What are the three types of mechanical seals?

Mechanical seals are used in rotary pumps, mixers, and other types of rotating equipment to prevent leakage of fluids. These seals come in different types and designs, each suitable for specific applications and operating conditions. In this article, we'll discuss the three most common types of mechanical seals and their features, advantages, and disadvantages.

1. Conventional Mechanical Seals

The conventional mechanical seal is the most basic and prevalent type of seal used in various industrial applications. It consists of two parts, a stationary seal face and a rotating seal face. The stationary seal face is mounted on the pump housing or gland, while the rotating seal face is attached to the pump shaft.

The two faces form a perfect seal when pressed together, creating a barrier that stops fluid from leaking out of the pump. The sealing faces are typically made of hard materials such as ceramic, tungsten carbide, or silicon carbide, which can withstand high pressures, temperatures, and corrosive chemicals.

The conventional mechanical seal relies on the friction between the two faces to create the seal. It requires a lubricating fluid, such as water or oil, to reduce the friction and dissipate the heat generated during operation. The fluid also helps to remove any debris or particulates that may be present on the sealing faces, preventing them from compromising the seal.

Advantages:

- Simple and easy to install

- Can handle a wide range of pressures and temperatures

- Compatible with a variety of fluids

- Economical in terms of cost and maintenance

Disadvantages:

- Requires a lubricating fluid, which can be costly and may contaminate the environment

- Susceptible to wear and abrasion, especially if the sealing faces are of different hardness or roughness

- May fail under extreme conditions, such as high temperatures, corrosive fluids, or excessive vibration

2. Cartridge Mechanical Seals

The cartridge mechanical seal is a more advanced type of seal that offers several benefits over the conventional seal. It consists of a pre-assembled unit that contains all the necessary components, including the sealing faces, springs, O-rings, and gland. The cartridge is then inserted into the pump housing or gland, making installation and replacement easier and quicker.

The cartridge design also eliminates the need for measuring and adjusting the seal components, ensuring proper alignment and seal face contact. It also reduces the risk of contaminants or debris entering the seal during assembly, improving reliability and performance.

The sealing faces in a cartridge seal can be arranged in different configurations, such as single, double, tandem, or back-to-back. Each configuration has its own advantages and limitations, depending on the application and operating conditions.

Advantages:

- Easy and quick to install and replace

- Minimizes the risk of contamination and misalignment

- Improves reliability and performance

- Offers a wide range of configurations to fit various applications

Disadvantages:

- May be more expensive than a conventional seal

- Requires cartridge-specific spares and tools for maintenance

- May not be compatible with some pump designs or sizes

3. Metal Bellows Seals

The metal bellows seal is a specialized type of seal that can accommodate axial and radial shaft movements. It consists of a thin-walled metallic bellows that flexes and expands axially or radially. The bellows is attached to the stationary seal face, while the rotating seal face is mounted on the pump shaft.

The bellows can compensate for thermal expansion, shaft misalignment, or axial movement, without affecting the sealing integrity. It also eliminates the need for O-rings or elastomers, which may degrade over time or fail under extreme conditions.

The sealing faces in a metal bellows seal can be made of the same or different materials, depending on the process fluid characteristics and temperature ranges. The bellows can also be designed for single, double, or tandem arrangements, depending on the level of containment required.

Advantages:

- Can handle axial and radial shaft movements

- Provides a reliable and long-lasting seal

- Resistant to corrosive and high-temperature fluids

- Minimizes the risk of leakage, even under extreme conditions

Disadvantages:

- May be more expensive than a conventional seal

- Requires precision manufacturing and assembly

- May be susceptible to fatigue or failure, depending on the bellows material and design

Conclusion

Mechanical seals play a crucial role in maintaining the performance and safety of rotary equipment. Choosing the right type of seal depends on the specific application and operating conditions. Conventional seals are suitable for simple and basic applications, while cartridge and metal bellows seals offer improved reliability and flexibility. Regardless of the seal type, proper installation, maintenance, and monitoring are essential to ensure optimal performance and safety of the equipment.