This text selects materials based on Chinese National Standards (GB). If American standards are required, ASME SA-516 material can be selected, while European standards adopt P355NL1. The corresponding material shall be selected in accordance with the regulations of the destination country.

Design and Production Scheme for Underground LPG Storage Tanks in Frigid Regions (Extreme Temperatures Below -30°C) Compliant with National Standards (GB)

Applicable Standards: GB/T 150.1~4-2024, GB/T 713.3, TSG 21-2016, GB 50156-2021, GB 3531

Core Dilemma: Under local extreme surface temperatures below -30°C and permafrost conditions, the tank shell, nozzles, saddles, and welds are concurrently exposed to low-temperature brittleness risks, compounded by underground soil corrosion. Compliance must be ensured throughout the entire process across six dimensions: design parameters, low-temperature material selection, low-temperature embrittlement-resistant structural design, anti-corrosion and cathodic protection, production and manufacturing, and civil engineering frost prevention.

I. Basic Design Temperature, Pressure, and Service Life Values (Mandatory for Drawings/Nameplates)

1. Design Temperature Range

• Maximum Design Temperature: Fixed at 50°C (retaining the standard LPG saturated vapor pressure verification).
• Minimum Design Temperature: -35°C (incorporating a 5°C safety margin to match the local extreme low temperature of -30°C).
• Unified Labeling: The nameplate, calculation sheets, and as-built documents must uniformly state: Design Temperature: -35°C to 50°C.

2. Design Pressure

• In strict compliance with GB 50156, a uniform pressure of 1.77 MPa shall be applied for underground tanks at refueling stations and commercial facilities (verified against the saturated vapor pressure of propane at 50°C).

3. Design Service Life

Due to the aggravated corrosion in frigid permafrost environments, low-end conventional 20-year configurations are prohibited. The standard configuration is set at 20 years, while permanent station projects shall be designed for 30 years, with enhanced internal and external corrosion allowances:

• 20-Year Service Life: Internal corrosion allowance ≥ 1.0 mm; external soil corrosion allowance ≥ 1.5 mm.
• 30-Year Service Life: Internal corrosion allowance ≥ 1.5 mm; external soil corrosion allowance ≥ 2.0 mm.

II. Material Selection for Low-Temperature Pressure Elements (Q345R Prohibited; Dedicated Low-Temperature Pressure Vessel Steel Mandatory)

1. Shell and Ellipsoidal Heads (Core Pressure Elements)

• Material: 16MnDR (Low-temperature vessel steel plate per GB/T 3531).
• Application Limit: Minimum allowable temperature is -40°C, fully covering the -35°C design lower limit.
• Mandatory Requirement: Charpy V-notch low-temperature impact tests must be performed on a plate-by-plate basis at -35°C, with an impact energy Akv ≥ 34 J.
• Prohibition of Q345R: The minimum allowable temperature for Q345R is only -20°C. It lacks low-temperature toughness below -20°C and is highly susceptible to low-temperature brittle fracture, which violates the low-temperature vessel clauses of GB 150.

2. Nozzles, Manholes, Flanges, and Reinforcing Rings

• Forgings: 16MnD forgings matching the base metal, subjected to low-temperature impact testing at -35°C.
• Small-Diameter Seamless Steel Pipe Nozzles: 16MnDG low-temperature steel pipes.

3. Saddles and Non-Pressure Elements

• The saddle base plate may use Q355B, but the saddle pad plate welded to the shell must be made of 16MnDR to prevent low-temperature brittle cracking caused by mismatched thermal characteristics at the weld.

4. Matching Welding Materials

• Welding Electrodes for 16MnDR: J507RH low-hydrogen, high-toughness electrodes.
• Submerged Arc Welding (SAW) Wire and Flux: Low-temperature matching combination H08Mn2E + SJ105, ensuring the weld metal passes the impact test at -35°C.

