What is the Application of the Double Flange Differential Pressure Transmitter in the Solvent Vaporation Area of the Petrochemical Plant
The differential pressure transmitter has been very widely used in industrial automation production. It also plays an increasingly important role in the automatic control system. With the continuous improvement of the automation level of petrochemical, steel, paper, food, and pharmaceutical enterprises, differential pressure transmitters are becoming more widespread. However, we also encounter more and more problems in production, coupled with differences in the level of installation, use, and maintenance personnel, making it impossible to quickly solve the problems that arise, which to a certain extent affects the normal conduct of production and even endangers production safety, and therefore places higher demands on the technical level of field instrumentation maintenance personnel.
The double flange differential pressure transmitter has the advantages of advanced technology, ease of use during installation, high measurement accuracy, low maintenance, and explosion-proof. However, under high adverse pressure conditions, the level is measured with a double flange differential pressure transmitter, which often results in abnormal level fluctuations and affects average production.
1. Process Status
In the solvent evaporation zone of the recovery unit of the ophthalmic spandex plant of Daqing Petrochemical Company, the five-effect heater 121-6.3 (hereinafter referred to as 121-6.3) uses a double-flange differential pressure transmitter for liquid level measurement, so that we can control the amount of regulating valve feed to maintain a constant liquid level of 121-6.3. When the liquid level is high, it is easy to cause production accidents such as running material; when the fluid level is low, the handling capacity is not enough to meet the production needs. Therefore the level measurement must be accurate and reliable. The installation of the 121-6.3 level transmitter is shown in the diagram below.
2. Principle of measurement and engineering
For the liquid in the container, the differential pressure and the level of the medium, the density of the following relationship: P = pgh
Under certain conditions, the medium density p is approximated as a constant. The double flange differential pressure transmitter consists of a sensitive pressure measuring diaphragm in contact with the process medium, a slight pressure capillary filled with conductive silicone oil, a differential capacitive diplomatic pressure measuring element, and an electronic amplifying converter. The liquid level transmitter is the five-effect heater detected by the transmitter’s positive and negative pressure side diaphragm.
The pressure up and down in the liquid level of 121-6.3 is then transmitted through the capillary tube to the differential capacitive sensitive pressure measuring element, which detects the change in differential pressure of the liquid level and realizes the level measurement.
3. Problems and analysis of causes
3.1 Existing problems
The 121-6.3 liquid level transmitter has experienced abnormal fluctuations and unstable liquid level control many times since the unit was operational, affecting average production. In addition, double-flange differential pressure transmitters are also used for liquid level measurement in the evaporation zone for the first to fourth effect evaporation heaters. However, since the commissioning of these four liquid level transmitters, fluid level control has been smooth, and the process operation has been good. For this reason, it is of great practical importance for us to find out the causes of abnormal fluctuations in the level transmitters by conducting tests on the 121-6.3 level transmitters.
3.2 Analysis of the causes of abnormal fluctuations
The same type of differential pressure transmitter, one effect to four effect heater level transmitter measurement is accurate. In comparison, the five effect heater level transmitter level appears abnormal fluctuations, one effect to five effect heater process parameters as shown in Table 1. Therefore, from the level transmitter measurement principle, it can be concluded that the process parameters related to the level measurement are pressure, temperature, density, and the measured medium.
Under certain working conditions, the measured medium and its density are constant, not influencing the liquid level measurement. As shown in Table 1, the temperature of the first to fifth effect heaters gradually decreases, and the pressure decreases. To determine the effect of pressure and temperature on the transmitter produced.
In 1999, when the equipment was overhauled, the following tests were carried out on the 121-6.3 level transmitter.
(1) We have removed the new 121-6.3 level transmitter in the range. We will use the precision digital manometer for calibration, and calibration results show that: the transmitter has zero-point drift, linearity becomes terrible. The transmitter input and output relationship is shown in Figure 2
(2) When we adjust the zero point of the transmitter, it is recalibrated within the range so that the transmitter’s accuracy is qualified and the linearity is good. The calibration results show that the above requirements can be met.
(3) We put the transmitter positive and negative pressure diaphragm box into the temperature of 90°C hot water simultaneously and then check the input and output. We found that the transmitter accuracy, linearity has not changed.
(4) We pumped the transmitter positive and negative pressure diaphragm box simultaneously. We kept the pressure of the negative pressure diaphragm box at 40 kPa, and the force of the positive pressure diaphragm box varied in the range of differential pressure measured by the transmitter. The test results show that the transmitter had zero-point drift, but the measurement accuracy qualified. Keep the pressure in the negative pressure diaphragm box at 10 kPa and vary the pressure in the positive pressure diaphragm box within the measuring range of the transmitter. The test results show that the transmitter zero-point produces a significant drift, linearity becomes terrible, has become unusable, and its input-output relationship is as follows
A combination of the above tests can be derived: The main process parameter affecting the 121-6.3 level transmitter is the pressure. 121-6.3 level transmitter differential pressure detection is conducted through the positive and negative pressure diaphragm box and the capillary tube within the conductive silicone oil.
When we install the transmitter positive and negative pressure diaphragm box on 121-6.3 equipment, long-term negative pressure in the adiabatic lO a state, the higher negative pressure dramatically reduces the temperature limit of the conductive silicone oil and make the diaphragm box within the conductive silicone oil part of the cracking, the formation of the original silicone oil flash point reduced light components. Flashpoint increased recombination if the delicate details in the negative pressure are higher, the temperature is lower when the evaporation, will make the conductive silicone oil can not be standard. Suppose the spare part evaporates at high negative pressure and low temperature. In that case, it will make the pressure-conducting silicone oil unable to conduct pressure properly, causing the zero point of the transmitter to drift and performance to drop. It cannot be used under high negative pressure.
Double flange differential pressure transmitters are not suitable for measuring liquid levels under high adverse pressure conditions. If used for a long time under such conditions, it will cause the zero point of the transmitter to drift and the performance to degrade until it cannot be adequately measured.
Suppose you need to measure the liquid level under high negative pressure in the design and production process. In that case, you should look for a new type of level meter instead of a double-flange differential pressure transmitter.