When a leak occurs inside the cold box of an air separation column, the refrigeration capacity available to maintain normal production becomes insufficient. Therefore, the primary indicator is a continuous drop in the liquid level of the main condenser. In the case of a significant gas leak, an increase in pressure inside the cold box can be observed. If the cold box is not adequately sealed, a large amount of cold gas will escape through the gaps. When cryogenic liquid leaks, however, there is no obvious pressure rise or gas emission; instead, a substantial decrease in the foundation temperature can often be detected.
In order to make a preliminary judgment of the leak location and the leaking substance before shutdown for maintenance-thereby shortening the downtime-many facilities have developed effective methods through practical experience. One such method is to analyze the purity of the gas escaping from the cold box. When nitrogen or liquid nitrogen leaks, the nitrogen volume fraction in the escaping cold gas can reach over 80%; when oxygen or liquid oxygen leaks, a significant increase in the oxygen volume fraction can be detected.
The second method is to observe the locations where "sweating" or "frosting" appears on the cold box wall. It should be noted that when cryogenic liquid leaks, the frosted area tends to be below the actual leak point.
The third method is to observe whether the escaping gas follows any regular pattern. This is mainly used to determine leaks in the switching channels of reversing heat exchangers. For alternately operated vessels, switching between them can help further identify the leak location.
The above judgment methods are often used in combination. To improve diagnostic accuracy, one should be familiar with the spatial positions of all vessels, pipelines, and valves inside the cold box, and continuously accumulate practical experience.
