【Help】Issues and precautions regarding the construction method of low-pressure vault tanks

Construction techniques for low-pressure dome tanks and special considerations compared to atmospheric storage tanks. Low-pressure dome tanks (typically those with a design internal pressure slightly higher than atmospheric pressure, such as 0.5 kPa to 30 kPa) share similarities in construction techniques with atmospheric storage tanks (whose design internal pressure is between -0.5 kPa and +0.5 kPa), but due to the higher pressure requirements, there are key differences and special considerations. The following are detailed explanations of the construction methods and special precautions: I. Construction method for low-pressure dome tanks 1. Construction preparation Design review: Ensure that the design documents comply with standards such as API 650 (Appendix F or G) or GB 50341, paying particular attention to design pressure, vent valve settings, and requirements for reinforcement plates. Material inspection: Steel plates, welding materials, and accessories (such as breather valves and emergency relief valves) must come with quality certificates and undergo re-inspection. Foundation inspection: The foundation’s elevation, levelness, and centering deviation must conform to the design specifications, and its load-bearing capacity must meet the required standards. 2. For the construction of the tank bottom plate, butt welding with backing strips is employed (not lap welding) to ensure full penetration of the welds; in addition, 100% vacuum leak testing or penetrant testing is conducted. Strictly control welding deformation to prevent bottom plate warping from affecting sealing. 3. Construction of tank wall panels: Panel arrangement: The thickness of the wall panels must meet the strength and stability requirements under the design pressure (negative pressure conditions must also be taken into account). Assembly and welding: Vertical assembly or flip-chip method is employed; longitudinal welds are joined by butt welding, while circumferential welds can be joined by butt welding or lap welding (depending on the design). Welding sequence: weld the longitudinal seams first and then the circumferential seams, control welding deformation, and use anti-deformation fixtures if necessary. Weld inspection: Longitudinal welds: 100% radiographic testing (RT) or ultrasonic testing (UT). Circumferential welds: At least 20% random inspection by RT/UT (the proportion is determined based on the design pressure). T-joint welds: Subject to thorough inspection to avoid stress concentration. 4. Vault construction – Roof assembly: The welds connecting the roof to the edge angle steel must achieve full penetration to ensure pressure-resistant sealing. Reinforcement structure: Radial or circumferential reinforcement members (such as ribs) are provided according to the design pressure to enhance compressive stability. Welding requirements: The welds on the top plate must be tight, and a gas-tightness test must be conducted. 5. Attachment installation: Breathing valve – the set pressure must be accurately calibrated, and it should be equipped with a flame arrester. Emergency relief valve: The pressure relief capacity must meet API 2000 requirements, and it shall be calibrated regularly. Reinforcement plates: The areas with openings (such as manholes and connection pipes) must be reinforced in accordance with standards, and a gas-tightness test shall be conducted after welding. Sealing device: All flanges and manholes must be sealed using pressure-resistant gaskets (such as metal wound gaskets). 6. Pressure testing: Positive pressure test: Inflate to the design pressure (e.g., 1.2 times the design pressure), maintain the pressure to inspect welds and sealing points, and use a foam agent or nitrogen for leak detection. Negative pressure test: Evacuate to the designed negative pressure (e.g., -0.5 kPa) to check the stability of the tank top and the welds. Settlement monitoring: Foundation settlement is monitored throughout the entire test. 7. Corrosion protection and insulation: The coating for the inner wall’s corrosion protection must be selected based on the properties of the medium (such as epoxy zinc-rich coatings). When insulating the exterior wall, the effect of deformation caused by pressure changes must be taken into account. II. Special precautions for the construction of atmospheric pressure storage tanks 1. Differences in design standards Atmospheric pressure tanks: Designed in accordance with the main text of API 650, with emphasis on hydrostatic loads. Low-pressure tank: It must comply with API 650 Appendix F (wind load and internal pressure) or Appendix G (higher internal pressure); the calculations must take into account both positive and negative pressure conditions. 2. Structural strength requirements: Wall and ceiling thicknesses: Low-pressure tanks require increased thickness or the installation of reinforcing structures to prevent instability. Edge banding angle steel: The cross-sectional dimensions need to be increased to ensure the compressive strength at the top wall joints. Anchoring design: Anchor bolts may be required for low-pressure tanks to resist the upward pulling force caused by internal pressure. 3. Welding and inspection standards: Higher requirements are placed on the weld quality for butt welds of low-pressure tanks (such as full penetration), and the inspection rate is often higher than that for normal-pressure tanks. Welding qualification: A welding qualification for pressure vessels or pressure-bearing equipment is required (such as ASME Section IX). 4. Attachment configuration: Breathing valve: Simple ventilation holes are commonly used in tanks at normal pressure, while precision breathing valves (with a setting error of ≤±5%) are required for tanks at low pressure. Relief device: A low-pressure tank must be equipped with an emergency relief valve to prevent overpressure. Instrument interface: A pressure monitoring port must be reserved for installing a pressure transmitter. 5. Test requirements: Atmospheric pressure tanks: Usually only a water filling test is conducted (for leak detection and settlement checks). Low-pressure tank: In addition to the water filling test, a positive/negative pressure airtightness test must be conducted, with a longer pressure retention time (e.g., over 1 hour). 6. Safety and Quality Control: Welding Procedure Qualification (PQR/WPS): It is necessary to cover pressure-bearing conditions. Material traceability: All stressed components must be traceable throughout the entire process. Lifting and assembly: Avoid forceful alignment to prevent residual stresses. III. Key risk control points: Overpressure risk: A clogged breathing valve can lead to overpressure; it is necessary to clean the pipes and test the valves during construction. Negative pressure instability: Monitor the deformation of the tank top during vacuuming to prevent it from collapsing. Weld seam leakage: Strictly control the welding process to avoid defects such as undercutting and lack of fusion. Uneven settlement of the foundation: Before testing, it is necessary to confirm that the foundation has been properly cured, and the settlement difference must be less than the value permitted by the design. IV. References for Standards and Specifications: International: API 650 (Appendices F/G), API 2000, ASME Section VIII; Domestic: GB 50341, GB 50128, NB/T 47003.1. Inspection standards: JB/T 4730, API 577. Summary: The construction of low-pressure dome tanks requires enhancing the design of the pressure-bearing structure based on normal-pressure tanks, raising the standards for welding and inspection, installing precise safety accessories, and strictly carrying out pressure tests. The construction team must have experience with pressure-bearing equipment, with a particular focus on compliance and safety in pressure-related aspects, to ensure that the storage tank can reliably withstand pressure during long-term operation.