FIELD OF INVENTION
The present invention generally relates to a method of sealing to effectively
prevent the leakage of hot coal air mixture to atmosphere by using non contact
type labyrinth seals in vertical spindle Bowl Mills used in fossil fuel power plants.
More particularly, the invention relates to any improved method of sealing in
vertical spindle Bowl Mills for preventing leakage of air coal mixture to the
atmosphere by adapting non-contact type labyrinth seals.
The invention further relates to a non-contact type sealing device in vertical bowl
mills.
BACKGRQUND OF INVENTION
Working of a Bowl Mill
In a bowl mill, the raw coal is fed through a center feed pipe. The input coal falls
at the center of a rotating bowl (08) which is lined with replaceable hard liners
(09) called bull ring segments (09). Due to centrifugal force, the coal starts
moving towards periphery of the bowl (08) in a spiraling fashion. In the process,
the coal comes between a grinding roll (10) and the bull ring segments (09) and
gets crushed. The crushed coal is carried by a stream of pressurized hot air to a
classifier section for further transportation to the boiler for firing. The drive for transportation of the crushed coal is provided from a horizontally
disposed motor, which is connected via a coupling to a pinion shaft in case of a
planetary drive, or a worm shaft in case of a worm drive. This in turn drives a
rotating table in case of the planetary drive or a vertical shaft in case of the
worm drive. In case of the planetary drive, the bowl hub (02) is connected to a
Rotary Table, and the Bowl Hub (02) drives the Bowl (08). In case of the worm
drive, the vertical shaft is connected to the bowl hub (02) which drives the bowl
(08). Grinding of coal takes place on the bowl (08). A gap is maintained between
the Bowl Hub (02) (rotating part) and an Air Inlet Housing (01) (stationary part)
which require to be sealed. The air inside the housing is at a higher temp and at
a higher pressure which facilitates transportation of the pulverised coal to the
boiler. Hence there is a tendency of the pressurized hot air to escape through
the gap to atmosphere if not sealed properly. This hot air is harmful to the drive
system, including the environment as the air is also laden with coal dust making
the area dirty.
Figures - 1, shows a prior art mechanical face seal to seal a gap between a
rotating Bowl Hub (02) and is driven by it. Seal air escapes at higher pressure
than the inside air pressure through a narrow gap between the Air Seal housing
(05) and an Air Seal Cover Runner (07), and thus forms a 'primary means to
prevent leakage. Metal to metal contact betWeenthe seal runner (64) and static
seal (03) provides a secondary sealing. The Seal runner (04) slides down to
compensatefor the wear. However, according to the prior art, during the operation, the studs (06)
generally get broken due to entrapment of the rejects, such as stones, pyrites
etc, and the air seal housing (05) being made of metal sheets gets distorted, and
further damaged due to entry of rocks/stones/metal rods etc. The Static seal
(03) made of graphite impregnated metals gets worn out due to rubbing action
and has a limited life. A Packing rope (11) provided between the seal runner (04)
and the Bowl Hub (02), sometimes restricts the movement of the seal runner
(04) downwards leading to a seal failure.
Further, the sealing arrangement is normally disposed at a lower elevation than
that of the housing (05) which normally forms a well effect. If the rejects, like
stones, coal particles etc make an entry into the zone, they get trapped in the
well and come in between the static seal (03) and the seal runner (04) and
ultimately damage the associated parts.
Another disadvantage of the prior art is that the sealing system takes long hours
for assembly, particularly because the access to the assembly location is qUite
cramped. Further, from operational point of view the assembly of the seal is
highly sensitive between the air seal cover (05) and air seal cover runner (07).
Both these parts are fabricated parts and locating those in accurate position is
indeed. Any misalignment causes rubbing of the rotating air seal cover runner
(07) with the static air seal housing (05) leading to seal failure. OBJECTS OF THE INVENTION
It is therefore, an object of the invention to propose improved method of sealing
in vertical spindle Bowl Mills for preventing leakage of air coal mixture to the
atmosphere by adapting non-contact type labyrinth seals, which eliminates the
disadvantagesof the prior art.
