Subject : The non metallic seal
A portion of the chemical industry uses non-metallic pumps. You'll
find them manufactured from Teflon®, graphite, Carbamate, and a
variety of other materials.
Common sense dictates that if the application engineer selected a
non metallic pump, he must of had a good reason for doing so, and it
would not be logical for you to equip it with a mechanical seal
containing metal components exposed to the product, unless you had
specific knowledge that doing so would make some kind of sense.
Most non-metallic seals are installed outside the stuffing box and
fall into two categories :
- The Teflon® bellows version.
- The O-ring version.
Both these designs have several features in
- These rotating seals are clamped, not set screwed to the
shaft. This is a necessary feature on glass shafts or any shaft
where the set screws would cause damage.
- Because of the clamping arrangement these seals are limited in
their pressure capability. Too high a stuffing box pressure can
cause them to slip on the shaft.
- The fluid is at the inside diameter of the seal. This means
that if there are solids present in the fluid (slurry) the solids
will be thrown into the lapped faces. It also means that as the
softer face wears, the rotating face will move towards the slurry
with a possibility of "hanging up" on the solids and opening the
- They are hydraulically balanced for fluctuating pressure, but
are limited by the clamping force on the shaft.
- They are both impossible to vent in a vertical application, so
you want to be aware of potential heat problems; especially at the
- Both versions should have a shroud install over them to
protect personnel in the area in the event of a seal failure.
- These seals can be used to "back up" an inside rotating seal.
When installed in this manner the resultant dual seal is refereed
to as "the face to face" version.
these seals are designed with no metal parts in the sealing fluid,
they are occasionally used in exotic metal pumps to save the
cost of purchasing an exotic metal seal.
- Because of the lack of set screws in the design, you should
install a clamp of some type behind the seal to prevent it from
sliding in higher pressure applications.
- Exotic metals are often used with dangerous products. You
really should be using dual seals made from the exotic metal and
not these seals, even though their price is attractive.
The following illustration describes the Teflon® version of a
non metallic seal :
- A glass filled Teflon® face is commonly fused
into the Teflon® bellows. This means that you are
somewhat limited in your choice of face materials.
- This version slips on the shaft easier than the
- Teflon's® cold flow properties also works against
you when you clamp it to the shaft. Unlike an O-ring,
Teflon® will relax under pressure.
- Teflon's chemical compatibility makes this version
more universal. You do not have to pick the correct
material for the O-ring.
The next illustration describes the O-ring version :
® DuPont Dow elastomer
- Although the dynamic O-ring is touching the shaft, it
will not frett the
shaft because the sliding or flexing takes place on the
O-ring outside diameter.
- The rotating carbon face should be banded as a safety
precaution because carbon is weak in tension
- In this design the metal holder can be drilled and
tapped to provide a better holding force if the seal is
used in "exotic metal" applications
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