GB1583244A - Electric arc steel processing unit including apparatus for detecting broken electrodes - Google Patents

Description

(54) ELECTRIC ARC STEEL PROCESSING UNIT INCLUDING APPARATUS FOR DETECTING BROKEN ELECTRODES (71) We, BRITISH STEEL CORPORATION, a Corporation incorporated and existing under the Iron and Steel Act 1967 whose principal office is at 33 Grosvenor Place, London, S.W. 1 do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an electric arc steel processing unit including apparatus for detecting a broken electrode and particularly, but not exclusively, relates to such apparatus for use in detecting broken electrodes in vacuum arc degassing units in steel processing.

In vacuum arc degassing units a ladle of molten metal, for example steel from an arc furnace, is deposited in a tank which is then sealed with a lid through which electrodes extend in vacuum4ight fashion-alternatively, the ladle itself may simply be sealed by a vacuum-tight cover. The melt is then heated by arcs stuck between the electrodes and the conductive melt, vacuum is applied and as the melt is stirred, for example by gas purging or by electromagnetic means, gaseous impurities are exhausted, refining the melt. Various alloying additions may be made at this time.

The unit may alternatively be employed simply as a holding furnace, the arc heating being utilised to maintain the melt within a specified temperature range.

The graphite or carbon electrodes are consumed to some degree during these processes and they are subjected to severe operational stresses, vibration etc. under the electromagnetic forces to which they are subjected. With the advent of long slender electrodes the possibility of breakages occurring as a result of these stresses is increased and this can be hazardous, particularly in vacuum arc degassing units where the electrodes are each enclosed within a telescopic sleeve. In such cases, the break can occur within the sleeve and, with control systems employed hitherto the loss of arc 'seen' by these systems can cause a reaction whereby the electrode is driven down to re-strike the arc on the top of the broken electrode section.In this event the arc is struck and re-established within the sleeve which will at best cause severe damage to the latter and at worst create an explosion.

It is an object of this invention to provide a broken electrode detector which would overcome this problem.

The present invention provides an electric arc steel processing unit, including apparatus for detecting a broken electrode comprising circuit means for monitoring current through the electrode and including a timing circuit operable in response to a loss of said current extending over a period in excess of a predetermined interval to provide a warning of electrode breakage and inhibit any drive on the electrode in a sense such as to re-establish the arc.

The timing circuit preferably includes first and second timers, the first having a delay interval such that normal current zeros during operation (due to the erratic nature of the arc) are ignored whilst the second has a delay interval of said pre-determined value indicative of electrode breakage.

In accordance with this invention then, a detector is provided which operates to monitor the current flow providing a simple, reliable and expendient solution to the problem and which is unaffected by the furnace environment.

In order that the invention can be fully understood, one embodiment thereof will now be described with reference to the accompanying drawing, in which: Figure 1 is a schematic illustration of a vacuum arc degassing plant; and Figure 2 is a schematic circuit diagram of the circuit means referred to.

Figure 1 shows a vacuum tank 1 having a lid 2 and in which is standing a ladle 3. The ladle contains a steel melt 4 and embodies a slide gate valve 5 for discharge. An inert gas is bubbled through the melt via a porous plug 6. Vacuum is drawn off through a port 7 in the tank and various de-oxidants, alloying additions etc. may be added through a hopper 8.

Three graphite electrodes 10 extend through the lid and each is carried by an arm 12 mounted in a sleeve 13 which screens the electrode, this sleeve 13 being telescopically movable over an inner sleeve 14, a sliding vacuum-tight seal being provided at 15.

The electrodes, and thus the sleeve 13, are progressively driven downwardly (or retracted), cf. arrow 16, by a driving mechanism (not shown) so as to maintain the arc established between the electrode and the melt.

A current transformer 17 monitors the current through each electrode separately.

Referring now to Figure 2, two timers T1 and T2 are each primed for operation by a supply source across them and are activated upon closure of contact CR responsive to current flowing through the current transformer 17 (Fig. 1).

