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By Sheila Kennedy
Seal OEMs are breaking new ground in their once-staid manufacturing practices. Historically, mechanical seals were designed with very flat, parallel faces, and nearly a century passed before this basic principle saw any notable change. During the past few decades, this tried and true sealing concept began evolving.
New and emerging manufacturing technologies provide greater flexibility in seal design, the goal being greater reliability, even in the most demanding applications.
Design considerations: Pressing two highly polished, optically flat sealing surfaces together forms the seal that minimizes process fluid leakage from between the rotating shaft and stationary casing. But things can go wrong and a mechanical seal can leak for a number of reasons. The seal face might chip or become pitted from improper wear or poor lubricants. It might warp under off-design conditions involving pressure or heat. Additionally, the state of the process fluid could change and clog or interfere with seal reliability.
Because downtime and repairs make leak-related maintenance so costly, seal manufacturers began experimenting with seal face geometry to minimize problems and improve performance.
New faces: Among the most common seal face modifications are hydropad grooves, a geometry that improves film formation between seal faces. This reduces heat generation and increases seal life. Microfeature face modifications such as groove technologies and wavy faces sculpt the seal surface to improve performance and extend the operating window. Laser machining technologies take the micromachining of seal topography to a new dimension. Features now can be contoured to sub-micron dimensions.
“Seal OEMs see laser technology as the engineer’s playground,” says Michael Huebner, a staff engineer at Flowserve Corp. “Features can now be designed that have the required accuracy with almost no limitation in the feature configuration. A surface can be altered to create areas that serve different operating functions, such as creating lift or surface tension, raising pressure or moving fluid. This was not possible even a few years ago.”
A vertical pump with an oil seal located at the bottom of the gearbox is an application that has proved challenging for years. Conditions such as oil viscosity and high speed aren’t conducive to extended flat seal face reliability. Flowserve reconciled this problem using laser micromachining to produce surface tension grooves and hydrodynamic lift. The surface tension bands form a hermetic seal. The faces are separated with a wavy face hydrodynamic lift that operates on atmospheric air. The result is a noncontacting seal that doesn’t leak, wear or generate heat.
Emerging innovations: “The future of mechanical seals will be very dynamic,” says Huebner. “The opportunity exists to move to smarter seals with a greater operating range and higher reliability. University research has created seal faces with built-in heat exchangers to efficiently dissipate seal heat. Nano-featured surface textures have been developed that show lowered resistance to fluid flow and may find their way into seal faces. Seals with passive and active control mechanisms controlled by nano-scale features are being considered.”
Compensating now: Until seal technology is perfected to a flawless state, the best you can do is mitigate your risks. Innovative system diagnostics and failure analysis tools are good for problem correction as well as incident prevention. In the event one of your seals leaks or fails mechanically, focus on treating the cause, not the symptoms. A novel approach to root cause analysis is the Cause Mapping technique ThinkReliability developed. Rather than developing a proprietary analysis tool, the company leverages software that already resides on virtually every business computer today —— Microsoft Excel.
Using a workshop setting, ThinkReliability teaches organizations how to use Excel to produce easily understood failure modes and effects analysis (FMEA) diagrams. This skill improves communication because the Cause Mapping results can be shared with others in the organization without the usual software and license constraints. The Cause Mapping approach empowers companies in any industry to diagnose seal system leaks, safety incidents, equipment failures and a host of other problems, then use this knowledge to prevent future failures.
E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at Sheila@addcomm.com.
PlantServices.com is an MRO (maintain, repair, replace, retrofit, overhaul and operations) resource site that features problem-solving articles and editorials for plant maintenance professionals.