Maintenance of Cryogenic Storage Tanks for Liquid Nitrogen

 
Cryogenic storage tanks are widely used for storing low-temperature liquids such as liquid nitrogen. Liquid nitrogen plays a crucial role in various industrial applications, including manufacturing, chemical processing, and cryopreservation. Due to its extremely low temperature (-196°C), the storage and handling of liquid nitrogen require strict adherence to safety and operational standards. Below is a detailed discussion on the usage and maintenance of cryogenic storage tanks for liquid nitrogen.

Cleaning and Maintenance

 
Regular Cleaning: Regular cleaning is essential to ensure the proper functioning and longevity of cryogenic storage tanks. An annual thorough cleaning is necessary to prevent corrosion of the tank walls by liquid nitrogen and to remove accumulated impurities that could affect the tank's performance.
Discharging Liquid Nitrogen: Prior to cleaning, all liquid nitrogen must be completely discharged from the tank. This must be done following professional procedures to ensure safety.
Using Neutral Detergents: During cleaning, neutral detergents should be used for an initial wash to effectively remove dirt and deposits from the tank walls.
Secondary Rinsing: Following the initial wash, a thorough rinse with a large volume of water is required to ensure that all detergent and debris are completely removed.
Drying: After cleaning, the tank must be thoroughly dried before use. The temperature of the cleaning and rinsing water should be kept below 40°C to avoid negatively impacting the tank’s low-temperature performance.

Pre-cooling and Filling

 
Pre-cooling Operation: Before storing large amounts of liquid nitrogen, a pre-cooling operation is necessary. This is to ensure the tank reaches a temperature suitable for liquid nitrogen storage. Directly filling a tank with large quantities of liquid nitrogen could cause thermal shock and damage to the tank material.
Initial Filling: During the pre-cooling operation, a small amount of liquid nitrogen should be introduced to gradually lower the tank temperature to the required level.
Environmental Control: The environment where the tank is located should be kept dry and well-ventilated throughout the filling process to prevent external moisture from affecting the tank’s performance.
Slow Filling: Once the tank reaches a stable low temperature, filling with liquid nitrogen can begin. This should be done slowly to minimize thermal shock.

Operational Precautions

 
Minimize Opening: During storage, the number of times the tank is opened should be minimized to reduce cold loss and prevent external air from entering the tank.
Inspect the Neck Seal: Regularly check the condition of the tank’s neck seal. If ice formation is observed, it needs to be removed promptly, as ice formation could indicate insulation issues affecting nitrogen storage efficiency.
Neck Inspection: If water droplets or frost appear at the neck, it may indicate insulation problems. A thorough inspection is necessary to ensure the tank can safely and effectively continue to store liquid nitrogen.
Marking and Recording: Detailed marking and recording of the tank’s filling times, volumes, and usage are essential. This helps manage potential issues promptly.

Transportation and Safety

 
Maintain Upright Position: During transportation, the tank should be kept upright to avoid severe vibrations or impacts.
Protective Measures: Liquid nitrogen’s extremely low temperature can cause frostbite if it comes into contact with skin. Special care is needed during filling and removal operations.
Filling Operation: When first filling the tank, due to its initial high temperature, it may take a long time to reach thermal equilibrium. It is recommended to initially fill with a small amount of liquid (around 20 liters) to pre-cool the tank, then continue with a slow fill to reduce ice blockage.

Long-Term Storage and Handling

 
Open Valve: When not in use for extended periods, ensure that the empty valve is open to prevent pressure build-up from damaging the tank. The storage area should be well-ventilated to avoid environmental impact from nitrogen leakage.
Sealing Operations: For long-term storage, empty the tank of liquid media and dry the interior with dry air before closing all valves. Prior to refilling, thoroughly dry the tank liner, valves, and pipelines with dry air to prevent ice blockages that could affect pressurization and fluid transfer.

Equipment Maintenance and Inspection

 
Routine Maintenance: As a highly vacuum-sealed device, cryogenic storage tanks require regular maintenance to ensure performance and extend their lifespan. Proper maintenance improves storage efficiency and reduces replacement frequency and operational costs.
Frost Formation: For pressurized nitrogen tanks, frost formation during pressurization is normal. However, if water or frost appears on the tank surface after the pressure valve is closed, it may indicate a loss of vacuum integrity. Contact a professional for repair or disposal.
Filling Maintenance: To reduce liquid loss, refill the tank promptly when only a small amount of liquid remains, or within 48 hours after depletion, to ensure effective utilization and maintenance.

Operational Guidelines

 
When operating cryogenic storage tanks, handle them gently to avoid damage. When opening valves, apply moderate force and avoid rapid actions. Especially when connecting metal hoses to inlet/outlet valves, avoid excessive force to prevent damage. Tighten connections slightly, ensuring a good seal, and support the valve body with one hand to ensure stability.
 
Proper management and maintenance of cryogenic storage tanks are crucial for the storage of liquid nitrogen. By adhering to the above guidelines, the safety, reliability, and longevity of the tanks can be significantly enhanced. Paying attention to every detail, from regular cleaning and pre-cooling to transportation, storage, and operation, is key to ensuring effective and safe liquid nitrogen storage. These precautions not only help reduce equipment failure and maintenance costs but also ensure a stable and safe supply of liquid nitrogen.