Analysis and Prevention of Water-cooling Cable Failure in LF Refining Furnace

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Analysis and Prevention of Water-cooling Cable Failure in LF Refining Furnace

Abstract: The principle and equipment composition of LF refining furnace are introduced, the water-cooling cable failure is analyzed, and finally the preventive measures for water-cooling cable failure are proposed.

Keywords: LF refining furnace; water-cooling cable; temperature; internal break

Introduction: In recent years, major domestic steel mills have carried out large-scale transformation of steelmaking equipment based on structural adjustment and product upgrading, combined with existing resources and basic conditions. As an important part of it, the furnace refining technology has made great progress. Among them, due to the wide adaptability of LF furnace and low equipment investment, steel mills have been equipped with LF ladle refining furnace in recent years. Practice has proved that converter-refining (LF)-continuous casting machine continuous rolling is an optimized process route with high efficiency, high quality and low consumption. However, due to the heating characteristics of the refining furnace, the periodic consumption and sudden accidents of water-cooling cables are also common. This paper proposes an analysis method and preventive measures for the internal break failure of LF furnace water-cooling cable.



1 Function of LF Refining Furnace

The Metallurgical Functions of LF Ladle Refining Furnace are as Follows:

1.1 Heating and Warming Up

Electric energy is transmitted to the electrode through a transformer (35KV 20MW), and the electrode arcs to convert the electric energy into heat energy and transfer it to the molten steel, thereby achieving the purpose of heating and warming up. Through reasonable electrical control, the LF furnace can automatically adjust according to different steel grades and refining stages to control the heating rate. Ultimately, the temperature of the molten steel will be controlled within the range required for continuous casting.

1.2 Composition Fine-tuning

The control system calculates the amount of material that needs to be added to the LF furnace according to the difference between the sampling analysis value and the target value of the steel grade, so as to achieve the purpose of composition fine-tuning.

1.3 Uniform Molten Steel Composition and Temperature

The LF furnace is equipped with a ladle bottom blowing argon stirring system. Argon can be blown into the ladle through the air-permeable bricks at the bottom of the ladle, so that the molten steel turns in the ladle, thereby uniforming the composition and temperature of the molten steel.



1.4 Desulfurization

Add slag to the ladle through the feeding system to make alkaline slag or feed CaSi wire, and achieve the purpose of desulfurization through argon blowing and stirring.

1.5 Deoxidation

Control the furnace pressure to maintain the reducing atmosphere in the ladle furnace to prevent secondary oxidation of molten steel. In the early stage of refining, Al wire is fed into the steel through the wire feeder to achieve the purpose of deoxidation. In the later stage of refining, when the sulfur content in the steel drops to a lower level, CaSi wire is fed into the molten steel for final deoxidation.

2 Analysis of the Cause of the Internal Break of the Water-cooled Cable

The LF furnace heating system consists of 35KV power supply to the primary side of the transformer, and the secondary side of the transformer is connected to the electrode through the furnace busbar system, water-cooled cable, and cross arm. The electrode is driven by the lifting of the cross arm to heat the molten steel in the steel ladle. Among these links, the failure rate of water-cooled cables is the highest. According to statistics of previous failures, 80% of the failures and fixed spare parts consumption of LF furnaces are caused by water-cooled cables. How to quickly judge the internal break of the cable becomes particularly important.



The following introduces the judgment method of the internal break of the water-cooled cable through the internal break of the water-cooled cable.



2.1 Failure Process



One day, the stainless steel short net of the fixed water-cooled cable of the east phase of the 1# LF furnace heated up and turned red. One of the water-cooled cables had a large temperature rise due to shaking. The power was immediately cut off and the water-cooled cable with a large temperature rise was replaced. After the replacement, steelmaking began, and the problem was still found. After replacing the other water-cooled cable, the equipment operated normally, but the subsequent continuous casting process was stopped due to the long processing time. By cutting open the two replaced water-cooled cables for inspection, we found that the internal cable of the first replaced water-cooled cable was normal and there was no internal break, while the second water-cooled cable had a serious internal break.



