Capital Repairs of a Blast Furnace
Capital Repairs of a Blast Furnace
Capital repairs of a blast furnace are those repairs which are taken up after the end of a campaign of the blast furnace. During the capital repairs the main jobs which are normally taken up are (i) relining of the furnace, (iii) repairs of the damaged portion of the shell, (iii) Major revisioning of all the equipment with replacement of the worn out components, (iv) replacement of those equipments, instruments and automation items which have become obsolete or completed their useful life, and (v) modification work the need of which was felt during the running of the blast furnace in the campaign which has just ended.
Since the campaign of a blast furnace lasts for a long time (10 years to 20 years), technology upgradation takes place during this period. Hence it becomes necessary to incorporate these technological upgradation features in the blast furnace during the capital repairs. This keeps the blast furnace up to date in its new campaign.
Capital repairs also provide opportunity to steel plant to improve the lining features of the furnace and to enhance the internal volume and hence the capacity of the blast furnace. The improvement in the lining features also help in the enhancement in the campaign life of the blast furnace.
Planning for the capital repairs
Capital repairs of the blast furnace are normally a time bound activity which is to be completed within the shortest possible time and in the assigned budget and hence accurate and close planning is very important for the successful completion of the capital repairs. Planning is required to be done to the minute levels. The achievement of the success in timely completion of the capital repairs within the estimated costs depends on how accurate and detailed planning has been done for the capital repairs.
The major activities which are to be planned for the blast furnace capital repairs include (i) preparatory jobs, (ii) engineering activities, (iii) blowing down, salamander tapping, and cooling of the furnace, (iv) making arrangement for entry of the men and materials into the furnace as well as arrangement for lighting, ventilation and working platforms for working inside the furnace, (v) dismantling of old refractories, worn out cooling plates/staves and jams along with the removal of debris, (vi) cutting and removal of damaged shell plates and procurement and welding of new shell plates, (vii) procurement and installation of furnace refractories, (viii) revisioning of equipment with the replacement of worn out components, (ix) complete replacement of outdated equipments, instruments and automation items including their procurement, (x) carrying out of modification work wherever needed, (xi) carrying out of revisioning, maintenance, and modification jobs in the cast houses, burden stock house, pump houses, gas cleaning plant, and slag granulation plant etc., (xii) pressure testing of the furnace and drying, pre- heating and cooling of the refractories, (xiii) blowing in of the furnace and making of the tap holes, and (xiv) tapping of first slag and metal in the new campaign etc.
Planning for men and materials including consumables as well as for hoisting equipment, tools and tackles are important planning activities. Planning for the men is very important since a large work force is needed for a short period of time. Amongst men masons, fitters and trained welders are the categories of people which are in great demand during the capital repairs. Generally these categories of the people are not available locally in the numbers required and are to be brought from outside.
Also important for the planning is the logistics of the materials since capital repairs involve movement of a large tonnage of a variety of materials in a short interval of time. Transport fleet is to match this requirement. Since a large amount of debris get generated during capital repairs, the debris dumping area is to be identified which is not very far from the capital repair site. Distance of dumping area has its influence on the requirement of the transport fleet.
Mechanization of the capital repairs work reduces the time of the furnace outage and has its effect on the planning. Planning for the repairs is to be done after taking a decision regarding the extent of mechanization during the repairs. Total mechanization of work will need availabilities of de-bricking machine of the size which can be accommodated inside the blast furnace, poclain and/or bob cats, portable conveyors, fork lifts, elevators for lifting materials etc.
Planning is also needed for the systematic storage of capital repair items so that these items are available at the time of need without loss of any time.
The time needed for the capital repairs reduces if certain jobs, which are possible to do parallelly, are planned in that way. However while planning the jobs in parallel, safety of the people are to be kept in mind. Sufficient numbers of safety items such as helmets, gloves, safety shoes, nose masks, welder goggles, breathing apparatus, CO monitors, and electrical safety items etc. are to be planned.
Planning activities of the capital repairs for the blast furnace starts a long time before the furnace is actually blown down for the capital repairs. It starts with the listing of all the major and minor items of work which are required to be taken up during the capital repairs. After the listing a rough estimate of the cost of the capital repairs is made and approvals for this rough estimate are taken so that the long lead items can be ordered to ensure their timely delivery. The cost of the capital repairs is updated with the completion of the planning.
When the detailed planning is completed, bar charts and pert charts are made for the close monitoring of the progress of the capital repairs work.
Preparatory and engineering activities
Preparatory activities are those activities which are carried out while the furnace is still running. These activities facilitate availabilities of materials in time during the capital repairs and avoid delays at that time. Examples are preparation for the salamander tapping, making available water of right pressure at furnace top for quenching and cooling of the furnace after salamander tapping, making available facilities for the installation of blowers and lighting at different levels of blast furnace, making available storage space and storing materials in those storages at different parts and levels of the blast furnace to reduce the movement of the materials at the time of the capital repairs, fabrication of working platforms to be used during capital repairs some of them may be hanging from the top, making facilities at the furnace top for hanging of the working platform to be used during the capital repairs, installation of hoisting equipments and tackles such as winches, telphers etc. which are to be used during capital repairs, temporary cabling if needed for the blowers, lighting and other temporary equipments to be used during the capital repairs, making available sufficient scaffolding materials which can be assembled quickly inside the furnace etc.
Some of the jobs mentioned above will need engineering activities like preparation of fabrication drawings, routing of temporary cables and water pipes, installation of booster pumps for water etc.
