Mechanical seals are used to prevent leakage between rotating and stationary parts in a wide range of applications. The mechanical seal in a pump prevents leakage of process fluid effectively by employing the following design principles.

1. Seal Face Contact: Mechanical seals rely on precise contact between two opposing seal faces i.e. stationary face and rotating face (they are considered as a primary sealing point between rotary and stationary parts), typically made from hard and wear-resistant materials like silicon carbide or tungsten carbide. These faces are carefully lapped to ensure a smooth, flat surface that promotes hydrodynamic lubrication and prevents leakage. Typically, mechanical seal faces are lapped to achieve a surface finish of Ra<0.2 µm or better. This level of smoothness ensures that the seal faces can maintain a tight seal while minimizing contact pressure and wear.
2. Hydrodynamic Lubrication: As the pump shaft rotates, a thin film of fluid is formed between the seal faces due to hydrodynamic lubrication. This fluid film prevents direct contact between the seal faces, reducing friction and wear, and maintaining a tight seal.
3. Spring Force: Mechanical seals employ springs or other mechanisms to maintain a constant force between the seal faces, ensuring proper contact and sealing even under varying pressure and thermal conditions. This spring force ensures that the seal faces remain in close contact, preventing leakage and maintaining sealing integrity.

Double mechanical seals are widely used in various industrial applications due to their superior sealing performance and enhanced reliability. Here are some of the primary reasons why double mechanical seals are preferred:

1. Zero Emission Compliance: A single mechanical seal permits the leakage of process fluids in the form of vapors, these vapors arise from the heat generated by friction at the primary seal face contact. But double mechanical seals offer a virtually leak-proof sealing solution, preventing the escape of hazardous or toxic substances into the environment. This is particularly crucial for applications involving volatile organic compounds (VOCs), hazardous chemicals, or other sensitive fluids.
2. Higher Reliability and Safety: Even if one seal fails, the process fluid will not leak into the atmosphere. Thus, double mechanical seals provide an extra layer of protection against leakage, ensuring the safe operation of equipment and preventing potential environmental contamination. This redundancy is essential in critical applications where even a minor leak could have severe consequences.
3. Extended Seal Life: The buffer/barrier fluid prevents the pumped fluid from reaching the outboard seal in case of a leak from the inboard seal, ensuring that the outboard seal remains clean and lubricated. Thus, double mechanical seals effectively manage the lubrication and cooling of the seal faces, extending their lifespan and reducing the frequency of seal replacements.
4. Versatility for Challenging Applications: Double mechanical seals can handle a wide range of fluids, including high-pressure, high-temperature, corrosive, and abrasive fluids. Their ability to withstand demanding conditions makes them suitable for various industrial applications.
5. Backup Seal in Case of Failure: In the event of an inboard seal failure, the outboard seal in a double mechanical seal configuration acts as a backup, preventing the leakage of the process fluid. This redundancy ensures continued operation and minimizes downtime in critical applications.
6. Alternative Seal for Unstable Lubrication: Double mechanical seals can provide an alternative sealing solution when the process fluid itself is not suitable for lubricating the seal faces. This is particularly useful for handling gaseous media, viscous fluids, non-settling slurries, or fluids prone to hardening.
7. Early Detection of Seal Failure: In some double mechanical seal designs, the buffer fluid can be monitored for leakage, providing an early indication of seal failure before it progresses to a catastrophic event. This early detection allows for timely maintenance and prevents potential damage to equipment.