Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED LABYRINTH SEAL FOR~STEAM~TURBINES~
~ ~ AC~GROUND OF THE INVENTION~
Fleld of~the Invention
The present invention~relates genèrally~to
honeycomb labyrinth seals and, more specifLcally, to
an improved seal which is used~for low-pressure
steam turbines.
Descriptlon of the Related Art
- A grooved honeycomb labyrinth seal for
steamed turbines~is described in U.S.~Patent No. ;~
4,41S`,457~(assigned to Westinghouse~Electric~
~Corporat1on). `~
;Referring to Figures~l and`2~heréin,~ the`~
honeycomb l~abyrinth;seal 3 des~ribed~in the~
15 ~ aforementioned~U.~S~.~;pa~ent;~is~fo ~ ~from~-à~
p1ura1i~y~of~arcuate~se ~ ènts~each~comp~lsing~à ase~
portion~7 and~a~plurality of rows~of;honeycomb~cells~
9~extending rad1alIy~inwardly from~;the~basè` ~ ion~
7~so that~èach cell 9 is~open~`adjacent~thè~b~lades 1.
20 ~ -~A~plurality~of~pa~ssages~or~grooves~ are~so~
di~posed t at`each~ce1~1~;9~:is connected to-at~1east~
one~;passage~,;while~the~maj`or1ty o~the cells~are
connected~to;~two passages or~grooves~ The~groove~
on the 1eft side~of the~drawin~ is~open~to~the~
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upstream steam while ~he groove 11 on the right side
of the drawing is open to the downstream and there
is an interconnection between the grooves ll and the
cells 9 allowing steam and water entràined in the
steam to flow across the base 7 of the seal from the
upstream to the downstream side of the~seal. The
cells 9~are generally hexagonal having~six~walls, a~
plurality of which are common with ad~acent cell
walls. '
An axially disposed gap 13~is~di~spoeed
between~the circumferentially adjacent honeycomb ;~
labyrinth seal segments which permits water
collected in the grooves ll to drain therefrom. An
appropriate drain, not shown, is also disposed in
the blade ring 5 to drain the water ~rom the seal
locating groove 4 in the blade ring 5.
Figure 3A;shows a typica 1 arrangement or
existing honeycomb seal designs in a steam turbine;~
blade path. A radla} step is~formed between~the~
seal inside diamèter and the outer~diameter;of the
downstream blade 16. ;~;A radi~al~ohannel~is~formed~
between surface 12 and sur~ac~e~l2A~for~`the~purpase~
of removing~steam~and moisture~rom~thé~blade'~path;~
flow. ~ It'~is~difficul~;to~achleve~;~e~icient~turning~
~ of the~flow~leaving the~rotating blade~ip~region~
under the'~sea~ ;and~thus~it~is~difficu1t'for~ the;~
moisture t'o~enter~;the~extraction~slot.~ This~ ,an~
lead~to disturbed~fIow conditions~Ln this~regL
It~is~postulated that moisture~drai~age'~
~ ~from~the~sea~ may follow~a;generally axial~path~and~
become entrained~into the~main do ~ stream~flow
' through stationary~blade 16, leadlng to lnoreased
.
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moisture erosion damage to the downstream rotating
blades.
By observing flow field behavior~and
moisture erosion characteristics downstream~of a
typical hon ycomb seal installation, it~has been
determined ~hat a disturbed flow pattern (standing
vortex) can exist in associatlon wltù the~radial
s~ep between the inne~ diameter of the seal~and the
outer diameter surface at the inlet to the~
downstream stationary blade row. Also, moisture
collected by ~the seal may not be flowing into the
downstream;moisture~removal or extraction~slot as
intended~. Both~of these phenomena~may be attributed
to a limited a~ility of the extra`ction steam and
water flow to negotiate the sharp 90~corner at the
seal exit edge. ~ ~ :
SUMMARY OF THE INVENTION
~ An object o~ the present invention is to ~;~
provide an improved honeyc~omb Iabyrinth~seal in
which the drainage path~for the moisture collected
~by the seal~direc;ts the~exit~water~f~low ~in~an~
outward radial dirèction~instead of~axial.
Another~ob~ect~of the~presént inYention ~is
to provide~an improved~honeyaomb~labyrinth~seal~in~
~ which the~centrifugal;a~fects~of the~rotating;~blade~
are utilized~to promote~an~outward~radial fIow~o~
both s~eam~and condensed~;watèr~from`the~ blade`~t1p
region near the~éX~t~edge.
