Discussion by H. F. Muecke

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4 66 Open Hearth Proceedings, 1965 industry; they are looking for repeat business. As long as a material is marketable, there is little incentive for a supplier to change to a newer material. This means the consumer must supply the pressure. Many refractory companies offer technical assistance in the use of gunning and other special materials for which they are to be commended. However, gunning products will only improve at a rate relative to the pressure brought to bear by the consumer. CONCLUSIONS As stated earlier, this paper is presented only to stimulate interest in the bonding principles for gunning. Whether we have the optimum bond is beside the point. The main thing is that additional work needs to be done. The great concern we have is that gunning may come into disrepute from misuse and misap- Cold gunning of open hearth furnaces at South Works has been well established as a standard practice. Four furnace areas are gunned-uptakes, slag pockets, checker chambers, and the flues. The purpose of the application is to prolong the service life of the refractories and to reduce air infiltration. The gunning is done by mason department crews during regularly-scheduled shutdowns for brickwork repair which normally occur at 300 heats, 525 heats, and at 725 heats. UPTAKE GUNNING The furnace uptakes are of suspended construction with basic brick in the upper zones and high-alumina brick in the lower zones. Since the area of most severe wear occurs in the upper zones, most of the gunning in the uptakes is limited to the basic brick sections. Chrome or chrome-magnesite materials are used with either hydraulic setting properties or sodium silicate as a bonding agent. Both types have worked successfully with little apparent difference, either in application or results; however, the sodium-silicate types seem to be more prone to slump during application. The thickness of the brickwork in the uptake and the age of the furnace are the criteria for determining whether the brickwork will be gunned or replaced. If 200 heats are expected before the next shutdown and a thickness of 5 in. of the original 15 in. of brick remains, the uptake is gunned, applying 2 to 4 in. of material. If the old brickwork is less than 5 in., it will be replaced. H. F. MUECKE is assistant superintendent, masonry, South Works, U. S. Steel Corp., Chicago, III. plication. Gunning as a whole may be thrown out as uneconomical, although the trouble may be in the materials and practices used. The background on castables was similar back in the 1940's. At that time anyone who could mix material together with early strength hydraulic cement and put it in a bag could sell it as castable. There were many failures, and castables as a whole had a bad name to live down. Fortunately, the marginal operator went out of the picture during a depressed business era and refractory companies with improved products were able to regain a good name for castabies. We would not like to see gunning go through a similar period. At the moment no refractory material is in sight which is going to be the answer to all steel mill refractory problems. With the processes available today, operators have the Discussion by H. F. Muecke Fig. 1 shows the partially gunned suspended nose on the endwall side of a furnace uptake. Some tuckpointing can be seen in the lower center area of the picture. This is done prior to gunning to fill large joint openings which occasionally occur at the shoe elevation. Fig. 2 is a picture of the same uptake showing gunned material on Fig. I-Outer nose of the uptake partially gunned. Table I. Results of Uptake Gunning. Average Heats on Uptakes three sides. The brick in the fantail nose in the center of the picture was about 6 in. thick, and some 4 in. of material was gunned into place. The light colored area on the left is the burned surface of 60 % alumina brick in the sloping chill wall. Heavy sections or new brickwork are not gunned. Prior to the gunning program, brickwork less than 7 in. thick was usually replaced. The life of many thin uptakes has been ex- Ex- Gunned tension at Life in Life 21 Furnaces 4~ Minimum Extension Maximum Extension ability to burn down any refractory structure we can provide. Basically, we must learn to live with the raw materials we have. We must learn to put them together better and to our best advantage. This can best be accomplished through a cooperative program, some of it necessarily being trial and error. In almost all instances, refractories are a compromise as one physical property is generally obtained at the expense of another. The compromises which fit one application, design, or operation, may be wrong for another. This is the reason we have such a variety of refractories available today. There is a great deal of interest in gunning at the present time, both in practice and in materials. If we can keep it pointed in the right direction, always keeping in mind the economic viewpoint, the steel industry can profit greatly. tended to the next shutdown by gunning to avoid the more costly method of replacement. Table I shows the results of uptake gunning in 21 open hearth furnaces. The gunning was done when the furnaces had an average of 422 heats. This extended the average life of the uptakes to 625 heats, resulting in an average increase of 203 heats in uptake life. The minimum extension in life was 103 heats and the maximum extension was 324 heats. When uptakes become thin during the furnace campaign, hot gunning is scheduled on a weekly basis. This is done by open hearth personnel using a chrome-based refractory material with a slurry-type gun. Several uptake wall failures have occurred prematurely. In these cases the failed areas were gunned to keep the furnace operating until a convenient shutdown period occurred. Trials are now in progress using a sequestered phosphatic solution as a bonding agent with a chrome-based gunning material which has very good gunning characteristics. Due to its rapid set, a thickness of 6 in. which appears to obtain a hard set throughout, can easily be achieved. Fig. 2-Uptake, partially gunned.

