Flanges for Pressure Vessels: An Essential Component

 
Flanges are crucial components in pressure vessels, serving as vital connectors between various parts. The design and selection of flanges play a key role in the overall performance, sealing effectiveness, and safety of pressure vessels. Typically, a flange consists of a flange plate, gasket, bolts or studs, and nuts, all aimed at ensuring a tight connection between different pressurized elements to prevent leaks. As industrialization advances, the use of pressure vessels in industries like petrochemicals, power generation, pharmaceuticals, and food production has increased, making the design and selection of flanges particularly important.

Structure of Pressure Vessel Flanges

Flanges are significant connecting components within pressure vessels, primarily used to link different parts together while ensuring sealing and structural stability. The main structural components include the following.

1. Flange Plate

The flange plate is the primary part that connects and secures various components. Its surface has a specific structure and dimensions to maintain a stable connection under high temperature and pressure conditions. Flange plates are designed according to international standards like ANSI and ASME to meet different operational requirements.

2. Gasket

 
The gasket is positioned between the flange plates to fill the gaps and ensure a seal. The material and design of the gasket directly influence sealing performance. Common materials include rubber, metal, and composite materials, which must have high-temperature resistance, corrosion resistance, and good elasticity. The thickness and compression ratio of the gasket are also crucial factors affecting sealing performance.

3. Bolts and Nuts

Bolts and nuts are used to secure the flange plates together, applying pre-tension to ensure the sealing surfaces are pressed tightly. The material, size, and pre-tension of the bolts significantly impact the sealing effectiveness. Appropriate bolt materials, such as stainless steel or alloy steel, should be selected based on operational conditions to enhance corrosion resistance and mechanical strength.

Sealing Types of Pressure Vessel Flanges

The sealing design of pressure vessel flanges is critical, directly affecting their sealing performance and safety. The sealing types can be categorized mainly as follows.

1. Forced Sealing

Forced sealing relies on fasteners (like bolts) to compress the flanges, achieving a seal. This sealing method is typically used for flat gaskets and spiral wound gaskets, depending on external pressure to maintain the seal. It is suitable for conditions with minimal temperature and pressure fluctuations. While forced sealing is reliable, it may lead to gasket fatigue in high-temperature and high-pressure environments.

2. Self-tightening Sealing

Self-tightening sealing uses the internal pressure of the container to create compressive force on the sealing surfaces. As the pressure of the medium increases, so does the sealing force, ensuring reliable sealing in high-pressure conditions. Common forms of self-tightening sealing include combined seals, O-rings, C-rings, B-rings, and wedge seals. This design is simple, easy to maintain, and ideal for high-temperature and high-pressure applications.

3. Semi-self-tightening Sealing

Semi-self-tightening sealing combines the advantages of forced and self-tightening seals, utilizing both the internal medium pressure and fasteners to generate compressive force on the sealing surfaces. The double-cone seal is a representative example, suitable for various applications, effectively resisting the impact of medium pressure. This sealing method maintains good sealing performance even with significant load fluctuations.

Working Principle of Pressure Vessel Flanges

The sealing principle of pressure vessel flanges is based on the pre-tension applied to the bolts. When pre-tension is applied to the flange bolts, they compress the gasket, forcing it to deform and fill the small imperfections between the flange sealing surfaces and the gasket. This process creates initial sealing conditions to prevent fluid leakage. The compressive force on the effective sealing area of the gasket at this point is known as the initial sealing compressive pressure.

When the equipment or pipeline is subjected to medium pressure, the bolts experience tensile stress and elongate, causing the flange sealing surfaces to move apart. This reduces the compressive force between the sealing surfaces and the gasket, leading to a decrease in gasket compression and a decline in the pre-tension sealing compressive pressure. If the gasket has sufficient elasticity, its deformation can compensate for the separation between the bolts and sealing surfaces, maintaining the pre-tension sealing compressive pressure above the working sealing compressive pressure, thus ensuring a good sealing state.

Key Considerations in Design and Selection

When designing and selecting pressure vessel flanges, consider the following aspects.

1. Sealing Performance

Overall sealing performance is the most important factor in flange design. It is essential to select suitable gasket materials and sealing forms to ensure effective sealing in high-temperature and high-pressure environments.

2. Material Selection

The flange material should have good corrosion resistance and mechanical strength to handle the chemical properties and working conditions of the medium. Common materials include stainless steel, carbon steel, and alloys, and material selection should be evaluated based on specific operational conditions.

3. Control of Pre-tension

Proper pre-tension not only helps achieve effective sealing but also prevents flange failure due to excessive or insufficient pre-tension. Pre-tension should be calculated and controlled based on specific conditions to ensure flange stability during operation.

4. Standardization and Regulation

Flange design and manufacturing should comply with relevant national standards and industry norms (like ASME and ISO) to ensure safety and reliability. Regular inspection and maintenance are also crucial for ensuring flange performance.

Conclusion

As a vital component of pressure vessels, the design, selection, and installation of flanges are critical for overall safety and stability. By carefully choosing sealing forms, materials, and pre-tension, and by following relevant standards, the sealing performance of flanges can be significantly improved, ensuring the safe operation of pressure vessels in high-temperature and high-pressure environments. A thorough understanding and correct application of flanges help reduce leakage risks, protect equipment and personnel, and enhance the reliability and efficiency of the entire system.