Method and device to control the force applied to the electrode-bearing arms of an electric arc furnace

Abstract: Method and device to control the force applied to the electrode-bearing arms of an electric arc furnace, the vertical regulation of the electrodes being achieved with an electrohydraulic or electropneumatic system consisting of an operational cylinder actuated by a fluid under pressure so as to raise and lower the electrodes, the control means of the system being adjustable by means of an electrical device, whereby the working pressure of the operational cylinder is measured by a pressure transducer (15) and is processed by a processor system (16) so as to obtain the weight of the electrodes and the condition of static pressure, the processor system (16) governing a quick-response servovalve (18) and being governed by an interface (19) of the system that adjusts the electrical current fed to the electrodes (11).

Description: This invention concerns a method and relative device to control the force applied to the electrode-bearing arms of an electric arc furnace. To be more exact, the invention concerns a method and device suitable to control constantly the force applied to the electrode-bearing arms of an electric arc furnace, the force being suitable to lift and position the electrode in a required manner in a vertical direction. By means of the invention it is possible also to monitor the mass of the electrode at any time during smelting so as to know the relative wear of the electrode and to be able to act accordingly. Document DE-A-2.312.603 (and therewith also FR-A-2.402.993) is known and concerns a device to prevent breakage of electrodes in electric furnaces having electrical, electrohydraulic or electropneumatic regulation, in particular for smelting of scrap; this device consists of an operational cylinder actuated by a fluid under pressure so as to lift and lower the electrodes, the control means of the cylinder being adjustable by means of an electrical commutator device which responds in the event of excursions of the current of the electrodes from the nominal value. In the above document a pressure switch is connected to the circuit under pressure of the operational cylinder and transmits by means of its switch to the control means a signal to lift the electrodes in the event of a drop of pressure in the operational cylinder when the electrodes contact non-conductive materials and when a given response pressure is reached in the operational cylinder. The circuit is also connected to a switch which, upon reduction of pressure in the operational cylinder, interrupts simultaneously the current fed to the electrical regulation circuit and, upon attainment of the response pressure, restarts the current, which in turn transmits a signal for lowering of the electrodes. However, the contents of the above document involve a plurality of unsuitable indications. The circuit cannot monitor constantly and continuously by means of the pressure switch the variable value of the static pressure in the cylinder and therefore can perform neither an automatic control of the value of wear of the electrodes nor the memorisation of the value of the weight of the arm at every speedy descent of the electrodes for the beginning of smelting, nor regulation in a closed circuit with a servovalve having a speedy response for control of the electrodes by means of a pressure control; the circuit is not even able to carry out automatic regulation with an electronic system to regulate the electrodes so as to prevent any short circuits at the electic arc during smelting, nor can it prevent automatically an accidental breakage of the electrodes. The above known document discloses operation by means of a timer and a relay and therefore provides for regulation in steps, which in itself is unstable, slow and discontinuous. Moreover, the known document employs a pressure switch, which of necessity has to be set with a given minus step as compared to the minimum actual value; and therefore when the pressure switch senses the pressure of actuation, there is the danger that the electrode may be already broken since this step of interval may be too great for the typical requirements of the electrode. Furthermore, the above known document does not disclose acting by perforating mechanically any insulating material found between the electrode and the metallic mass to be melted. The prior art document DE-A-3.242.934 provides for measurement of the static pressure, when the adjustment cycle begins, and provides for the use of that pressure for the whole adjustment cycle. This value is adjusted to a value correlated with the current and has the purpose of achieving a determined speed of descent of the electrode. It is stated expressly in the document that the measurement of the static pressure takes place only once at the beginning of the cycle. DE-A-3.242.934 is not able to control the consumption of the electrode nor to study the electrode nor to compare its consumption on the basis of the curves of consumption; it is not even able to check whether there have been variations in the weight of the electrode-bearing arm nor what type of variations have taken place. Moreover, this prior art document is not able to know whether the electrode, when rested on the scrap, should be raised or should be thrust towards the scrap. The present applicant has tackled the problem of obviating the above shortcomings and has designed, tested and embodied a device according to the invention. The method and device to control the force applied to the electrode-bearing arms of an electric arc furnace are set forth and characterized in the main claims, while the dependent claims describe variants of the idea of the solution. The method and device according to the invention enable the value of wear of the electrode to be controlled automatically,that is to say, they make an automatic control possible on the basis of the variation in the weight of the electrode. According to the invention it thus becomes possible to process the variation in the weight of the electrode during the various steps of the smelting. Moreover, the invention makes possible the monitoring and storage of the value of the weight of the arm and of the electrode applied to that arm at each swift descent of the electrodes when the smelting begins. Furthermore, the invention enables regulation to take place in a closed circuit for control of the electrodes by means of a constant and continuous control of the pressure. Besides, the invention permits graduation of the weight of the electrodes on any insulating material present between the electrode and the metallic mass to be melted so as to perforate the material mechanically and thus to make possible the striking of the electric arc. The invention permits also an automatic regulation with an electronic electrode-regulation system to prevent any short-circuits at the electric arc during smelting. According to the invention the control of the position of the electrodes takes place by means of a limit switch or by linear position transducers, whether the latter be continuous or be positioned at the critical points involved. According to the invention, by monitoring continuously the variation in pressure in the circuit and by controlling that pressure with the pressure corresponding to the weight of the