Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to improvements in and
relating to electric furnaces, of the continuous electric
arc type. Such furnaces are widely used for the manufacture
of phosphorus by the thermal method, and also for making
such products as carbide, ferrosilicon and ferromanganese.
Typically furnaces of the foregoing type employ
downwardly disposed car~on electrodes which gradually erode
at the tip. In order to maintain a constant arc length the
electrode is gradually lowered further into the furnace.
The electrode is constructed of segments, each new segment
being attached in situ to the upper end of the existing
electrode, as required to maintain the operation of the
Eurnace. The usual method of assembly is to provide each
electrode with a screw threaded socket at either end.
Successive segments are usually ~oined by a correspondingly
screw threaded cylindrical connector, known as a nipple,
part of which is screwed into each of the segments joined
thereby. Normally when the segments of a continuous electric
arc urnace electrode are assembled, they are screwed
together under a generally compressive force.
A pxoblem which has been encountered, particularl~
in larger installations, is a tendency for the electrode to
break, resulting in a serious loss of production every time
it has to be replaced. We now believe that a major cause
of stress in the electrode, leading to breakages, has in the
past been poor connection between the faces of the screw
thread on the nipple and the segment respectively. This ~;
arises because these members are constructed from graphite
or other forms of carbon whose mechanical properties make
it impractical to machine the thread with sufficient
accuracy to ensure good contact between each of the faces.
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3S
The usual method of assembly, done under a compressive
force, does not improve this condition because during such
an assembly only the outer face of each screw thread wil~
abrade. Particles abraded from these outer faces may
collect between the inner faces of the threads further de-
creasing the area of contact. Yet, it is these inner faces
that act as load bearing faces when the electrode is
suspended in the rurnace. Thus, the weight of the assembled
electrode may cause col~apse of the surface of the thread at
the highly loaded points of contact and separation of the
abutting end faces of the successive segments. As a
consequence, the whole current then passes throu~h ~he
narrow area of contact between the threads, producing
~ocalized overheating, thermal stresses and ultimately
breakage.
The present invention is directed to an improved
method of assembly of the electrode segments, which overcomes
the prior art problems. It was found that, surprisingly,
stronger and more consistent joints are produced with
resulting decreased electrode breakages and thus improved
overall efficiency of the furnace operation by following a
novel method of assembly.
The method of the present invention comprises a
preliminary step of honing the mating threads of the joint
under tension, followed by the actual addition of the new
segment to the electrode, commonly termed "heading up" the
electrode, also done under tension~
With ~hese operations carried out under tension,
the inner load bearing faces of the screw threads of a
member in the joint are honed by friction with the correspon-
ding faces of the threads on the other member. Thus, an
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135
increased area of contact between the inner faces of the
threads is achieved. This improves ~he load hearing char-
acteristics of the threads thus minimizing the chance of
their collapse. In addition, the much improved contact of
the inner faces of the threads honed together improves
electrical conductivity through the electrode, further
decreasing thereby the chances for differential thermal
stresses and hence breakage.
The initial step involved in the method of the
present invention involves a honing operation, wherein
the nipple is screwed together with the successive electrode
members one or more times while maintaining tension between
the nipple and the electrode member.
Tension between the nipple and the electrode segment
during the honing step, and also during the assembly step,
may be maintained in various ways. One convenient method
is to apply the tension hydraulically. For example, the
electrode segment may be clamped in a fixed vertical position
and the nipple screwed into it ac~ainst the pull of the
hydraulic tensioning device. For example, the tension in
a hydraulic piston may be maintained at the desired level by
a suitably modified bleed valve. In this way, the inner
faces of the screw threads of the socket are honed with
the correspondin~ faces of the threads on the nipple end. `~
The minimum tension for honing in and for assembly
under tension is not critical in that some advantage can be
obtained even at very low loads, but for practical purposes
on large co~mercial installations only relatively trivial
improvements are likely to be obtained at less than 500 lb
tension. The maximum permissible tension for any installation
i5 the maximum load that the threads can carry without the
~0~ 35
risk of breaking. In practice, however, on large installations
the maximum convenient load may be determlned by the amount of
work required to screw home the members. For a commercial
scale operation we have obtained particularly satisfactory
results between 2000 lb and 3000 lb tension, although, of
course, on other plants, higher or lower loads may be
preferred, and can easily be selected.