III. Key Structural Design Points for Low-Temperature Brittle Fracture Prevention (Mandatory per GB 150 Appendix E for Low-Temperature Vessels)

1. Elimination of Stress Concentration (The Most Critical Failure Point at Low Temperatures)

• All openings must utilize integral forging reinforcement. Reinforcing rings are strictly prohibited (due to stress concentration at the corners of reinforcing rings, which easily cracks at low temperatures).
• Nozzles must have full-radiused transitions to the shell/head, with internal and external fillets R ≥ 8 mm.
• Butt welds of the heads and shell must be ground smooth, free of weld flash, with misalignment strictly limited to $\le$ 10% of the wall thickness and $\le$ 2 mm.

2. Upgrade of Joint Efficiency and Nondestructive Testing (NDT)

• The welded joint efficiency shall be taken as Φ = 1.0 (upgraded from the Φ = 0.85 used for conventional underground tanks due to the frigid environment).
• All Class A and B longitudinal and circumferential butt welds must undergo 100% Radiographic Testing (RT) with Grade II compliance. Class C and D nozzle fillet welds must undergo 100% Magnetic Particle Testing (MT) to screen for microcracks (which propagate rapidly under low temperatures).

3. Prevention of Frost Heaving and Cryogenic Contraction Cracking of Nozzles

• All discharge/outlet piping on the tank top must be fitted with metal bellows expansion joints to counteract the tensile stress generated by permafrost frost heaving and cryogenic tank contraction.
• Nozzles for level gauges, pressure gauges, and emergency shut-off valves must be extended above the permafrost layer, ensuring valve bodies are not buried within frozen soil.
• The drain valve must be located at the lowest point of the tank, with full-length heat tracing/insulation applied to prevent accumulated water from freezing and cracking the valve and pipeline.

4. Tank Slope

• The tank body must slope toward the drainage/drain end at a gradient of 3‰ to 5‰ to ensure total evacuation of bottom water, preventing internal ice formation from cracking the shell due to localized internal pressure.

IV. Underground Double-Layer Corrosion Protection + Long-Term Cathodic Protection (Mitigating Aggravated Permafrost Stray Current and Acidic Soil Corrosion)

Conventional epoxy coal tar pitch coatings cannot withstand the 20/30-year lifecycle under frigid permafrost conditions. A dual system comprising an external coating + sacrificial anode cathodic protection must be implemented:

1. External Anti-Corrosion Coating (Choice between 3PE and FBE)

• Option 1 (Recommended): Three-layer polyethylene (3PE) anti-corrosion coating, with a total thickness ≥ 2.5 mm, capable of withstanding low temperatures down to -40°C without cracking or blistering.
• Option 2: Fusion-bonded epoxy (FBE) coating, with a dry film thickness ≥ 800µm and compliant low-temperature toughness.
• Note: Thin epoxy coal tar pitch is prohibited as it easily embrittles and delaminates in frozen soil.

2. Mandatory Sacrificial Anode Cathodic Protection System (Mandatory per GB 50494 for Underground Gas Tanks)

• Magnesium alloy sacrificial anode groups shall be arranged uniformly along both sides of the tank shell.
• Two long-term copper sulfate (Cu/CuSO4) reference electrodes shall be installed, with testing posts reserved. The protection potential measured upon completion must range from -0.85 V to -1.15 V.
• Insulating gaskets and sleeves must be installed on all nozzles and manhole flanges to isolate stray currents in the pipeline and prevent galvanic corrosion from accelerating tank perforation.

3. Frost-Resistant Backfill Medium

• The perimeter of the tank must be backfilled with dry, neutral, fine river sand (frozen soil, clay, and construction waste are strictly prohibited), with a sand layer thickness ≥ 500 mm. The sand layer isolates the tank from direct compression by frozen soil lumps and reduces soil corrosivity.

V. Special Low-Temperature Process Control During Production and Manufacturing (Low-Temperature Impact Reports Must Accompany the Factory Clearance)

1. Re-inspection of Materials upon Warehouse Entry

• Each heat/lot of 16MnDR steel plates must undergo re-inspection for chemical composition, mechanical properties, and low-temperature impact testing at -35°C. Cutting is prohibited without a compliant re-inspection report.