Thes~e~and other objeats~of the~pr~sènt~
30~ ~ 1nvention~are~met by~provldin~ a~honeycom~ labyrlnth~
sea} cooperat1vely associated with rotatable~steam
turb1ne~blades~ànd 1nc1ud1ng ~;ba~e~gor~10n hav1ng
2 ~
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an inlet edge and an outlet edge, ~ plurality of
rows of honeycomb cells extending radially inwardly
from the~base portion so that each cell is open
adjacent the blade, a plurality of~passages so
disposed that each cell is connected to`at:least one
passage:and at least one passa~e is open~upstream of
the rows of honeycomb cells and at least one passage:
is opened downstream of the rows of honeycomb cells,
thereby permitting steam to 10w through the ~
passages and cells from the upstream side~:to the
downstream side of the honeycomb labyrinth seal,
wherein a downstream most row of honeycomb~cells ;~
: extends beyond the exit edge of the backing plate,
and wherein an exit edge of the rotati~g blades ~ ~;
extends beyond the honeycomb labyrinth seal~
: These and other objects and àdvantages of
the honeycomb labyrinth seal of the present
invention will become more apparent with reference
to the following de~ailed descriptlon and;drawings. :~
20 ~ :BRIEF DESCR~PTION OF THE~DRAWINGS
Fig.~ l :is a:partial sectlonal view of a
known honeycomb labyrinth seal disposed in a~portion~
of a steàmed tùxbine ~takèn~along line~ I o~FLg.:2;~
Fig.: 2 1s a plan view taken from~the~
25; ~: dire~tion;~of~:line:II -~II
Fig. 3(a): is a~:partial cut away view~
showing the~honeycomb labyrinth::seal used~prior to ;;~
the~.present~ nv~ntion;~
Fig.~ 3(b:)~is~:a partial cut-away vièw~
~30 ~ ~showing the~honey~mb~labyrlnth~eeal~according~to a;~
preferred embodl~ent~o~:~the present~Lnvenelon;~
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Fig. 4 is an enlarged top view o~ a
portion of the labyrinth seal of Fig. 3;
Fig. 5 is a cross-sectional view taken
along line IV of Fig. 4; and
Fig. 6 is a cross-sectional view taken
along line VI of Fig. 4.
.
DETAILED DESCRIPTION OF THE PREFERRED E~ODIMENTS
Referring to Figs. 3-6, the honey~omb
labyrinth seal 3 is connected ~o a seal support ring
12 by any conventional means through a ~ase portion
7. A moisture removal/extraction slo~ opening 14 is
formed between downstream stationary blade 16 and
the seal support ring 12 ad;acent the upstream
stationary blade 18. The rotating blade 1 ro~ates
lS in a direction perpendicular to the ~low direction~
The base portion 7 is provided with a
plurality of passages by grooves 11. The labyrinth
seal 3 is formed from a plurality of arcuate
segments each comprising a base portion 7 and a
plurality of rows of honeycomb cell~ 9 extending
radially inwardly from the base portion 7 so that
each cell 9 is open adjacent the blade 1. ~ach cell
9 is connected to at least one passage 11.
The base portion 7 has an exi~ side edge
20 which in the known seal described in Figs. 1 and
2 would have extended fur~her than the honeycomb
cells, so that the downstream-most row o~ honeycomb
cells would be centered over the downstream-most
groove 11, with the downstream-most side of those
3Q cells suppor~ed on the base portion 7.
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According to the present invention, the
downstream-most row 9A of honeycomb cells S is
centered over a strip 7A of the base portion at the
exit side ~the strip 7A having a width substantially
the same as the width of the grooves 11) so that a
portion of the cells in row ~A overhang the base
portion 7. The arrangement promotes an internal
moisture drainage path for those alternating groups
of cells having circum~erentially oriented cell wall
elements centered over the drainage grooves 11. The
exit drainage flow is thus directed in a radial
direction as it leaves the honeycomb cells at the
exit side. The radial drainage path is between the
exit edge 20 of the base portion 7 and the outermost
lS wall 22 of the cells 9 in row 9A. Moisture droplet
flow centrifuged outwardly from the rotating blade
tip and impinging on the open face of the honeycomb
cells 9 will promote a radially outward path for the
seal exit drain flow.
Fig. 6 illustrates the circumferential-
axial drainage path communication for all groups of
cells adjacent to those shown in Fig. 5.
Generally, the desired internal water
dxainage characteristics for the seal 3 are
determined by the inlet ~o exi~ pressure drop acxoss
the rotating blade row and the outward pressure
generated by moisture droplets leaving the blade tip
and entering the open face of the honeycomb cell
structure.
Preferably, the downstream-most or exi~
side row of honeycomb cells extends b~yond the exit :
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edge 20 of the base portion 7 by about one half of a
groove width.
Referring to Fig. 3, it is noted that the
blade 1 preferably extends beyond the seal 3, rather
than being centered under the seal as in the known
seal of Figs. 1 and 2. Preferabl~, the amount of
extension is determined by-the tip section width,
with optimum overhang of the blade 1 baing
determined in relation to the width of throat
opening. The overhang of the rotating blade exit
edge promotes a seal exit drainage flow path in an
outward radial direction due to pressure from
centrifuged moisture droplet flow leaving the blade
tip. Also, the rotating blade exit edge overhang
also promotes radial expansion of steam flow
immediately downstream of the seal 3. This tends to
direct flow towards the moisture removal or
extraction slot 14 and provide well behaved (low
loss) flow conditions near the outer diameter
surface at the inlet to the downstream stationary
blade row.
The present invention allows for increased
width of downstream extraction or moisture removal
slots, or alternatively, allows reduced blade path
axial spacing if the extraction slot is a critical
restraint, due to upstream movement of the seal and
blade ring exit faces 20 and 12 ~Fig. 3) xelative to
the rotating blade.
Numerous modifications and adaptations of
the present invention will be apparent to those so
skilled in the art and thus, it is intended by the
following claims to covex all such modifications and
:
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adaptations which fall within the true spirit and
scope of the invention.
