Note: Descriptions are shown in the official language in which they were submitted.
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CRACK RESISTANT VALVE PLATE ASSEMBLY
FOR A MOLTEN METAL SLIDE GATE VALVE
Background Of The Invention
This invention generally relates to valve plates for use in slide gate
valves for controlling a flow of molten metal, and is specifically concerned
with a valve plate assembly that is resistant to cracks caused from thermal
stresses.
Slide gate valves are commonly used to control a flow of molten
metal in steel making and other metallurgical processes. Such valves
comprise a support frame, an upper stationary valve plate having an orifice
in registry with a tundish or ladle nozzle for conducting a flow of molten
metal, and a throttle plate likewise having a metal conducting orifice that
is slidably movable under the stationary valve plate. In slide gate valves
used in conjunction with continuous casting molds, a lower stationary valve
plate is provided beneath the movable throttle plate which likewise has a
flow conducting orifice that is substantially aligned with the orifice of the
upper stationary plate. The rate of flow of molten metal is dependent upon
the degree of overlap of the orifice of the slidably movable throttle plate
with the orifice of the upper stationary plate. The movable throttle plate
is usually longer than the stationary throttle plates in order to give it the
capacity of throttling the flow of molten metal from both the front and back
edges of its own orifice, as well as the ability to shut off the flow
altogether by bringing its orifice completely outside of any overlap with the
orifices of the stationary plate. Typically, the throttle plate is slidably
manipulated between the stationary plates by means of a hydraulic linkage.
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Both the throttle plate and the stationary plates of such slide gate valves
are formed from heat and erosion resistance refractory materials,
such as aluminum oxide, alumina-carbon, zirconium oxide. However,
despite the heat and erosion resistance of such refractory materials, the
severe thermal stresses that they are subjected to ultimately causes some
degree of cracking to occur. For example, in steelmaking, each valve plate
is subjected to temperatures of approximately 2900 in the area
immediately surrounding its flow-conducting orifice, while its exterior
edges are experiencing only ambient temperature. The resulting large
thermal gradient creates large amounts of mechanical stress as the area of
each plate immediately surrounding its orifice expands at a substantially
greater rate than the balance of the plate. These stresses cause cracks to
form which radiate outwardly from the orifice of the plate. If nothing is
done to contain the spread of these cracks, they can extend all the way to
the outer edges of the plate, causing it to break.
To prevent the spreading of such cracks and the consequent breakage
of the valve plates, various clamping mechanisms have been developed in
the prior art. The purpose of these mechanisms is to apply sufficient
pressure around the perimeter of the plate so that cracks emanating from
the orifice do not spread to the edges of the plate. In one such mechanism,
a steel band is stretched around the perimeter of each of the valve plates.
Unfortunately, the applicants have observed that there are at least three
disadvantages associated with the use of such band-type clamping
mechanisms. First, because the steel that forms such bands is a superior
thermal conductor to the air that would otherwise surround the plate edges,
the use of a steel band actually increases the thermal gradient across the
lengthwise and widthwise axes of plate, thereby encouraging even more
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cracking to occur. Secondly, as the steel band heats up as a result of being
in the vicinity of molten metal, it expands much faster than the refractory
material forming the valve plates, which in turn causes it to relax the
compressive forces that it needs to apply around the plate in order to
discourage the spread of cracks. Thirdly, if the cornerb of the plate are not
rounded, such clamping bands can apply localized mechanical stresses onto
the corners of the plates, which in turn can cause unwanted cracldng in
these areas.
