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Solving Shaft Leakage Problems on Top-Entry Agitators |
June 28, 2006 |
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Posted by Kay Mann at June 28, 2006 09:34 AM |
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Usually, the driveshaft is suspended from a coupling at the top, and ends at the bottom, inside the vessel. This lower end of the shaft is an inherent maintenance problem: if a lower bearing exists at all, it is practically inaccessible and is rarely, if ever, maintained. Lower bearing wear, or lack of a bearing at all, allows the shaft to wobble as it turns. This radial shaft misalignment causes seals at the top of the vessel to fail.
Most of these vessels have a packing gland at the top of the vessel, which also acts as a stabilizing bearing, limiting shaft runout. However, you can imagine that as the packing becomes worn and compressed, shaft runout increases, creating leak paths. This can be an environmental or safety issue when the product contains harmful vapors and these escape.
Inaccessibility of the shaft seal can only make things worse.
One processor of shampoo had a vertical shaft vacuum mixer with lip seals at the top. The seals themselves were inexpensive enough and the product was not hazardous by any means. But about every four months a seal would fail and it would take two workers two days to lift the motor and gearbox up off the shaft coupling and slip a new lip seal over the shaft, then replace the whole assembly. (n.b. These guys even had to cut a hole in the cement ceiling to allow them to lift the motor up high enough to clear the coupling!).
Another problem inherent with vertical shafts is shaft floatation. This often results from interaction between the agitator and process material, causing axial (vertical) motion of the shaft. Thermal growth and shrinkage cycles can also produce axial motion, which compromises packing and most seals.
There is hope, however. Meco has recently patented its OFS-series seal, which permits radial and axial misalignments without compromising the integrity of the seal face. (Yes, it is a single-face seal). A positive drive mechanism allows driveshafts to float axially through the seal without affecting the seal face, while maintaining spring force on the seal.
The seal's design also permits radial eccentricity, while a companion steady bushing limits angular misalignment. The OFS has proven effective in containing vacuums and a variety of gases, vapors and fumes, even at high speeds and temperatures.
Best of all, these seals are available fully-split. No more holes in the ceiling.Photo Caption: A Meco OFS seal spinning
on a vertical agitator shaft, containing
chemical vapors.
Kay Mann
Meco Seals
