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The common causes are as follows: (1) Abnormal operating conditions inside the tower, resulting in flooding or liquid suspension. The hazard of liquid slugging is that the oxygen and nitrogen purity cannot be kept within the specified range, remaining in a state of fluctuation. The reason for this is that the liquid on the tray finds it difficult to flow down along the overflow funnel, causing the liquid level in the overflow funnel to rise higher and higher until it reaches the same level as the liquid level on the tray. As a result, the liquid cannot flow downward, and this leads to a condition known as \"liquid suspension\" or \"liquid flooding\". The reason for the rise in the liquid level inside the overflow hopper is due to two factors: an increase in the resistance of the tray and an increase in the resistance of the overflow hopper itself. 1. The magnitude of the tray resistance depends on the amount of vapor rising, the resistance of the sieve pores, and the thickness of the liquid layer on the tray. The reasons for the increased tray resistance are: 1) An increase in air volume. This occurs especially when the valve is opened too quickly, causing a sudden increase in the amount of air supplied to the tower and thus a sudden rise in the gas velocity ； 2) The sieve pores of the tray are blocked by solid carbon dioxide or silica gel powder, etc. II. The magnitude of the resistance in the overflow hopper depends on the volume of liquid flowing downstream and the structural characteristics at the outlet of the hopper (such as the size of the outlet channel). The reasons for the increased resistance in the overflow hopper are: 1) Excessive amount of liquid flowing downstream. Especially when the liquid air and liquid nitrogen throttling valves are operated too quickly, it causes a sudden increase in the amount of liquid flowing downward within the tower ； 2) Incorrect processing and assembly of the overflow hopper or deformation of the tray. This causes the flow channel at the outlet of the overflow hopper to narrow. Therefore, flooding during normal production usually occurs when the valve moves too abruptly or after a very long operating cycle. Therefore, operating the valve carefully and slowly is one of the key points in using an oxygen generator. (2) The heat transfer temperature difference of the main condensation evaporator increases. In an air separation unit, the pressure in the lower column depends on the temperature difference of the main cooler; a larger temperature difference in the main cooler results in a higher pressure in the lower column. As a result, under otherwise identical conditions, the exhaust pressure of the compressor increases, leading to higher energy consumption. To this end, air separation units always keep the main cooling temperature difference low in order to reduce energy consumption. In actual operation, it is always desirable for the temperature difference across the main cooler to be as close as possible to the design value. However, during production, the heat transfer surface of the main cooler can see its heat transfer coefficient decrease due to contamination; moreover, when there is an excessive amount of solid carbon dioxide in the liquid oxygen, the liquid oxygen becomes more viscous, which also leads to a reduction in the heat transfer coefficient. In liquid oxygen operations, an excessively low liquid level or excessive blockages in the pipes, as well as the accumulation of non-condensable gases on the nitrogen side, all result in a reduced heat transfer area. The heat load of the main cooler is proportional to the heat transfer area, heat transfer coefficient, and heat transfer temperature difference. To maintain the primary cooling and heating load required for production, when the heat transfer coefficient and heat transfer area decrease, it is necessary to increase the heat transfer temperature difference. An increase in the heat transfer temperature difference necessarily requires an increase in the pressure in the lower column; as this pressure rises, the energy consumption of the compressor increases. At the same time, a higher pressure in the lower column results in less air entering the column, thereby reducing the oxygen production. To ensure a normal heat load for the main cooler while preventing an increase in the heat transfer temperature difference, it is necessary to pay attention to the adsorption efficiency of the molecular sieve purifier, so as to effectively prevent carbon dioxide from entering the upper column ； Periodically purge non-condensable gases such as neon and helium ； Pay attention to the liquid oxygen level to prevent it from dropping too low ； Once overload operation is required, it is necessary to check whether the heat transfer surface of the main cooler has sufficient capacity; overload operation is only possible if the main cooler has enough heat transfer surface. (3) Main cooling system leakage. The reasons for the leakage are as follows: 1) The pipes wear against each other due to vibration. For the long-tube condensation-evaporation evaporator, it is equipped with tens of thousands of copper tubes with a diameter of only 10 mm and a length of 8 m, and the spacing between these tubes is very small. During operation, due to the impact of air currents and vibrations, the pipes are prone to bending deformation in the middle section, causing them to rub against each other; over time, this can lead to leakage. 2) Freezing and cracking due to water accumulation inside the pipe. When the heating is incomplete, especially when small tubes become blocked and this creates an opportunity for water to accumulate, and that water cannot be blown away during heating, it freezes into ice at low temperatures; the expansion of volume can then cause the small tubes to crack. 