arm at the time of each speedy descent, it is possible to maintain a position of balance of the weight of the electrode with variations of its weight and to lift the electrode swiftly in the event of a quick and undesired variation of the weight burdening the arm. The invention positions directly on the operational cylinders a continuous pressure transducer, which detects the pressure and converts it into analogue or digital values that are received by a processor system according to the regulation requirements. The pressure transducer is fitted directly on the cylinder to keep the value of dynamic pressure as close as possible to the value of static pressure. The transducer fitted to the cylinders is also equipped internally with a temperature compensator so as to keep always accurate the signal of monitoring of the hydraulic pressure. The processor system is connected to a servovalve, known in the state of the art and having a speedy response, which controls the operational cylinder and receives information from and supplies it to the system of regulation of the electrodes. A possible pressure storage vessel and a system of a known type to deliver fluid under pressure are included. According to the invention the pressure transducer fitted directly to the cylinder remains always active in the circuit and monitors swiftly and accurately the dynamic pressure and can therefore detect and manage continuously the pressure existing in the circuit. By means of the invention, when the electrode is in its raised position before its swift descent, the pressure is measured and memorised, and therefore the weight of the electrode is evaluated and taken as a reference point so as to have a precise reference element at each swift descent. When the electrode descends by some centimetres of its travel from its raised position, the pressure transducer fitted to the cylinder reads the dynamic pressure and memorises the static pressure plus the differential of dynamic pressure. The transducer supplies this information to the processor system, which sets a control point value for corrective intervention differing by less than a preset percentage (about 1%) from the dynamic value memorised. If the electrode is rested on the scrap, a reduction takes place in the dynamic pressure since the system is displaced speedily towards the static pressure and thereafter also towards a further very quick reduction in the static pressure. In this case the pressure control restores the system at once by means of the electrohydraulic servovalve having a swift response, and the system causes the electrode to be raised at once and keeps it at the set point value indicated by the regulation system. If an arc condition exists, the raising of the electrode can be carried out to restore the desired pressure conditions; otherwise, if the electric arc has not fired, the electrode is kept resting on the scrap, an overpressure possibly being provided, so that the force of the weight of the electrode will cause perforation of the insulating layer which prevents the arc firing. In this way detrimental short-circuits at the electric arc and accidental breakages of the electrodes are avoided. The response times of the system are of the order of 50-80 milliseconds; the system normally responds in a maximum time of 100 milliseconds. Owing to the very quick response time and to the interval of corrective intervention of about 1% of the value memorised, the proposed system obviates accidental breakages of the electrode and accidental short-circuits. Let us now see a preferred embodiment of the invention with the help of the attached figure, which is given as a non-restrictive example. The figure shows a diagram of an electric furnace with a relative electrode and an actuation and control circuit according to the invention. In the figure an electric arc furnace 10 with scrap 17 to be smelted within it cooperates with electrodes 11 borne by an arm 12 actuated vertically by an operational cylinder 13. Each electrode 11 is fed by its own high-voltage line 14 fed and controlled by its own regulation system. The operational cylinder 13 is fed by a system 21 which delivers fluid under pressure and which may cooperate with a pressure storage vessel 20. The operational cylinder 13 is fed through a quick-response control servovalve 18, and a continuous pressure transducer 15 is comprised in the immediate vicinity of the operational cylinder 13 and may be of a thermally compensated type. This continuous pressure transducer 15, which is substantially installed on the cylinder 13, provides continuous real information for a processor system 16, which on the one hand governs the servovalve 18 and on the other hand receives information from and supplies it to an interface 19 of the system which regulates the electrical current fed to the electrodes 11. According to a variant the processor system 16 receives information from the pressure transducer 15 at preset and predeterminable intervals. As we said earlier, systems with a linear transducer or a limit switch 22 may be comprised to control the terminal positions of the arm 12 that bears the electrodes 11. When the arm 12 is in a raised position, the pressure transducer 15 memorises the static pressure and supplies information to the processor system 16, which calculates the weight of the system consisting of the electrode, the arm and the burdening part of the operational cylinder 13. When the electrode 11 has begun its descent from its raised position along a preset segment, such as a length of some centimetres for instance, the pressure transducer 15 reads the dynamic pressure and supplies this information to the processor system 16, which stores the static pressure plus the dynamic pressure differential. The processor system 16 applies an intervention value differing by less than 1% (but any percentage intervention threshold could be selected) from the dynamic pressure value received from the pressure transducer 15 acting on the cylinder 13. When the electrode 11, continuing its descent, rests on the scrap 17, the dynamic pressure is converted swiftly into simple static pressure and, if no corrective intervention takes place, the static pressure too diminishes quickly. The pressure transducer 15, monitoring continuously this variation in dynamic and static pressures, supplies this information to the processor system 16, which takes corrective action on the quick-response servovalve 18, which causes the electrode 11 to be lifted and keeps it at a static pressure value read by the pressure transducer 15 and required by the processor system 16. In this way, in view of the swiftness of corrective intervention, short-circuits at the electric arc are obviated and accidental breakages of electrodes are avoided. If, however, the interface 19 gives warning that the arc has still not struck between the electrode and the metallic mass, thus indicating the existence of an insulating layer, the processor system 16 does not intervene but lets the force of the weight of the electrode act upon and perforate the insulating layer. If the processor system 16 detects a too long time of static pressure, such time indicating possible problems in perforating the insulating layer, the processor system 16 can also apply a pressure to act on the electrode to assist the latter to perforate the insulating layer.

