Alarm Experience on Leakage of Intermediate Frequency Furnace
Leakage alarm device is an important protection device of crucible induction intermediate frequency furnace, which is used to judge the usage of furnace lining and to prevent leakage accident caused by lining cracking. In the daily operation on electric furnace, how to distinguish normal furnace leakage alarm system; when the system sends out signal, how to accurately determine whether it is true. These are not only related to the length of the furnace life, but also related to the safety of electric equipment. The following detailed analysis of several common methods between detection and judgment.
2 The principle of leakage alarm system
Our company has a ten-ton and two twenty-ton intermediate frequency furnace, all with contact type furnace leakage alarm device. Its principle is that burying stainless steel wire in the bottom of a furnace as an electrode, which is connected to a molten iron in the furnace. Between the induction coil inside mica plate and asbestos board, there installs the stainless steel wire mesh as another electrode, whose height is slightly higher than that of hot metal with distribution shape as “C”. Supplying low voltage dc power between two electrodes. On normal occasion, due to the lining material and asbestos board has a lot of insulation resistance, alarm circuit current is very small. When the furnace lining erosion, and hot metal penetration coming to the stainless steel wire mesh, electrode on the bottom conducts to the steel wire mesh, and alarm circuit current increases rapidly. After reaching the set values, the alarm system sends out signals.
In order to ensure accuracy and reliability, the following conditions must be met:
2.1 the furnace bottom line and the molten iron to maintain good electrical contact.
2.2 the furnace bottom line and warning network can’t connect to ground at the same time.
2.3 alarm ammeter setting value is less than short-circuit current value of furnace bottom line and warning network.
So as to ensure that when the molten iron is going to come into contacting with the warning network, alarm system has already sent out alarm signals in advance for providing the enough time for emptying the hot metal. It is to ensure the safe operation of electric equipment.
3 Daily inspection method for furnace leakage alarm system
In the process of furnace operation,the system must be checked every day. Method is as follows:(note: this method is applicable to the measurement before the system sending out signals )
3.1 confirm that the alarm system’s power is on,and furnace selection switch(SA2) in the closed position.
3.2 turn the control knob of electric furnace power in anticlockwise direction to the end, rotating the inverter to stop. Then, the furnace stops working.
3.3 the short circuit of furnace bottom line and warning network outlet should arouse the warning signals. Demolition of shorting stub can make the alarm current returns to the original value. If it doesn’t warn, the dc power supply, alarm current meter, current limiting resistor and its connecting lines should be checked.
3.4 prepare a dry and solid iron solder. Connect one side with wire to the alarm cables “320” in the ark and another side should be inserted into a liquid metal in the furnace (note：iron solder does not contact the furnace shell and ground). Then, to observe whether the alarm on ammeter pointer turns towards the right.
3.4.1 If the alarm ammeter pointer toward the right more than the set values for the alarm and warns with light and sound, it means that furnace bottom line contact with the molten iron well.
3.4.2 If alarm on the alarm ammeter pointer not be the setting value, furnace bottom line and the molted iron are in bad contact. The setting value needs to adjust to the left till the alarm sends out warning.
3.4.3 If the ammeter pointer is fixed, the furnace bottom line and molted iron are completely separated.
3.4.4 Under the condition of the failure of leakage alarm, connect one side with wire to furnace bottom line “321” in the ark and another side should be inserted into a liquid metal in the furnace (note：iron solder does not contact the furnace shell and ground). Then, to observe the value of current value.
- If the ammeter pointer is fixed, it means that the furnace lining is in good condition.
- If the ammeter pointer toward the right , it means that the furnace lining is in bad condition. The more value deflects, the more serious that the phenomenon of lining erosion and cracking comes.
- If the ammeter pointer toward the set values with sound and light warning, it means that the lining has aroused the furnace leakage alarm.
3.5 connect one side of wire to furnace bottom line “321” in the ark and another contacts to the furnace shell. To observe whether alarm ammeter pointer toward the right.
3.5.1 If the ammeter pointer is fixed, it accounts for the good insulation of alarm cables against ground.
- 5.2If the ammeter pointer toward the right ,it accounts for the bad insulation of alarm cables against ground.The more value deflects, the more serious that the phenomenon of grounding of the alarm cables comes.
3.6 connect one side of wire to the alarm cables “320” in the ark and another to furnace shell. To observe whether alarm ammeter pointer toward the right.
3.6.1 If the ammeter pointer is fixed, it accounts for the good insulation of furnace bottom line and the molted iron against ground.
- 6.2 f the ammeter pointer toward the right ,it accounts for the bad insulation of furnace bottom line and the molted iron against ground. The more value deflects, the more serious that the phenomenon of grounding of furnace bottom line and the molted iron comes.