Application of Differential Pressure Transmitter in Measuring Liquid Level and Analysis of Common Faults
In industrial production, the level of liquid level directly affects the safety of production, so the continuous monitoring of liquid level is especially important. The differential pressure transmitter is due to the characteristics of uninterrupted measurement, and is suitable for various complicated working conditions. The parameter adjustment is simple and convenient, the requirements on the field environment are low, the measurement height can be flexibly adjusted, and the measurement liquid level is widely obtained. application. Especially when the boiler is in use, the water level is an extremely important indicator. Too high or too low is dangerous, so the water level should be automatically controlled. At present, the relatively mature measured water level is measured by a differential pressure transmitter, keeping the water level within a safe range and simultaneously converting the water level into a standard current output.
1 Working principle
The basic principle of the differential pressure transmitter is to divide a space with sensitive components (such as a bellows) into two pressure chambers, namely a positive pressure chamber and a negative pressure chamber; when pressure is introduced into the two pressure chambers respectively, the sensor is Under the joint action of positive pressure and negative pressure, a certain displacement or displacement trend is generated. This displacement (displacement trend) is proportional to the pressure difference between the two pressure chambers, and the displacement amount is converted into the reaction differential pressure by the internal conversion circuit. Standard signal output, usually output signal is the standard 4 ~ 20mA current signal.
In the actual construction, there are many kinds of structures, conversion methods, standard signals, and the like of the sensitive components. Structural aspects: resistance strain gauge, piezoresistive, inductive, capacitive, resonant, etc., but generally use piezoresistive differential pressure transmitter, because it has low price, good linearity, easy maintenance, migration Large amount, high precision and other characteristics. Conversion method: There are different conversion methods depending on the different sensors, but it is certain that they are converted into electrical signals. Standard signal: There are two kinds of current signal and voltage signal in current production, usually using 4~20mA current signal.
2 Migration method
In the actual measurement, the installation position of the transmitter is often not at the same position as the lowest liquid level of the liquid to be tested. There are three types of positive migration, negative migration and no migration depending on the installation position of the differential pressure transmitter or the measurement container. .
2.1 Positive migration
The positive transfer of the transmitter is suitable for the measurement of the liquid level in open or non-open containers. During installation, the positive pressure chamber is connected to the container interface, and the negative pressure chamber is connected to the atmosphere or the low pressure side. Assuming that the container is an open container, the position of the differential pressure transmitter is lower than the minimum liquid level by h, assuming that the liquid density in the container is ρ, the liquid level in the container is H, and ΔP is the pressure applied to the transmitter. ΔP = ρgH + ρgh.
When H=0, ¡÷P=ρgh, there is a static pressure in the positive pressure chamber of the differential pressure transmitter, the pressure is ρgh, and the output current signal is greater than 4mA.
When H=Hmax, ¡÷P=ρgH+ρgh, the transmitter output is greater than 20mA, so the static pressure ρgh must be eliminated to ensure that the measured liquid level is consistent with the actual liquid level, so the transmitter must be Tuning.
Assume that the transmitter has a range of 40KPa and a migration of 20KPa. The normal adjustment method is as follows: the transmitter is placed horizontally on the ground, the positive pressure side is connected to the signal, the negative pressure is connected to the atmosphere, and the 24V DC power supply is used. If necessary, a 250Ω resistor can be connected in the loop. When the transmitter pressure is 20Kpa, the output signal size is 4mA. When the pressure is 60Kpa, the output signal is 20mA. Since the field instruments are mostly intelligent instruments that support the Hart protocol, the parameters can be changed directly using the handheld device.
2.2 Negative migration
For a closed container, the internal liquid should not be leaked while measuring the liquid level. At this time, negative migration measures should be taken to achieve the liquid level measurement. Taking a steam drum as an example, in order to prevent impurities in the steam drum from clogging the pressure guiding tube or air bubbles entering the positive and negative pressure chamber, an isolation tank should be installed in the middle of the transmitter and the measured container (the length of the isolation tank is the same as the distance from the pressure point) ) and flush into the water (if the measured medium is corrosive, it can be flushed into the isolation tank). Assume that the positive and negative pressure chamber of the transmitter is at the same level, the density of water is ρ, the pressure inside the drum is F, and the distance between the positive and negative pressure of the steam drum is H (ie the maximum liquid level measured). The static pressure applied to the transmitter is ΔP (positive and negative pressure chamber pressure difference).
