Pressure Vessel Shell Design and Construction

The shell of a pressure vessel is one of its key components, designed to provide the necessary space for pressure containment. The inner diameter and volume of the shell are typically determined through engineering calculations to ensure they meet specific process requirements. In practice, cylindrical and spherical shells are the most common structures of pressure vessel shells, each with unique properties and suitable applications.

Basic Functions of the Shell

The primary function of the shell is to withstand the pressure generated by the internal medium while also resisting external environmental impacts. Several key factors must be considered during the design and manufacturing processes.

1. Pressure Resistance

The shell must be designed to withstand changes in internal and external pressure. Using appropriate wall thickness and materials can enhance its pressure-bearing capability.

2. Corrosion Resistance

In certain processes, the shell material must have good corrosion resistance to prolong its service life. Stainless steel, alloy steel, or corrosion-resistant coatings are often used in corrosive environments.

3. Weldability

The welding characteristics of different materials must be carefully considered to ensure the quality and strength of the welds. High-quality welding techniques can significantly improve the overall strength and seal integrity of the shell.

Types of Shell Structures

The structure of a pressure vessel's shell plays a crucial role in its pressure performance and applicability. Based on the shape, size, and manufacturing process, shells can be categorized into several types, each with unique advantages suitable for different industries.

1. Cylindrical Shells

Cylindrical shells are the most common shape used in pressure vessels. The manufacturing method varies based on the shell's diameter:

Diameter Less Than 500 mm: Seamless steel pipes are used, which have no longitudinal weld seams. This method is simple and cost-effective.
Diameter Greater Than 500 mm: Typically made from steel plates using methods such as:
Rolled Cylinders: Steel plates are rolled into cylindrical shapes, suitable for most medium-sized pressure vessels.
Welded Half-Cylinders: Steel plates are first formed into two half-cylinders using a hydraulic press and then welded together with longitudinal seams. The number of seams usually increases with the shell's diameter.

2. Single-Layer Shells

Single-layer shells consist of a single material throughout the wall. The main manufacturing methods include:
Single-Layer Rolled and Welded: This is the most widely used shell design due to its cost-effectiveness and process flexibility, common in the oil and chemical industries.
Integral Forged: Suitable for high-pressure vessels, this type features a dense microstructure and high strength, ideal for welding difficult high-strength steels. It is typically used for small pressure vessels with diameters between 300 mm and 800 mm, such as polyethylene reactors.
Forged-Welded: Primarily used for high-strength applications that require specific processing needs.

3. Composite Shells

Composite shells are made from two or more layers of discontinuous materials. Their structure and manufacturing methods include:
Multi-Layer: This type increases strength and pressure resistance through layering, making it suitable for high-pressure and temperature applications.
Wound: Using the properties of wound materials enhances overall stiffness and load-bearing capacity. This design is common in composite pressure vessels, ideal for aerospace, automotive, and high-performance industrial applications.

Connection Methods

For short pressure vessels, heads can be directly connected to both ends of the cylinder, forming a sealed pressure space. However, for long shells, steel plates are typically rolled and welded into multiple shell sections (known as shell segments), which are then welded together to achieve the desired length.
Circumferential Welds: The welds between shell segments and between the shell and end heads are oriented perpendicular to the shell's axis. These circumferential welds significantly affect the overall strength and sealing of the shell.

Design and Manufacturing Considerations

When designing and manufacturing pressure vessel shells, several factors must be considered.

Material Selection: Choose suitable materials based on operating conditions to ensure stability under high pressure and corrosive environments. Common materials include carbon steel, stainless steel, alloy steel, and composites.
Manufacturing Process: Select manufacturing methods wisely to enhance production efficiency and reduce costs. Different processes will impact the shell's performance and should be chosen based on actual needs.
Safety Standards: Adhere to relevant industry standards and regulations to ensure the safety and reliability of pressure vessels. Preventive measures should be incorporated during the design phase to mitigate potential safety risks.

Maintenance and Inspection

Regular maintenance and inspection are essential for ensuring the long-term safe operation of pressure vessels. Key activities include:

Regular Checks: Inspect welds, connections, and corrosion protection layers periodically to identify and address potential issues promptly.
Pressure Testing: Conduct pressure tests regularly to verify the shell's pressure capacity and sealing performance.
Corrosion Monitoring: Monitor corrosive factors in the operating environment and take protective measures as needed to extend the shell's service life.

Conclusion

The design and construction of a pressure vessel's shell directly impact its performance and applications. By carefully selecting the shell structure and welding methods, it is possible to effectively meet various process pressure requirements, ensuring the safety and reliability of pressure vessels during operation. As materials science and manufacturing technologies continue to advance, the diversity of pressure vessel shell structures will increase, adapting to more complex industrial needs. Staying informed about new technologies will lead to higher safety standards and efficiency in the pressure vessel industry.