Advantages of Self-Aligning 
Flexible Stator Seals

I want to discuss some of the inherent benefits of Flexible Stator Seals.

Mechanical seals require an energizing force to keep the seal faces closed. This can be supplied by metal bellows, a single large spring, or a number of smaller springs located circumferentially around one seal face. The choice of an energizing device depends on several factors, but there is also the design choice of whether to energize the rotating or stationary elements in the seal assembly.

Many traditional mechanical seals have their springs mounted on the rotation portion of the seal assembly. While this often allowed the mechanical seals to be designed to fit in very small stuffing boxes, they also need meticulous angular alignment to function optimally (angular misalignment in flexible rotor seals leads to excessive cycling of the energizing element and failure due to fatigue).

Newer pumps tend to have much larger seal chambers, making using a flexible stator design more practical.

Correct equipment alignment is always critical to the long life of any seal. However, operational conditions can negatively affect equipment alignment after start-up (heat buildup, vibration, etc…). Any feature in a mechanical seal that can mitigate these problems is worthy of consideration.

Mechanical seals with Flexible stators adapt to minor angular misalignment. This means that these seals maintain effective sealing performance even when perfect alignment is challenging. This reduces the likelihood of spring failure through fatigue and helps ensure a stable fluid film.

Thanks to their self-aligning nature, these seals mitigate the impact of vibration and misalignment, reducing wear and tear. This extends the seals’ lifespan and enhances the Mean Time Between Failure (MTBF).

Fluctuations in operating conditions can cause equipment movement. Self-aligning flexible stator seals accommodate these movements while maintaining a reliable seal, preserving product integrity and preventing leakage.

API 682 4th edition discusses both design choices and states that, for the purposes of the standard, either option is technically equivalent*. However, it does highlight some areas where a flexible stator design may give better performance, such as:

• High shaft speed > 23m/s (4500ft/min)
• The balance diameter is greater than 115mm (4.5”).
• Distortion of pump case or gland plate for several reasons.
• Perpendicularity of the seal mounting surface and shaft is a problem.
• Seal chamber runouts described in section 6.1.2.13 or ASME B73.1 / 73.2 cannot 
  be met.

Keep the above points in mind the next time you are reviewing a seal failure and ask if it's possible that one or more of them could be present in your application. Then look at your seal design and ask if maybe there is a different solution…

Talk to the experts at A.R. Thomson Group today.

(Note: This post is intended for educational purposes and is not specific to any particular situation or company. Always consult with professionals for accurate advice.)

Specific sections are referenced in API682 4th edition.

*6.1.1.2 (Technically equivalent)

*6.1.1.5 (areas to consider stationary flexible element)

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