The critical role of capacitive level transmitters in synthetic level measurement and control systems cannot be overlooked, as even a minor change can lead to inaccurate level detection responses. For example:
A factory installed a level measurement and control system for ammonia synthesis separation and cooling exchange. The level gauge selected was our pneumatic high-pressure float level control transmitter UQZ-113, with the control valve being MATS-320KDg6C=0.1, along with a pneumatic control recorder and bypass panel, forming a high-pressure control system for ammonia separation and cooling exchange in the synthesis section. To facilitate maintenance, each system used a total of 7 Dg10 high-pressure valves, two standard tees, one elbow, 15 pairs of high-pressure flanges, 48 high-pressure bolts, 90 high-pressure nuts, and 5M φ24×6 high-pressure pipes. To extend the life of the high-pressure control valve and ensure adjustment effectiveness, a series connection scheme with 2 high-pressure control valves was adopted. The entire system was complex, requiring substantial installation effort, resulting in a cluttered site, and each system cost over 15,000 yuan (several years ago). However, after these two systems were installed, they could not be put into normal use due to inaccurate level detection responses.
**Cause Analysis**
In the ammonia production at our factory, the level detection devices designed and installed in the conventional way using communicating pipe forms (such as float-type level gauges) for high-pressure containers like ammonia separators and cold exchangers failed to work properly. According to investigations, this phenomenon is common in small fertilizer factories.
The reason for this phenomenon is that high-pressure gas-liquid separators such as ammonia separators and cold exchangers are not simple empty cylindrical containers. Their working pressure can reach up to 32 MPa, and they all contain differently structured internal components for separation. Additionally, the outer cylinders of these high-pressure vessels are made by multi-layer plate welding, which does not allow holes to be drilled in the cylinder body. Therefore, the reserved interfaces for installing level detection instruments—both gas-phase and liquid-phase—are located on the upper and lower heads of the integrally forged high-pressure vessels: the gas-phase interface on the upper head and the liquid-phase interface on the lower head.
It is well known that after high-pressure media pass through equipment, due to the resistance of the equipment, there will be a pressure difference between the inlet and outlet of the equipment. The size of this pressure difference depends on factors such as the structure of the equipment, the cross-sectional area of flow, and the medium flow rate, and it changes with the variation of the medium flow rate. Taking our ammonia separator as an example, after the medium enters the container, it flows through the internal separation components for gas-liquid separation. Liquid ammonia accumulates at the bottom of the ammonia separator as a product and is discharged, while the gas is led out from the upper outlet of the container and enters the next-stage device. The level detection interfaces (gas-phase and liquid-phase) for our ammonia separator and cold exchanger are precisely located on either side of the separation section. The pressure difference generated by the separation section directly acts upon the gas-phase and liquid-phase interfaces of the level gauge. Measurements show that the pressure difference in the separation section can reach several meters to dozens of meters of water column under full load. Depending on the structure of the ammonia separator and cold exchanger equipment, the direction of the medium inlet and its flow within the separation section, two scenarios may occur: one where the positive pressure side of the separation segment's pressure difference acts on the liquid above the level, while the negative pressure side acts on the gas-phase interface of the level gauge, causing the gas-phase and liquid-phase readings of the level gauge to reflect the actual level in the container plus the pressure difference generated by the separation segment. Since the measurement range of general float-type level transmitters is only within 1 meter, as long as there is a slight amount of liquid at the bottom of the container, the connecting pipe of the external level gauge (float) quickly fills up under the pressure difference, causing the level gauge to display a full level. Unless the liquid level in the container is completely drained, regardless of how the level inside changes, the external float-type level gauge always shows a full scale. In another scenario, the positive pressure side of the internal component's pressure difference acts on the gas-phase interface of the level gauge, while the negative pressure side acts on the liquid above the level in the container, causing the internal component's pressure difference to subtract from the actual level value. Due to the effect of the internal component's pressure difference, regardless of how the level in the container changes—even if the container is completely filled with liquid—the level gauge always displays zero.
The reason why the original pneumatic float-type level measurement system for ammonia separation and cold exchange at our factory was inaccurate still falls under the first type.
**Improvement Measures**
To quickly achieve energy saving and consumption reduction, our factory decided to re-engineer the self-control system for ammonia separation and cold exchange level detection in ammonia synthesis. We adopted the MAT-320C internally mounted capacitive high-pressure level transmitter promoted by the "Fertilizer Industry" and "Small Nitrogen Fertilizer" editorial department, inserting it directly into the liquid phase hole of the ammonia separator and cold exchanger to detect the level. A plasma double-column digital display instrument (with upper and lower limit alarms) was used to display level changes, with the upper and lower limit alarms and control valve positions adjusted by a simple electronic controller. As the existing pneumatic high-pressure control valves had never been used, to save investment, we planned to temporarily continue using them for regulation.
Our factory completed the installation of both sets of level gauges in less than half a day during a minor repair. After commissioning and use, the level detection part quickly began working normally, putting the ammonia release operation under liquid level indication into operation. Subsequently, we debugged the self-control system and found that the original pneumatic control valve MATS had serious internal leakage. Although the two valves installed in series on the cold exchanger were fully closed and their flow capacity was selected to be very small, it was still difficult to raise the level, indicating that the control valve was no longer suitable for high-pressure level self-control needs. Even so, the gas loss was significantly reduced compared to manual ammonia release, and the CH4 content in the circulating gas also increased. Our factory then quickly purchased two new dedicated automatic ammonia release valves for replacement, using them respectively in the ammonia separation and cold exchanger systems to achieve automatic ammonia release, allowing the level self-control to quickly return to normal and solving this key production issue in a short time, achieving automatic control of high-pressure level measurement in ammonia synthesis separation and cold exchange.
**Implementation Effects**
Since October 2007, the ammonia synthesis separation and cold exchange level measurement and control system primarily based on the internally mounted capacitive high-pressure level transmitter at a certain factory has been officially operational for more than 2 years. Practical experience shows that the implementation of this project has produced significant results and benefits. According to statistics, it generates direct annual benefits of 800,000 yuan.