Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND
This invention relates to the thermochemical desur-
facing of metal workpieces, commonly called scarfing. More
specifically, this invention comprises a method and apparatus
for preheating the surface of a metal wDrkpiece where a
scarfing reaction is to be started.
A complete scarfing cycle usually consists of three
steps: (1) positioning the workpiece in register with the
scarfing units, (2) preheating the workpiece to form a molten
puddle, and (3) carrying out the scarfing reaction with a
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stream of scarfing oxygen while causing relative motion between
the workpiece and the scarfing unit or units. This invention
is concerned principally with the preheating step.
The prior art discloses several methods for performing
the preheating step. Jones et al, in U.S. Patent No. 2,267,405,
discloses preheating with a flame produced by co~bining oxygen
and fuel gas within a torch and igniting the gas mixture as
it leaves the torch. The problem with combining oxygen and
fuel gas within a torch, hereinafter referred to as "pre-
10 mix~ng", is that the explosive mixture is subject to flashback,
i.e. ignition inside the torch, which can damage the torch
and become a safety hazard.
An improvement in the pre-mixed flame was disclosed
by Jones et al in U.S. Patent No. 2,356,197, in which oxygen
and fuel gas are combined just prior to being discharged from
the nozzle. While this was an improvement in the state of
the art, the apparatus was still subject to flashback. If
the outer nozzle were to be plugged, e.g. with spattered
metal, while the oxygen and fuel gas holes inside the unit
remained ~pen, the two gases could mix inside the unit, there-
by creating an explosive mixture subject to flashbac~.
Allmang's U.S. Patent No. 3,231,431 discloses post-
mixed preheating apparatus wherein the oxygen and fuel gas
are combined outside the unit, thereby completely el~minating
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the possibility of flashback. However, the intensity of the
flame produced by this post-mixed apparatus is limited.
While Allmang's method can be used to preheat hot workpieces,
its low intensity flame requires an unacceptably long time
to preheat cold workpieces.
Lytle's U.S. Patent No. 3,752,460 discloses post-
mixed preheating apparatus that uses a stream of "trap" oxygen
to decrease preheating time. While Lytle's invention is an
improvement over Allmang, Lytle's apparatus is not capable of
preheating relatively cold workpieces fast enough for commer-
cial operations.
Engel's U.S. Patent No. 3,966,503 discloses a method
for making an instantaneous scarfing start, that reduces the
time required for preheating the workpiece virtually to zero.
Engel's method is faster than the method of the present inven-
tion; however, Engel's method requires a rod feed mechanism
and a high intensity jet of oxygen, not required by the present
invention. Hence, the present invention is advantageous when
an instantaneous scarfing start is not required, but a fast
start on cold steel is desired.
Until the present invention, it has not been
~ possible to rapidly preheat a portion of the surface of a
relatively cold metal workpiece to scarfing temperature, using
10791~1
a flame, without danger of flashback, or without using rods,
high intensity blowpipes or other adjuvant material.
OBJECTS
Accordingly, it is an object of this invention to
provide a method as well as apparatus for scarfing the sur-
face of a workpiece that provides acceptably short preheat
times for scarfing relatively cold workpieces, without being
sub~ect to flashback, and without requiring adjuvant material.
It is another object of the present invention to
provide a method as well as apparatus for producing a post-
mixed scarfing preheating flame that is more intense than
those produced by the prior art.
SIJMMARY OF l~IE INVENTION
The above and other objects, which will readily be
apparent to those skilled in the art, are achieved by the
present invention, one aspect of which comprises:
A process for thermochemically scarfing a metal
workpiece comprising:
(1) preheating a spot on the surface of the
workpiece where the scarfing reaction is to begin by directing
a post-mixed preheating flame at said spot, said preheating
flame being formed by:
(a) disc~ rging at least one stream of
preheat oxidizing gas and at least one stream of preheat fuel
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gas from separate ports in such a manner that said streams
impinge external to their discharge ports, above the work
surface and in such manner that the axes of said streams form
an acute included angle between them, and
(b) stabilizing said preheating flame by
discharging a low-intensity stream of oxidizing gas, the
direction of said stabilizing stream being in the same general
direction as the direction of said flame and prox~mate to the
impingement of said preheat oxidizing gas and preheat fuel
gas streams, and
(c) continuing steps (a) and (b) until said
spot reaches its oxidizing gas ignition temperature, and
thereafter
(2) directing a stream of scarfing oxidizing gas
at an acute angle to the work surface at said preheated spot,
while simultaneously
(3) causing relative movement bétween said
8carfing oxidizing gas stream and said work surface, thereby
producing a scarfing cut.