To overcome these and other shortcomings, clamping systems have
been developed that comprise a frame having screw-operated wedges which
engage corners of the plate that have been truncated in an angle that is
complementary to the angle of the wedges. While such frame and wedge
type clamping mechanisms constitute a clear advance over the mere use of
steel banding around the perimeter of the plates, the inventors have further
noted at least two shortcomings with this design that prevent it from
achieving its full, crack-retarding potential. In all of the variations of
this
design that the applicants are aware of, the angle of each of the truncated
corners with respect to either the lengthwise or widthwise edge of the plate
is the same, regardless of the position of the orifice along the longitudinal
center line of the plate. Consequently, in plates where the orifice is offset
along the longitudinal center line of the plate (which includes virtually all
valve plates), the clamping forces cannot be uniformly focused where the
maximum amount of cracking occurs, i.e., in the vicinity of the orifice
where the greatest amount of thermal stresses are present. Moreover, even
in instances where the orifice is centraIly located in the valve plate, the
applicants have observed that the angular orientation of the truncated
corners in such plates does not optimally prevent the spreading of cracks,
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as previously thought. Such non-optimality results from the face that crack
formation is not uniformly distributed 360 around the orifice, but instead
is biased along the longitudinal center line of all valve plates whether
stationary or movable. Such an asymmetrical distribution of cracks around
the plate orifices is believed to occur as a result of the longitudinal
sliding
action of the throttle plate across the faces of the stationary plates. Still
another shortcoming associated with prior art clamping mechanisms is their
use, in some cases, of angles shallower than 20 with respect to the
longitudinal edges of the plate. In addition to providing inadequate
clamping forces to close up cracks along the transverse axis of the plate,
the use of such shallow angles generates large localized stresses due to the
large amount of compression that the clamping wedges apply to the
truncated corners. Such localized stresses can result in cracking and
fissuring in the corner regions of the valve plates, which is directly
contrary to the overall purpose of the clamping mechanism. A final
shortcoming associated with such valve plates in general is their lack of any
optimization of the length of the truncated corners, or the lengths and
widths of the plate with respect to the diameter of its orifice. While the
corner lengths should be of a certain minimal size in order to avoid the
production of unwanted localized mechanical stresses in these regions of
the plate, they should not be made overly large, either.
Clearly, there is a need for a valve plate whose corners are truncated
at angles that optimally focus the clamping forces in the most crack-prone
areas of the plate in order to maximally retard the lengthening of any such
cracks. Ideally, the corners should have a length sufficient to avoid the
production of unwanted localized mechanical stresses in the corners.
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Summarrv Of The Invention
Generally speaking, the invention is a crack resistant valve plate
assembly for use in a slide gate valve that overcomes or at least ameliorates
all of the disadvantages associated with the prior art. The assembly
comprises a refractory valve plate having a orifice for conducting molten
metal that is positioned along a center line of the plate, and truncated
corners for focusing a clamping force toward the center line in the vicinity
of the orifice to prevent the formation and spreading of cracks therein,
wherein the angular orientation of each of the truncated corners varies with
the position of the orifice along the center line. The assembly further
comprises a clamping frame for applying the required clamping force to
each of the truncated corners.
To achieve the aforementioned force focusing objective, each of the
truncated corners is orthogonal to a line falling within an angle whose
vertex is defined by a point tangent to the orifice diameter. One side of the
angle is defined by a line extending from the tangent point across the center
line, and through a point where converging edges of the plate would
intersect but for the presence of the truncated corner. The other side of the
angle is defined by a line extending from the tangent point across the center
line, and through an intersection of lines drawn parallel to the converging
plate edges that are spaced from these edges a distance equal to the orifice
diameter.
In the preferred embodiment, each of the truncated corners is
orthogonal to a line extending between the tangent point to an orifice
having the maximum diameter that the plate can operate with, across the
center line, and through an intersection of lines drawn parallel to the
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converging plate edges that is spaced from the edges a distance equal to {
one-half of a maximum orifice diameter. When the plate is rectangular in
shape, each of the corners is truncated along a line that is orthogonal to the
aforementioned line and which extends through an intersection of one of
the parallel lines and the lengthwise edge of the plate.
The plate assembly may be movable along an axis within the slide
gate valve that is coincident with its longitudinal center line, or it may be
stationary with respect to the slide gate valve. In either case, the plate
includes an orifice along one of its center lines, and truncated corners cut
in accordance with the same geometric formula as previously described
with respect to the first valve plate. In either case, a clamping frame is
provided for applying the required clamping force onto the truncated
corners.
