Research and Application of Automatic Cleaning Device for High-Pressure Water Boiler Doors

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Research and Application of Automatic Door Cleaning Device for High-Pressure Water Boilers Zhu Yinhua, Cheng Leyi, Jin Bao (Baosteel Co., Ltd., Baosteel Branch, Shanghai 200941). The M-type coke ovens in Baosteel’s first phase were 6m coke ovens introduced from Nippon Steel Corporation in the early 1980s; to remove the tar and graphite accumulated on the sides of the oven doors as well as at their edges, a door cleaning device was installed on the coke pusher. The original furnace door cleaner consists of a hydraulic motor, reduction gears, a chain-shaped milling cutter, and a scraper for cleaning the edges. During the process of cleaning the furnace door, the cleaning scraper comes into metal contact with the furnace door’s web and its edges. Due to variations in the position of the furnace door or improper adjustment of the scraper’s angle, the cleaner can get stuck, and in some cases, the scraper may even damage the furnace door’s web and edges. Later, manual shovels were used to clean the furnace door, which not only increased the workload of the workers working in front of the furnace but also easily damaged the edges of the furnace door blade, resulting in severe smoking from the furnace door. In 2006, the Baosteel ironmaking plant, in collaboration with domestic design firms, developed a new type of automatic high-pressure water door cleaning device for the No. 3 coke pusher by employing domestic design and installation along with the import of some equipment from abroad, achieving good results. 1 Problems existing in the original furnace door cleaning operation: (1) The cleaning effect is unsatisfactory, and the sealing performance of the furnace door is poor. Due to the elastic edge structure of the coke oven door, the existing scrapers used for cleaning the door cannot make firm contact with the areas that need cleaning; they can only clean the central part of the edge. The corners of the grooves in the door edge are areas that cannot be cleaned, and the tar and graphite accumulated there reduce the sealability of the door. This makes it difficult to lift the door using the door lifting mechanism, and smoking and flaming occur at the door, which not only causes damage to the door but also pollutes the environment and even affects its service life. (2) High failure rate, frequent maintenance. Foreign objects can easily enter the drive gears and are difficult to remove, often causing overload failures during operation. Since the gap between the furnace door brick groove and the combined blade is set at 5 mm, once secondary coke and tar fill this gap during cleaning, the cleaner is prone to overloading and shutting down. (3) Manual cleaning is labor-intensive, and the furnace doors are severely damaged. Due to the unsatisfactory results of mechanical cleaning, manual shoveling was used as a substitute. This method involves high labor intensity, low efficiency, and extremely harsh working conditions; moreover, since the shovel comes into direct contact with the edge of the furnace door, it can easily damage that edge and reduce the service life of the furnace door. 2 Main equipment and technical parameters of the furnace door cleaning device. The high-pressure water furnace door cleaning device is mainly composed of the furnace door cleaner unit itself, the high-pressure water system, the hanging mechanism, the hydraulic system, and the electrical control system, as shown in Figure 1. This device works in conjunction with a door-taking machine; the cleaning mechanism is connected to the trolley via a monorail-type suspension system, and the trolley is driven by cylinders to move back and forth along fixed tracks. The height of the cleaning mechanism can be adjusted appropriately through the suspension system. 2.1 Furnace Door Cleaner The cleaner’s main body consists of a horizontally moving top cleaning device, a horizontally moving bottom cleaning device, and a vertically moving side cleaning device, as shown in Figure 2. The top is cleaned by a mobile cart equipped with high-pressure water nozzles, while the bottom is cleaned by a bottom cleaning cart equipped with spiral cutters and high-pressure water nozzles. The side section is mainly composed of a side cleaning cart, a chain traction device, and a planetary reducer equipped with an inverter motor, which drive the spiral milling cutter and high-pressure water nozzles to carry out cleaning. When the cleaner is in operation, the side cleaning carriage, driven by a hydraulic motor, moves the spiral cutters on both sides at a distance of 10 mm from the side of the furnace door. Then, under the action of the pulling strip, it