- Nov 30, 2017 -
Zero point migration, that is, the starting point of the range of positive and negative migration, to adapt to different needs of the production site, in the actual production and application, the migration can be divided into no migration, negative migration and positive migration of three.
Zero migration concept
Changes in the output value due to some influence when the input is at the lower limit of the range. When the lower limit is not zero, also known as the starting point of migration (offset).
With the instrument's input signal (measured) relative to the range as the abscissa, the output signal relative to the signal range as the ordinate, you can draw the instrument's input / output characteristic curve, set it as a straight line. The characteristic curve is shifted while the slope of the curve is constant, called zero point shift.
Transmitter measurement range is equal to the range + migration, that is, the measurement range = range + migration. Positive migration moves in the positive direction, negative migration moves in the negative direction, and the movement distance is the migration amount. Therefore, the essence of positive and negative migration is to adjust the transmitter to change the upper and lower limits of the range, The same size. It can also be remembered that when the input pressure is zero (0%), the output current signal is also zero (0%), there is no migration; when the input pressure is zero (0%), the output current signal is positive Negative migration; When the input pressure is zero (0%), the output current signal is negative, less than the zero point for positive migration;
Application of differential pressure transmitter to measure the liquid level, if the differential pressure transmitter positive and negative pressure chamber and container take the pressure level at the same level, you do not need to migrate. In practice, out of equipment installation location and easy maintenance and other considerations, the measuring instruments may not always be able to take the pressure point at the same level; Another example is the measured medium is a highly corrosive or heavy viscosity of the liquid, Can not be directly introduced into the media pressure measurement instrument, the isolation tank must be installed, the isolation liquid to transmit pressure signals to prevent the measured instrument is corroded. At this time we must consider the medium and the liquid column of the liquid pressure on the manometer readings.
Industrial production, often need to do for the following circumstances zero migration.
Measure closed container level problems
Pressure transmitter with a closed container to measure the level, if the gas-phase medium is easy to condense, then into the transmitter pressure tube and negative pressure chamber are condensate, negative pressure chamber to change the height of the liquid column, causing measurement error . Therefore, it is common practice to install a balance vessel in front of the duct above the negative pressure chamber and fill the balance vessel with condensate. For the measurement of harmful, corrosive media, in order to prevent the media into the transmitter, but also need to install the isolator and the pressure guide tube is also full of isolation fluid. In both of these measurements, the signal from the negative pressure chamber of the transmitter will be greater than the positive pressure chamber signal due to the presence of condensate, and measurement will not be possible. If you reverse the transmitter positive and negative pressure port is the method can be used, but when the liquid level rises, the transmitter output instead of falling, which easily lead to illusion, and the connection with the control system is not convenient.
To solve the above problems, it is necessary to use negative migration, so that the transmitter zero pressure from the negative pressure began to meet the requirements of the.
Transmitter below the pressure point when the problem
Transmitters are generally installed below the pressure point, the pressure pipe filled with condensed water. Due to the influence of hydrostatic pressure, the output current of the transmitter contains the hydrostatic pressure of the water column. The greater the vertical distance between the pressure point and the transmitter, the greater the error. Measurement of open container level, if the transmitter installed in the bottom of the container, due to the measured media liquid column static pressure measurement error will result, the result is the same as above.
To solve the above problem, it is necessary to use the positive migration, the transmitter output to zero (4mA), so that is equal to minus the value of liquid column static pressure. After the migration of the transmitter output value is the actual working pressure value.
Increase the measurement accuracy by migrating the compression range
For example, a Taiwan 35t / h industrial boiler, the gas consumption has always been 25t / h or more, in order to improve the accuracy of flow measurement, you can consider changing the measurement range of the display instrument to 20 ~ 40t / h, the transmitter through the positive Migrate the compression range to work with.
It is known that when any mass flow rate is 40t / h and any differential pressure is 60kPa, the following method can be used to calculate:
Arbitrary differential pressure = (square of arbitrary mass flow / square of mass flow of instrument at upper limit of differential pressure) × Upper differential pressure of instrument
According to the calculation results, when the mass flow rate is 20t / h, the corresponding differential pressure is 15kPa; when the mass flow rate is 40t / h, the corresponding differential pressure is 60kPa; then the range of this transmitter is reduced to 0-45kPa Is the migration so that the transmitter measurement range of 15 ~ 60kPa, the corresponding flow of 20t / h ~ 40t / h, thus improving the measurement accuracy.
For example, in the production of a measured temperature range is 500 ~ 600 ℃, if you choose the range of 0 ~ 800 ℃, accuracy of 0.5 temperature transmitter, the maximum measurement error of ± 4 ℃. However, if the positive displacement is used to compress the range, a temperature transmitter with the same accuracy of 400 to 800 ° C is selected. The maximum measurement error is ± 2 ° C, and the measurement accuracy is improved after the transfer. To explain is that only some of the temperature transmitter with the above functions, so the selection should pay attention to choose.
The following problems occur when the migration volume is improperly set
Positive migration fault: to determine the differential pressure transmitter is moving in the field during the measurement is accurate, you should first close the differential pressure transmitter three valves positive and negative pressure measurement chamber, open the balance valve and meter vent plug, The meter output should be below 4mA at this time. If the output is not less than 4mA, positive pressure chamber leads may be some block or three valve block. Second, close the positive pressure chamber to take pressure points, open the vent switch, then the output should be 4mA. If the output is less than 4mA, the migration may be smaller or zero low; if filled with a spacer, the spacer may not be filled or missing from the side; if the output is higher than 4mA, indicating that the migration of larger or zero High.
Negative Migration Failure: To determine whether the differential pressure transmitter with negative migration is accurate during field use, first turn off the positive and negative pressure measurement chambers of the three-valve differential pressure transmitter, open the balance valve and the meter vent plug, The output should be 20mA. Second, close the positive and negative pressure chamber take pressure point, open the vent switch, this time, the instrument output should be 4mA, if not 20mA or 4mA, positive and negative pressure chamber should check the lead is blocked, the migration is changed, zero Is accurate, the isolation fluid is lost and so on.
Taking the C-101 liquid level control system of ketone-benzene plant in a refinery as an example, when measuring the measuring range of the EJA intelligent dual-flanged transmitter in the ketone-benzene plant of the plant, it was found that there was a design calculation error in the measuring range of the transmitter. LICA-1201 and other transmitters in the DCS engineer station to check their range, found no double flange span migration, the calculation of the modified range, the instrument is running normally. Therefore, only by the correct calculation method and reference migration to be calculated in order to ensure the accuracy of the instrument range.
How to achieve the migration of the transmitter?
First, before the product is shipped, the manufacturer calculates the amount of migration and the minimum liquid level we provide to the manufacturer so that they can complete the migration after the migration calculation is completed and the calibration range is completed.
The second is to use the terminal to complete the migration;
Third, the actual measurement site, at the zero position to the transmitter migration.
In short, the production site, due to the ever-changing measurement parameters and equipment, the measurement requirements vary, which requires the transmitter has a migration function, and user-friendly in the field according to the actual migration to meet the production of different measurement needs .