It is preferred to hone together the nipple with each `
new electrode segment and with the next successive segment
before adding the former to the existing electrode, and to
carry out as much of the assembling as possible o the
furnace to decrease the downtime of the furnace,~ so that
the furnace need only be shut down during actual assembly
with the existing electrode. For example, after the appro-
priate honing operation, it is pxeferred that the nipple
should be screwed into the new segment initially and that
the segment and nipple should then be screwed together into
the existing electrode with a tension being maintained
between the two during the assembly, the socket of the
existing electrode having been previously honed.
It is also possible to hone nipples with the socket
in each end of a se~ment, prior to adding the latter to
the existing electrode, so that the upper socket of the new
electrode segment is prehoned for assembly with a further
electrode segment at an appropriate time. Desirably ater
the new nipple has been finally screwed into the new segment,
the assembly is protected by a steel skirt having a flange
to support the base o the segment and extending beyond
the nipple to preven1: accidental damage thereto during
storage and handling.
To help ensure a tight fit between the successive
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electrode segments when they are finally assembled it is
preferrea to lubricate the abutting faces with an electrically
conductive lubricant, such as, petroleum jelly. Preferably,
when the electrode is finally assembled under tension a
su~ficient torque is applied to the successive segments to
ensure adequate contact between the abutting faces of the
segments.
Although the preferred way to assemble an
electrode according to the present invention is by joining
successive segments by nipples, as described above, it is
also possible to construct an electrode by another embodiment
of the invention from segments which have male or female -
threaded portions at either and respectively, enabling them
to be screwed together directly.
The invention is illustrated by the following
example: '
Examples
~ .
Electrode segments for a phosphorus ~urnace com-
prised carbon cylinders each pro~rided with a threaded
2~ coaxial bore a~ either end. The segments were joined by
correspondingly threaded graphite nipples.
1. Comparative
The former method of "heading up" the electrode
comprised shutting down the furnace, screwing the new nipple
into the upper end of the top segment o the furnace, lower-
ing the new segment onto the protruding upper end of the
QW nipple and screwing it into position. A final torque
was then applied in two stages of 30,000 and 40,000 lbs
ft respectively. The relative angular movemPnt between the
two segments at each stage of torquing was noted and used
as an index of the adequacy of the joint.
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With the above procedure, the joints were generally
inadequate, variable and unpredictable. Down time on the
furnace for heading up averaged 40 minutes and electrode
breakages were frequent. In comparative tests, the torque
required to break the joint was found to be about 30,000
lb ft whIch is substantially lower than the tightening
torque, indicating a poor joint. ~-
2. The Invention
. .
In this example of the invention the honing load on
successive members was maintained during each honing operatio~
by securing the lower member in a fixed vertical position
and suspending the upper member in a ~reely rotatable
suspension rom a hydraulic tensioning device adjusted to
maintain a constant tension by a controlled bleed o fluid
to the cylinder while the upper member was screwed into
position.
In each case honing compxised: cleaning the relevant
threads; screwing the upper member two turns under its own
weight; applying the honing load and screwing the upper
member home; partially unscrewing the upper member and
repeating the sequence. According to this examplè each
hea~ng up sequence comprised:
~a) honing an end (Nla) of the next nipple (Nl) to a
bore ~Sla) of the next segment (Sl);
(b) screwing the nipple Nl into place in segment Sl and
placing over the nipple Nl a protective annular skirt having
an inwardly directed flange at one end, adapted to engage
the end face of the segment, and an outwardly directed
flange at the other;
(c) inverting the segment Sl, with the nipple Nl and the
skirt in position;
... ; - :
3~
(d) honing one end (N2b) of the next-but-one nipple N2 to
the bore tSlb) at the other end of segment Sl and removing
the nipple N2, for subsequent attachment to the next-but-
one segment S2;
(e) shutting down the furnace;
(f~ removing the skirt from segment Sl and applying
vaseline to the lower face of the latter;
(g) placing the segment Sl in position over the electrode
and rotating it to screw the protruding end (Nl~) of the
nipple Nl two turns into the exposed bore of the electrode
(to which it was already honed from step ~D) of the previous `
heading up sèquence); and
(h) applying the honing load and rotating segment Sl
to screw the nipple Nl home: and
(i) applying the final torque~
In comparison with e~ample 1, the joints were
stronger and more consistent. A torque of over 40,000 lb
was required to break the joint. Down time on the furnace
for heading up was only 2Q minutes. In trial runs using the
novel method of heading up, no electrode breakages were
observed over a total period sufficient to produce, on
average, at least six breakages under the previously used
system.
