Low temperature liquid storage tanks are specialized equipment used to store low-temperature liquids such as liquefied natural gas (LNG), liquid oxygen, liquid nitrogen, and liquid argon. Their safe operation directly affects the safety of personnel, the environment, and property. Due to the flammable and explosive properties of low-temperature liquids (such as LNG), strong oxidizing properties (such as liquid oxygen), or low-temperature frostbite, the inspection and use of storage tanks must strictly comply with regulations. The following are the key points for inspection and use of low-temperature liquid storage tanks:
1、 Key points for inspection of low-temperature liquid storage tanks
1. Inspection cycle and classification
Regular inspection:
First inspection: Conduct a comprehensive inspection within one year after use.
Follow up inspection: Depending on the type, medium, and operating conditions of the storage tank, a comprehensive inspection is usually conducted every 3-6 years, with at least one external inspection conducted annually.
Special circumstances: If the storage tank experiences deformation, leakage, foundation settlement, or other abnormalities, it must be immediately stopped and inspected.
Inspection classification:
External inspection: Check the appearance of the tank, insulation layer, pipeline connections, valve sealing, etc.
Internal inspection: Check for corrosion on the inner wall of the tank, weld cracks, and settlement of pearl sand (vacuum insulated storage tank).
Strength test: Verify the pressure bearing capacity of the tank body through water or air pressure tests (usually once every 10 years).
Airtightness test: Check the sealing of the tank and pipeline to prevent medium leakage.
2. Key inspection items
Tank structure:
Check whether the tank material meets the low-temperature requirements (such as austenitic stainless steel, aluminum alloy) to avoid low-temperature brittle fracture.
Verify the quality of welds, with a focus on inspecting butt welds, fillet welds, and reinforcement plate welds.
Thermal insulation system:
Vacuum powder insulated storage tank: Check the vacuum degree (usually ≤ 10Pa). If the vacuum degree decreases, it needs to be vacuumed again or supplemented with pearl sand.
High vacuum multi-layer winding insulated storage tank: Check the integrity of the winding layer to avoid insulation performance degradation.
Security attachments:
Pressure gauges, level gauges, and thermometers: Verify accuracy to ensure accurate readings.
Safety valve and bursting disc: Check whether they are within the valid calibration period and whether the operating pressure meets the design requirements.
Emergency cut-off device: Test remote control function to ensure quick isolation in case of leakage.
Pipelines and valves:
Check if the flexible connections of low-temperature pipelines (such as corrugated pipes) are intact to avoid thermal expansion and contraction that may cause breakage.
Verify valve sealing, with a focus on checking the main valve, safety valve, and discharge valve.
Foundation and Support:
Check whether the foundation of the storage tank has settled or cracked, whether the supporting structure is stable, and prevent the tank from tilting.
3. Inspection methods and tools
Non destructive testing:
Ultrasonic testing (UT): detecting thinning of tank wall thickness or internal defects.
Magnetic particle testing (MT) or penetrant testing (PT): detecting surface cracks.
Radiographic testing (RT): Verify the internal quality of the weld seam.
Vacuum degree test: Use a vacuum gauge to measure the vacuum degree of the insulation layer.
Airtightness test: Use helium mass spectrometer leak detector or soapy water to detect minor leaks.
Infrared thermography: detects the temperature distribution on the surface of the tank and assists in determining insulation layer defects.
2、 Specification for the use of low-temperature liquid storage tanks
1. Preparation before operation
Personnel training: Operators need to undergo professional training to be familiar with the characteristics of low-temperature liquids, emergency response, and equipment operation procedures.
Equipment inspection:
Confirm that the pressure, liquid level, and temperature of the storage tank are within the normal range.
Check if the safety accessories (such as safety valves and pressure gauges) are within the valid calibration period.
Verify pipeline connections, valve status, and emergency shut-off device functionality.
Environmental confirmation:
Ensure that there are no flammable materials around the storage tank and that there is good ventilation (especially for liquid oxygen storage tanks, oxygen rich environments should be avoided).