In order to provide a movable valve plate assembly with all the
desirable capabilities of a shut-off stroke, and front and back throttle
strokes that is formed from a minimum amount of ceramic material, the
plate of the movable assembly is preferably rectangular in shape, having
a length of between amount 5.5 and 5.75 orifice diameters, and a width of
between about 2.9 and 3.1 orifice diameters. In the preferred embodiment,
the length and width of the movable plate are 5.66 and 3.0 orifice
diameters, respectively. In order to provide a stationary valve plate
assembly that cooperates with a moving valve plate to provide a shut-off
capability, and front and back side throttle strokes, the plate of the
stationary assembly is likewise preferably rectangular in shape, having a
length of between about 4.5 and 4.75 orifice diameters, and a width of
between about 2.9 and 3.1 orifice diameters. In the preferred embodiment
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the length and width of the stationary valve plate is 4.66 and 3.0 orifice
diameters, respectively.
Whether movable or stationary, the valve plate assembly of the
invention provides a crack-resistant valve plate having shut-off and front
and back throttling capabilities with a minimum amount wasted ceramic
material.
Brief Description Of The Several Figures
Figure 1 is a schematized, cross-sectional side view of a slide gate
valve installed in a tundish that utilizes valve plate assembly of the
invention;
Figure 2 is a top plan view of the throttle plate assembly of the
invention;
Figure 3 is a top plan view of the lower stationary plate assembly
of the invention;
Figures 4 through 10 are bottom plan views of the plate used in the
lower stationary plate assembly of the invention, illustrating a preferable
method of proportioning this plate, and how the angles of the truncated
corners are determined;
Figures 11, 12, and 13 are top plan view of the plate used in the
throttle plate assembly, illustrating a preferable method of proportioning
this plate, and how the angles of its truncated corners are determined;
Figure 14 is a top plan view of the upper stationary plate
superimposed over the throttle plate in a shut-off position, and
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Figure 15 is the same top plan view of the plates illustrated in
Figure 14 wherein the throttle plate has been longitudinally slid into a back
throttling position with respect to the stationary top plate.
Detailed Description Of The Preferred Embodiment
With reference now to Figure 1, wherein like numbers designate like
components throughout all the several figures, the invention includes both
movable and stationary valve plate assemblies 1 for use in a slide gate
valve 2 of the type used to regulate a flow of molten steel or other metal
from a tundish 3. The slide gate valve 1 is secured onto a mounting plate
5 which in turn is connected to the tundish shell 7 by a mounting structure
not illustrated. Valve 1 includes a nozzle 9 formed from a ceramic
material having a funnel-shaped bore 10 for directing a cylindrically-shaped
flow of molten metal out of the tundish 3. The nozzle 9 is mechanically
mounted in the bottom wall of the tundish 3 by way of a packing of heat
resistant, particulate ramming material 11.
The principal purpose of the valve plate assembly 1 of the invention
is to modulate the flow of molten metal exiting the bore 10 of the nozzle
9. To this end, the invention includes upper and lower stationary plate
assemblies 13 and 17, with a slidably movable throttle plate assembly 23
sandwiched therebetween. The upper stationary plate assembly 13 includes
a stationary plate 14 of ceramic material having a circular orifice 15 for
conducting a flow of molten metal from the bore 10. The lower stationary
plate assembly 17 likewise has a stationary plate 18 of refractory material
with an orifice 19 that is the same size as, and is concentrically aligned
with the orifice 15 of the upper stationary plate 14. Preferably, both the
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upper and lower stationary plates 14, 18 have the same length and width.
