The role of capacitive level transmitter in synthetic high-pressure liquid level measurement and control

1 The original state of the system

More than ten years ago, according to the traditional general design and conventional selection, our factory had installed a liquid level measurement and control system for synthetic ammonia separation and cold exchange. The liquid level meter selected the pneumatic high-pressure float liquid level adjustment transmitter UTQ-113 of Shanghai Automation Instrument Factory No. 5, and the regulating valve used ZMBS-320KDg6C=0.1, equipped with a pneumatic adjustment recorder and a sub-line board, to form a synthetic section ammonia separation and cold exchange high-pressure measurement and control system. In order to facilitate maintenance and repair. Each system uses a total of 7 Dg10 high-pressure valves, two standard tees, 1 elbow, 15 pairs of high-pressure flanges, 48 ​​high-pressure bolts, 90 high-pressure nuts and 5Mφ 24×6 high-pressure pipes. In order to extend the service life of the high-pressure regulating valve and ensure the regulation effect, a series connection scheme of 2 high-pressure regulating valves is adopted. The entire system is cumbersome, the installation volume is large, the site is messy, and the investment in each system is more than 15,000 yuan (the price a few years ago). However, after the installation of these two systems, they have not been put into normal use due to inaccurate liquid level detection response. 2 Cause Analysis The liquid level detection in high-pressure containers such as ammonia separators and cold exchangers in the production of synthetic ammonia in our plant cannot work properly. According to the investigation, it is common in small fertilizer plants. The reason for this phenomenon is that high-pressure gas-liquid separators such as ammonia separators and cold exchangers are not simple empty cylinder containers. The working pressure is as high as 32MPa. They are equipped with separation internals with different structural forms. The outer cylinders of these high-pressure containers are welded with multi-layer coils, and holes are not allowed on the cylinder. Therefore, the gas phase and liquid phase reserved interfaces installed as liquid level detection instruments are opened on the upper and lower heads of the integrally forged high-pressure container, that is, the gas phase interface is on the upper head and the liquid phase interface is on the lower head. As we all know, after the high-pressure medium flows through the equipment, due to the influence of the equipment resistance, a pressure difference will appear at the inlet and outlet of the equipment. The size of the pressure difference is related to factors such as equipment structure, flow section, medium flow rate, etc., and changes with the medium flow rate. Taking our ammonia separator as an example, after the medium enters the container, it flows through the separation internals to separate gas and liquid. Liquid ammonia accumulates at the bottom of the ammonia fraction and is discharged as a product, and the gas is drawn out of the ammonia fraction from the outlet at the upper end of the container and enters the next-level device. The liquid level detection interfaces (liquid phase and gas phase) of our ammonia fraction and cold exchange are located on both sides of the separation section, and the pressure difference generated by the separation section also acts on the gas-liquid phase interface of the liquid level gauge. According to measurements, the pressure difference of the separation section can be as high as several meters to more than ten meters of water column at full load. Depending on the structure of the ammonia fraction and cold exchange equipment, the position of the medium inlet, and the flow direction in the separation section, two situations may occur; one is that the positive pressure side of the pressure difference value of the separation section acts on the liquid level and the negative pressure side acts on the gas phase interface of the liquid level gauge. At this time, what is drawn out by the gas phase and liquid phase of the liquid level gauge is the positive superposition of the actual liquid level in the container and the pressure difference generated by the separation section. The measurement range of the general float level transmitter is only within 1 meter. Therefore, as long as there is a little liquid at the bottom of the container, it will quickly fill the connecting pipe of the external liquid level gauge (float) under the action of the pressure difference, so that the liquid level gauge will display the full liquid level. Unless the liquid level in the container is completely emptied, the external float level gauge will always display the full scale regardless of how the liquid level changes. Another situation is that the positive pressure side of the pressure difference of the internal parts acts on the gas phase interface of the liquid level gauge, and the negative pressure side acts on the liquid level in the container. The internal pressure difference