Discussion on the safety of air separation cryogenic tanks
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Recently, I have seen some accidents in which cold air leakage inside the cryogenic tank caused the tank to expand and burst. There is serious concern regarding the safety of cryogenic tanks. In accordance with Article 99, Paragraph (2) of the Regulations on the Safety Supervision of Special Equipment (amended by Order No. 549 of 2009), a pressure vessel refers to a closed device used to hold gases or liquids and subjected to certain pressure; such devices include fixed and mobile containers in which the maximum operating pressure is 0.1 MPa (gauge pressure) or higher, and the product of pressure and volume is 2.5 MPa·L or higher, for gases, liquefied gases, and liquids whose maximum operating temperature is equal to or higher than their standard boiling point ； Gas cylinders, liquefied gas cylinders, and cylinders for liquids with a standard boiling point of 0°C or lower, whose nominal working pressure at full charge is greater than or equal to 0.2 MPa (gauge pressure), and whose product of pressure and volume is greater than or equal to 1.0 MPa·L ； Oxygen chambers, etc. The fine print under the regulations specifies that for certain special pressure vessels, it is not necessary to carry out regular inspections in accordance with the regulations; however, other requirements (such as those related to design and manufacturing) must still be met. To quote: 3. Pressure vessels that are not ** in refrigeration units, pressure vessels in direct-fired absorption refrigeration systems, and cryogenic tanks in air separation equipment: Cryogenic tanks are classified as special pressure vessels and do not require regular inspections, but their design and manufacturing must still comply with the regulatory requirements. However, due to its fragile wall structure, the cold box does not have the capacity to withstand high pressures (especially when exposed to low temperatures as well). Yet in reality, there are situations where liquid leaks cause vaporization, subjecting the cold box to both high pressure and low temperatures. The cold box itself lacks the safety pressure relief devices found in pressure vessels; it has no pressure gauges or pressure transmitters for remote monitoring and alarm detection. Therefore, it is only possible to determine whether there are leaks or other serious issues inside the cold box through regular visual inspections and analysis of various operational parameters, which often results in missing the optimal time to address the problems. The fragile structure of the cold box can be damaged in a short period of time, leading to significant dangers and losses. From the manufacturer’s perspective, there are problems with the installation first; so is the design reasonable? From the user’s perspective, are questions such as how to conduct daily checks, detect, address, and resolve the issue caused by pearlescent sand in the cold box interfering with pressure release things we need to consider? I hope everyone can discuss their experiences regarding the routine inspections and safety settings of cold boxes. Thank you
The cryogenic tanks designed by our company are equipped with safety relief devices; the pressure released through these devices is only a few KPa. There is also a pressure gauge to monitor the pressure inside the cryogenic tank. I believe most air separation plant manufacturers should have these two features. In addition, many manufacturers install thermistors at the bottom of the cryogenic tank to measure temperature changes there; in the event of severe leakage, the temperature readout from these thermistors will also change
Most cold boxes are equipped with ventilation ports, which allow for pressure balance regardless of whether the pressure is high or low. Cold boxes generally have a certain level of pressure, but the issue mentioned by the original poster does exist. Due to the large size of these cold boxes, there is sometimes only one pressure gauge, and there are also significant problems with the distribution of the sealing gas within them. The manufacturers responsible for their design should carry out modifications
It is important to inspect the breathing vents on the top of the cold box; sometimes rust is found, and the vents cannot be opened due to installation issues
Judging from the accidents that have occurred, whether it is air separation units with a capacity of over 20,000 or those with a capacity of just a few thousand, the breather valves located on top of the cryogenic tanks failed to perform their function of ensuring safe pressure release. In many cases, the low-temperature liquids leaked and vaporized, causing rapid pressure increases; moreover, the perlite present inside these units likely acted as an obstacle to pressure release, resulting in the breather valves becoming clogged in a very short time. Cold boxes are still prone to the risk of pressure buildup. I have a question for everyone. A few days ago, I saw the 81700 oxygen production air separation unit manufactured by the American company AP; its main cooling tank features a circular design. What is the purpose of such a design? Thank you