A second aspect of the invention comprises:
Scarfing apparatus comprising: (a) means for forming
a post-mixed preheating flame, (b) means for discharging a
8tream of scarfing oxidizing gas through a scarfing nozzle,
and (c) means for producing relative motion between the
scarfing oxidizing gas and a workpiece, characterized in that
said means for forming said preheating flame comprises:
(1) orifice means for discharging a stream of
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preheat fuel gas, the axis of said orifice means being directed
toward the workpiece to be scarfed,
(2) orifice means for discharging a stream of
preheat oxidizing gas, the axis of said orifice being directed
to intersect the axis of said preheat fuel gas orifice means
at an acute included angle, external to- said orifices and
above the surface of the workpiece, and
(3) means for discharging a low-intensity stream
of stabilizing oxidizing gas, said means comprising an orifice,
the axis of which is directed proximate to the intersection of
the axes of said preheat fuel gas orifice and said preheat
oxidizing gas orifice and directed in the same general direc-
tion as the resultant of said axes.
In other embodiments of the invention, the low-
inten8ity stream of oxidizing gas may pass through the
impingement of the-two-preheat streams and/or the low-
intensity stream may form an angle at from 10 to 90
with the forward axis of the flame.
The preferred oxidizing gas is oxygen and the pre-
ferred included angle of impingement between the preheat gasstreams is from 5 to 50. The preferred embodiment uses the
~ame orifice to discharge the preheat stabilizing oxygen stream
as well as the scarfing oxygen stream.
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The term "oxidizing gas" as used throughout the
present specification and claims is used to mean a gas con-
taining an oxidizing agent. The preferred oxidizing gas is
commercially pure oxygen, and for simplicity the term "oxygen"
is used hereafter throughout the specification. However, the
invention may be practiced using oxidizing gases other than
pure oxygen. For example, the oxidizing gas for scarfLng and
stabilizing can be oxygen having a purity as low as 99 percent,
or lower. However, the results will be poorer with impure
oxygen, especially with oxygen of less than 99 percent purity.
The preheat oxidizing gas can contain as low as 21 percent
oxygen, i.e. it can be air, but preheat times will increase
with decreasing oxygen percentage in the preheat oxidizing gas
stream.
The term "preheat" is used to mean bringing a por-
tion of the surface of a workpiece to its oxidizing gas igni-
tion temperature; that is, the temperature at which the work-
piece will ignite when in an atmosphere of oxidizing gas.
IN THE DRAWINGS
Figure 1 is a side view of a scarfing unit illustrat-
ing a preferred embodiment of the present invention.
Figure 2 ~s a sectional view of Figure 1 viewed
along line 2-2.
Figure 3 is an enlarged side view of Figure 1
illustrating the key elements of the invention.
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Figure 4 illustrates the preferred location of the
molten puddle with respect to the scarfing oxygen stream for
scarfing starts on the flat portion of a work surface.
Figure 5 illustrates a start on the end of a work
surface.
Figure 6 graphic~lly compares the preheat time
obtained by practice of the present invention with prior art
methods of preheating the work surface.
Figure 7 illustrates an embodiment of the invention
in which the preheat streams are discharged from the lower pre-
heat block of the scarfing ap~aratus.
Figure 8 is a side view of apparatus having separate
stabilizing oxygen and scarfing oxygen ports.
Figure 9 is a front view of the apparatus of Figure
8 viewed along the lines 9-9.
Figure 10 is a front view of an alternate method
of constructing the appara~us of Figure 8.
Figure 11 is a side view of apparatus having
separate stabilizing oxygen and scarfing oxygen ports
wherein the stabilizing and preheat streams impinge on
a common locus.
Figure 12 is a side view of apparatus similar
to that of Figure 3, but with the stabilizing and preheat
streams impinging on a common locus.