When H=0, P+=F, P_=ρgH+F, ¡÷P=P+-P_=F-ρgHF=-ρgH, there is a static pressure on the negative pressure side of the transmitter, and the pressure is ρgH, which makes the transmitter The output is less than 4mA.
When H=Hmax, P+=ρgH+F, P_=ρgH+F, ¡÷P=P+-P_=ρgH+F-ρgH-F=0, the differential pressure on both sides of the transmitter is 0, output 20mA;
Although the full scale corresponds to the actual liquid level, when the liquid level height H < Hmax (maximum liquid level height), the pressure on the negative pressure side of the transmitter is always greater than the pressure on the positive pressure side, so that the output of the transmitter is always smaller than the actual The signal output corresponding to the liquid level, and the linear relationship between the transmitter and the liquid level is broken. In order to match the output signal with the actual liquid level, it is necessary to eliminate the static pressure on the negative pressure side, that is, negatively transfer the transmitter, the migration amount is ρgH; under normal circumstances, the negative pressure side is filled with liquid (generally water), assuming When the full liquid level is 60cm, the negative migration is -6Kpa. In the absence of liquid level, the pressure change shows -6Kpa, ie P+=0, P_=6Kpa, ¡÷P=P+-P_=0-6=-6Kpa; When the liquid level of the container reaches 60cm, the pressure change shows 0Kpa, P+=6Kpa, P_=6Kpa, ¡÷P=P+-P_=6-6=0Kpa (the gas pressure in the upper part of the steam drum acts on the positive pressure negative pressure chamber is the same) of).
2.3 No migration
Under normal production, the installation location on site cannot be in the same position as the positive and negative pressure of the pressure change, and the non-migration in the production process rarely meets the complex production situation. So the use of no migration is less than the other two, so I won't go into too much here.
3 common fault analysis and solutions
1) The display is high or low. If the displayed liquid level is lower than the actual liquid level, it indicates that the positive pressure of the container has not reached the transmitter bellows, and there is a possibility that the positive pressure side and the pressure take-up point are blocked, resulting in a low measured value. In this case, it is normal to remove the impurities in the positive pressure portion. If it is too high, it means that the liquid inside the negative pressure pressure pipe does not reach the set negative migration amount (such as normal 6Kpa, now only 5Kpa), as long as the valve is adjusted so that the pressure tube of the negative pressure part is filled with liquid, reaching 6Kpa. can.
2) Display up and down fluctuations frequently. There is a fluctuation in the up and down, it is very likely that there is a steam drum in the negative pressure chamber, which causes the pressure in the negative pressure part to be unstable. Just discharge the air bubbles and refill the liquid.
3) Zero drift. Since the production site environment may cause some interference to the transmitter, resulting in changes in the parameters set inside the transmitter, if the problem cannot be solved after repeating 1), 2), it is necessary to stop the meter to check the transmitter. Zero and correct. A slight drift does not affect the actual measured value.
Accurate measurement is the premise of stable operation of industrial production. Only by fully grasping and understanding the working principle, migration mode and common faults of differential pressure transmitter, can it be flexibly used in production, find and solve the fault of field instrument in time, and quickly eliminate it. Due to the production stoppage caused by the instrument failure, the relevant control scheme can be further improved in time. In industrial production, the level of liquid level directly affects the safety of production, so the continuous monitoring of liquid level is especially important. The differential pressure transmitter is due to the characteristics of uninterrupted measurement, and is suitable for various complicated working conditions. The parameter adjustment is simple and convenient, the requirements on the field environment are low, the measurement height can be flexibly adjusted, and the measurement liquid level is widely obtained. application. Especially when the boiler is in use, the water level is an extremely important indicator. Too high or too low is dangerous, so the water level should be automatically controlled. At present, the relatively mature measured water level is measured by a differential pressure transmitter, keeping the water level within a safe range and simultaneously converting the water level into a standard current output.