Operating pressure of liquid nitrogen storage tank in air separation unit
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The author recently updated the liquid nitrogen pump of the air separation unit and learned that the inlet pressure of the liquid nitrogen pump is 0.85 MPa, with a temperature of -196°C. Something doesn’t seem right, as it seems that liquid nitrogen can be stored at normal pressure at -196°C. However, upon seeing that the maximum operating pressure of the liquid nitrogen storage tank is 0.85 MPa at -196°C, I am also puzzled. After reviewing some literature, I learned that liquid nitrogen storage tanks are equipped with external expansion vaporizers, which cause a portion of the liquid nitrogen to vaporize and return to the top of the tank, thereby regulating the pressure inside the storage tank. The author has some questions: 1. Is this regulating pressure a constant value? How does it change as the liquid level drops? What measures should be taken regarding this pressure interlock when the storage tank is emptied? ； 2. What is the basis for determining this working pressure? For example, the maximum working pressure of the storage tank here is 0.85 MPa – is this due to the need to take into account the effective net positive suction head of the liquid nitrogen pump? ； 3. Are there any other relevant standards, literature, or books recommended? I hope the experts here can discuss and provide answers.
1. The control pressure is generally not a fixed value; it may be adjusted according to the actual liquid level in the liquid nitrogen tank and the operating conditions. When the liquid level drops, the pressure may decrease due to the increase in the gas space inside the tank; at this point, an external vaporizer can maintain the pressure by adjusting the conversion from liquid to gas state. When the storage tank is emptied, the system needs appropriate safety measures to address potential negative pressure risks and prevent damage to the tank structure. 2. The set operating pressure is typically determined based on various factors, including but not limited to the pump’s net positive suction head requirement, the evaporation rate of liquid nitrogen, and the system’s safety standards. In the situation you mentioned, the maximum operating pressure of 0.85 MPa is likely set to ensure the effective operation of the liquid nitrogen pump, as well as to guarantee safety and stability during the storage and transportation of liquid nitrogen. 3. Relevant standards, literature, or books can provide information on cryogenic engineering, chemical processing machinery, and pressure vessel design. For example, international standards such as the ASME Boiler and Pressure Vessel Code and ISO 21009 \"Cryogenic vessels – Static vacuum insulated vessels\" provide detailed information and guidance on the design and operation of cryogenic storage tanks and equipment. Professional books such as \"Low-Temperature Engineering Technology\" can also be referred to for a deeper understanding and data support. .
Let me reply myself: 1. The pressure remains within a fixed range, which is the pressure of nitrogen in the utility systems. Generally, liquid nitrogen does not need to be pumped to the vaporizer; instead, it flows directly from the storage tank to the vaporizer and then to the utility systems. Therefore, the pressure in the storage tank is generally maintained at 0.7–0.8 MPa. There won’t be time, unless for maintenance. 2. As mentioned in point 1, this refers to the pressure in the utility systems; of course, when the storage tanks are not in use, the pressure maintained may be around 0.2–0.3 MPa. At the site, I saw 6 storage tanks, 5 of which had a pressure of around 0.7 MPa, while one had a pressure of around 0.35 MPa. 3. The book doesn’t go into as much detail; just ask about the on-site process directly