Curious about the components that make up a mechanical seal?
In this blog post, we’ll break down the key parts of a mechanical seal and explain their functions.
By the end, you’ll have a clear understanding of how these critical sealing devices work.
Primary Sealing Components
The primary sealing components of a mechanical seal are the seal faces, also known as seal rings. These precision-machined components create the main sealing interface that prevents fluid leakage between the rotating shaft and stationary housing.
The seal faces consist of two main parts:
- Stationary Face (Primary Ring): This component is fixed to the stationary housing of the equipment, such as a pump casing. The stationary face remains in a fixed position and does not rotate with the shaft.
- Rotating Face (Mating Ring): This component is attached to and rotates with the shaft. The rotating face is spring-loaded to maintain contact with the stationary face, accommodating any small shaft movements or misalignments.
Seal Face Arrangements
- Balanced seals: These have a smaller area exposed to the sealed fluid pressure, reducing the closing force and resulting in lower heat generation and wear.
- Unbalanced seals: These have a larger area exposed to the sealed fluid pressure, creating a higher closing force. They are simpler in design but may have higher wear rates.
- Tandem seals: These feature two sets of seal faces arranged in series, providing a backup seal in case the primary seal fails.
- Double seals: These consist of two sets of seal faces with a barrier fluid between them, isolating the process fluid from the atmosphere.
Fluid Film
A thin fluid film, usually from the process fluid itself or an external source, is maintained between the faces to reduce friction, heat generation, and wear. The fluid film also helps to carry away any heat generated due to the contact between the rotating and stationary faces.
The seal faces are lapped to a very high flatness, often within a few light bands, to minimize leakage. The small gap between the faces, typically around 1 micron, allows a controlled amount of fluid to pass through for lubrication while keeping the leakage to a minimum which is 10 drops per hour.
Secondary Sealing Components
O-Rings
O-rings are the most common type of secondary seal used in mechanical seals. The O-ring sits in a groove and gets compressed between two surfaces, forming a tight seal that prevents leakage.
O-rings are used in both static and dynamic applications within the mechanical seal. For static sealing between stationary components, the O-ring is compressed to zero clearance so no fluid can pass through. In dynamic sealing, the O-ring allows some movement while maintaining a seal.
Wedges and V-Rings
Wedges and V-rings are specially shaped elastomeric seals used in some mechanical seal designs as alternatives to O-rings. As their names imply, wedges have a triangular cross-section while V-rings look like a “V”.
These custom profiles allow wedges and V-rings to seal more effectively in certain situations compared to O-rings. The angled surfaces create a tighter seal when compressed. Wedges are often used for unidirectional sealing, where they are energized by the sealed pressure itself. V-rings can provide bidirectional sealing.
Gaskets
Gaskets are the simplest types of secondary seals. They are usually just flat pieces of a sealing material compressed between two stationary surfaces.
Gaskets are die-cut from sheet materials and used for larger sealing areas. Gaskets are common for sealing the outer joints of a mechanical seal assembly.
Mechanical Loading and Drive Components
Springs
Springs are commonly used in mechanical seals to provide the closing force that keeps the seal faces in contact. There are a few main types:
- Single coil springs: A simple design using one large coil spring to load the seal. While economical, single springs can be prone to clogging and don’t provide even loading.
- Multiple coil springs: Several smaller springs are used instead of one large spring. This allows for more uniform loading on the seal faces and reduces clogging. The springs can also be made from materials resistant to corrosion and chemicals.
- Wave springs: A single wave-shaped spring that takes up less axial space than coil springs. Wave springs provide a more even load but require special manufacturing.
Bellows
Bellows are another option for loading mechanical seals. Metal bellows are welded assemblies that act like a spring, but also serve as the secondary seal between the rotating and stationary components.
Bellows provide excellent response to shaft movement and eliminate dynamic O-rings that can hang up and cause damage. They are ideal for high temperature, high pressure and chemically harsh environments.
Drive Mechanisms
In addition to springs or bellows providing the closing force, mechanical seals need a way to drive the rotation of the seal components. Common drive mechanisms include:
- Elastomer drive: Uses a flexible rubber part to transmit rotation. Simple and economical but limited to lower speeds and temperatures.
- Positive drive: Pins, lugs or keys lock the seal components together for rotation. Allows higher speeds and temperatures than elastomeric drives.
- Spring drive: Drive pins or lugs are incorporated into the springs themselves. Eliminates separate drive components but requires careful design.
Metal Hardware and Structural Components
Gland and Seal Chamber
The gland is the metal component that holds the stationary parts of the mechanical seal in place against the seal chamber or pump housing. The gland has ports to allow for circulation of a barrier fluid or to connect a flush plan to the seal chamber.
The seal chamber is the cavity in the pump housing or a separate component where the mechanical seal is installed. It contains the process fluid on one side of the seal faces. The seal chamber must be designed to promote proper circulation of the process fluid at the seal faces to carry away heat and any solids.