Figure 13 is a s~de view of apparatus in which
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the Qtabilizing stream is directed proximate to the
lmpingement of the preheat streams, but not in the same
general direction as the flame.
Figure 14 ~s a side view of apparatus similar
to that of figure 13, but with the stabilizing stream
passing through the impingement of the preheat streams.
DETAILED DESCRIPTION OF THE INVENTION
Figures 1, 2 and 3 illustrate a preferred embodLment
of the invention. A typical scarfing unit is comprised of an
upper preheat block 1, a lower preheat block 2, a head 3, and
a shoe 4. The blocks 2 and 3 are called preheat blocks because
preheating flames are discharged from these blocks in conven-
tional apparatus. However, in the apparatus illustrated in
Figures 1, 2 and 3, only the flames discharged from the upper
preheat block are used for preheating. A slot-like scarfing
nozzle 16, from which a sheet-like stream of scarfing oxygen
is discharged, is formed by the lower surface 20 of the upper
preheat block 1 and the upper surface 21 of the lower preheat
block 2. The lower preheat block 2 is provided with a row
of fuel gas ports 19, communicating with conventional suitable
gas passages (not shown). Oxygen and fuel gas are supplied
to head 3 through pipes (not shown) and then to the respective
gas passages by means well known in the art. The shoe 4 rides
on the surface of the workp~ece ~ during scarfing to keep the
scarfing nozzle positioned a constant distance Z (Figure 3)
from the wor~ surface. The scarfing reaction is carried out
by lmpinging on a molten puddle a sheet-like stream of scarfing
10 .
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oxygen discharged from nozzle 16 at an ac~te angle to the work
surface, while relative motion is caused to take place between
the workpiece and the scarfing unit.
In accordance with the invention, the upper preheat
block is provided with a row of preheat fuel gas ports 17 and
a row of preheat oxygen ports 18, each of said ports communicat-
ing with supply passages (not shown) for fuel and oxygen,
respectively. While the drawing shows the preheat oxygen ports
18 located above the preheat fuel gas ports 17~ the reverse
arrangement, although not preferred, will also work. More
generally, it is preferred that the preheat fuel gas ports
be located between the preheat oxygen ports and the
below-described stabilizing oxygen port, but different
arrangements are operable.
The apparatus functions as follows. Preheat oxygen
8treams 9 from ports 18 and preheat fuel gas streams 10 from
ports 17 impinge forming a combustible mixture. The impinge-
ment appears as a point 30 in Figure 3. Upon ignition, the
com~ustible mixture forms a flame 14, having a low intensity
zone 13 and a high intensity zone 12. It has been found that
the high intensity zone 12 may be lengthened so that its tip
27 is ~ust above the surface of the workpiece W, thereby pro-
- ducing a longer, more intense flame, by stabilizing the preheat
flame by providing a low intensity stream of oxygen that passes
proximate to the point of impingement 30 and in the same
general direction as the flame 14. By passing the low inten-
8ity stream 15 "proxi~ate tc" the point of impingement is meant
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that the stream should pass close to the point of impingement
30, but not through it. While the term "point of impingement"
has been used, it should be recognized that the term "locus
of impingement" would be re ?ccurate, since there are many
intersecting streams, hence many points of impingement; and
furthermore, since the streams have thickness, the inter-
sections are areas rather than merely points. Hence, for
brevity, the term "impingement" is used throughout the
specification and claims to mean the locus of the areas of
impingement of the preheat fuel gas and preheat oxidizing gas
streams. The preferred source of the stabilizing oxygen
stream 15 is scarfing nozzle 16. Conventional valve means
(not shown) are provided for producing the low intensity stream
of oxygen 15 (lower in intensity than a scarfing oxygen stream)
through scarfing nozzle 16.
The stream 15 should be directed in the same general
direction as the flame. That is, if stream 15 were resolved
into two vector components, one parallel to and one perpendicu-
lar to the direction of the flame, the vector component
parallel to the flame should point in the same direction asthe flame. When practicing the embodiment of the invention
illustrated in Figure 3 with the preferred values of
Tables I and II, the flæme will be close to the resultant
or (or more correctly, the bisector of the angle formed
by) the axes of the ?reheat oxygen and preheat fuel gas
ports. Preferably, the projections of the axes of stream
15 and flame 14 cross forming an acute included angle
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as illustrated in Figures 1 and 3. It is also preferable
that the axis of the stabilizing stream lS be parallel
to that of the preheat fuel gas stream, as also shown in
Figures 1 and 3.