Check if the fire-fighting facilities (such as dry powder fire extinguishers and sprinkler systems) are operational.
2. Filling and discharge operations
Filling restrictions:
It is strictly prohibited to overfill (usually not exceeding 90% of the tank capacity) to prevent liquid expansion from causing excessive pressure.
Before filling, it is necessary to confirm that the type of medium is consistent with the design medium of the tank body (such as liquid oxygen tanks cannot be filled with liquid nitrogen).
Filling speed control:
Slowly fill to avoid rapid cooling that may cause stress concentration in the tank or damage to the insulation layer.
Emission operation:
Wear protective face shields and gloves when discharging low-temperature liquids to prevent frostbite.
The discharge outlet should face a safe area to avoid liquid splashing onto personnel or equipment.
Residual liquid treatment:
Before long-term shutdown, the residual liquid in the tank should be emptied and replaced with nitrogen to prevent oxidation or corrosion of the medium.
3. Operation monitoring and maintenance
Daily monitoring:
Record changes in tank pressure, liquid level, and temperature, and immediately investigate any abnormalities found.
Regularly inspect the surface of the tank for frost and ice (which may indicate insulation failure).
Regular maintenance:
Clean or replace the filter to prevent impurities from blocking the pipeline.
Lubricate the moving parts of the valve to ensure flexible opening and closing.
Check if the electrical system (such as level gauges and alarm devices) is properly grounded.
Insulation layer maintenance:
Vacuum insulated storage tanks need to be regularly checked for vacuum degree, and if necessary, pearl sand should be added or vacuum should be re drawn.
Avoid surface collision of the tank and prevent damage to the insulation layer.
4. Emergency response
Leakage handling:
Immediately activate the emergency shutdown device to isolate the leakage source.
Evacuate personnel to a safe area and set up a cordon.
Take measures based on the characteristics of the medium:
LNG leakage: Dilute combustible gases with water mist to avoid ignition sources.
Liquid oxygen leakage: eliminate surrounding fire sources and prevent oxygen enrichment for combustion assistance.
Fire handling:
Use dry powder or carbon dioxide fire extinguishers, and do not use water to extinguish fires (liquid oxygen may intensify combustion when in contact with water).
Personnel frostbite:
Immediately soak the frostbitten area in warm water (40-50 ℃), do not rub or snow rub.
3、 Common Problems and Prevention of Low Temperature Liquid Storage Tanks
Corrosion of tank body:
Reason: The medium contains impurities (such as chloride ions) or humid air entering the tank.
Prevention: Use pure media and regularly drain the condensed water in the tank.
Failure of insulation layer:
Reason: Decreased vacuum degree, settlement of pearl sand, or external damage.
Prevention: Regularly check the vacuum level to avoid tank collision.
Frequent tripping of safety valve:
Reason: Overfilling, rising ambient temperature, or pressure gauge malfunction.
Prevention: Strictly control the filling amount and install pressure buffering devices.
Valve freeze:
Reason: Low temperature medium causes ice formation inside the valve.
Prevention: Choose low-temperature dedicated valves and regularly move the valves to prevent jamming.
4、 Regulatory and standard basis
Domestic standards:
GB/T18442 "Fixed Vacuum Insulated Cryogenic Pressure Vessels"
TSG21 Safety Technical Supervision Regulations for Fixed Pressure Vessels
GB25204 "Safety Technical Supervision Regulations for Mobile Pressure Vessels" (applicable to tank trucks)
International standards:
ASMEBPVC Section VIII (American Society of Mechanical Engineers Pressure Vessel Standard)
EN13458 "Low temperature vessels" series standards (EU)
summary
The inspection and use of low-temperature liquid storage tanks should prioritize safety, and reduce risks through regular inspections, standardized operations, and emergency management. Users should develop inspection plans based on equipment manuals, regulatory standards, and actual operating conditions, and strengthen personnel training to ensure the safe operation of storage tanks throughout their entire lifecycle.