Key Points for Spherical Tank Selection and Design: Medium Characteristics, Volume Calculation, and Pressure Rating Matching

Key Points for Spherical Tank Selection and Design: Medium Characteristics, Volume Calculation, and Pressure Rating Matching

The selection and design of spherical tanks must focus on safe storage, integrating medium characteristics, actual storage requirements, and regulatory standards. Precise control of three key links—medium adaptation, volume calculation, and pressure rating matching—is essential to ensure long-term stable operation of the equipment.

Medium characteristics serve as the primary basis for selection and design, requiring targeted matching of materials and structures. When storing liquefied natural gas (LNG), due to the low-temperature property (-162℃) of the medium, 9Ni steel with excellent low-temperature toughness must be used. The tank wall needs a multi-layer thermal insulation structure (e.g., perlite + vacuum layer) to prevent structural brittle fracture caused by low-temperature leakage. For crude oil storage, if the sulfur content of the medium exceeds 0.5%, sulfur-resistant carbon steel (such as Q345R(HIC)) should be selected, and an internal anti-corrosion coating (e.g., epoxy coal tar pitch) should be designed to avoid sulfur corrosion. Additionally, the volatility of media (e.g., propane, butane) must be considered in seal structure selection—double-sealing face flanges are preferred to reduce leakage risks.

Volume calculation must align with storage needs and regulatory requirements, following a two-step process. The first step is to determine the theoretical volume: based on daily storage demand (e.g., a chemical plant’s daily crude oil consumption of 500m³), calculate using the formula "Theoretical Volume = Daily Consumption × Safety Factor (1.2-1.5)". For example, 500m³ × 1.3 = 650m³. The second step accounts for structural losses: the actual effective volume of the spherical tank must exclude the top gas phase space (approximately 5%-8%) and the bottom sewage discharge space (approximately 2%-3%). Thus, a theoretical volume of 650m³ requires a designed volume of 700-720m³ to meet actual storage needs. Meanwhile, compliance with the GB 12337 standard is mandatory—dual safety valves must be installed for tanks with a single-tank volume exceeding 1000m³.

Pressure rating matching must balance medium pressure and structural strength. First, determine the design pressure: based on the medium’s saturated vapor pressure (e.g., LNG has a vapor pressure of 0.1MPa at -162℃), add a 10%-20% safety margin, resulting in a design pressure of 0.12MPa. For compressed natural gas (CNG) storage, calculate using the actual working pressure (20MPa) + 15% margin, setting the design pressure to 23MPa. Second, match the wall thickness using the formula: "Wall Thickness = (Design Pressure × Spherical Tank Inner Diameter) / (4 × Allowable Stress × Weld Efficiency)". For example, for a Q345R steel spherical tank (inner diameter 12m, design pressure 0.8MPa) with an allowable stress of 189MPa and a weld efficiency of 0.9, the calculated wall thickness is approximately 13mm. This must also meet the minimum wall thickness requirement (not less than 10mm) to ensure structural strength compliance.