5 Basic Open Hearth Operations 67 Although the trials are in a stage too early for proper evaluation of service life, the material seems to have merit as an uptake gunning material. SLAG POCKET AND CHECKER CHAMBER GUNNING The slag pockets are constructed of high-alumina brick of the 60% and 70 % grades. Gunning in this area is done primarily to seal cracks and joints, although some thin main walls have been gunned to successfully prolong service life. Sealing is done with a pyrophyllitic material, and a hydraulic setting high-duty fireclay material is used to build up walls. The pyrophyllitic material is applied as a sealing agent in the checker chambers. Joints and cracks, primarily at the springline, are cleaned by scraping and gunned. Air infiltration has been reduced to a range of 8 to 12 % compared to the previous 12 to 16%. FLUE GUNNING Flue arches and walls in the shop have been gunned with a type C castable gun mix, which has been very effective in preventing deterio- Fig. 3-Gunned stack flue after 18 months service. ration of the brickwork. Gun placed material in service for 18 months still appears to be good. Fig. 3 is a view of the furnace flues leading into the stack and boiler. The material on the flue arch was solid. Some peeling can be seen on the jambs where the gunned material was thin and the surface relati vely smooth. Fig. 4 shows a gunned flue arch after 18 months of service. Although The economical operation of an open hearth shop is dependent in part upon low maintenance cost and long operating life of each furnace. It is necessary that during the campaign of a furnace its productivity rate expressed in tons per hour be A. V. LEUN is assistant superintendent, labor and construction dept., Bethlehem Steel Co., Bethlehem, Pa. Fig. 4-Gunned flue arch after 18 months service. irregular in appearance, it was solid and only minor cracking and spalling was evident. HOT METAL MIXER GUNNING Two hot metal mixers of 600 ton capacities have been maintained during the past three years by a combination of relines and cold gunning applications. After a mixer is relined, it is maintained by taking it out of service at five month intervals for gunning. The mixers are lined with a super-duty firebrick and the gunning material used is a first quality clay type with hydraulic setting properties. From 2 to 5 in. of material is gunned on the eroded areas of the walls, using an average of 13 tons on each job. The best results are obtained when the surface to be gunned is rough and irregular. The five month shutdown interval corresponds with a shutdown needed to repair the brick rings of the charging hole, and the jambs, bottom, and walls of the discharge spout. Eroded parts of the jambs and spout are not gunned, but are rammed with an 80% alumina material. The gunning practice has more than doubled the period of time between relines. Previously the average mixer life between relines was 10 months. By gunning the life has been extended to 22 months. CREWS AND EQUIPMENT The quality of gun placed material is dependent upon the ability of the gun crew. Well-trained people are essential for good results and only time and experience will produce an Discussion by A. V. Leun maintained at a high level and not be allowed to drop appreciably as the campaign progresses. With the continued drive for increased production and maximum furnace availability, the problem of maintaining a furnace in efficient operating condition has become more important than heretofore. Problems such as (1) faster heating up after rebuild, (2) maintenance of air preheat and of low air infiltration, and (3) the refractory patching during expert nozzleman. Our crew consists of a nozzelman, a gun operator and two helpers. The nozzleman and the gun operator are capable of handling either job and alternate positions periodically. The equipment in use is an Allentown Pneumatic gun-type N-l shown in Fig. 5. This is a nozzle-mix type gun which has performed quite satisfactorily. An air operated water pump is used in the water supply line to insure adequate and uniform pressure. During a normal turn of work, which involves moving the equipment to different furnace areas and hooking up air and water hoses, about 200 bags of material each weighing 100 lb can be gunned. Fig. 5-Gunning equipment. An essential part of the gunning practice is cleaning and maintenance of the gun. The equipment must be lubricated daily and properly cleaned after each use. About every two weeks the gun must be completely dismantled, inspected for worn parts, cleaned, and lubricated. This responsibility has been assigned directly to the gun crew, and has proven very effective in keeping the gun in good operating condition. CONCLUSIONS The gunning practice has proven to be an effective means of prolonging refractory service life and reducing air infiltration. However, considerable judgement is required in determining gunning application possibilities as its use is not intended to replace brickwork, but rather to add thickness to thin brickwork, thereby adding a calculated number of heats of service. operation, especially in those places not accessible to normal repair, etc. have brought the use of shooting to the fore. Shooting can best be defined as the application of a monolithic material mixed with the proper amount of water to an area or surface by means of a machine operated by air and designed for that purpose. The material shot can be a refractory nature, an insulating medium, a sandcement mixture, or just sand.