Another object of the invention is to propose improved method of sealing in
vertical spindle Bowl Mills for preventing leakage of air coal mixture to the
atmosphere by adapting non-contact type labyrinth seals, which is capable to
provide sealing of emitting air-coal mixture corresponding to the speed and
temperature under which the sealing device operates.
A further object of the invention is to propose a non-contact type sealing device
in vertical spindle bowl mills for preventing leakage of air-cool mixture to the
atmosphere, which adapts labyrinth seals having less wearing-out property.
A still further object of the invention is to propose a non-contact type sealing
device in vertical spindle bowl mills for preventing leakage of air-cool mixture to
the atmosphere, which allows in-situ assembly of the device without requiring
the associatedassembliesto be removed. A still another object of the invention is to propose improved method of sealing
in vertical spindle Bowl Mills for preventing leakage of air coal mixture to the
atmosphere by adapting non-contact type labyrinth seals, which prevents the
damage of the costly bowl hub due to accidental rubbing of the seals during
rotation of the bowl hub.
SUMMARY OF INVENTION
Accordingly, there is provided an improved method of sealing in vertical spindle
bowl mills for preventing leakage of air-coal mixture to the atmosphere by
adapting non-contact type labyrinth seals, the method comprising the steps of
providing a non-contact type sealing device comprising at least two labyrinth
seals haVinga plurality of grooves and serrations on their sealing faces, the first
seal provided inside a rotating bowl hub and facing the dirty coal-air mixture; at
least one air chamber disposed next to the first seal for supplying pressurized
seal air; the second seal located after the air chamber and exposed to the
atmosphere; the at least-two labyrinth seals connected to an air inlet housing; a
close radial clearance provided between the labyrinth seals and the rotating bowl
hub; rotating by means of an electric drive the shafts of the mills which produces
a non-contact type sealing action by the labyrinth seals, and controlling the fluid
passagevia a plurality of chambers by the centrifugal motion including formation of controlled fluid vortices; increasing the speed of rotation of the shafts which
forces the fluid towards the outside and away from the third passages;
entrapping the escaped air from the main chamber in a labyrinth chamber and
forcing the air to form a vortex motion which preventing the escape of the
entrapped air including repelling any other fluid; and providing sufficient groove
width to knife edge width ratio which in combination with the grooves of the
frictionless labyrinth seals with clearance to ensure leakage-proof and effective
sealing.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - shows a ball mill drive with contact type seal according to prior art.
Figure 2 - A non-contact type labyrinth seal device in bowl mill according to
the present invention.
DETAIL DESCRIPTION OF THE INVENTION
As shown in figure-2 the prior art mechanical face seal is replaced with a noncontact
labyrinth seal device. The non-contact labyrinth seal is an annular ring
with a plurality of grooves, and having serrations cut on the sealing face. Atleast two labyrinth seals (12) are provided. The first seal (12) is provided at the inside
the bowl (02) facing the dirty coal plus air mixture. Then a seal air chamber is
provided. The second labyrinth seal (2) is provided just after the seal air
chamber is provided. The second labyrinth seal (12) is provided just after the
seal air chamber. The second labyrinth (12) is exposed to the atmosphere. The
labyrinths (12) are connected to an Air Inlet Housing (01) via fasteners (13, 14).
In between the two Labyrinths seals (12), the air chamber is formed to provided
the pressurized seal air. A close radial clearance is provided between the
labyrinth seals (12) and rotating bowl hub (02). As per tribological literature,
there has to be difference of 1:4 between the hardness of the two components
which are likely to come into contact and hence a soft material is selected as the
material for labyrinth seals (12). This will avoid the damage of the heavy,
expensive Bowl Hub (item 02). Even if there is an damage due to aCCident
rubbing, it is shall be restricted to the soft seals only, and save the Bowl Hub
(02). The Labyrinth seals (12) are designed in split form, so that these can be
assembledin-situ without haVingto remove the other associated assembly.