Timer T1 has a time-constant of 2 seconds and is therefore de-activated should CR open for a period in excess of two seconds, and similarly timer T2 is de-activated should CR open for a period in excess of 5 seconds, the latter period being adjudged to be indicative of electrode breakage whilst the former accommodates normal arc interruptions during operation.

With T1 operative, contact T1 1 closes energizing a setting relay SR through normally closed contact AR1 of an alarm relay AR. This circuit is held through the setting relay's own contact SR1 and the normally closed conact RR1 of a 'raise electrode' relay (not shown). At the same time, operation of T2 causes contact T21 to open (contact SR2 of the setting relay is 'slugged' (delayed) for closure) isolating the alarm relay under these circumstances. Alarm relay AR is latched-on when energized, thus, to de-latch this element a separate circuit is provided throngh a re-set button RS.

In operation, contact CR opens and closes frequently during normal arc interruptions but both timers T1 and T2 remain activated unless the circuit interruptions should exceed 2 seconds. The delay provided by the timer T1 prevents the relay contact T1 1 from chattering on and off in sympathy with the CR contact. Should CR be open in excess of 2 seconds however, contact Tl 1 opens. This will close again should CR not remain open for a further 3 seconds.

Should CR be open in excess of 5 seconds contact T21 closes latching on the relay AR which effects an audible and visual alarm and arrests the driving mechanism for lowering the electrode: Contact AR1 opens deenergizing the setting relay SR. This is indicative of electrode breakage.

The electrode is then raised whereupon the raise relay is energized, opening its contact RR1. Relay AR remains latched on until delatched by manual operation of the re-set button RS whereupon the ciruit returns to normal, contact Tl 1 of course having been opened upon the initial 2 second delay following the cessation of current.

Raising the electrode during normal operation of the arc will automatically reset the circuit since contact RR1 will open and the setting relay contacts SR1 and SR2 will open as soon as the current has been interrupted in excess of two seconds.

The circuit is thus set upon the striking of an arc at the start of the process, normal current zeros, due to the erratic nature of the arc, are ignored and a loss of arc power for a defined period (5 seconds) is registered as a broken electrode. The downward motion of the electrode is then automatically arrested and alarms are sounded. The circuit must be manually reset following a broken electrode; the circuit is automatically reset if the electrodes are raised under manual control.

Although the invention has been described with reference to the specific embodiment illustrated, it is to be understood that various modifications may be made without departing from the scope of this invention. For example, the particular time intervals mentioned are not critical and may well be varied depending on practice. Other circuits may also be provided consistent with the principle of detecting loss of arc current in excess of a predetermined period.

WHAT WE CLAIM IS 1. An electric arc steel processing unit, including apparatus for detecting a broken electrode comprising circuit means for monitoring current through the electrode and including a timing circuit operable in response to a loss of said current extending over a period in excess of a predetermined interval to provide a warning of electrode breakage and inhibit any drive on the electrode in a sense such as to re-establish the arc.

2. A unit according to claim 1, wherein the timing circuit includes first and second timers, the first timer providing a delay period such that normal zeros in the current passing through the electrode are inoperative to activate the detection sequence, and the second timer provides a delay period of said predetermined interval.

3. A unit according to claim 1 or claim 2, wherein the timing circuit includes contacts connected in sub-circuit loops with control relay means is primed for operation upon an arc being stuck from the electrodes in the steel processing unit.

4. A unit according to any one of claims 1 to 3, wherein the circuit means is manually resettable following detection of electrode breakage and automatically re-settable in