2.2 We have made the following analysis on how to judge the situation of cable internal break fault:

The factory values of the three-phase cross arm water-cooling cables are the same, and the water pipes are unobstructed. Let's take the East phase cross arm water-cooling cable as an example: If the temperature value of the East phase cross arm water-cooling cable is higher than that of the other two phases in the temperature detection, then there may be a problem with the East phase cross arm water-cooling cable. There are three types of faults. Let's look at Figure 2:

2.2.1 If one of the East phase cross arm water-cooling cables is broken and the other one is running normally, then the resistance of the water-cooling cable that is running normally remains normal. Set the resistance value R1=2 ohms. The resistance of the water-cooling cable with a broken wire increases. Set the resistance value R2 =10 ohms, the short nets of the two water-cooled cables are the same, and the resistance is R=2 ohms. After calculation, the main current I=6.25A passing through the circuit, the current I short net=I/2=3.125A passing through the normal water-cooled cable, the current I1=5.2A passing through the broken wire water-cooled cable, the current I2=1.05A, and in order to ensure the current distribution, the current I supplements the current distribution through the stainless steel short net of the fixed water-cooled cable. According to Joule's law, the heat Q=I²Rt, then the normal water-cooled cable has a large heat and high temperature, and the damaged and broken water-cooled cable has a small heat and low temperature.



1）The inspector who measures the temperature mechanically needs to measure the temperature of the three points of water inlet C, water outlet C, and short pipe C. If the temperature of water inlet C is 35℃, short pipe C is 50℃, and water outlet C is 55℃, then the temperature difference of the water-cooling cable on the right side of the left figure is large, which is 15℃, and its heat generation is large. The temperature difference of the water-cooling cable on the left side is small, which is 5℃, and the heat generation is small. According to the above analysis, there is a problem with the outlet of the water-cooling cable on the left. 

2) The inspector who measures the temperature electrically needs to pay close attention to the temperature of the stainless steel short net that fixes the water-cooling cable. According to the above analysis, if a water-cooling cable is damaged, in order to achieve current distribution, the stainless steel short net of the fixed water-cooling cable has current I compensation passing through it, which will generate work and heat. If the temperature at this point increases, the water-cooling cable may be faulty.



2.2.2 If there is no obvious temperature difference at the four temperature measurement points, it does not necessarily mean that the water-cooling cable is operating normally. Both water-cooling cables may have slight wire breaks.



2.2.3 There is no obvious temperature difference at the four temperature measurement points, two water-cooled cables have serious wire breakage, and the current in the cross-sectional area cannot reach the rated value of 4000A (or 5500A), but can only reach a certain value.



3 Preventive Measures for LF Refining Furnace Water-cooled Cable Failure

Through the summary and analysis of LF refining furnace water-cooled cable failure, the following preventive measures have been formulated:

3.1 Drain the cooling water in the water-cooled cable, and use the maintenance time to backwash the water-cooled cable to ensure that the water channel is unobstructed and the water-cooled cable is well cooled.



3.2 Establish a thermometer current report for the water-cooled cable, measure the temperature of the cable connection and cooling water every day, and record the current passing through the water-cooled cable. Use the trend function of the report to realize real-time monitoring of cable temperature and current, analyze the abnormal data found, and make early repairs.



3.3 Do a good job in the life management of water-cooled cables. For the repaired water-cooled cables, the repair unit is required to provide feedback on the resistance and broken strands of the cables, and the cables with many broken strands should be scrapped.

References:

[1] Guo Zonghua, Song Hongguang, Yin Zhihui. Design and application improvement of static reactive power compensator for 140t refining furnace [J]. Electric World, 2017, 58 (10): 3-7.

[2] Dong Chuanjun, Zhou Weibiao, Li Xuliang, et al. Application of SVC device in LF refining furnace of Hangzhou Iron and Steel [J]. National Annual Conference on Metallurgical Power Supply and Consumption, 2013 (5)