Safety Inspection Specifications for Spherical Storage Tanks: Key Steps in Wall Thickness Testing, Weld Inspection, and Pressure Testing

Safety Inspection Specifications for Spherical Storage Tanks: Key Steps in Wall Thickness Testing, Weld Inspection, and Pressure Testing

As core equipment for storing flammable, explosive, or high-pressure media in the chemical and energy sectors, the safety inspection of spherical storage tanks is directly related to production safety. Three key links—wall thickness testing, weld inspection, and pressure testing—must be closely monitored, with strict adherence to standardized procedures to identify potential risks.

Wall thickness testing serves as the foundation for evaluating tank corrosion and structural strength, requiring a combination of "comprehensive sampling + focused monitoring". Before testing, the anti-corrosion coating and impurities on the tank wall surface must be removed. An ultrasonic thickness gauge (with an accuracy of ±0.1mm) is used, and the tank is divided into inspection zones including the "equatorial belt, upper and lower polar plates, and pillar connection points". The sampling ratio for each zone shall not be less than 20% of the total area, with focused testing on corrosion-thinned areas in contact with the medium (such as the bottom of the liquid phase zone and the top of the gas phase zone). If local wall thickness is found to be less than 80% of the designed value, the inspection scope shall be expanded; if the minimum wall thickness is less than 70% of the designed thickness, the tank's load-bearing capacity must be evaluated, and shutdown for maintenance shall be conducted if necessary. Test data shall be recorded in real-time, and a wall thickness change ledger shall be established to compare historical data and determine the corrosion rate.

Weld inspection focuses on identifying welding quality hazards, prioritizing the "combined non-destructive testing method". For butt welds, penetrant testing (PT) is first used to detect surface cracks and pores. Before testing, the weld surface shall be ground to a roughness of Ra ≤ 25μm. After applying the penetrant and allowing it to stand for 10-15 minutes, a developer is used to reveal defects. For internal defect detection, radiographic testing (RT) or ultrasonic testing (UT) is adopted. RT shall cover all circumferential and longitudinal welds, with a sensitivity reaching Level AB. UT focuses on rechecking areas not covered by RT. If internal weld defects such as cracks longer than 10mm or inclusions larger than 50mm² are found, the defect locations shall be marked for repair. After repair, 100% re-inspection shall be conducted to ensure a 100% weld qualification rate.

Pressure testing verifies the tank's sealing and pressure-bearing performance, requiring strict control of the "pressure rise - pressure holding - pressure reduction" process. The preferred test medium is clean water (to avoid contaminating the tank inner wall). For tanks storing flammable media, dry compressed air (with a dew point ≤ -40℃) shall be used as the test medium. Pressure shall be increased slowly in stages, with each stage not exceeding 20% of the designed pressure. Each stage shall be held for 30 minutes to check for tank deformation or leakage. When the pressure rises to 1.25 times the designed pressure, it shall be held for 60 minutes. During this period, soapy water shall be applied to the sealing surfaces of flanges and valves to check for bubbles. The pressure reduction rate shall not exceed 0.1MPa/min. After reducing to the designed pressure, the sealing status shall be checked again. If sudden pressure drop or obvious tank deformation occurs during the test, pressure shall be released immediately and the tank shut down to identify and repair leakage points. After repair, the pressure test shall be repeated to ensure test data complies with the requirements of GB 12337 Steel Spherical Storage Tanks.