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. in the tank and various de-oxidants, alloying additions etc. may be added through a hopper 8. Three graphite electrodes 10 extend through the lid and each is carried by an arm 12 mounted in a sleeve 13 which screens the electrode, this sleeve 13 being telescopically movable over an inner sleeve 14, a sliding vacuum-tight seal being provided at 15. The electrodes, and thus the sleeve 13, are progressively driven downwardly (or retracted), cf. arrow 16, by a driving mechanism (not shown) so as to maintain the arc established between the electrode and the melt. A current transformer 17 monitors the current through each electrode separately. Referring now to Figure 2, two timers T1 and T2 are each primed for operation by a supply source across them and are activated upon closure of contact CR responsive to current flowing through the current transformer 17 (Fig. 1). Timer T1 has a time-constant of 2 seconds and is therefore de-activated should CR open for a period in excess of two seconds, and similarly timer T2 is de-activated should CR open for a period in excess of 5 seconds, the latter period being adjudged to be indicative of electrode breakage whilst the former accommodates normal arc interruptions during operation. With T1 operative, contact T1 1 closes energizing a setting relay SR through normally closed contact AR1 of an alarm relay AR. This circuit is held through the setting relay's own contact SR1 and the normally closed conact RR1 of a 'raise electrode' relay (not shown). At the same time, operation of T2 causes contact T21 to open (contact SR2 of the setting relay is 'slugged' (delayed) for closure) isolating the alarm relay under these circumstances. Alarm relay AR is latched-on when energized, thus, to de-latch this element a separate circuit is provided throngh a re-set button RS. In operation, contact CR opens and closes frequently during normal arc interruptions but both timers T1 and T2 remain activated unless the circuit interruptions should exceed 2 seconds. The delay provided by the timer T1 prevents the relay contact T1 1 from chattering on and off in sympathy with the CR contact. Should CR be open in excess of 2 seconds however, contact Tl 1 opens. This will close again should CR not remain open for a further 3 seconds. Should CR be open in excess of 5 seconds contact T21 closes latching on the relay AR which effects an audible and visual alarm and arrests the driving mechanism for lowering the electrode: Contact AR1 opens deenergizing the setting relay SR. This is indicative of electrode breakage. The electrode is then raised whereupon the raise relay is energized, opening its contact RR1. Relay AR remains latched on until delatched by manual operation of the re-set button RS whereupon the ciruit returns to normal, contact Tl 1 of course having been opened upon the initial 2 second delay following the cessation of current. Raising the electrode during normal operation of the arc will automatically reset the circuit since contact RR1 will open and the setting relay contacts SR1 and SR2 will open as soon as the current has been interrupted in excess of two seconds. The circuit is thus set upon the striking of an arc at the start of the process, normal current zeros, due to the erratic nature of the arc, are ignored and a loss of arc power for a defined period (5 seconds) is registered as a broken electrode. The downward motion of the electrode is then automatically arrested and alarms are sounded. The circuit must be manually reset following a broken electrode; the circuit is automatically reset if the electrodes are raised under manual control. Although the invention has been described with reference to the specific embodiment illustrated, it is to be understood that various modifications may be made without departing from the scope of this invention. For example, the particular time intervals mentioned are not critical and may well be varied depending on practice. Other circuits may also be provided consistent with the principle of detecting loss of arc current in excess of a predetermined period. WHAT WE CLAIM IS

1. An electric arc steel processing unit, including apparatus for detecting a broken electrode comprising circuit means for monitoring current through the electrode and including a timing circuit operable in response to a loss of said current extending over a period in excess of a predetermined interval to provide a warning of electrode breakage and inhibit any drive on the electrode in a sense such as to re-establish the arc.

2. A unit according to claim 1, wherein the timing circuit includes first and second timers, the first timer providing a delay period such that normal zeros in the current passing through the electrode are inoperative to activate the detection sequence, and the second timer provides a delay period of said predetermined interval.

3. A unit according to claim 1 or claim 2, wherein the timing circuit includes contacts connected in sub-circuit loops with control relay means is primed for operation upon an arc being stuck from the electrodes in the steel processing unit.

4. A unit according to any one of claims 1 to 3, wherein the circuit means is manually resettable following detection of electrode breakage and automatically re-settable in

response to raising of the electrodes during normal operation.

5. A unit according to any one of claims 1 to 4, wherein the said warning is visual and/or audible.

6. A unit according to any one of claims 1 to 5, wherein the unit is a vacuum arc degasser.

7. An electric arc steel processing unit, including apparatus for detecting a broken electrode substantially as herein described with reference to the accompanying drawings.