———the lowest measured liquid level to the height between the center line of the low-end flange of the container; p 1 ———tested The specific gravity of the liquid medium. The transmitter calibration range is:
P = △P 1 ～ △P 2 = A ～ H·p 1 + A
3. 2 Transmitter installed in closed container level measurement
The high pressure chamber flange is connected to the high end flange of the container, and the low pressure chamber flange is connected to the lower end flange of the container.
Connection of the high pressure chamber flange to the high end flange of the container
Then the transmitter range is: L = H·p
The zero migration is: A = h 3 · p 2 - h 1 · p 1
When the liquid level is measured at the lowest level, the equivalent static pressure difference acting on the transmitter is: ΔP 1 = h 3 · p 2 - h 1 · p 1 = A
When the liquid level is measured at the highest level, the equivalent static pressure difference acting on the transmitter is: P 2 = h 3 · p 2 - h 1 · p 1 - H · p 1 = A - H·p 1
Where: H 3 ———the height between the centerline of the high and low end flanges of the container.
The transmitter calibration range is:
P = △P 1 ～ △P 2 = A ～ A - H·p 1
The result measured by the double flange differential pressure transmitter is the pressure difference, ie ΔP = ρg Δh. Since the tank is often cylindrical and the area S of the cross-section circle is constant, the gravity G = ΔP · S = ρg △ h · S, S is constant, and G is proportional to ΔP. That is, as long as the ΔP value is accurately detected, it is inversely proportional to the height Δh. When the temperature changes, although the liquid volume expands or contracts, the actual liquid level rises or falls, and the detected pressure remains constant. If the actual liquid level needs to be displayed at the production site, media temperature compensation can also be introduced to solve the problem.
4. 1 General double flange differential pressure transmitter calibration method
First adjust the damping to zero state, first adjust the zero point, then adjust the full-scale pressure to full scale, so that the output is 20 mA. It is fast in the field adjustment, here is the quick adjustment method of zero point and range. Zero adjustment has almost no effect on fullness, but it has an effect on zero when adjusting fullness. When there is no migration, the effect is about 1/5 of the range adjustment amount, that is, the range is adjusted upward by 1 mA, and the zero point will move upward by about 0. . 2 mA and vice versa. For example, enter a full-scale pressure of 100 kPa and the reading is 19. 900 mA, the range potentiometer makes the output 19. 900 + (20. 000 -19. 900) × 1. 25 = 20. 025 mA. The range is increased by 0. 125 mA, then the zero point increases by 1/5 × 0. 125 = 0. 025. Zero point potentiometer makes the output 20. 000 mA. After the zero and full scale adjustments are normal, check the middle scales to see if they are out of tolerance and fine tune if necessary. Then carry out the migration, linear, and damping adjustment work.
4. 2 Calibration of intelligent differential pressure transmitter
It is not acceptable to calibrate the smart transmitter using the conventional method described above, as this is determined by the structural principle of the HART transmitter. Because the smart transmitter is between the input pressure source and the generated 4 mA ~ 20 mA current signal, in addition to the mechanical, circuit, and micro-processing chip to calculate the input data, so the calibration is different from the conventional method. In fact, the manufacturer also has a description of the calibration of the smart transmitter. For example, the EJA transmitter has the following functions: “set range”, “heavy quantitation” and “fine adjustment”. The “set range” operation is mainly through the digital setting of LRV and URV to complete the configuration work, while the “re-quantitative” operation requires the transmitter to be connected to the standard.
On the quasi-pressure source, guided by a series of instructions, the transmitter directly senses the actual pressure and sets the value. The initial and final settings of the range are directly dependent on the actual pressure input value. However, it should be noted that although the analog output of the transmitter is correctly related to the input value used, the digital reading of the process value will display a slightly different value, which can be calibrated by fine-tuning the item. Since the parts must be adjusted separately and must be adjusted, the actual steps can be followed by the following steps:
(1) Perform a 4 mA to 20 mA trimming to correct the D/A converter inside the transmitter. Since it does not involve sensing components, no external pressure source is required.
(2) Make another full-scale fine-tuning so that the 4 mA to 20 mA, digital reading matches the actual applied pressure signal, so a pressure signal source is required.
(3) Finally, the heavy-quantity process is performed, and the analog output 4 mA ~ 20 mA is matched with the applied pressure signal source, and its function is adjusted with the zero (Z) and range (R) switches on the transmitter housing. The effect is exactly the same.
The installation position and calibration analysis described above for the double flange transmitter are the methods used by Huaheng Instrument's customer in a salt company.