What Are Metal Bellow Mechanical Seals

What Are Metal Bellow Mechanical Seals

Metal bellow mechanical seals are specialized sealing devices designed to prevent leakage of fluids or gases in rotating equipment, such as pumps, compressors, and mixers. These seals consist of a metal bellows, typically made from high-quality stainless steel, attached to a stationary and a rotating face. The metal bellows provides a flexible, pressure-responsive seal that can accommodate axial and angular misalignment while maintaining a stable sealing interface.

The primary components of a metal bellow mechanical seal include the metal bellows, the stationary face (usually made of a hard material like silicon carbide or tungsten carbide), the rotating face (often made of carbon or a similar material), and the secondary sealing elements, such as O-rings or gaskets. The metal bellows acts as a spring, providing the necessary closing force to keep the sealing faces in contact and maintain a leak-tight seal.

How Metal Bellow Mechanical Seals Work

Metal bellow mechanical seals operate on the principle of maintaining a thin fluid film between the stationary and rotating faces. The metal bellows, which is welded to the stationary face, flexes to accommodate axial and angular movement while maintaining a constant closing force on the sealing faces. This closing force is generated by the pressure of the sealed fluid acting on the effective area of the bellows and the spring force of the bellows itself.

As the rotating equipment operates, the rotating face moves with the shaft while the stationary face remains fixed. The fluid film between the faces lubricates and cools the sealing interface, minimizing wear and preventing direct contact between the faces. The metal bellows continuously adjusts to maintain the optimal sealing face contact, even under varying operating conditions, such as changes in pressure, temperature, or shaft position.

Design of Metal Bellow Mechanical Seals

1. Bellows material: The metal bellows is typically made from high-quality stainless steel, such as 316L or Hastelloy C, to provide excellent corrosion resistance, flexibility, and durability.
2. Sealing face materials: The stationary and rotating faces are selected based on their compatibility with the sealed fluid and their ability to withstand the operating conditions. Common face material combinations include silicon carbide against carbon, tungsten carbide against carbon, and silicon carbide against silicon carbide.
3. Bellows configuration: The number of convolutions, wall thickness, and overall length of the metal bellows are designed to provide the required spring force and flexibility for the specific application.
4. Secondary sealing elements: O-rings, gaskets, and other secondary sealing elements are selected based on their compatibility with the sealed fluid and their ability to provide reliable static sealing between the mechanical seal components and the equipment.

Advantages of Metal Bellow Mechanical Seals

• Excellent sealing performance: The pressure-responsive design of the metal bellows ensures that the sealing faces maintain optimal contact, resulting in a highly efficient, leak-tight seal.
• Durability and reliability: The all-metal construction of the bellows provides superior durability and resistance to fatigue, corrosion, and high temperatures compared to elastomeric bellows or other sealing arrangements.
• Low maintenance: Metal bellow mechanical seals require minimal maintenance due to their robust design and the absence of wear-prone components like springs or dynamic O-rings.
• Compact design: The integrated bellows and sealing faces allow for a compact, space-saving design that can be easily installed in a variety of equipment configurations.
• Wide range of applications: Metal bellow mechanical seals can be used in a diverse range of industries and applications, including high-pressure, high-temperature, and chemically aggressive environments.

What Are Metal Bellow Mechanical Seals Used For

• Pumps: Metal bellow seals are commonly used in centrifugal pumps, positive displacement pumps, and cryogenic pumps to prevent leakage of liquids or gases.
• Compressors: In compressor applications, metal bellow seals are used to seal high-pressure gases and prevent contamination of the compressed media.
• Mixers and agitators: Metal bellow seals are employed in mixing and agitation equipment to contain process fluids and prevent shaft leakage.
• Refineries and petrochemical plants: In the oil and gas industry, metal bellow seals are used in pumps, compressors, and other rotating equipment to handle hydrocarbons, solvents, and other aggressive fluids.
• Chemical processing: Metal bellow seals are utilized in chemical manufacturing plants to seal pumps and other equipment handling corrosive or toxic chemicals.
• Power generation: In power plants, metal bellow seals are used in boiler feed pumps, circulating water pumps, and other critical equipment to ensure reliable operation and prevent leakage.
• Pharmaceutical and food processing: Metal bellow seals are employed in hygienic and aseptic applications to maintain product purity and prevent contamination.

Factors to Consider When Choosing Metal Bellow Mechanical Seals

Plate Shape

The shape of the metal plates used in the bellows can significantly impact the seal’s flexibility and durability. Flat plates offer simplicity but may be more prone to fatigue, while formed plates can provide enhanced flexibility and stress distribution.

Angle

The angle at which the metal plates are welded together affects the bellow’s ability to compress and expand. A larger angle allows for greater compression but may reduce the seal’s stability. Conversely, a smaller angle provides increased stability but limits the compression range.

Weld Integrity

Poor weld quality can lead to leaks, corrosion, and premature failure. Ensure that the metal bellow mechanical seals you choose have high-quality, consistent welds that can withstand the demands of your application.

Plate Thickness (Thin Plates)

Thin metal plates offer the advantage of increased flexibility, allowing the bellows to compress and expand more easily. However, thinner plates may be more susceptible to damage and wear.

Controlling Vibrations

Vibrations can cause significant stress on metal bellow mechanical seals, leading to fatigue and failure. Implementing vibration control measures, such as using dampening materials or designing the seal with vibration-resistant features, can help prolong the seal’s life and maintain its performance in high-vibration environments.

Double-ply Bellows

For applications requiring enhanced strength and durability, double-ply bellows can be an excellent choice. These bellows feature two layers of metal plates, providing increased resistance to pressure and temperature fluctuations. Double-ply bellows are particularly well-suited for high-pressure or high-temperature applications where single-ply bellows may not suffice.

Pressure and Seal Balance

Achieving the proper balance between pressure and seal performance is critical for the success of metal bellow mechanical seals. Over-pressurization can cause the bellows to deform or fail, while under-pressurization may result in leaks.