3) Slight localized explosion in the main cooler. Under certain conditions, an explosion may occur in a localized area of the main coolant due to the accumulation of acetylene or hydrocarbons. When such a minor explosion occurs, there is no external indication and no sound is heard; it is often impossible to detect at first. This can only happen when the oxygen purity changes automatically and cannot be adjusted. In the event of a severe leak in the main cooler, large amounts of nitrogen at higher pressure leak into the low-pressure oxygen side, resulting in significant changes in the pressures of the upper and lower columns as well as in the purity of the product, until normal operation can no longer be maintained and the plant has to be shut down. When there is a slight leak in the main cooler, it usually does not cause significant changes in the pressures of the upper and lower columns, nor does it lead to a significant decrease in the purity of liquid oxygen inside the main cooler. It is a common phenomenon that there is a significant difference in purity between gaseous oxygen and liquid oxygen, with the concentration in the gas phase being lower than the value corresponding to equilibrium with liquid oxygen. For example, a factory’s tests showed that the concentration of liquid oxygen was 99% and that of gaseous oxygen was 96%; however, during maintenance work, it was discovered that 7 main cooling tubes were leaking. Therefore, during normal operation, large adjustments that could cause airflow shock should be avoided ； During system heating for thawing, thorough heating is required, with the dew point temperature at the outlet meeting the specified requirements ； Pay attention to the operating condition of the molecular sieve purifier; keep the main cooler inactive, and regularly discharge liquid oxygen to prevent the accumulation of hydrocarbons inside the main cooler. (4) Plate deformation. At the air inlet of the distillation tower, if the air flow velocity is too high and the strength of the tray is insufficient, the tray will deform under the impact of the airflow. Adjust the inlet position to be appropriate, and be careful to control the gas flow rate entering the tower during operation (it must be done slowly) to prevent deformation of the tower trays. (5) The oxygen purity does not meet the requirements. Factors affecting oxygen purity include: 1) Excessive amount of oxygen drawn off. From a material balance perspective, when the amount of air processed remains constant and the purity of nitrogen is fixed, if a certain purity of oxygen is required, only a specific quantity of oxygen can be extracted. If too much is removed, the purity will inevitably fail to meet the requirements. From a distillation perspective, if too much oxygen is removed, the amount of vapor rising in the stripping section of the upper column decreases, the reflux ratio increases, and nitrogen (argon) in the liquid does not evaporate sufficiently, which will lead to a decrease in oxygen purity. At this time, the oxygen supply valve should be adjusted to a lower setting in order to reduce the amount of oxygen supplied, while the nitrogen supply valve should be opened wider to maintain a constant pressure in the upper column. 2) The oxygen purity in the liquid air is too low. A low oxygen purity in Liquid Air necessarily means an excessive amount of Liquid Air. On the one hand, it increases the separation burden in the stripping section of the upper tower; on the other hand, the large amount of reflux makes it difficult to fully evaporate the nitrogen (argon) components, resulting in a decrease in oxygen purity. At this point, the distillation conditions in the lower column should be adjusted to increase the oxygen content in the liquid-air mixture appropriately. 3) The liquid oxygen level in the condensation evaporator is too high. When the level of the main cryogenic liquid oxygen rises, it indicates that the amount of liquid flowing downstream is greater than the amount evaporating; as a result, the reflux ratio in the stripping section increases, leading to a decrease in oxygen purity. At this point, the expansion machine can be reduced in size. When the liquid oxygen level is high and the oxygen purity is poor, making it difficult to make adjustments promptly, some of the liquid oxygen can be discharged to fully utilize the heat exchange area of the condensation evaporator, after which adjustments can be made again. The rise in the level of liquid oxygen in the main cooling system may also be due to a large amount of solid carbon dioxide being present in the liquid oxygen, which impairs heat transfer; as a result, the liquid oxygen cannot evaporate, forcing a reduction in the amount of oxygen that can be extracted. If necessary, the vehicle has to be stopped to heat up. 4) The tray efficiency decreases. If the distillation tray deforms, the tower tilts, or the sieve pores are blocked by solid impurities, it will affect the gas-liquid mass transfer on each tray, resulting in a decrease in purity. If the operating cycle has been long and the tray resistance has increased, the unit should be shut down for heating. 5) Abnormal distillation conditions. When conditions such as flooding or liquid leakage occur in the distillation tower, it disrupts the normal distillation process, resulting in a decrease in purity. At this point, measures must be taken based on the specific circumstances to eliminate abnormal operating conditions. 6) Main cooling leak. When the brazing quality of the condensation evaporator is poor, or the evaporation tubes wear and leak, or there is a local explosion that results in minor leaks, high-pressure gaseous nitrogen can leak into the lower-pressure oxygen side, leading to a decrease in oxygen purity. When the purity drops only slightly, it can be determined by analyzing the difference in purity between liquid oxygen and gaseous oxygen. When the difference between the two exceeds the normal gas-liquid equilibrium concentration difference, it is often due to a leak, requiring the plant to be shut down for maintenance. I hope experts can offer some guidance and additions; thank you all