,'.~
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3:;
SUl?PLEMENTARY rlSCLOSURE
In the principal disclosure of this application,
the~e is described an improved method of assembly of
electrode joints for electric furnaces which comprises a
preliminary step of honing the mating threads of the
joint under tension, followed by the actual addition of
a new segment to the electrode, also done under tension.
This supplementary disclosure is directed to improve-
ments in the procedure of the principal disclosure to achieve
an overall improvement in joint quality. The improved
features of this supplementary disclosure will be
appreciated from a typical operation for adding a new
electrode segment to the upper end of an existing electrode
in a furnace.
As set forth in the principal disclosure, the first
stage of the assembly is the preparation of the joint and
comprises an initial honing operation.
The new electrode segment is held securely in a
vertical position by a suitable support. In this position
the electroae se~ment is in identical orientation to that
it will assume, after assembly, at the top of the electrode,
i.e., the threaded socket at the top of the segment will
be the top socket on the electrode after installation. The
top socket of the segment is identiied with a suitable
mark and a corresponding identifying mark is placed at one
end of a thre~ded nipple. This particular end of the
selected nipple is the one to be honed in the socket at the
top end o the new electrode segment. The corresponding
identifying marks serve to ensure that in a future step in
the sequence o "heading up" the electrode in the furnace,
after the new segment has been installed and become part
,
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o~ the electrode and the next new segment is to be acded
to it, this particularly marked pre-honed nipple end will
be the one used as the connector member for its correspon-
dingly marked socket which at that time will be the top
socket on the existing electrode in the furnace.
The threads in the electrode socket and those on
the nipple end usually are first smoothed off by, for
example, wire brushing, followed by cleaning the threads
by, for example, blowing off loose particles with compressed
air.
In accordance with this supplementary disclosure,
a thin disc, typically about 1/4" thick, is then placed in
the base o~ the socket. The disc is dimensioned to engage
the lower end of the nipple screwed into the socket and has
a diameter up to about 70%, preferably up to about 45% of
the diameter of the socket. The disc typically is made of
plywood, although other matèrial with similar properties,
~Q~
such as,-~6~Ls~may be used.
The functions of this di3c during the initial honing
operation are to keep the face o:E the nipple end and that
of the socket base from slamming together when the nipple
is screwed home as is described below and to minimize the
chances of interference between the respective faces of the
nipple and the socket base, which may be caused by possible
excèssive ~evenness of these faces.
The nipple is now suspended over the socket by
suitable means, for example, a chainfall secured to a crane,
in such a way that the end of the nipple, marked as
described above, faces the socket identified with the
corresponding mark.
The nipple is carefully centered over the socket,
-- 10 --
.
L35
preferably by a suitable centralization device, and then it
is screwed home until it snug fi:ts against the disc at the
bottom of the socket while tension is being applied between
the nipple and the electrode segment. Tension during the
screwing in operation can be maintained in various ways.
One convenient method which is described in the principal
disclosure is to apply the tension hydraulically so that
while the new electrode segment is held securely in a fixed
vertical position the nipple is screwed into it against the
pull of a hydraulic tensioning device, for example, secured
to the crane employed for suspending the nipple over the
socket. During this operation the inner faces of the screw
threads of the socket are honed with the corresponding
faces of the threads on the nipple end.