2. Low-Temperature Welding Procedures

• When welding is performed at ambient temperatures below 5°C, the shell and nozzles must be preheated to ≥ 80°C, with an interpass temperature maintained between 80°C and 200°C.
• Post-weld insulation and slow cooling must be applied to prevent hardening and cold cracking caused by rapid cooling of the weld.
• Upon completion of all welding, the entire tank must undergo overall post-weld heat treatment (PWHT) for stress relief (holding at 600°C to 640°C) to eliminate residual welding stresses and significantly mitigate low-temperature brittle fracture risks. (While optional for conventional southern tanks, PWHT is mandatory for frigid-zone low-temperature applications).

3. Temperature Requirements for Pressure Testing

• The water temperature during hydrostatic and airtightness testing must be ≥ 5°C. Pressure testing with low-temperature icy water is prohibited to prevent brittle fracture. The hydrostatic test pressure shall be 1.25 times the design pressure, and the airtightness test shall be 1.0 times the design pressure.

4. Special Archiving of Factory Documentation

The documentation package must include: 16MnDR steel plate low-temperature impact reports, forging impact reports, weld RT/MT inspection reports, heat treatment curves, anti-corrosion layer inspection reports, cathodic protection design calculation sheets, corrosion allowance strength calculation sheets, and the -35°C design temperature strength verification sheet.

VI. Civil Engineering and Installation Design for Frost Heaving Prevention (Core Permafrost Risk Management)

1. Burial Depth Exceeding Conventional 0.5 m Requirements

• Assuming the local frost depth is $H$, the earth cover thickness above the tank top must be ≥ H + 0.5m. This ensures the entire tank is positioned within the constant-temperature layer below the permafrost line, preventing cyclic thermal stresses from repeated freezing and thawing.
• If a thick earth cover is unfeasible, an insulated tank pit must be constructed, with the pit walls filled with insulating rock wool.

2. Frost-Heaving Prevention and Anti-Floating Foundation

• The base of the reinforced concrete foundation must be situated below the frost depth. A 300 mm thick gravel frost-prevention layer must be laid beneath the foundation, and C30 concrete must be formulated with anti-freezing admixtures.
• Heavy-duty anti-floating anchoring must be provided. In frigid areas with high groundwater levels, anti-floating piles must be added to prevent springtime snowmelt groundwater buoyancy from lifting the tank and rupturing the connected nozzles.

3. Tank Pit Drainage

• The bottom of the tank pit must feature a 3‰ slope, a drainage ditch, and an explosion-proof pumping well to promptly evacuate snowmelt and groundwater, preventing long-term immersion of the tank exterior in ice water, which accelerates corrosion.

VII. Low-Temperature Adaptation Requirements for Safety Accessories

1. Safety Valves and Emergency Shut-Off Valves

• Low-temperature dedicated valve bodies made of WCB-L low-temperature carbon steel must be selected, with a minimum allowable temperature of -40°C. Valve inlet pipes must be insulated and heat-traced to prevent residual LPG liquid from freezing and jamming the valve cavity.

2. Level and Pressure Transmitters

• Explosion-proof, low-temperature rated variants must be used. Transmitters shall be installed in instrument wells above the permafrost layer, and the instrument wells must be insulated and sealed.

3. Lightning Protection and Anti-Static Grounding

• Both ends of the tank must be grounded, with grounding electrodes deeply buried below the permafrost layer, ensuring a grounding resistance ≤ 10 Ω. All metallic pipelines must be bridged for static electricity prevention.

Minimal Compliance Checklist (Meeting All Points Ensures Complete GB Compliance)

1. Material: Shell and heads made of 16MnDR, verified by passing low-temperature impact testing at -35°C.
2. Design Parameters: Range of -35°C to 50°C, 1.77 MPa, with corrosion allowances matched to a 20/30-year service life.
3. Welds: 100% RT Grade II compliance, followed by overall post-weld heat treatment (PWHT) for stress relief.
4. External Corrosion Protection: Thick-coat 3PE/FBE coating coupled with magnesium alloy sacrificial anode cathodic protection.
5. Civil Engineering: Tank fully buried below the permafrost line, supported by a frost-heaving resistant foundation and a comprehensive drainage system.
6. Accessories: All valves and instruments selected from low-temperature adapted product lines, with expansion compensation provided at nozzle connections.