Mounted on the lower surface of the lower stationary plate 18 is a tube
fixture 20 which may be used, for example, for directing a flow of molten
steel into a continuous casting mold. The tube fixture 20 includes a tube
mounting plate 21 that is integrally connected to a tube shroud 22. A
mounting assembly (not shown) secures the plate 21 of the tube fixture 20
into the position illustrated in Figure 1. The tube fixture 20 isolates the
modulated flow of liquid metal exiting the valve plate system 1 from
ambient air in order to prevent _ambient oxygen from reacting with the
molten metal. The throttle plate assembly 23 is slidably mounted between
the upper and lower stationary plate assemblies 13 and 17. The throttle
plate assembly 23 likewise includes a plate 24 formed of a ceramic material
having an orifice 25 which may be circular, and the same diameter as the
orifice 15 of the upper stationary plates 14. The orifice 18 of the lower
stationary plate 18 is larger than that of the orifices 25 and 15 to avoid
trapping steel in the throttle plate 24 during a shut-off operation.
However, in order to provide the valve plate system 1 with a back edge
throttling capability as well as a shut-off and front edge throttling
capability, the throttle plate 23 is longer than the upper and lower
stationary plates 13 and 17. During the operation of the slide gate valve
2, the throttle plate 24 is slidably and reciprocally moved by means of a
hydraulic linkage (not shown) along an axis A that corresponds with the
longitudinal center lines of the plates 13, 17, and 24.
With reference now to Figure 2, truncated corners 30a-d are
provided on the generally rectangularly shaped throttle plate 24 in order to
focus clamping forces near the orifice 25 along the lengthwise center line
92 (which is collinear with the axis A shown in Figure 1). A hoop of steel
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banding 31 is provided in tension around the perimeter of the throttle plate
24 in order to enhance the integrity of the plate. Both the plate 24 and ,
banding 31 are surrounded by a clamping frame 33 that applies substantial
compressive clamping forces to the aforementioned truncated corners 30a-
d. For this purpose, the clamping frame 30 has a stationary clamping
member 35 with opposing clamping feet 37a,b that are aligned at the same
angle with respect to the truncated corners 30a,b on the left side of the
plate 34 to avoid the generation of localized stresses. Clamping frame 33
further includes a pair of spaced apart, paraIlel frame legs 39 to which a
movable clamping assembly 41 is attached. The assembly 41 includes a
movable clamping member 43 likewise having opposing clamping feet
45a,b that are disposed at the same angle as the truncated corners 30c,d
present on the right side of the plate 24. A clamping screw 49 that extends
through a bore (not shown) in the clamp support member 47 threadedly
engages another bore (also not shown) in the movable clamping member
43 such that, when the screw 49 is turned, the claniping feet 45a,b of the
movable clamping member 43 engage the truncated corners 30c,d on the
right side of the plate 34. This operation in turn causes the clamping feet
37a,b of the stationary clamping member 35 to apply clamping pressure
onto the truncated corners 30a,b on the left side of the plate 24.
Since both the upper and lower stationary plate assemblies 13 and
17 are substantially identical in all salient aspects, only the lower
stationary
plate assembly 17 will be described in order to avoid prolixity.
With reference now to Figure 3, the lower plate assembly 17
includes a lower stationary plate 18 having an orifice 19 which may be
circular and identical in diameter to the orifice 25 of the throttle plate 24.
Like the throttle plate 24, the stationary plate 18 has truncated corners 54a-
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d for focusing a clamping force along the longitudinal center line 70 of the
plate near the vicinity of the orifice 19. The lower stationary plate
assembly 17 further includes a clamping frame 58 for applying clamping
forces onto the truncated corner-s 54a-d. . To this end, the clamping frame
58 includes a rectangularly shaped frame assembly 59 (illustrated in
phantom) that contains, on its left end, a stationary clamping member 60
having clamping feet 62a,b which work in the same fashion as the feet
37a,b described with respect to clamping member 35. The frame assembly
59 further contains, on its right side, a movable clamping member 64.