moves up and down, removing tar and coke dust from the brick walls, while simultaneously using high-pressure water jets to clean the tar accumulated along the edges of the furnace door. At the same time, the cleaner at the bottom of the furnace door moves back and forth along the tracks driven by hydraulic cylinders; a screw milling tool is used to remove tar from the bottom of the furnace door bricks, while high-pressure water is used for rotating spray cleaning to remove tar from within the edges of the bottom part of the furnace door. High-pressure water is also used for rotating spray cleaning to remove tar from within the edges of the top part of the furnace door. The drive methods for various components of the sweeper are shown in Table 1. Table 1: Driving methods for various components of the sweeper
Component | Driving method | Stroke, mm | Speed, m/min
Mobile cart | Cylinder-driven | 900 | 5
Side cleaning unit | Electric reducer, sprocket and chain drive | 5800 | 3, 6, 10, 12
Lower cleaning units | Cylinder-driven | 440 | 1
Top cleaning unit | Hydraulic motor-driven | 440 | 1
2.2 High-pressure water system: The high-pressure water system is primarily composed of a water tank, high-pressure pump, high-pressure valves, high-pressure water pipes, and high-pressure nozzles. The high-pressure water pump station supplies water at a pressure of 60–100 MPa through high-pressure pump sets and high-pressure switching valves; this water is then ejected onto the surface to be cleaned on the furnace door by 4 sets of high-pressure rotary nozzles driven by oil motors, thereby achieving the cleaning purpose, as shown in Figure 3. Figure 3 Schematic diagram of the high-pressure water system piping. 2.2.1 Water supply and water quality requirements: Since rotating high-pressure water nozzles must withstand water pressures of over 60 MPa, high standards are required regarding water quality. Excessive impurities can cause rapid wear on the nozzles, thereby reducing the service life of the equipment. Therefore, the water quality requirements for using the cleaner are as follows: Turbidity: not more than 3 degrees ； pH value: 6.5～8.5 ； Total hardness: 450 mg/L; Total soluble solids: 1000 mg/L. Baosteel uses domestic water as the water source, which meets the water quality requirements. The water supply system is equipped with 2 water tanks, one on the top of the furnace and one on the coke pusher. Normally, the tank on the furnace top remains filled with water. The tank on the pusher is refilled by connecting a metal quick connector from the fire hose on that vehicle to the fixed connector on the drain pipe of the tank on the furnace top; the refilling process takes about 5 minutes. During cleaning, the average water consumption per furnace door is 100 L, allowing 20 furnace doors to be cleaned with a single supply of water. 2.2.2 Drainage system: Since the temperature of the coke oven doors is very high when they are removed, 40% of the water generated during the cleaning process with high-pressure water evaporates, while 60% flows toward conveyor No. 3 located below the oven doors. To this end, a guide channel is created between conveyor No. 3 and conveyor No. 1, allowing the water from conveyor No. 3 to flow into conveyor No. 1. This helps to compensate for the amount of water that evaporates when processing the ends of the coke on conveyor No. 1, prevents secondary contamination by wastewater after the furnace doors are cleaned, and reduces the amount of water needed for processing those ends of the coke. 3 Working Principle of the Cleaning Device 3.1 Working Principle of Spiral Milling Cutter Cleaning Spiral milling cutter cleaning is a form of mechanical cleaning; when the cleaning machine is in operation, the side cleaning carts, driven by cylinders, cause the spiral milling cutters on both sides to press against the two sides of the refractory brick grooves in the furnace door. The spiral milling cutters on both sides are held in place by the connecting cylinders, allowing the cleaning surface to make contact with the cutters and thus generating a stable and even cleaning force. By rotating, the tar and coke powder on the brick grooves on both sides of the furnace door are scraped off; meanwhile, the compression cylinder is equipped with a floating circuit to prevent overload. 