The preheat fuel gas and oxygen streams must impinge
at an acute angle, i.e. at an angle greater than OD but less
than 90. The preferred ~ange is 5 to 50, and the most
preferred impingement angle is 15.
The stabilizing oxygen stream lS from nozzle 16 must
be of low intensity, i.e. with a lower nozzle velocity than
that of the preheat oxygen and fuel gas from nozzles 18 and
17. Preferably, the nozzle velocity of the stabilizing oxygen
is about 10% of that of the preheat streams.
If the preheat flame were not stabilized as described
above, the length of the high intensity zone (from the
impingement 30 to tip 27) would be so short that the preheating
step could not be performed in acceptably short times unless
the stand-off distance Z were reduced. Reducing the stand-off
distsnce Z to bring the tip of the high intensity zone of an
unstabilized flame close to the workpiece would subject the
scsrfing unit to much more damage from spattered metal and
slag than occurs at normal stand-off distances.
Flames produced by fuel gas from lower ports 19
mixing with oxygen from nozzle 16, are used to sustain the
sc~rfing reaction. These flames are not necessary during pre-
heat, bu~ fuel gas should flow from ports 19 during preheat
to prevent their plugging.
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After the molten puddle forms at spot B, the valve
means controlling oxygen flow from slot 16 are adjusted to
increase the intensity of the oxygen flow from low intensity
to scarfing intensity, and relative motion between the work-
piece and the scarfing unit is started, thereby producing a
scarfing cut on the surface of the workpiece. During the
scarfing operation, the preheat flames formed by streams 9 and
10 are left on at a lower intensity th~n during preheat to
help sustain the scarfing reaction. A baffle 28 positioned
above preheat ports 17 and ~, is used to prevent blow-off of
the low intensity flame during scarfing.
There are several design variables to be decided upon
when fabricating the apparatus of the present invention, many
of which are not independent of each other. For conventional
scarfing apparatus, the following are usually fixed:
~1) G, the angle between the scarfing oxygen and
surface of the workpiece,
(2) X, the height of nozzle 16,
(3) Z, the standoff distance,
(4) U, the width of the scarfing unit, (see Figure 2),
(5) the type of fuel gas available,
(6) the type of oxidizing gas available.
For each set of values for the above parameters, there will be
an operable range and a preferred value for the variables used
in designing preheating apparatus in accordance with the inven-
tion.
_ 14 -
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The following two tables give examples of values that
have been found satisfactory for practicing the invention.
Table I lists a set of typical values of parameters for con-
ventional scarfing equipment,known to produce good scarfing.
TABLE I
G, scarfing oxygen angle ................... - 35~
X, height of nozzle 16 ..................... 5.6 mm
Z, standoff distance ....................... 25 mm
U, width of scarfing unit .................. 270 mm
10 fuel gas ..................................... natural gas
oxidizing gas .............................. oxygen
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Table II gives the operable range and the preferred
value of the variables found useful for practicing the inven-
tion when the fixed parameters are those shown in Table I.
