Blast Furnaces: Making a Good Thing Even Better

In recent years, the blast furnace has had tremendous upgrades. One of the major contributors to this is the improvement to the cooling systems and the refractory design of the furnace. In addition, new top designs and furnace structural design has enabled us to operate at much higher pressures. Along with these improvements, we now have more sophisticated equipment to monitor the furnace conditions and have better burden distribution capabilities. Our ability to record operating conditions and to operate from computers has been a tremendous advantage to maintain a consistent operating condition within the blast furnace. We have improved production capabilities through the use of injection systems at the tuyeres that result in higher production capabilities and better furnace operations as the injection systems can be adjusted much faster to better influence the furnace operation. Hot blast temperatures have been increased dramatically which is a tremendous fuel savings.

In order for the blast furnace to operate at these improved conditions, all the support systems must also be improved and upgraded. We must have better stockhouse controls and feeding systems to the furnace. We require better and higher pressure blowers that supply cold blast air to the stoves which heat the cold blast and produce the hot blast to the furnace. With the increased temperatures, the stoves and valves and hot blast main must also support these higher temperatures and pressures. We require better gas cleaning systems and heat recovery systems to enable us to conserve fuel and improve efficiency.

We also can not forget that we no longer have spare hot metal capacity in integrated facilities as we did in the past. We must have a more reliable and consistent operation because when the furnace does not operate, the steel plant downstream will not have hot metal and also must shut down. Some plants now only have one blast furnace in operation which makes these facilities very dependent on the reliability of the blast furnace.

One of the major items that becomes overlooked is the fact that all of the systems and the blast furnace itself will need to be shut down if a critical piece of equipment does not function. This article will address some of the major items that could cause a furnace shutdown and some of the major improvements in some of the equipment that are necessary to protect the furnace from an unexpected outage.
I will address equipment that is used primarily in two areas. These two areas will be the stove systems, including valves from the cold blast system, and blast furnace equipment. One of the major contributors to enable the development of blast furnace equipment is the improvement of hydraulic systems, piping, and components. These major improvements have provided the same contribution to equipment development that the improved refractories and cooling systems have provided in the development of the blast furnace and supporting systems.

Stove equipment

The equipment discussed here will be primarily the valves that are used in these systems. However, we should not forget the massive improvements in the stove designs including the modern use of ceramic burners in place of the mechanical burners from the past. Again, the progress in the new stove designs is a major result of the improvements to refractory. We have increased the capability of our stoves to the extent that we now are carrying hot blast temperatures in the range of 2200 F (1200 C) to 2300 F (1250 C) and even higher. In addition, the pressure of the system has also been increased dramatically to support the pressure requirements of the blast furnace. The in-line equipment must now be improved to operate at much higher pressures and also under much higher temperatures. The configuration of valves has also changed for a variety of these applications. For example, cold blast valves, chimney valves, gas valves and combustion air valves to the stove burners are now almost all made from a lever valve concept where in the past these valves were either gate type or flapper type construction. Hot blast valves are now gate valve construction where the old style hot blast valve was always a mushroom valve. All maintenance type valves were primarily either spectacle plate design or mechanical and or thermal goggle valves.

Meeting the challenges.

It is important that all the valves are reliable and long lasting with a minimum of maintenance. At best, failure of valves will result in reduced hot blast temperature due to possible outage of one stove. In the worst case scenario of hot blast valve failure, a furnace shut down will be needed to replace this valve.

There are definitely some important obstacles that require special consideration with the new valve construction. For example, lever valves have movable parts that can not be lubricated due to location. These parts must continue to work and not bind during the operation or the valve fails. It requires special materials for the pins and bushings and attention must be paid to clearances between these parts or they will freeze together due to the environment and temperatures they must withstand.

Also, hot blast valves must operate at very high temperatures (higher than hot blast temperature) and also withstand higher pressure than in the past. Hot blast valves must be reliable and long lasting as this valve requires a furnace shut down when it needs replaced. We must not forget that valve operators are now almost always hydraulic operators in place of the electromechanical operators of the past. Hydraulic operation has proven to be a much more reliable and more maintenance friendly than the electromechanical operators. In addition, hydraulic operators normally require much less space to deliver the same force requirements. Mechanical and thermal goggle valves are now replaced with hydraulically operated valves. Hydraulic operators are more reliable than electromechanical or pneumatic operators.

Blast furnace equipment

Two major areas of blast furnace equipment are critical to ensuring safe, continuous and efficient furnace operation. This involves the top equipment where all the raw materials enter the furnace and the cast house floor equipment where the hot metal exits the furnace. We must be able to tap the furnace in a timely fashion and also be able to plug the taphole at the required time. All the hot metal and slag must be channeled to the proper locations. Without proper operation of this equipment costly outages and significant safety hazards can result.