In a preferred way of carrying out the above
described honing operation, in accordance with this supple-
mentary disclosure, no tension is applied during the
engagement of the first or possi'bly the first two threads
in order to minimize t~e risk of breaking these threads
during tensioning, and tension thereafter is applied for
the remainder of the honing operation.
As set forth in the principal disclosure, the
minimum tension for honing under which only trivial improve-
ments are likely to be obtained is 500 lbs. The maximum
permissible honing tension for any installation is the
maximum load that the threads can carry without the risk
o~ breaking. This value can easily be determined for a
particular size and shape of electrode joint. In practice,
so lon~ as it is not higher than the maximum permissible
tension, as defined above, the desirable level of tension
load for honin~ is determined by the maximum effort the
~' ~
. ~ , .
operators carrying out the screwing in o~ the nipple, can
expend in overcoming the frictional resistance of the thread
surfaces while tension is being applied. For a large
commercial scale operation particularly satisfactory results
have been obtained between 2000 and 3000 lbs. tension,
although, of course, on other plants, higher or lower loads
may be preferred and can easily be selected. The magnitude
of the tension load during honing can be monitored, for
example, by measuring the oil pressure in the hydraulic
tensioning device.
One way o carrying out the screwing of the nipple
~nto the socket in accordance with this supp:lementary
disclosure is by attaching a suitable framework to it
equipped with a number of handles against which the operators
exert the required rotationary force.
~ ~t is particularly prefe~red that the operators car-
r~ing out the honing operation maintain a constant and
uniform effort throughout the full entry of the nipple.
Ensuring this can be ~acilitated~ for ~xample, by thè use
of torque wrenches attached to the turning handles.
After completion of the honing operation as des-
cribed a~ove, the honing tension is determined and the
nippla is unscrawed either completely from the end of the
socket, or according to the preferred method o~ operation
in accordance with this supplementary disclosura only to
the first, or alternatively, the second thread from the
top end of the socket. Then the honing tension is re-applied
and the nipple is screwed home again, exactly as described
above for the first honing operation.
At the completion of the second honing operation,
the honing tension is again terminated and the nipple is
- 12 -
~0~ 35
unscrewed all the way out of the socket. This nipple, the
marked end of which is now double-honed to the correspondingly
marked socket a~ the top end of the new electrode segment
is now supplied with a suitable protective cover and set
aside for later use in a future step when adding the next
new segment to the electrode after the present new segment
had been installed and become part of the electrode. The
disc is removed from the socket.
While the preferred way of carrying out the
invention in accordance with this supplementary disclosure
is by a double step of honing as described above, tha
invention encompasses also assembly operations that employ
only a single honing step, as described in the principal
disclosure. While the smoothness and contact area of the
inner th~ead surfaces is much improved after only a single
honing under tension, substantia:L further improvement is
achieved by a second honing step. However, the benefit
obtained after more than two hon:ing operations is only
negligible.
20 ` The new electrode segmen~ is now removed from its
securely held vertical position and is reversed, end Eor
end. The socket at the top of the segment now is the one
that will face the top socket on the existing electrode
when in a later step of the operation shortly to follow,
the se~ment is again turned end for end and made ready for
"heading up" the electrode.
~he segment is fixed securely in the vertical
position. A nipple, one end of which is marked as havin~
been honed to the socket on the top of the existing elec-
trode in the furnace, in a previous honing operation, is
removed from storage and is suspended over the socket of
- 13 -
` ' : ``` : ` ~ ` ` ~ `
the segment in such a way t~t the end of the nipple
opposite its marked end faces the socket.
This end of the nipple and the corresponding socket,
are put through the same steps of operation as described
previously for the socket at the opposite end of the
electrode segment and corresponding nipple end. Thus,
the smoothing of thP threads, the placement of the disc in
the socket, suspending and centrali2ing the nipple, honing
under tension, pxeferably done twice, are carried out exactly
as described above. However, on completion o the honing
the nipple is not unscrewed but is left in position in the
socket.