TABLE II
Preferred Approximate
Variable Value Operable Range
Diameter of preheat fuel
gas ports 17: 1.0 mm 0.7 - 1.7 mm
Preheat fuel gas flow rate
per port: 1.7 SCMH 1 - 3.5 SCMH
Spacing of preheat fuel gas
ports (dimension Y, in Fig. 2) 6.0 mm 3 - 16 mm
Diameter of preheat oxygen
ports 18 1.6 mm 1 - 2.3 mm
Preheat oxygen flow rate per
port: 3.7 SCMH 1.5 - 6 SCMH
Spacing of preheat oxygen gas
ports (dimension Y, in Fig. 2) 6.0 mm 3 - 16 mm
Impingement angle between axes
of preheat fuel por~s and pre-
heat oxygen gas ports (angle
D, in Figure 3) 15 5 _ 50
Distance 26 between surface
20 and preheat fuel gas
ports 16 10 mm 3 - 15 mm
Angle between preheat oxygen
port axis and workpiece
(angle H, in Fig. 3) 50 40 - 75
Distances 31 and 32 from
impingement 30 to preheat
ports 15 mm 3 - 22 mm
Distances 29 (see Fig. 2)
between center lines of port
17 and port 18 4 mm 1.5 - 6 mm
Stabilizing Oxygen flow rate
from slot 16 during preheating,
per cm of slot width 6 SCMH 3 - 10 SCMH
-16-
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The variables shown in Table ~I are dependent upon
each other. Therefore, if any are made significantly different
from the preferred value, the preferred value and operable
range of other variables may change. Of course, if any of
the fixed parameters of Table I are changed, the preferred
and operable ranges of some of the variables in Table II may
change. Those skilled in the art will recognize that an
a~most limitless number of combinations of values for Tables I
and II will give satisfactory results.
The preferred shape of ports 17 and 18 is circular,
but other shapes will work. For example, the ports could be
square or rectangular. A single elongated preheat oxygen
nozzle could be used with a single elongated preheat fuel gas
nozzle, although such an arrangement is not preferred. The
invention works best, however, if a plurality of oxygen and
fuel gas ports are provided, arranged in rows opposite each
other as illustrated in Figures 2 and 10. If a plurality of
preheat ports are used, the port spacing, dimension y in
Figure 2, should be uniform. Each oxygen port 18 should be
directly opposite a fuel gas port 17. This preferred arrange-
ment gives the most uniform and fastest preheat, however, non-
uniform port spacing or staggered fuel gas and oxygen ports
- or both will also work.
The flame angle F, i.e. the angle formed by the axis
of the flame 14 with respect to the surface of the workpiece W
should be be~ween 40~ and 55, for a standoff distance Z of
25 mm. If a~gle F exceeds 55, the flame tends to gouge the
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workpiece. If angle F is less than 40, the tip 27 of the
high intensity zone 12 will be too far from the work surface
T to give desirably short preheat times. The flame angle F
is determined by the values of the parameters in Tables I
and II. The above-enumerated preferred values of the variables
will give a satisfactory flame angle, but those skilled in
the art will recognize that many other successful combinations
are possible.
The invention works best if the preheat fuel gas
and oxygen ports are as close to each other as possible with-
out the gases converging within the unit, thereby creating
the possibility of premixing and flashback.
Figure 4 illustrates the preferred location of
starter puddle B with respect to the center line pro~ection
of axi~ 15 of scarfing oxygen nozzle 16, when making starts
on the top surface T of a workpiece W. As illustrated in
Figure 4, the oxygen stream from slot 16 should impinge on
~he rear end C of starting puddle B with respect to the direc-
tion of the scarfing cut indicated by arrow J. This location
of the starting puddle allows all the molten material from
the puddle to be blown forward, thereby leaving none to form
ridges or fins at the rear of the cut.
If a start on the end of the workpiece W is to be
made, as shown in Figure 5, then the results will be satis-
factory if the scarfing oxygen stream from slot 16 impinges
on any part of the starting puddle, since there is no work sur-
face T to the rear of the puddle on which ridges may form
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and it does not mstter if fins are formed on the end surface E.
Figures 7 through 14 illustrate alternative embodi-
ments of the invention which, although not the preferred, are
nevertheless capable of producing a stabilized post-mixed
preheat flame.
Figure 7 is a side view of a scarfing unit that is
similar to that shown in Figures 1, 2 and 3, except that the
preheat oxygen and fuel ~as ports, 18 and 17, respectively, are
located in the lower preheat block 2. The apparatus functions
similarly to the apparatus of Figures 1, 2 and 3.
Figures 8 and 9 show an arrangement in which the
stabiliz~ng oxygen is supplied from port 16' separate from
scarfing oxygen slot 16. Here a stream of preheat oxygen 9 from
port 18, impinges upon a stream of preheat fuel gas 10 from port
17 to form a post-mixed flame 14. The flame is stabilized
by a low-intensity stream of oxygen 15' from port 16', directed
proximate to the impingement 30, and in the general direction
of the flame. The preheat and stabilizing ports 17, 18 and
16' are shown located in upper preheat block 1. They could
also have been located in lower preheat block 2. ~fter pre-
heating is accomplished, a stream of scarfing oxygen from
slot 16 is turned on to scarf the workpiece. As described
previously, fuel gas discharged from port 19 helps sustain
the scarfing reaction.