6 68 Open Hearth Proceedings, 1965 It is not the purpose of this discussion to compare the merits of the various types of machines now used for this purpose. It is merely our desire to bring out the many applications where such equipment can be used to good advantage and which will result in better cost and longer furnace operating hours. In those open hearth furnaces built of silica brick in slag pockets, uptakes, and roofs, too fast a heating rate in the early stages of light-up after a rebuild will often result in excessive spalling of the brickwork. In many instances it has been found that although the roof had been satisfactorily brought up to 1200 F, the fantails and slag pocket arches were still in the critical spalling range for silica brick, with spalling, sometimes excessive, occurring in these parts. Fig. I-Applying thin application of insulation to slag pocket arches and fantails during rebuilding. This condition was corrected by shooting an insulating material on all brickwork below floor line. The material applied to a thickness not exceeding 3fs in., by retarding heat flow to and from the protected brickwork, minimizes the danger from thermal shock. This procedure has eliminated spalling of brickwork in the slag pocket arches, fantails, and uptakes. Fig. 1 shows shooting as described above. We have found this insulating procedure to be very advantageous when applied to the central section of silica roofs before lite-up. This application retards the heat flow into the brickwork, preventing too rapid heating and subsequent early expansion through the key of the roof. External sealing of open hearth furnaces to minimize air infiltration is of paramount importance. This sealing must be done as soon as a furnace is put in operation with follow-up shooting taking place when expansion of the walls has been completed. This external sealing of the walls, while doing an excellent job, is sometimes not too effective, particularly in the areas of the flues, slag pockets, and checkers. Besides being difficult to seal, some sections of the open hearth system are inaccessible during operation. Reactions occurring in an open hearth furnace often create excessive furnace pressures for short periods of time. The relief for these pressure periods occurs through brick joints in the walls. Too often, insulation applied externally becomes loosened and increased air infiltration develops. While recognizing that sealing of walls externally is important, the sealing of these walls internally is also important, especially where outside sealing is impossible. When the brick joints of walls are closed with a refractory material on the inside of the furnace system, such as the checker chamber walls, flues, seals, etc., it is extremely difficult to loosen this material with pressure increases within the system. Air infiltration is thus kept to a minimum for a longer period of time. With the added installation of steam flue blower systems in the flues, necessitating a system of trenches in the flue floors and slots of various depths in the walls, it is most economical to repair these trenches by shooting. The shooting of all such flues, etc., accomplishes the job in about one-sixth of the time, besides effecting a tremendous decrease in labor cost. When an open hearth furnace is in operation there is naturally more rapid wear on certain sections than on others. This wear can be from a slag attack or high temperature or a combination of both. Basic roofs will spall or peel more rapidly in some sections than in others. Furnace backwalls, particularly those with little slope will become thin. uptake chill walls adj acent to the burner blocks will scour and in some cases cut through, endwalls will require patching. Here again, in all of these sections the application of shooting is of great value. The life of a basic roof should be tied to the campaign cycle of the furnace which includes the uptakes, slag pockets, checkers, and flues. Since we can get a minimum of 600 heats from basic uptakes which in turn approximates two furnace campaigns in slag removal and checker and flue cleaning, it follows that the Fig. 2-Backwall being repaired. Fig. 3-Building up badly eroded block on operating furnace. economical life of the basic roof must be in units of these or 600 heats minimum. Therefore the maintenance of basic open hearth roofs in order to obtain the maximum economical life per roof is of tremendous importance. This maintenance necessitates the daily shooting of a basic material to those sections of the roof needing repair. This shooting requires about 15 min per day. The maintenance of basic open hearth backwalls has eliminated the necessity of patching during the campaign. This repairing of the backwall with brick necessitated the shu tting down of the furnace. The shooting of the wall is accomplished with the fuel on the furnace. Where backwall repairs were sometimes necessary at 70 to 80 heats before shooting, since its application campaigns in excess of 225 heats have been possible with no repairs to the backwalls other than shooting maintenance. Fig. 2 shows such an application. Open hearth uptake chill walls are inaccessible to patching with brick while the furnace is in operation. To repair this section requires the furnace to be shut down and cooled sufficiently to allow for men to work. Chill walls are often eroded by liquid slag from the chills. This erosion generally occurs in the corners next to the block and uptake walls. Unless repaired the erosion will continue and ultimately cut the arches supporting the walls. Further, the gullies cut into the brickwork will deepen until they have cut through the walls exposing the

