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Vacorda tell you Why we need to do the zero-point migration for transmitter

Author:vacorda    Source:vacorda.com    Date: 2018-05-07 16:44:18

The zero-point migration, that is, from the starting point of the scale, is to carry out the migration in the positive and negative directions to meet the different needs of the production site. In practical production applications, the migration can be divided into three types: no migration, negative migration, and positive migration.

vacorda instruments differential pressure transmitter

In industrial production, it is often necessary to do zero migration for the following situations:

1. Measuring the level of closed containers:

Measuring the level of closed containers

Use the differential pressure transmitter to measure the level of the closed container. If the gas-phase medium is easy to condense, the condensate flows into the pressure guiding tube and the negative pressure chamber of the transmitter, which causes the height of the liquid column in the negative pressure chamber to change and causes measurement errors. . Therefore, it is common to install a balance vessel in front of the conduit above the negative pressure chamber and fill the balance vessel with condensate. For measuring harmful and corrosive media, in order to prevent the media from entering the transmitter, it is also necessary to install the isolator and to fill the pressure guiding tube with the spacer fluid. In the above two kinds of measurement, because of the existence of the condensate, will make the signal of the negative pressure chamber of the transmitter greater than the signal of the positive pressure room, will not be able to carry on the measurement. If the method of reversely connecting the positive and negative pressure ports of the transmitter can be used, but the output of the transmitter is decreased when the liquid level rises, it is easy to cause illusions, and the connection with the control system is also inconvenient.

To solve the above problems, we must use negative migration, so that the transmitter zero-point starts from the negative differential pressure, and it can meet the requirements for use.

2. When the transmitter is below the pressure pickup point

The general transmitter is installed under the pressure point, and the pressure tube is full of condensation water. Due to the static pressure of the water column, the output current of the transmitter contains the static pressure of the water column. The greater the vertical distance between the pressure pickup point and the transmitter, the greater the error caused. When measuring the level of the open container, if the transmitter is installed under the container, the measurement error will also occur due to the static pressure of the measured liquid column, and the result is the same as above.

To solve the above problem, we must use positive migration to adjust the output value of the transmitter to zero (4mA), which is equivalent to subtracting the liquid column static pressure value. The output value of the transmitter after being migrated is the actual working pressure value.

3. Improve the measurement accuracy by migrating the compression range

For example, there is a 35t/h industrial boiler whose gas consumption is always above 25t/h. In order to improve the accuracy of flow measurement, the measurement range of the display instrument can be changed to 20~40t/h, and the transmitter passes through positively migrate the compression range to match it.

It is known that when any mass flow is 40t/h, any differential pressure is 60kPa, then it can be calculated as follows:

Any differential pressure = (the square of any mass flow / the square of the meter mass flow when the differential pressure is upper limit) × the upper limit of the differential pressure of the meter.

According to the calculation result, when any mass flow is 20t/h, the corresponding differential pressure is 15kPa; when any mass flow is 40t/h, the corresponding differential pressure is 60kPa; then the range of this transmitter is reduced to 0~45kPa. Moving forward, the measuring range of the transmitter is 15~60kPa, and the corresponding flow rate is 20t/h~40t/h, which improves the measurement accuracy.

For example, in the production of a measured temperature change range is 500 ~ 600 °C, if you choose a range of 0 ~ 800 °C, accuracy of 0.5 temperature transmitter, the maximum measurement error of ± 4 °C. However, if you use positive migration to compress the range and select a temperature transmitter with the same accuracy from 400 to 800°C, the maximum measurement error at this time is ±2°C, which improves the measurement accuracy after migration. It should be noted that only some of the temperature transmitters have the above-mentioned functions, so care should be taken when choosing a type.

4. The amount of migration is not properly set up may cause the following problems:

Positive migration failure: To judge whether the differential pressure transmitter being migrated is accurate during field use, the positive and negative pressure measurement chambers of the three-pressure differential pressure transmitter group should be closed first, and the balance valve and instrument venting plug should be opened. The meter output should be less than 4mA at this time. If the output is not lower than 4mA, it may be a positive pressure chamber lead or a three-valve block. Second, close the pressure chamber pressure point, open the vent switch, then the output should be 4mA. If the output is lower than 4mA, the migration may become smaller or the zero may be lower. If the spacer fluid is filled, the spacer fluid may not be filled or may leak from the side; if the output is higher than 4mA, the migration amount becomes larger or smaller. High.

Negative transfer failure: to judge whether the negative pressure differential pressure transmitter is accurate during field use, first close the positive and negative pressure measurement chambers of the three-pressure differential pressure transmitter group, open the balance valve and the instrument discharge plug, and the instrument The output should be 20mA. Secondly, close the positive and negative pressure chamber to take pressure point, open the vent switch, this time, the instrument output should be 4mA, if not 20mA or 4mA, you should check the positive and negative pressure chamber lead is blocked, the migration amount is changed, zero position Whether it is accurate, whether the spacer fluid is lost or not.

Taking a kerosene and benzene plant C-101 liquid level control system of a refinery as an example, when measuring the measuring range of the EJA intelligent double flange transmitter in the ketone benzene plant of the oil refinery, the transmitter range was found to have a design calculation error. When the LICA-1201 and other transmitters examined their ranges on the DCS engineer station, they found that there was no migration of the double flange range. After calculating the modified range, the meter operation was normal. Therefore, the accuracy of the instrument's range can only be guaranteed if it is calculated in accordance with the correct calculation method and the referenced migration amount.

How to realize the migration of the transmitter?

First, the migration amount need to be calculated by the manufacturer and the minimum level value is provided by the customer to the manufacturer before shipment. After they have completed the migration calculation, the migration will be completed when the calibration is completed.

Second, use a manual operator to complete the migration;

Third, make the actual measurement at the site and the transmitter is migrated at the zero-point.

In short, in the production site, due to the ever-changing measurement parameters and equipment, the requirements for measurement are different. This requires the transmitter to have the function of migration, and can facilitate the user to carry out the actual migration in the site to meet the different measurement needs in actual production.

Sichuan Vacorda Instruments Manufacturing Co., Ltd

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