Clamping member 64 includes clamping feet 66a,b which may be
compressively engaged against the truncated corners 54c,d of the plate 18
by the turning of a clamping screw 68 that operates in the same fashion as
the previously described screw 49. In all cases, the angles of the truncated
corners 54a-d and the clamping feet 62a,b and 68a,b are the same so that
broad area contact is made between these components, thereby avoiding
localized stresses which could cause unwanted cracks in the plate 18 in the
corner locations.
Figure 4 illustrates how the lengthwise and widthwise dimensions of
the preferred embodiments of each of the stationary plates 14 and 18 are
determined as a function of the maximum diameter D of the orifice 19 that
the plates can (as a practical matter) operate with. In order to
accommodate a shut-off position with respect to the throttle plate 24, the
length of the upper half of the plate 18 from the center point of the orifice
19 must be able to accommodate a shut-off stroke Ss of at least 1.5 orifice
diameters. While it is theoretically possible for a shut-off stroke to be only
a little larger than a single orifice diameter, such a sizing scheme would
not take into consideration the substantial elongation that occurs of the
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orifice 19 along the lengthwise center line 70 of the plate 18 due to
erosion. Hence, as a practical matter, the shut-off stroke must be at least
1.5 orifice diameters. Such a stroke would position the orifice 25 of the
throttle plate 24 in the position illustrated in phantom on top of the plate
18. In order to have a sufficient amount of plate in the lengthwise
direction to support the throttle plate 24 when it has arrives at the shut-off
position illustrated in phantom, it is necessary to have an additional length
D of stationary plate beyond the center point of the orifice 25, making the
total length of the plate 18 from the center of the orifice 19 to be 1.5D +
1D=2.5D.
In determining the remaining length of the stationary plate 18 from
the center of the orifice 19, it is necessary to consider only a back
throttling position between the orifice 19 of the stationary plate, and the
orifice 25' of the throttle plate 24 as it is not necessary for the plate 18
to
accommodate two separate shut-off strokes. Accordingly, the length of the
bottom half of the plate 18 is .66D (which allows the throttle plate orifice
25' to arrive in the maximum back throttling position illustrated in Figure
4) plus an additional length of plate equal to 1.5 orifice diameters so that
the stationary plate 18 provides sufficient support for the throttle plate 24
within the structure of the slide gate valve 2. Hence the bottom half of the
plate should be a total of .66D + 1.5D = 2.16D. Combining the two
halves of the plate, the total length of the stationary plate 18 (as well as
the
upper stationary plate 14) should be 2.16D + 2.5D = 4.66D. In order for
the stationary plate 18 to have a width sufficient within the slide gate valve
2 to have a structural strength sufficient to withstand the mechanical
stresses applied to it from a stream of molten steel, and to provide adequate
surface for a tube plate or a well nozzle, the width of the plate 18 should
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be 1.5 orifice diameters on either side from the center line of the orifice
19, making for a total width of 1.5D + 1.5D = 3.0D. While the lengths
and widths of the plates have been expressed in terms of maximum orifice
diameter, the same methodology could be used to express these dimensions
in terms of maximum orifice width in cases where the orifices are not
circular.
'hir.ning now to Figures 5, 6, and 7, and a description of the method
used in determining the angles of the truncated corners 54a-d, the first step
of this angle determining method is the provision of construction lines
along the inner perimeter of the plate 18 that are parallel but spaced apart
from the outer edges of this plate a distance of one-half of an orifice, or
.5D. These construction lines are illustrated in Figure 5 as lines 72a-d.
These lines intersect at corners 74a-d as shown. Figure 6 illustrates the
next step of the angle determining method. Here, lines 78a-d are drawn
between the corners 74a-d of the construction lines, and tangent points 76a-
d with an orifice 19 of maximum diameter, wherein each of the lines 78a-d
crosses the lengthwise center line 70. The next step of the method
determines not only the angle, but the length of the truncated corners. As
shown in Figure 7, in this step, lines 80a-d are drawn which are both
perpendicular to tangent lines 78a-d and which intersect with the
horizontal constructions lines 72b,d. These lines 80a-d are used as guides
for a corner cutting operation of the rectangular refractory plate 18 to
arrive at truncated corners 54a-d.
