What Is Mechanical Seal
A mechanical seal is a device used to seal the rotating shaft of a pump, mixer, or other rotating equipment to prevent leakage of process fluid. It consists of a stationary part (gland) and a rotating part (seal ring) that are held together by spring force and fluid pressure to create a seal between the two faces.
Mechanical seals are widely used in various industries, including chemical processing, oil and gas, pharmaceuticals, and water treatment.
How Do Mechanical Seals Work
The basic principle of a mechanical seal is to create a barrier between the process fluid and the atmosphere.
The seal faces are perpendicular to the shaft and are held together by a combination of spring force and fluid pressure. The rotating seal ring is attached to the shaft, while the stationary seal ring is fixed to the pump housing.
As the shaft rotates, a thin fluid film is created between the seal faces, which prevents direct contact and minimizes wear. The fluid film also helps to cool and lubricate the seal faces, ensuring reliable operation and extended seal life.
Design of Mechanical Seal
Parts of Mechanical Seal
A typical mechanical seal consists of several key components:
- Stationary part (gland): Houses the stationary seal ring and provides a connection to the pump housing.
- Rotating part (seal ring): Attached to the rotating shaft and creates the primary seal against the stationary seal ring.
- Secondary seals: O-rings, gaskets, or bellows that prevent leakage between the stationary and rotating components.
- Springs: Provide axial force to maintain contact between the seal faces.
- Drive mechanism: Transmits rotational force from the shaft to the rotating seal ring.
Sealing Points of Mechanical Seal
Mechanical seals have two main sealing points:
- The interface between the rotating and stationary seal faces.
- The secondary seals between the rotating components and the shaft.
- The secondary seals between the stationary components and the equipment housing.
The Fluid Film of Mechanical Seal
The fluid film is a thin layer of liquid that separates the stationary and rotating seal faces. It is maintained by a balance of hydraulic pressure, spring force, and centrifugal force. The fluid film helps to lubricate the seal faces, reduce wear, and dissipate heat generated by friction.mechanical seal.
Advantages of Mechanical Seal
Mechanical seals offer several advantages over other sealing methods, such as gland packing:
- Reduced leakage: Mechanical seals provide a more effective seal, minimizing fluid loss and environmental impact.
- Longer service life: With proper maintenance, mechanical seals can last longer than gland packing, reducing downtime and maintenance costs.
- Improved efficiency: By reducing leakage and friction, mechanical seals contribute to improved system efficiency.
- Enhanced safety: Mechanical seals help contain hazardous substances, protecting personnel and the environment.
Types of Mechanical Seals
Balanced and Unbalanced Seals
Mechanical seals can be classified as balanced or unbalanced, based on how the hydraulic forces are managed:
- Balanced seals: Designed to minimize the impact of hydraulic forces on the seal faces, allowing for more consistent performance and longer service life.
- Unbalanced seals: Simpler in design but more susceptible to the effects of hydraulic forces, which can lead to increased wear and shorter service life.
Pusher and Non-pusher Seals
Pusher and non-pusher seals differ in how the rotating seal ring is attached to the shaft:
- Pusher seals: The rotating seal ring is mounted on the shaft using springs and secondary seals, allowing for axial movement to compensate for wear and shaft deflection.
- Non-pusher seals: The rotating seal ring is fixed to the shaft, providing a more compact design but with less flexibility to accommodate wear and shaft movement.
Cartridge Seals
Cartridge seals are pre-assembled units that combine the stationary and rotating components, springs, and secondary seals into a single, easy-to-install package.
What Is the Difference Between Gland Packing and Mechanical Seal
Gland packing is a type of seal used to prevent leaks around rotating shafts. It is made of soft, pliable material compressed around the shaft. The packing is held in place by a gland follower and compressed by adjusting nuts. Gland packing is less expensive than mechanical seals. It handles shaft misalignment better. However, it has higher friction and requires more maintenance. Mechanical seals use flat, pre-shaped components instead of pliable packing. They have lower friction and leakage. But they are more expensive and less tolerant of misalignment compared to gland packing.
What Is the Difference Between Oil Seal and Mechanical Seal
An oil seal is a type of seal that prevents oil leakage. It is made of rubber or elastomer. Oil seals are used in rotating shafts. They are less complex than mechanical seals. Mechanical seals are used for more demanding applications. They have multiple components and require lubrication. Mechanical seals can handle higher pressures and temperatures than oil seals. Oil seals are cheaper and easier to replace. They are suitable for less critical applications.
FAQs
What is the gap between the faces of a mechanical seal
The gap between the faces of a mechanical seal is typically very small, on the order of 1 microns (millionths of a meter). This tiny gap allows a thin film of fluid to flow between the seal faces for lubrication.
How many mechanical seals does a pump have
Most pumps have one mechanical seal. Some pumps may have two seals for added protection against leaks. Dual seals are often used in harsh environments or with hazardous fluids. Single seals are sufficient for most standard pumping applications.