7 Basic Open Hearth Operations 69 chills to excessive temperature and the furnace to higher air infiltration. Shooting of these gullies through openings in the endwalls has proven successful and has eliminated the necessity of taking the furnace off for repairing. This shooting is done while the furnace is operating. It is the best procedure to lock the checker reversal so that the furnace is working out of the end opposite to the one being repaired. If the erosion is deep, the shooting should be accomplished in several intervals to allow for release of steam between applications. Silica endwall and uptake wall patching during the campaign of a furnace sometimes becomes difficult because of uneven foundation where the patch is to start. The furnace is too hot to allow for careful level off, this location often being well below floor line. To provide a foundation thick enough to carry the new patch it is sometimes necessary to shoot the section, in order to bring the foundation to a size from which brick can be installed. This procedure, while not necessary at all times, is invaluable on occasions where gullying has taken place too far below floor line to allow for brick repairs. At times the burner blocks erode too rapidly or wash adjacent to the endwalls. To realize the maximum campaign life of a furnace under such conditions, it is advantageous to repair this erosion by shooting. Fig. 3 shows such a repair being accomplished through an opening in the furnace uptake wall while the furnace is in operation. During the rebuild of an open hearth furnace, many problems occur. It is important to tear out and replace a minimum of brick since each brick represents 50 to 60 in tear out and replacement cost plus the cost of the refractory. Fig. 4-Repairing block by shooting. Shooting was resorted to, as shown in Fig. 4 with the finished repair shown in Fig. 5. This reinforcing of the block allowed for subsequent removal of the bottom ring, its replacement, and the building out of the block. When it is necessary to remove and replace tap holes during rebuild shooting is resorted to. The refractory material is uniformly applied, using the proper pressure with satisfactory results. Backwalls can be shot at the end of a rebuild. Basic endwalls, allowed to remain for another campaign are rough and often peeled. It is good practice to apply a shot coating of a chrome magnesite mix to a depth of Ijz to 2 in. in thickness to stabilize any peeled particles and to close up all cavities, increasing the life accordingly (see Fig. 6). The repairing of tap hole piers is easily accomplished with the aid of shooting. This type of repair can be accomplished rapidly with a minimum of lost time. ~. Fig. 5-Block after shooting. In hot metal shops where transfer ladles are required to carry the metal from the mixer to the furnaces, the lining of these transfer ladles with a refractory can be applied monolithically by shooting. It is important that the water content of the applied material be kept to a minimum conducive with reasonable rebound. Such shot ladles must be dried before being put into service and this drying will be more effective the less the content of water. Too high a water content and too rapid drying of the ladle will result in unnecessary shrinkage cracks and peeling of the lining. Ladles properly lined give excellent life besides a lower initial cost of lining. In addition to the many applications of shooting in the open hearth already described, the use of this procedure is invaluable when bottom or front bank repairs are necessary during a furnace campaign, especially after a break-out. Where it is difficult for such repairs to be made by the normal procedure, the shooting of the job is resorted to. Using a bent pipe to guide the material, it is shot into the proper location requiring refractory repair. This method of repairing will produce a denser, more solidly packed patch than if the material was shoveled in by hand. Monolithic applications of refractory bodies to reduce cost is very effective in many other furnaces throughout a steel plant, in addition to the open hearth. Soaking pit covers can generally be repaired by shooting when failure is located in the corners or adjacent to the framework. When rebuilding a row of soaking pits, it is often good practice to install the top 18 in. of pit wall monolithically by shooting. This eliminates brick joints, making the curb section more rigid. Buggies of car bottom or hea ting furnaces often lend themselves to installation of the refractory by shooting. Hot metal cars used to transport metal from the blast furnace to the open hearth are lined with refractory. Under constant usage, erosion often occurs in the lining, mostly in the vicinity of the spout. Very often the life of such cars is extended by the shooting of refractory into the worn sections. In forge or mill furnaces, burner walls are generally best installed by shooting with a resulting faster installation and more uniform finished job. Occasionally when a blast furnace is taken ou t of production for a bosh or top repair it becomes evident that a scoured out doughnut shaped erosion has occurred in the inwall section. This type of repair has been effectively made by shooting. A repair on such a stock lining, where only 8 in. of lining remained at its thinnest section, required three days, working day shift only. Approximately 100 tons of material was used. A few of the many repair jobs accomplished by shooting have been discussed in this paper. Its applications are limitless. In these days of continual striving for a minimum of cost w ith an increased life of furnace units, the shooting of refractory materials to existing furnace walls or sections is of tre- Fig. 6-Applying refractory coating to inner face of open hearth endwall during rebuild. mendous importance. While such shooting of refractories is accomplished, in many cases, on specific sections during the rebuilding of a furnace or section of a furnace, its value is greatest when used in the maintenance field of furnace operation. It is here that the greatest savings have been made. There is hardly a job that cannot be repaired by shooting.