1 Introduction
The leakage alarm device is an important protection device for the induction medium frequency furnace. It is a safety device for judging the use of the furnace lining and preventing the furnace lining from cracking. In the daily operation of the electric furnace, how to judge whether the leak alarm system is normal; how to accurately judge whether it is a true alarm when the leak alarm system issues an alarm signal, which is not only related to the length of the furnace, but also related to the safety of the electric furnace equipment. The following detailed analysis of several commonly used methods of detection and judgment.
2 Principle of the leak alarm system
Our company has one 10 ton and two 20 ton intermediate frequency furnaces, all of which use contact leakage alarm devices. The principle is to embed stainless steel wire as an electrode (referred to as the bottom line) at the bottom of the furnace, which is connected to the molten iron in the furnace. Between the mica board and the asbestos board inside the induction coil, a stainless steel wire mesh is installed as another electrode (referred to as an alarm net), and its height is slightly higher than that of the molten iron in the furnace, and is distributed in a "C" shape. A low-voltage DC power supply is added between the two electrodes. Under normal conditions, since the lining material and the asbestos board have large insulation resistance, the alarm loop current is small. When the furnace lining is eroded to a certain extent, when the molten iron penetrates into the stainless steel mesh, the bottom electrode and the stainless steel wire mesh are turned on, and the alarm loop current increases rapidly. After the alarm setting value is reached, the alarm system sends an alarm signal.
In order to ensure that the alarm system is accurate and reliable, the following conditions must be met:
2.1 The bottom line maintains good electrical contact with the molten iron.
2.2 The bottom line and the alarm network cannot be grounded at the same time.
2.3 The setting value of the alarm ammeter is less than the value of the short circuit current of the bottom line and the alarm net.
In this way, when the molten iron is about to contact the alarm network, the alarm system has already issued an alarm signal in advance, which provides sufficient time for the molten iron in the furnace to be timely to ensure the safe operation of the electric furnace equipment.
3 Daily inspection method for the leak alarm system
During the operation of the electric furnace, the alarm system must be tested every day. The method is as follows: (Note: This method is suitable for the measurement before the alarm system sends an alarm signal)
3.1 Confirm that the alarm system control power is in operation and the furnace selection switch (SA2) is in the closed position.
3.2 Turn the power control knob of the electric furnace counterclockwise to the bottom and rotate the inverter stop knob to stop the electric furnace.
3.3 Short-circuit the bottom line of the furnace and the outlet line of the alarm network. The alarm system should issue an alarm signal. After the short wiring is removed, the alarm current returns to its original value. If there is no alarm, check the DC power supply, alarm current meter, current limiting resistor and its connecting line.
3.4 Prepare a dry solid iron brazing wire, connect the iron brazing wire to the alarm cable “320” in the alarm cabinet, and insert the other end into the molten metal of the furnace (note: the iron brazing cannot contact the furnace shell and ground). Observe whether the pointer on the alarm ammeter is deflected to the right.
3.4.1 If the pointer on the alarm ammeter is deflected to the right and exceeds the alarm setting value, an audible and visual alarm is issued, indicating that the bottom line is in good contact with the molten iron.
3.4.2 If the pointer on the alarm ammeter is deflected to the right and the alarm setting value is not reached, the bottom line is not in good contact with the molten iron. The alarm setting value needs to be adjusted to the left until an audible and visual alarm is issued.
3.4.3 If the pointer on the alarm ammeter does not move, the bottom line is completely disconnected from the molten iron. The leak alarm system is invalid.
3.4.4 In the case of failure of the leak alarm system, connect one end of the iron to the bottom line “321” of the alarm cabinet, and insert the other end into the molten metal of the furnace (note: the iron cannot touch the shell and Ground), observe the value on the alarm ammeter. This value reflects the true alarm current value.
(2) If the alarm ammeter pointer does not move, the lining is good.
(2) If the alarm ammeter pointer is deflected to the right, the lining is not good. The larger the deflection value, the more serious the erosion and cracking of the lining.
(3) If the alarm current meter pointer reaches the alarm setting value, an audible and visual alarm will be issued, indicating that the furnace lining has leaked the furnace alarm.
3.5 Connect one end of the wire to the bottom line “321” in the alarm cabinet, and the other end of the wire contacts the furnace shell. Observe whether the pointer on the alarm ammeter is deflected to the right.
3.5.1 If the alarm ammeter pointer does not move, the alarm cable is well insulated from the ground.
3.5.2 If the alarm ammeter pointer is deflected to the right, the alarm cable is not well insulated from the ground. The larger the deflection value, the more serious the grounding of the alarm cable.
3.6 Connect one end of the wire to the alarm cable “320” in the alarm cabinet, and the other end of the wire contacts the furnace shell. Observe whether the pointer on the alarm ammeter is deflected to the right.
3.6.1 If the indicator of the alarm ammeter does not move, it indicates that the bottom line and molten iron are well insulated from the ground.
3.6.2 If the alarm ammeter pointer is deflected to the right, the bottom line and molten iron are not well insulated from the ground. The larger the deflection value, the more serious the grounding of the hearth and the molten iron.