Figure lO is the same as Figure 9, except that the
~tabilizing oxygen originates from an elongated, slot-like
nozzle 16". T~e preheat oxygen and fuel could also be supplied
-19-
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fro~ elongated, slot-like nozzles, although such an
arrangement is not preferred.
Figure 11 iB a side view of apparatus having
stabilizing oxygen ports 16' separate from scarfing oxygen
port 16, similar to figure 8. However, the stabilizing
oxygen stream 15' passes through the impingement 30 of
preheat oxygen stream 9 and preheat fuel gas stream 10.
It has been found that baffles 28 and 28', while not
absolutely necessary, increase the range over which the
flow rates of the preheat and stabilizing streams may
be varied and still produce a stabilized flame, If the
fuel gas port is not between the preheat and stabilizing
oxygen port, a baffle near to the fuel gas port is
especially helpful,
Figure 12 is a side view of apparatus in which
both the stabilizing and scarfing oxygen are discharged
from the same nozzle, nozzle 16, as in figure 3, How-
ever, in figure 12, the stabilizing oxygen passes through
the ~mpingement of the preheat streams. This arrange-
ment, although not as preferable as that of figure 3,is Also capable of producing a stabilized preheat flame,
~ provided that impingement 30 is located above the workpiece
(not ~hown).
It i9 preferred that the stabilizing oxygen
stream be directed in the same general direction as the
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~079~81
flame. As stated previously, this means that, if the
8tabilizing oxygen stream use resolved into two vector
components, one parallel to and one perpendicular to the
direction of the flame, the vector component parallel to
the flame will point in the same direction as the flame.
However, it has been found that a stabilized flame, capable
of producing short preheat time, can be formed if the
stabilizing oxygen stream ~s directed perpendicular to
the flame or even opposite to the general direction of
the flame.
Figure 13 is a side view of apparatus in which
the stabilizing oxygen stream 15' is not directed in the
same general direction as the flame, Here preheat oxygen
stream 9 and preheat fuel gas stream 10 impinge at impinge-
ment 30 as described previously, forming post mixed flame
14 having forward axis 22, defined as the line segment of
the central axis of the flame lying ahead of the point
(23) at which the stabilizing stream crosses the central
~i8 of the flame, The term "ahead of" is intended to
mean the sam~ direction pointed to by the '~" formed by
the ~mpingement of the preheat streams. Hence, if the
stabilizing oxygen stream forms an angle A of more than
90~ with the forward axis of the flame, the stabiliz$ng
stream is directed in the same general direction as the
flame. However, it has been found that even if angle A
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i8 between 10 and 90, a stabilized flame may still be
produced. It should be noted that the location of the
flame with respect to the forward resultant of (i.e.
the bisector of the angle between) the preheat port
axes is influenced by the direction, velocity, and
flow rate of the stabilizing oxygen. In figure 13,
the stabilizing oxygen stream 15' passes proximate to
the impingement of the preheat streams ahead of the
impingement, Satisfactory results may also be achieved
if the stabilizing stream is directed behind the impinge-
ment,
Figure 14 is a side view of apparatus similar
to that of figure 13 except that stabilizing oxygen stream
15' passes through the impingement of the preheat streams.
This srrangement i8 also capable of producing satisfactory
results,
While not wishing to be tied to a particular
theory, the following explanation of how the invention
achieves ~horter preheat times is submitted. It has
been observed that an unstabilized post mixed flame
-2~-
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formed by the impingement of preheat fuel gas and pre-
heat oxygen only, tends to have a relatively large low-
lntensity zone and a very small high-intensity zone.
In some cases, no high intensity zone can be perceived.
Moreover an unstabilized post mixed flame tends to flutter.