The Labyrinth seals (12) on the rotating shafts provide a non-contact sealing
action by controlling the passage of fluid through a variety of chambers by the
centrifugal motion, including via formation of controlled fluid vortices. At higher
speeds, the centrifugal motion forces the liquid towards the outside and
therefore away from any passage.The escapedair from the main chamber becomesentrapped in a labyrinth chamber, where it is forced into a vortex-like
motion, which prevent its escape, and further repels any other fluid. As the
labyrinth seals (12) are non-contact type, they do not wear out.
Unlike other rotating type lip seals, the Labyrinth Seal is frictionless and will not
damage the shafts and has a virtually unlimited life. Sealing depends on the form
of the labyrinth gap and the length of the leakage path. At least three groves are
provided to ensure adequate sealing with clearance depending on the speed and
temperature under which the seal is operating. Adequate groove width to knife
edge width ratio is maintained for effective sealing action.
Advantages of the invention:
1) Non contact type sealing device is provided and hence expected to have a
very long life.
2) In contact type seals of the prior art, the power is lost due to friction and
also it generates heat.
3) In the new deVice, the assembly becomes easy and time is saved in
assembly at site.
WE CLAIM
1. An improved method of sealing in vertical spindle bowl mills for preventing
leakage of air-coal mixture to the atmosphere by adapting non-contact
type labyrinth seals, the method comprising the steps of:
_ providing a non-contact type sealing device comprising at least two
labyrinth seals (12) having a plurality of grooves and serrations on
their sealing faces, the first seal (12) provided inside a rotating
bowl hub (02) and facing the dirty coal-air mixture; at least one air
chamber disposed next to the first seal (12) for supplying
pressurized seal air; the second seal (12) located after the air
chamber and exposed to the atmosphere; the at least two labyrinth
seals (12) connected to an air inlet housing (01); a close radial
clearance provided between the labyrinth seals (12) and the
rotating bowl hub (02);
- rotating by means of an electric drive the shafts of the mills which
produces a non-contact type sealing action by the labyrinth seals
(12), and controlling the fluid passage via a plurality of chambers
by the centrifugal motion including formation of controlled fluid
vortices;
- increasing the speed of rotation of the shafts which forces the fluid
towards the outside and away from the fluid passages;
- entrapping the escaped air from the main chamber in a labyrinth
chamber and forcing the air to from a vortex motion which
preventing the escape of the entrapped air including repelling any
other fluid; and
- providing sufficient groove width to knife edge width ration which
in combination with the grooves of the frictions labyrinth seals with
clearance to ensure leakage-proof and effective sealing.
2. A non-contact type sealing device in vertical spindle bowl mills for
preventing leakage of air-coal mixture to the atmosphere by adapting
non-contact type labyrinth seals, the device comprising:
- at least two labyrinth seals (12) having a plurality of grooves and
serrations on their sealing faces, the first seal (12) provided inside
a rotating bowl hub (02) and facing the dirty coal-air mixture; at
least one air chamber disposed next to the first seal (12) for
supplying pressurized seal air; the second seal (12) located after
the air chamber and exposed to the atmosphere; the at least two
labyrinth seals (12) connected to an air inlet housing (01); a close
radial clearance provided between the labyrinth seals (12) and the
rotating bowl hub (02);
3. The device as claimed in claim 2, wherein the labyrinth seals (12) are
formed of soft material to prevent damage of the bowl hub (2) due to
accidental rubbing.
4. The device as claimed in claim 2 or 3, wherein the labyrinth seals (12) are
configured in split form.
5. The device as claimed in as claimed in any of claims 2 to 4, wherein the
labyrinth seals (12) are friction less.
6. An improved method of sealing in vertical spindle bowl mills for preventing
leakage of air-coal mixture to the atmosphere by adapting non-contact
type labyrinth seals as substantially described and illustrated herein with
reference to the accompanying drawings.
7. A non-contact type sealing device in vertical spindle bowl mills for
preventing leakage of air-coal mixture to the atmosphere by adapting
non-contact type labyrinth seals as substantially described and illustrated
herein with reference to the accompanying drawings.