Claims: Method to control the force applied to the electrode-bearing arms (12) of an electric arc furnace (10), the vertical regulation of the electrodes (11) being achieved with an electrohydraulic or electro-pneumatic system (20-21) consisting of an operational cylinder (13) actuated by fluid under pressure so as to raise and lower the electrodes (11), the control means of the operational cylinder (13) being adjustable by means of an electrical device (18), in which method the working pressure of the operational cylinder (13) is measured by a pressure transducer (15) and is correlated by a processor system (16) with the weight of the electrodes (11) and with the condition of static pressure, the processor system (16) governing a quick-response servovalve (18) and being governed by an interface (19) of the system that adjusts the electrical current fed to the electrodes (11), in which method the static pressure is read and stored whenever the electrode (11) is in its raised position, whereas when the electrode (11) passes from its raised position to its working position along a first initial segment of its travel, the pressure transducer (15) reads the dynamic pressure and supplies this value to the processor system (16), which stores this value of dynamic pressure and adds it to the value of static pressure, the processor system (16) providing an intervention value which is defined by a desired threshold of deviation in relation to the stored value. Method as in Claim 1, whereby the processor system (16) causes the electrode (11) to be lifted, unless the interface (19) gives a contrary warning, when the dynamic pressure is within a determined range. Method as in Claim 1 or 2, whereby, whenever the interface (19) gives warning that the arc has not fired, the processor system (16) governs an overpressure on the electrode (11) if the static pressure drops quickly below the value of static pressure stored in relation to the condition of the electrode (11) in a raised position. Method as in any claim hereinbefore, whereby when the electrode (11) rests on the scrap (17), the pressure transducer (15) warns the processor system (16), which governs the servovalve (18) and causes the electrode (11) to be lifted and kept at the pre-set value of pressure if the interface (19) advises that the arc has fired. Device to control the force applied to the electrode-bearing arms (12) of an electric arc furnace (10), vertical adjustment of the electrodes (11) being achieved with an electrohydraulic or electro-pneumatic system (20-21) consisting of an operational cylinder (13) actuated by a fluid under pressure so as to raise and lower the electrodes (11), the control means of the operational cylinder (13) being adjustable by means of an electrical device (18), this device being suitable to perform the method of the claims hereinbefore, whereby a quick-response servovalve (18) is positioned on the line of feed of fluid under pressure to the operational cylinder (13), while means to read the pressure relative to the operational cylinder (13) are arranged on the operational cylinder (13), these means being of a pressure transducer (15) type, the pressure transducer (15) being connected to a processor system (16), which supplies information to, and receives information from, an interface (19) of the system which governs the adjustment of the electric current fed to the electrodes (11) and also governs the quick-response servovalve (18), the device being characterised in that the processor system (16) is associated with the means (15) reading the pressure relative to the operational cylinder (13) and comprises means to store the weight of the electrode (11) when the latter (11) is in any raised position, together with means to store the weight of the arm (12) plus the burdening weight of the operational cylinder (13) and also comprises means to store the dynamic pressure whenever the electrode (11) begins descending from its raised position, and is also characterised in that the pressure transducer (15) is installed in the vicinity of the operational cylinder (13). Device as in Claim 5, in which the pressure transducer (15) comprises means for thermal compensation. Device as in Claim 5 or 6, in which the pressure transducer (15) supplies a measurement of the working pressure to the processor system (16) periodically. Device as in Claim 5 or 6, in which the pressure transducer (15) supplies a measurement of the working pressure to the processor system (16) continuously.

Inventors: GENSINI GIANNI, NIGRIS GIOVANNI, GENSINI, GIANNI, NIGRIS, GIOVANNI
Applicants: DANIELI OFF MECC, DANIELI & C. OFFICINE MECCANICHE S.P.A.
Application year: 1991
Main classification: H05B7
IPC codes: H05B7