and the liquid level value are reversely superimposed. Due to the pressure difference of the internal parts, no matter how the liquid level in the container changes, even if the liquid level has filled the container or even contains liquid, the liquid level gauge always displays zero. The reason why the original ammonia and cold exchange pneumatic float liquid level measurement system of our factory is inaccurate is that it still belongs to the former type. 3 Improvement measures In order to achieve energy saving and consumption reduction as soon as possible, our factory is determined to reconstruct the synthetic ammonia and cold exchange liquid level detection and control system. The UYB-320C built-in capacitive high-pressure liquid level transmitter, which is valued and promoted by the editorial departments of "Fertilizer Industry" and "Small Nitrogen Fertilizer", is used. The liquid level is directly inserted into the equipment from the liquid phase hole of ammonia separation and cold exchange. The plasma double-light column digital display (with upper and lower limit alarms) displays the liquid level changes. The upper and lower limit alarms of the liquid level and the regulating valve position are self-adjusted by a simple electronic regulator. Since the original pneumatic high-pressure regulating valve has never been used after installation, in order to save investment, it is planned to be used temporarily for regulation. Our factory used minor repairs and installed two sets of liquid level gauges in less than half a day. After commissioning and use, the liquid level detection part soon worked normally and was put into operation to realize the ammonia release operation with liquid level indication. Subsequently, the automatic control system was debugged and it was found that the original pneumatic regulating valve ZMBS had serious internal leakage. Although the two valves installed in series with the cold exchange were in a state of being completely closed and their flow capacity was also selected to be very small, the liquid level was still difficult to raise, indicating that the regulating valve was difficult to adapt to the needs of high-pressure liquid level automatic control. Even so, the amount of gas leakage has been significantly reduced compared with manual ammonia release, and the CH4 content in the circulating gas has also increased. Our factory quickly purchased two new special automatic ammonia release valves for replacement, which were used for automatic ammonia release in the ammonia separation and cold exchange systems, respectively, so that the liquid level automatic control quickly returned to normal. Finally, this key production problem of our factory was solved in a short period of time, and the automatic control of high-pressure liquid level measurement of synthetic ammonia separation and cold exchange was realized. 4 Implementation effect Our factory's synthetic ammonia separation and cold exchange liquid level measurement and control system, which is mainly based on built-in capacitive high-pressure liquid level transmitter, has been officially put into operation for more than 2 years since September 1993. Practice has proved that the implementation effect of this project is obvious and has generated significant benefits. According to statistics, it directly generates 800,000 yuan in annual profits. (1) After the high-pressure liquid level detection automatic control system was put into use, the effect was significant (see table). The CH4 content in the circulating gas has increased significantly. Our factory is a heavy oil gasification plant, and it has increased from 4%~6% before the installation of the liquid level meter to 11%~14%, effectively reducing the effective gas venting and leakage of the synthesis system.

Table High-pressure liquid level detection automatic control system before and after commissioning
Comparison table of CH4 content in synthetic station circulating gas Unit (%)

(2) The system can accurately measure the liquid level of ammonia separation and cold exchange, and realizes automatic or manual ammonia release under the condition of liquid level detection, eliminating manual ammonia release under the condition of no liquid level display, and effectively realizing safe production.

(3) The use of the liquid level automatic control system frees operators from frequent manual adjustments, greatly reducing the labor intensity of workers, stabilizing the operation, and the catalyst temperature of the synthesis tower is stable, almost in a straight line.

(4) The liquid level meter is extremely easy to install, with a very light structure and a total weight of less than 2Kg. It only needs a flange, 3 bolts, and 3 nuts. The liquid level meter measurement and control system works reliably and stably. Our factory has basically not been repaired in the two years since it was put into operation.

The implementation of this project was guided and helped by Li Dezhong, a senior engineer of Dadi Industrial Control Company, Tuzuoqi, Inner Mongolia, and we would like to express our sincere gratitude.