The new electrode segment with the nipple in pos~tion
i9 now again removed from its support and is again turned,
end for end. It is transported by suitable means to near
the top of the furnace and made ready for addition to the
existing electrode.
In the second stage of t~le assembly, the "heading
up" of the electrode, the furnace is shut down, i.e t ~ the
electric power supply is disconnected.
The new electrode segment with the nipple protruding
downwardly from the now bottom socket, is suspended by
suitable means, for example, a chainfall secured to a crane
; over the existing electrode, in such a way that the protruding
end of the nipple, marked as having been honed to the socket
on top o the existing electrode in a previous honing
oper~tion, faces t~i~; correspondingly marked socket.
The threads in the electrode are cleaned and a
disc, identical to the one used in the initial honing
operation, is placed in the base of the socket in accordance
with this supplementary disclosure. This disc, in addition
:
.,
- 14 - ~-
3L35
to the function described before, has an added funct~on
here, i.e., that of providing a "take-up" for the differ-
ential expansion between the nipple, usually made of graphite
and the electrode, usually made of carbon, once the electrode
is in operation.
The new electrode segment is lowered and the
protruding nipple is carefully centered over the socket,
preferably by a suitable centralization device. The
intended abutting faces of the existing electrode and the
new electrode segment, as described in the principal
disclosure, are now preferably coated with an electrically
conductive lubricant, such as, petroleum jelIy, in order
to help ensure a tight fit between the successive segmènts.
The new electrode segment with protruding nipple is
then screwed home until the nipple snug fits against the
disc at the bottom of the socket. Tension is applied during
the screwing operation. Tension may be maintained, for
example, by the same type of hydraulic tensioning device
that was descri~ed previously in connection with tensioning
during the initial honing operation or by other convenient
means. The magnitude of tension and its limits are the
same as used during the honing operation.
In a preferred way of carrying out the step o~
screwing home the electrode segment in accordance with
this supplementary disclosure, no tension is applied during
the engagement of the first or possibly the ~irst two
~hreads in order to minimi~e the risk of breaking these
threads during tensioning. ;
One way o~ c~ying out the screwing of the nipple
with the new electrode segment into the socket on top of
the existing furnace electrode in accordance with this
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.
.. . . . ..
3~;i
supplementary d;sclos~re is by attaching a sui~able band
to the segment equipped with a number of handles against
which the operators exert the required rotationary force.
A slow steady push applied uniformly is particularly pre-
ferred. Monitoring this, can be done, for example, by the
use of torque wrenches attached to the turning handles.
Preferably, when the electrode is finally assembled,
a chalk mark or other indication is placed across the join
and as described in the principal disclosure, suEficient
torque is applied to ensure adequate contact between the
abutting faces of the segments. The distance of relative
movement of the segments upon application o~ full torque,
a~ determined by the relative positions of the chalk marks
after completion of the torquing~ may be used to judge the
eEficiency with ~hich the joint has been assembled.
As outlined in the princ:;pal disclosure, a preferred
way of practicing the invention is to do as much of the
honing and assembly work as can be practically carried out
without having to shut down the Eurnace. Thus, nipples
are honed to particular sockets of electrode segments as
can be arr~nged conveniently before their use in the
assembly and aEter identifying the corresponding members
~ith corresponding marks these nipples are stor2d, ready
~or use. Also, nipples can be honed to a socket of an
electrode segment, leEt in position and then this combination
of segment and nipple can be stored as a unit, ready for
use. In the embodiment of the invention hereinbefore
described, the Eurnace is shut down only during the final
stage o~ the assembly process. This represents an additional
advantage o~er the prior art of assembling under compression
where every step oE the operation has to be carried out
- 16 -
3~i -
while the furnace is shut down. Loss of furnace production
time, therefore, may be decreased using the present
invention.
The invention is particularly useful in minimizing
the difficulties which result from the use of electrode
support systems which impose stresses on the electrode
joints.
A particularly preferred way of carrying out the `
invention i9 to use it in conjunction with the novel
electrode nipple described in our U.S. Patent No.
4,167,643 issued September 11, 1979.
~ 17
