Best Practices for Cleaning Cryogenic Liquid Storage Tanks

 
In industrial production and storage, cryogenic liquid storage tanks play a critical role in safely storing liquefied gases, chemicals, and other low-temperature liquids. However, over time, these storage tanks can accumulate dirt and impurities, which may compromise their operational efficiency and safety. Therefore, regular cleaning of cryogenic liquid storage tanks is essential. Below, we will outline the cleaning process and quality checks for these storage tanks.
 

Cleaning Process

 
Maintenance of cryogenic liquid storage tanks entails a crucial step: cleaning. Over time, various dirt and impurities can accumulate inside the tanks, potentially affecting the quality and safety of the stored liquids. Therefore, regular cleaning ensures the interior of the tanks remains clean, thereby maintaining equipment operation and safety. Here's a detailed overview of the cleaning process.
 
1. Pre-cleaning Degreasing Treatment

Before conducting water and air pressure tests, the inner surface of the tank must undergo degreasing cleaning after passing a vacuum box leak test. This step ensures there is no grease or other contaminants on the inner tank surface, providing a clean foundation for subsequent cleaning.
 
2. Impurity and Dirt Removal

During the cleaning process, it's crucial to thoroughly remove all solid impurities such as welding slag, splashes, welding wires, sand, coatings, rust scars, and grease. Residual impurities inside the tank can potentially compromise the quality and safety of the stored liquids.
 
3. Pipeline Pre-treatment

Before assembling pipelines on-site, surfaces requiring degreasing should be wiped with lint-free cloth soaked in cleaning agent to ensure thorough cleaning and contamination prevention. This step aims to guarantee the cleanliness of pipelines and prevent contamination of the inner tank.
 
4. Spray Cleaning Method

Cleaning the inner tank involves using a spray cleaning method where a cleaning machine evenly sprays the cleaning agent onto the degreasing surface. This method effectively covers the entire surface, ensuring the cleaning agent acts fully.
 
5. Cleaning Agent and Conditions

Cleaning agents such as trisodium phosphate detergent or sodium silicate are used, with concentrations of 15g/l and 20g/l respectively. The cleaning agent temperature ranges from 60 to 80°C, with the cleaning machine pressurizing the cleaning agent to 7.5MPa. These conditions maximize cleaning effectiveness and ensure thorough tank interior cleaning.
 
6. Continuous Cleaning Time and Flow Rate

The continuous time of the cleaning agent on the degreasing surface should be no less than 5 minutes. The flow rate of the cleaning agent sprayed from the nozzle should not be less than 15L/min, with a spraying density of no less than 25L/m². These parameters ensure the thoroughness and efficiency of the cleaning.
 
7. Comprehensive Cleaning

All inner tank connecting pipe inner surfaces and outer surfaces of the inner tank cavity connecting pipes must be sprayed with cleaning agent. This step ensures every corner of the inner tank receives proper cleaning, avoiding the presence of residual contaminants.
 
8. Local Cleaning Treatment

If any areas are found inadequately cleaned during the spray cleaning process, they can be wiped clean with a cloth dipped in hot cleaning agent. This meticulous treatment ensures the thoroughness and integrity of the entire cleaning process.
 

Quality Inspection of Cleaning

 
Inspecting the quality of cleaning is a crucial step in ensuring the thoroughness and effectiveness of the cleaning process. Even after completing the cleaning process, without quality inspection, it's impossible to guarantee the thorough cleanliness of the tank interior. Therefore, a series of inspections is necessary to verify whether the cleaning meets standard requirements. Here's an outline of the quality inspection process.
 

• Final Inspection: The final inspection of cleaning quality should be conducted after pressure testing is completed. This step is essential to ensure that the cleaning process meets standard requirements.
• Visual Inspection: Under suitable lighting, the surface should not exhibit moisture, cleaning agent residue, welding residues, organic matter, or scattered particles with a diameter exceeding 2mm. These detailed observations ensure the thoroughness and quality of the cleaning.
• Ultraviolet Inspection: For surfaces visible to the naked eye, ultraviolet light with a wavelength of 3200-3800 angstroms is used to inspect degreased surfaces. The absence of oil fluorescence indicates a pass. This step helps detect contaminants not easily visible to the naked eye, ensuring surface cleanliness.
• Filter Paper Wipe Inspection: For surfaces not visible to the naked eye, clean dry white filter paper is used to wipe the degreased surface. The absence of oil traces on the paper indicates a pass. This inspection method further confirms surface cleanliness.
• Oil-Free Vapor Blowing Inspection: For degreased surfaces where the above methods cannot be used, oil-free vapor blowing is employed, and condensate should be placed on camphor tablets with a diameter smaller than 1mm, with the tablets rotating continuously indicating a pass. This step detects tiny contaminants, ensuring the integrity and thoroughness of the cleaning.
• Pollution Prevention Measures: After passing inspection, measures should be taken to prevent contamination, and the tank should be sealed with oil-free dry nitrogen. These measures ensure the cleanliness of the tank after cleaning and prevent secondary pollution.

 
The cleaning process of cryogenic liquid storage tanks is not just a simple task but a crucial operation concerning safety, quality, and environmental protection. Through proper cleaning procedures and rigorous quality inspection, the cleanliness and safety of the tank interior can be ensured, providing reliable protection for industrial production. Therefore, in daily maintenance and management, the cleaning of cryogenic liquid storage tanks should be highly prioritized, ensuring adherence to standard procedures to safeguard the normal operation and production safety of equipment.