3.2 Working principle of high-pressure water cleaning High-pressure water jets are a new technology that has seen rapid development in recent years, and are mainly used in areas such as cleaning, cutting, and crushing. High-pressure water jets use water as the working medium; through pressure-boosting equipment and nozzles of fixed shape, they generate jet streams with high flow rates and high energy density. These jets possess extremely high energy densities and impact kinetic energy, which strike the surface to be cleaned in a forward or tangential direction, continuously performing actions such as striking, erosion, stripping, cutting, and cleaning in order to remove dirt. 4 Main Features and Implementation Results The high-pressure water boiler door cleaning device was put into use on Baosteel’s No. 3 coke pusher in July 2006, achieving good results. (1) The cleaning effect is significant. By combining mechanical cleaning with high-pressure water cleaning, and in particular by using high-pressure water jets, it is easy to remove the tar from the edges of the furnace door and from the brick grooves in the corners of the furnace door ； The spiral milling cutter exerts a large feed force while rotating, thus enabling a strong cleaning force. (2) The cleaning process is stable. The spiral mills on both sides use connecting cylinders to clamp the furnace door, allowing the cleaning surface to make contact with the mills and thus achieving a stable and even cleaning force. Since the clamping cylinders are equipped with a floating circuit, overload can be prevented. (3) Adjustment convenience. A hydraulic motor is used to drive the spiral milling cutter to rotate; by adjusting the amount of oil and oil pressure, it is possible to easily control the cleaning force and cleaning density. The cleaner can swing freely in front-back and left-right directions, and even when the cleaner is not aligned with the furnace door to be cleaned, the spiral milling cutter can remain closely attached to the cleaning surface. (4) Protect the furnace door equipment. High-pressure water does not easily damage the edges of the furnace door blades, which helps ensure smooth airflow in the grooves of the furnace door. This not only extends the service life of the furnace door and maintains the sealing quality of those blade edges, but also reduces the workload of the workers working in front of the furnace, thereby improving the working environment. 5 Areas for Improvement: There are very few successful cases of this device being used in China, so there is a lack of experience that can serve as a reference. Based on the transformation process and its performance over half a year, there are still some issues that need to be addressed. (1) Reduce the pressure of the high-pressure water pump. Since the furnace door cleaning operation is carried out while the furnace is in operation, the newly removed furnace door is still hot, and the tar and soot adhering to it are at a very high temperature and in a somewhat softened state. Practice has shown that a pressure of 40–50 MPa from a high-pressure water nozzle is sufficient to remove the tar completely, thereby reducing the load on the system, minimizing water consumption, and extending the service life of the equipment. (2) Optimization of the nozzle spraying distance. Practice has shown that there is an optimal value for the spraying distance; when the cleaning capacity of the water jet meets the requirements, the spraying distance can be increased appropriately, which not only improves the cleaning area per unit time but also enhances the safety of the operation. When the furnace door is cold and tar is difficult to clean, the spraying distance can be appropriately reduced to fully utilize the water wedge effect and improve the cleaning efficiency. The optimal spray distance mainly depends on the jet pressure and the diameter of the nozzle, and can be determined using the following formula. In the formula Lopt=99.7d0.9 (p/100), Lopt represents the optimal injection distance in mm; p is the jet pressure in MPa; and d is the diameter of the nozzle outlet in mm. (3) The pressure of the hydraulic system should be appropriately increased. Since the high-pressure water cleaning device utilizes the existing hydraulic system of the coke pusher, and the opening and closing actions of the small furnace door take place simultaneously during door cleaning, the pressure and flow rate of the hydraulic system decrease, which affects the smooth operation of these opening and closing actions; as a result, the small furnace door often fails to open or close properly. 6 Conclusion The use of the high-pressure water furnace door cleaning device has yielded very significant results; its automated control system operates stably and provides excellent cleaning performance, which has led to a notable improvement in the smoking issue associated with the furnace doors of Baosteel’s first-phase coke ovens. At the same time, it **reduced the workload of those responsible for operating the furnace doors and the workers working in front of the furnace, improved the working conditions, extended the service life of the furnace doors, and brought about positive social and economic benefits.**