Sleeve and Shaft
The sleeve is a cylindrical component that fits over the pump shaft in the area where the mechanical seal is installed. Its purpose is to protect the shaft from wear and corrosion by the process fluid. It has a precision-machined surface to provide a smooth running surface for the rotating seal face.
In some designs, the mechanical seal rotary face is mounted directly on the pump shaft rather than on a sleeve. However, sleeves are more commonly used, as they allow for easier seal maintenance without needing to remove the shaft.
Collars, Flanges, and Lock Rings
Collars, flanges, and lock rings are used to axially position and secure the rotating parts of the mechanical seal on the shaft or sleeve.
A collar is a cylindrical component that slides over the sleeve or shaft and is locked in place, usually by set screws. It provides a surface for the rotary face assembly to be mounted and also transmits torque to drive the rotary face.
Flanges are ring-shaped components that perform a similar function to collars in positioning and driving the rotary seal components. They are typically sealed to the shaft or sleeve with O-rings.
Lock rings are used to axially position components like the seal gland on the shaft. They fit into machined grooves on the shaft or sleeve and lock in place to prevent axial movement.
Seal Cartridges and Pre-assembled Units
Many mechanical seals today come as pre-assembled cartridges rather than as individual components. A seal cartridge contains all the seal parts, including the gland, sleeve, faces, secondary seals, and hardware pre-assembled and set at the factory.
Cartridge seals allow for easier and more precise installation, as the critical seal dimensions and face loading are preset. The complete unit can be installed onto the shaft as a single component.
Some mechanical seals are also available as complete pre-assembled units with the pump housing, known as seal chamber assemblies or SCAs. These drop-in units contain the mechanical seal and seal chamber with all piping and accessories pre-plumbed.
Seal Part Materials and Selection Criteria
Face Materials and Tribological Properties
The seal faces are the primary sealing surfaces that prevent leakage. They are typically made of hard, wear-resistant materials. The two main types of face material combinations are:
- Hard-Hard Face Combinations : In this setup, both the stationary and rotating faces are made of hard materials like silicon carbide, tungsten carbide, or ceramic. Hard-hard combinations provide excellent wear resistance and heat dissipation. They are suitable for high-speed, high-pressure, and high-temperature applications.
- Hard-Soft Face Combinations: Here, one face is made of a hard material while the other is a softer material like carbon graphite. The soft face conforms to the hard face, creating a tight seal. Hard-soft combinations have lower heat generation and are better for low-pressure and low-speed applications.
The faces must have a low coefficient of friction and high wear resistance to minimize heat generation and extend seal life.
The PV (Pressure-Velocity) limit is a key parameter that indicates the maximum combination of pressure and velocity the face materials can withstand without excessive wear or heat generation.
Secondary Seal Elastomers and Compatibility
Elastomers like nitrile rubber (NBR), fluoroelastomer (FKM), and ethylene propylene diene monomer (EPDM) are common choices for secondary seals.
The selection of elastomer depends on its compatibility with the process fluid, temperature range, and chemical resistance.
For example, EPDM is suitable for water and mild chemicals, while FKM is better for high-temperature and aggressive chemical applications.
Metal Component Materials and Corrosion Resistance
The metal components are typically made of stainless steel, brass, or other corrosion-resistant alloys.
The choice of metal depends on the corrosiveness of the process fluid and the operating environment. For example, 316 stainless steel is a common choice for general-purpose applications, while Hastelloy or titanium may be necessary for highly corrosive chemicals.
Temperature, Pressure, Speed, and Media Considerations
High temperatures can degrade elastomers and cause thermal expansion of metal parts.
High pressures can cause deformation and leakage if the materials are not sufficiently strong.
The speed of the rotating equipment affects the PV limit and heat generation of the seal faces.
The chemical composition, viscosity, and abrasiveness of the media also influence material compatibility and wear resistance.
What Are the Points of a Mechanical Seal
A mechanical seal has three main sealing points: between the stationary and rotating seal faces, between the stationary seal component and the pump housing, and between the rotating seal component and the shaft.
Can Tantalum Spring Be Used in a Mechanical Seal
Yes, tantalum spring can be used in a mechanical seal. Tantalum is a strong, corrosion-resistant metal that maintains its elasticity at high temperatures, making it suitable for use as a spring material in mechanical seals.
Does a Stationary or Rotary Part Remain in Contact with Pump Fluid in a Mechanical Seal
In a mechanical seal, the stationary part remains in constant contact with the pump fluid. The rotary part, on the other hand, is connected to the rotating shaft and does not directly touch the fluid.
Why Is a Mechanical Seal Used in a Pump at the Impeller Side
A mechanical seal is installed on the impeller side of a pump to prevent fluid leakage along the rotating shaft.
The seal maintains the pressure boundary between the pump interior and the atmosphere, ensuring efficient operation and preventing contamination.
Conclusion
In summary, understanding the parts of a mechanical seal is crucial for ensuring the integrity and efficiency of a sealing system in various applications.
Explore more about how these components work together to prevent leakage and maintain system pressure by visiting our detailed guide on mechanical seals.