If an attempt is made to increase the intensity of an
unstab~lized flame by inereasing the flow of the preheat
oxygen and preheat fuel gas, fluttering becomes more
pronounced. Finally the unstabilized flame is blown
away from the preheat discharge ports by the increased
gas flow and is extinguished. It has been observed that
baffles help hold the flame in position and allow some-
what higher preheat gas flow rates before the flame is
extinguished,
When a post-mixed flame is stabil zed by a low-
intensity stream of oxygen, in accordance with the inven-
tion, the ~tabilized flame very quickly develops a long,
distinctive high intensity zone and fluttering stops,
The flame remAins stable even though the flows of preheat
fuel gas and preheat oxygen are increased to rates higher
than those which extinguished the unstabilized flame,
The beneficial effect of the stabilizing stream, partic-
ularly when directed as illustrated in Figure 3, is
~elieved to be achieved because:
(1) Since the stabilizing oxygen is added
with a low-intensity stream, it does not interfere with
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the external mixing of the preheat oxygen and preheat
fuel gas 8treams. Yet it adds oxygen that helps support
combustion and provides an oxygen atmosphere surrounding
the high-intensity zone of the flame. This oxygen
atmosphere provides an excellent medium for the flame to
propagate back toward the preheat discharge ports, ignit-
ing unburnt fuel gas closer to said ports.
(2) The stabilizing oxygen stream also form6
a shield to protect the flame from air, which does not
provide as good a flame propagation medium as oxygen
and causes the flame to become unstable and blow away
from the preheat discharge ports.
EXAMPLES BASED ON TABLES I AND II
Scarfing starts on the top surface of a workpiece,
as shown in Figure 4, were made, in the laboratory, using
apparatus having the values set forth in Table I and the pre-
ferred values of Table II. The test results are graphically
represented by curve X in Figure 6, in which the initial
temperature (TC), of the workpiece is plotted along one axis,
2Q while the preheat time required (t) in seconds, is plotted
along the other axis. For purposes of comparison, curve Y
shows the results obtained under comparable conditions using
the scarfing apparatus disclosed by Lytle in U.S. Patent No.
3,752,460, while curve Z shows the results obtainable by a
conventional post-mixed preheat flame formed by oxygen dis-
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charged from the scarfing nozzle and fuel jets, such as dis-
closed by Allmang in U.S. Patent No. 3,231,431.
As illustrated by Figure 6, for cold workpieces, the
present invention is a significant improvement over prior art
preheating methods, giving preheating times less than half
that required by the Lytle method for workpieces above 200C.
For workpieces below 20QC, the present invention requires
significantly less than half the preheat time of the Lytle
method. Note that the graph indicates that the trap oxygen
method of Lytle is unable to achieve preheat times below 20
seconds for workpieces below 250C, while the present
invention requires less than 20 seconds to preheat a work-
piece at 0C.
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EXAMPLE RASED ON FIGURE 11
A post-mixed stabilized flame was produced by
impinging the two preheat streams and a stabilizing stream
at a common locus as shown in Figure 11. Table III gives
an operable range and preferred values of variables
useful for practicing-the invention. As in Table II, the
variables are dependant on eachother. Deviation from the
preferred value of one may change the operable range and
preferred values of other variables.
ABLE III
Preferred Approximate
Variable Value Operable Range
Angle C (Fig.ll) 25 5-90
Distance from impinge-
ment 30 to preheat end
~tsl~lllzing o~qgen ports 15mm 3-22mm
Stabilizing Oxygen port 15'
Diameter 2mm 1-6mm
Flow ~ate per port 1.3 SCMH 1-4 SCMH
The preferred values of variables not listed
in Table III are the same as those listed in Table II.
-26-
1079i81
Example Based on Fi~ure 13
_ Table IV gives an operable range and pre-
ferred values of variables useful ~r practicing the
invention in accordance with figure 13.
TABLE IV
Variable Preferred Approximate
Value Operable Ran~e
Distances from imping- 15mm 3-22mm
ment 30 to preheat and
8tabilizing oxygen ports
Stabilizing Oxygen port 15'
Diameter 2mm 1-6
Flow Rate per Port 1.3 SC~H 1-4 SCMH
Angle A 80- 10-90
As with ~he previous Tables, the values are
dependent on each other. Changing one value may change
the range of others. The preferred values of variables
not listed in Table IV are the same as those listed in
Table II.
