Gene Vogel
Pump & Vibration Specialist
Electrical Apparatus Service Association
St. Louis, MO
In his paper presented at the EASA Convention 2017, Gene Vogel provides an in-depth overview of mechanical seals used in centrifugal pumps, highlighting their importance, design, and common issues. Mechanical seals are crucial for preventing liquid from escaping where the shaft enters the pump housing. They consist of two main components: one attached to the shaft and one to the pump housing, each with extremely smooth mating seal faces held together by mechanical and hydraulic pressure.
The flatness and smoothness of seal faces are critical for their functionality, typically measured in "helium light bands," which equates to around 0.00003 inches (0.00075 mm). This extreme flatness allows the seal faces to develop a hydrodynamic seal, with the liquid being pumped acting as a lubricant and coolant. Mechanical seals are not designed to run dry, as the liquid film between the faces reduces friction and wicks away heat.
Seal face material selection is vital, considering factors like heat capacity, abrasive resistance, corrosion resistance, mechanical strength, and cost. Common materials include carbon, ceramic, tungsten carbide, silicon carbide, chrome oxide, and Ni-resist (nickel-iron). Each material has specific properties that make it suitable for different applications, especially those involving abrasive or corrosive pumpage.
Mechanical seals also feature secondary seals, typically elastomeric, which seal the stationary element to the housing and the rotating element to the shaft. The choice of elastomer material depends on chemical compatibility with the pumpage and temperature tolerance.
Vogel discusses various seal designs, including pusher and non-pusher seals. Pusher seals use springs to apply pressure, requiring the secondary seal to move on the shaft. Non-pusher seals use a bellows or diaphragm, allowing the seal face to move independently of the secondary seal, preventing issues if the elastomer sticks to the shaft or if corrosion inhibits movement.
Dual seals are another innovation, providing a chamber between two seals filled with barrier fluid. This fluid cools and lubricates the seal faces, even when the pump is not operating with pumpage, and protects against corrosive or abrasive materials.
Cartridge seals simplify installation, reducing the risk of seal failure due to improper assembly by maintenance staff. Balanced seals control hydraulic force on the seal faces, using a recess cut on the front of the seal face to balance pressure.
Heat and vibration are major causes of seal failure. Vibration can result from pump conditions like bearing issues, shaft misalignment, unbalance, cavitation, or misalignment of seal components. Heat is generated by friction between seal faces, and the pumpage or barrier fluid must wick away this heat to prevent damage.
Vogel concludes by emphasizing the importance of understanding the specific application requirements and seeking expert advice for challenging seal applications.
Key Points Covered:
- Importance and function of mechanical seals in centrifugal pumps
- Critical factors for seal face material selection
- Design features of mechanical seals: pusher vs. non-pusher, dual seals, cartridge seals, balanced seals
- Causes of seal failure: heat and vibration
- Role of secondary seals and elastomer material selection
Key Takeaways:
- Mechanical seals prevent liquid from escaping the pump housing
- Seal face flatness and smoothness are crucial for functionality
- Material selection depends on application requirements
- Dual seals and cartridge seals offer enhanced performance and ease of installation
- Heat and vibration are common causes of seal failure
- Expert advice is essential for challenging seal applications
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