Blowing down, salamander tapping, and cooling of the blast furnace
The process of blowing down the blast furnace is explained in a separate article available under link http://www.ispatguru.com/methods-of-shutting-down-a-blast-furnace/. One precaution is necessary which is to be taken during the blowing down. Water and steam which is used during blowing down for cooling the top gases gets dissociated into hydrogen and oxygen. Increased quantity of hydrogen and oxygen can form explosive mixture and hence continuous monitoring of the top gas analysis is essential. Top gas is normally analyzed in a gas chromatograph.
The process of salamander tapping is explained in detail in the articles available under links http://www.ispatguru.com/methods-of-shutting-down-a-blast-furnace/ and http://www.ispatguru.com/salamander-tapping-for-capital-repairs-of-blast-furnace/.
Quenching and cooling of the furnace with water sprayed from the furnace top is an important activity which is taken up after the salamander tapping. This is done to speed up the cooling of the furnace to the ambient temperature. However the quantity of the water sprayed in the beginning is to be controlled so as to avoid forming of the explosive mixtures because of dissociation of water into hydrogen and oxygen. Water quantity can be increased after the temperature of the furnace comes down to a level when the water dissociation reaction does not take place.
After the furnace is cooled down, the furnace is purged with nitrogen for the removal of harmful gases (carbon mono oxide etc.) which may be present in the furnace.
Capital repairs activities
Capital repairs of the furnace start after the furnace is cooled down. It starts with the blanking of blast furnace gas line, opening of the permanent openings provided in the furnace, removal of the tuyeres, coolers and blow pipes and dismantling of the top equipment for revisioning. Furnace shell is cut in a planned way at appropriate places for entry of men, materials and machinery. Arrangements are made for lighting and ventilation inside the furnace for people to work inside. After this the hearth bottom is inspected to ensure that the entire salamander has been removed and the furnace has clean hearth. If salamander is remaining it is to be removed by making it loose. Sometimes lancing or use of explosives is needed for the removal of the left out salamander.
After ensuring of a clean hearth, the dismantling work for the old lining starts. To save time of dismantling, de-bricking machine mounted on movable suspended platform is used in some furnaces. The dismantling is followed by the debris removal and cleaning which is usually done by deploying a poclain and temporary conveyor. After the cleaning is over then the actual repairs work starts as per planning which include shell repairs, erection of throat lining plates and cooling staves, and refractory lining.
Refractory lining work is a massive work and is the main work which is to be carried out inside the furnace. Normally it is carried out parallelly at several fronts using hanging platforms. It is very important that debris of the lining of the upper level platform must not fall on the lower level platforms. Also it is important that it is carried out under the close supervision of refractory engineers round the clock to ensure the integrity of the refractory lining. Presence of the refractory engineer is also needed for the taking on the spot decisions.
Activities after completion of the refractory lining
After the refractory lining is complete all the lining debris is to be removed and the lining is to be inspected by a team of executives along with the contractor to ensure the quality and integrity of the lining work. After this is ascertained, the erection materials need to be removed from the furnace. Then the shell plates cut for the entry are welded back and lining is carried out in those areas again under the supervision of a refractory engineer. The top equipments are assembled back and the permanent openings are closed. After this the coolers, blow pipes and tuyeres are put back and the furnace is pressure tested. After this the furnace is ready for drying, preheating and cooling of the refractories which is to be carried out as per the drying, preheating and cooling schedule. After cooling there is a need for the inspection of the refractories. These days in many furnaces to speed up the process, the preheating and inspection parts of these activities are dispensed with. There are some cases where people have gone directly for blowing in after the pressure testing. However, it is not desirable.
After this the furnace is blown in. The details of furnace drying and the blowing in process are given in a separate article under the link http://www.ispatguru.com/the-process-of-blowing-in-of-a-blast-furnace/.
Capital repairs are a gigantic job which is to be carried out in a limited space. A large work force belonging to a large number of contractors works at a time. Also work in a big way is carried out simultaneously in pump houses, burden stock house, slag granulation plant, and work shop etc. The refractory lining work in the confined space at the blast furnace and the simultaneous work at other places need effective and close co-ordination and supervision to avoid any confusion and misunderstanding. Communications during the capital repairs are to be perfect and all the documents which need reference are to be available at the site. Hence the site office of the capital repairs is a nerve point of the capital repairs and need to be strengthened by round the clock with posting of those senior executives who can take decisions at the time of need. Also during capital repairs time any issues related to industrial relations are to be avoided in the plant.
Safety and emergency services
Safety of the people, materials and machinery is the most important necessity during the capital repairs. Even a small accident can delay the completion of the repairs by several days and a big mishap can cause a catastrophe. Besides accidents have a big demoralizing effect on the work force. Hence round the clock safety supervision by safety inspectors with powers to stop the work in case of safety violation is very important. It is better to pre-inform the factory inspector about the capital repairs being taken up at the blast furnace.
It is also important that adequate fire-fighting facilities are available at all the floors of the blast furnace. Even a small fire during capital repairs has got the potential to upset the complete schedule of the capital repairs. Bigger fires can cause catastrophe. Round the clock availability of the fire tenders and fire service men are needed during the capital repairs.
The blast furnace capital repairs site must have the first aid facilities and round the clock availability of ambulance to take care of emergencies.
Given below are some of the photographs (Fig 1, 2, and 3) taken during the capital repairs of BF 1 of Visakhapatnam Steel Plant during 2013.
Fig 1 Photographs of capital repairs of BF 1 at Visakhapatnam Steel Plant
Fig 2 Photographs of capital repairs of BF 1 at Visakhapatnam Steel Plant
Fig 3 Photographs of capital repairs of BF 1 at Visakhapatnam Steel Plant