CA1088135A - Assembly of furnace electrodes - Google Patents

Abstract

IMPROVEMENTS IN AND RELATING TO ELECTRIC FURNACES
ABSTRACT

Electric furnaces of the continuous electric arc type employ carbon electrodes which gradually erode at the tip. The electrodes are constructed of segments, and new segments are attached in situ to the upper ends of existing electrodes, as required to maintain operation of the furnace. The segments are usually screwed together. A problem which has been encount-ered, particularly in larger installations, is a tendency for the electrodes to break causing serious production loss. It has been found, in accordance with this invention, that electrode breakages may be reduced and the over-all efficiency of the operation improved by screwing the electrode segments together under tension.

Description

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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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 - . . .

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;

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(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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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 , _ 9 _ . ~;, .. .~ .
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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, .

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 ~' ~
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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 ~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 ` ' : ``` : ` ~ ` ` ~ `

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 - ~-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 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.

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Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of adding a new electrode segment to an elongate electrode suspended in a furnace wherein the electrode is formed by joining segments having threaded sockets at either end by a correspondingly threaded nipple, the improvement which comprises the following steps:
(a) one end of said nipple is screwed into the socket of a first electrode segment consti-tuting the top end of said electrode at least once while maintaining a tension load between said nipple and said electrode segment, (b) the other end of said nipple is screwed into one of the sockets of a second electrode segment constituting said new electrode segment at least once while maintaining a tension load between said nipple and said second electrode segment, said steps (a) and (b) being effected to result in a combination of said nipple screwed into either one of said electrode segments but not both, in the socket into which said nipple was previously screwed under tension, and (c) said combination is screwed into said socket of the other of said electrode segments while maintaining a tension load between said combination and said other of said electrode segments.

2. In a method of adding a new electrode segment to an elongate electrode suspended in a furnace wherein the electrode is formed by joining segments having threaded sockets at either end by a correspondingly threaded nipple, the improvement which comprises the following sequence of steps:
(a) the said nipple is screwed into the socket at the top of the segment constituting the top end of said electrode at least once while main-taining a tension load between said nipple and said electrode and is subsequently removed, (b) the other end of said nipple is thereafter screwed into one of the sockets of said new electrode segment at least once while maintaining a tension load between said nipple and said new electrode segment, and (c) the combination of said nipple and said new electrode segment resulting from step (b) with the end of said nipple protruding from said combination is screwed into said socket at the top end of said electrode while maintaining a tension load between said combination and said electrode.

3. The method of claim 2 wherein step (a) is carried out prior to said segment constituting the top end of said electrode.

4. The method of claim 2 wherein said tension load is from about 500 lbs. up to that tension load which the threads of said joining members can carry without breaking.

CLAIMS SUPPORTED BY SUPPLEMENTARY DISCLOSURE

5. The method of claim 2 wherein said tension load is applied during steps (a), (b) and (c) only after the engagement of the first threads respectively of the joining members.

6. The method of claim 5 wherein said first threads are the first two threads.

7. The method of claim 2 wherein the screwing in operation of said nipple is effected twice consecutively in both steps (a) and (b).

8. The method of claim 2 wherein a thin spacer disc is placed into each of said sockets prior to each of steps (a), (b) and (c), said disc being dimensioned to engage the lower end of the nipple screwed into the socket and having a diameter up to about 70% of the nominal diameter of said socket.

9. The method of claim 8 wherein the diameter of said spacer disc is up to about 45% of the nominal diameter of said socket.