Figure 6 also illustrates a more generalized method whereby the
angle of the truncated corners 54a-d may be determined. In this
generalized method, construction segments 82a,b (each of which is one
orifice diameter D in length whether the orifice used in the plate is the
maximum diameter or not) are drawn at right angles to the length and
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width of the plate to form a square as shown. In this more generalized
method step, the angle of the truncated corners 54a-d is any line falling
within the angle B defined at its vertex by tangent point 76c, and on one
of its sides by a line extending through the intersection 84 of the
aforementioned segments 82a,b and on its other side by the intersection 86
of the converging lengthwise and widthwise edges of the plate 18 prior to
truncation. Any of the lines within the angle B may be used to create a
truncation angle by the construction of a line at right angle to any one of
these family of lines. Each such right-angled line should extend through
the intersection of the horizontal construction lines 72b,d so that the length
of the truncated corners may be determined as well as the angle.
Figure 8 illustrates a plate 18 whose corners 54a-d have been
truncated in accordance with the more specific embodiment of the method,
wherein lines at right angles to the tangent lines 78a-d are used to
determine the specific truncation angles. After the corners have been so
truncated, they are preferably radiused at their ends as 90 as is illustrated
in Figure 9. Such rounding of the corners helps to prevent the generation
of localized stresses in the corner regions of the plate 18.
Figure 10 illustrates the final product of the proportioning and
corner truncation method of the invention. In particular, it should be noted
how the 3.OD width of the plate 18 allows it to accommodate the tube
fixture 20, which has a mounting plate 21 which is 2.5 orifice diameters
in both length and width.
Figures 11 through 13 illustrate a method that may be used to
determine both the length and width proportions of the throttle plate 24
relative to maximum orifice diameters D, as well as the angle that the
corners 30a-d should be truncated. With respect to the upper half of the
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throttle plate 24, the shut-off stroke requires, as it did in the case of the
stationary plate 18, at least one and one-half maximum orifice diameters
or 1.5D. An additional 1.50D of refractory plate must be added beyond
the center of the position of the orifice 19 of the stationary plate 18 in the
shut-off position to provide enough plate length for the hydraulic linkage
to engage and manipulate. Accordingly, the length of the upper half of the
throttle plate 24 must be 1.5D + 1.5D = 3.0D. Turning now to the lower
half of the plate 24, in order to accommodate a back throttling position, at
least two-thirds of an orifice diameter or .66D is required. Moreover, at
least 2 diameters of length are required beyond the back throttling position,
both for adequate support as well as for an adequate seal-up surface for
the prevention of unwanted aspiration between the plates. Hence the total
length of the bottom half of the throttling plate must be .66D + 2.OD for
a total of 2.66D. Adding the upper and the lower halves of the throttle
plate 24 together, the total length comes 3.OD + 2.66D = 5.66D. The
width of the plate is determined in the same manner as the stationary plate
18 for manufacturing compatibility and expediency. Accordingly, the
width of the throttle plate 24 is 1.5D + 1.5D = 3.OD.
Turning now to Figure 13, the angle of the truncated corners 30a-d
of the throttle plate 24 are determined by precisely the same methodology
as described with respect to the stationary plate 18 (and in particular,
Figure 6). Accordingly, there is no need to repeat the details of this step
of the method. It should be noted that, in addition to the specific method
described with respect to Figure 6 wherein the angle of the truncated
corners is determined by lines constructed at right angles to the previously
described tangent lines 78a-d, the generalized method step described with
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respect to the upper right hand corner of the plate 18 in Figure 6 may also
be applied to the corners of the throttle plate 24. While this invention has
been described in the context of a single
preferred embodiment, various modifications, additions, and variations will
become evident to persons of skiIl in the art. All such modifications,
additions, and variations are intended to fall with the scope of this
invention, which is limited only by the claims appended hereto.
