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MECO has developed its OFS-style
Outside Face Seal for service on a Sterns & Rogers white
pan mixer.
The mixer is a hot, (nearly 300ºF) negative-pressure
vessel for removing water from sugar beet juice. Steam
is bubbled through the vessel as it slowly (60 rpm)
mixes the sugar juice. A vacuum is induced, saturating
the steam with water from the sugar juice. The greater
the vacuum, the greater the efficiency with which water
is removed.
The vessel is 30 feet high and 15 feet in diameter.
The six-inch diameter shaft is top-entry, with a four-sided
stave bearing at the bottom of the vessel. A coupling
is approximately 10" above the packed stuffing box.
The drive is mounted above the vessel and rocks in every
direction from excessive wear. The mixer arrangement
requires the packing to also serve as a steady bushing.
As the packing is crushed radially, the vacuum is
reduced, decreasing the efficiency. When the vacuum
falls below -18 PSIG, packing adjustment is required.
The air and vessel temperatures on top are often too
high for humans to work and the vessel must be allowed
to cool, slowing or stopping production.
MECO developed the OFS seal specifically for the white
pan mixer. The requirements were:
- materials capable of surviving 400º Fahrenheit,
- containment of 20 PSIG or more of vacuum,
- a driving collar and seal tolerant of 3/8"
of axial float,
- 10 plus degrees of angular misalignment, and
- 5/16" of TIR runout.
A split steady bushing made of MECO3000 polymer-filled
PTFE is slid into the stuffing box. The bushing's inner
diameter is 3/16" greater than the shaft diameter.
The large difference helps control the shaft without
generating excessive heat.
The OFS is a fully-split, single-face mechanical seal.
The face is kept closed using the force of the vacuum,
(6) compression springs and the weight of the stainless
steel rotating seal face.
The first OFS was installed in Minnesota on September
3, 1994. It has been under continuous service during
the plant's production campaigns, holding a vacuum of
-23-24 PSIG.
During routine maintenance in 1995, the overhead gearbox
was accidentally dropped onto the seal. Minor damage
occurred, but the seal continued to operate without
compromise throughout the campaign. As of December,
2001, the seal has been rebuilt twice during Summer
shutdowns.
The OFS seal has since been installed on a variety of top-entry and horizontal-shaft machines, and has proven effective in sealing many different materials.
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