The refractory improvements and designs have enabled us to operate with less blast furnace maintenance and for longer periods of time. We now have covers for the iron and slag runners with emission evacuation.

The improved refractory and taphole mixes for plugging tapholes have become harder and the taphole lengths have become longer and smaller in diameter which has required a great improvement in clay guns and taphole drills. The application of hydraulics has enabled us to improve tapping equipment to meet these more difficult conditions.

It is now common practice to mount clay guns and taphole drills on the same side of the iron trough. This equipment must be low profile equipment in order to operate beneath the tuyere platforms. Hydraulic components have made this possible.

Clay gun improvements

Clay guns have been dramatically improved over the years and continual improvements are incorporated into this equipment. In modern furnaces, we use almost exclusively hydraulic operated clay guns. These were developed due to the fact that we needed to provide a stronger clay gun without increasing the height of the clay gun because, with modern improvements of blast furnaces, we now have a tuyere platform over the clay gun. This need has led to the development of a clay gun that is mounted on a heavy base with a tilted post. The swing mechanism is designed to swing the clay gun into the taphole with one motion and then the clay is extruded into the taphole. We can provide as strong a clay gun as required for the new modern taphole mixes with a hydraulic clay gun but not with the old electro mechanical construction. In addition, we are able to provide better protection of the clay gun at the taphole with hydraulics because now there is not any requirement for any electrical devices on the clay gun. Modern clay guns are now using a minimum hydraulic nozzle pressure of 250 Bars (3600 psi) and it will certainly require higher pressures in the near future. Many special features are incorporated into these designs such as cooling and/or heating of clay barrels.

The modernization of taphole drills

Taphole drills have been modernized from the lighter duty construction and pneumatic operators into the now all hydraulic drills. Woodings Industrial Corporation has developed a heavy duty all hydraulic taphole drill that has been designed with the same features as the new hydraulic clay guns. It has a heavy duty base and jib arm arrangement that swings on a tilted post with one motion to the taphole. There are no blast furnace attachments required as this jib arm is held into position with a hydraulic cylinder, the same as a modern clay gun. The hydraulic percussion hammers that are replacing the pneumatic percussion hammers provide a much stronger percussion hammer, in a smaller area. To open a taphole under the new conditions, it requires a very strong rotational torque with a high frequency low impact energy hammer. Any high impact energy will damage the refractory of the taphole or the mushroom. In addition, hydraulic feed motors are used and they are much stronger and fit into smaller space.

We now are using automatic operations for the clay gun and taphole drill including the monitoring of taphole length and clay gun volumes. Some operations incorporate the automation of the trough cover removal along with the clay gun and taphole drill. Record- keeping through the PLC of these systems provides invaluable information and the automation increases the reliability of the equipment.

New top designs handle the pressure

The blast furnace top design has improved dramatically to enable furnaces to operate at higher pressures. The development of seal valves which enabled us to operate at higher pressures has made a major contribution to this effort. We have used seal valves with bell type tops with improved distribution systems including movable armor. This advancement enabled the development of the traditional bell-less design which incorporates the use of material hoppers and the rotating gearbox with a distribution chute. The design of a traditional bell-less type top has two material hoppers at the top of the blast furnace. One material hopper can discharge material to the blast furnace while the other material hopper is being filled. These material hoppers can operate in that way as a result of the seal valves which either isolate one hopper from the blast furnace while it is being filled and the other hopper is isolated from the atmosphere while it is filling the blast furnace. The rotating gearbox with a tilting chute distributes the material into the furnace in the most desirable locations for best furnace operating conditions. There are also variations of the two material hopper design which include one or even three material hoppers. These designs are predicated on the furnace output requirements.

The original design of a bell-less type top incorporated electromechanical drives for operation of the rotation and tilting mechanism. There is now a new development (the Hydraulic Distributor) that incorporates a hydraulic system for the tilting mechanism which results in a more reliable and predictable tilt angle for the chute and provides an improved cooling system and refractory protection for the bottom of the rotating gear box. This design eliminates almost all the gearing that is required for the electromechanical design and has an improved sealing system from the blast furnace.

The new design eliminates all the maintenance requirements for all the gear boxes of the traditional bell-less design. The Hydraulic Distributor design has fewer moving parts and with better cooling and refractory protection, there will be less maintenance and improved life with this system.

Conclusion

As a result of blast furnace improvements, equipment used for various operations must also be improved to handle the more difficult application. Any unexpected or premature equipment failure will result in furnace outages and increased operations costs.
Hydraulics have played a major role to enable the development of stronger and more reliable equipment similar to the way developments in refractory quality have led to the improvements in blast furnace performance and longevity. The bottom line is that the operation of a blast furnace is only as good as the systems and equipment that services the blast furnace and its related systems.