The Welding Process for Large Vertical Storage Tanks

Welding is a crucial aspect of manufacturing large vertical storage tanks. It not only affects the structural integrity and safety of the tank but also ensures the safe and reliable storage of materials. This article provides a comprehensive overview of the welding process for large vertical storage tanks, covering foundation requirements, prefabrication and assembly, welding techniques, special material welding, welding sequences, quality control, and environmental impacts.

Foundation Requirements for Storage Tanks

The foundation of a storage tank is vital as it bears the weight of the entire structure. The quality of the foundation directly impacts the tank's stability and lifespan. The following requirements should be followed during construction.

1. Design Standards

The foundation design must adhere to national and local standards. Soil tests and load calculations are typically required to ensure that the foundation can support the tank's weight.

2. Inspection and Acceptance

Before installing the tank, a thorough inspection of the foundation is necessary to confirm that the elevation, flatness, and strength meet design specifications.

3. Material Requirements

The foundation materials must comply with regulations, using concrete and steel bars that ensure pressure resistance and crack prevention. The concrete strength should generally be no lower than C30 and meet additional requirements such as impermeability and freeze resistance.

Prefabrication and Assembly of the Tank

Large vertical storage tanks are typically manufactured using a modular prefabrication and assembly approach to enhance production efficiency and reduce costs. The specific steps include:

1. Material Inspection

All selected materials and components must be re-inspected, including the chemical composition and mechanical properties of the steel plates, to ensure compliance with design standards.

2. Edge Preparation

Steel plates need to be cleaned and prepared to create edges that guarantee welding quality. The edge design should match the welding process requirements, with common types including V-shaped and U-shaped edges.

3. Component Assembly

The prefabricated components include the bottom plate, wall plates, floating roof, internal floating roof, and fixed top plate. These components should be assembled in the factory or on-site, ensuring verticality, horizontality, and proper gaps during assembly.

Welding Process Requirements

The quality of the welding process significantly influences the structural stability and sealing of the tank. Key welding requirements include:

1. Welding Preparation

Ensure that the welding fixtures are securely fixed to prevent movement during the welding process. Before welding, any dirt, rust, or oil within a 20mm range of the edges must be cleaned. The welding machine and materials should be checked to confirm they are functioning correctly.

2. Welding Specifications

For initial welding, the length of the weld seam for ordinary carbon steel and low-alloy steel should be at least 50mm, while it should be no less than 80mm for low-alloy steel with a yield strength greater than 390MPa. Appropriate welding methods such as arc welding or gas shielded welding should be selected based on material characteristics.

3. Welding Process Control

During welding, control the speed and temperature to avoid excessive stress and deformation. Regular temperature checks on the weld seam should be conducted to prevent thermal cracking due to temperature changes. For multi-layer welding, ensure that interlayer joints are staggered to enhance weld strength.

Special Considerations for Welding Unique Materials

When welding special materials such as stainless steel, the following points are particularly important.

1. Surface Cleanliness

All water, oil, and dirt within a 20mm range of the edges must be completely removed. For stainless steel, specialized cleaning agents should be used to ensure no contaminants are present.

2. Welding Techniques

Use low energy input, short arcs, and multi-layer techniques to ensure good penetration and fusion. Consider employing argon shielding during welding to improve the corrosion resistance of the weld.

3. Protective Measures

During welding, protective coatings should be used to prevent environmental contamination. After welding, promptly perform acid cleaning and passivation to ensure corrosion resistance. Additionally, monitor smoke and gas produced during welding to ensure good ventilation.

Importance of Welding Sequence

The sequence of welding is critical for minimizing distortion and controlling stress. The following principles should be observed.

1. Symmetrical Welding

Use a symmetrical approach in the welding process to reduce stress and distortion. Specifically, alternate between opposing weld seams to balance deformation caused by welding shrinkage.

2. Segmented Welding

For corner welds between the tank bottom and wall, welding should be done from both inside and outside to ensure quality and structural strength. This method helps reduce localized stress concentration and improves the overall performance of the weld.

3. Shrinkage Joint Management

Welders should work in a single direction and use segmented or skip welding methods to control changes in shrinkage joints. Post-welding cooling is necessary to reduce residual stress in the weld.

Quality Control and Inspection

After welding, strict quality control and inspection are essential to ensure the weld quality and structural stability. Quality control measures include:

1. Non-Destructive Testing

Perform non-destructive tests such as penetrant testing and ultrasonic testing to identify defects within the weld. Testing should adhere to relevant standards to ensure accurate and reliable results.

2. Weld Inspection

Check the appearance, dimensions, and strength of the weld to confirm compliance with design specifications. Measuring tools should be used to assess the height, width, and gap of the weld.

3. Documentation and Feedback

Record all parameters during the welding process to create a complete welding archive for future reference and tracing. Issues discovered during welding should be addressed promptly and documented.

Environmental Impact and Safety Measures

Welding can impact both the environment and personnel, so appropriate safety measures should be implemented.

1. Protective Equipment

Operators should wear suitable protective gear, such as safety glasses, gloves, and protective clothing, to minimize injuries during welding.

2. Ventilation Management

Welding areas should be well-ventilated to prevent harmful gases and smoke accumulation. If necessary, exhaust systems should be installed to ensure air quality during the welding process.

3. Waste Disposal

Waste and emissions generated during welding must be handled in accordance with environmental regulations to prevent pollution. Companies should establish waste management protocols to ensure compliance in waste disposal.

Conclusion

The welding process for large vertical storage tanks is crucial for ensuring the safety, stability, and reliability of the tanks. Throughout the welding process, it is essential to adhere to various technical standards and requirements to ensure quality and structural strength. By employing scientifically sound welding practices and strict quality control, the overall quality of large vertical storage tanks can be enhanced, ensuring their safety and reliability during use. Additionally, good safety measures and environmental management lay a foundation for sustainable development in the welding industry.