EP2371186A1 - Method of monitoring the wear of at least one of the electrodes of a plasma torch - Google Patents

Abstract

The invention relates to a method of monitoring the wear of at least one of the electrodes (1, 2) of a plasma torch comprising two electrodes (1, 2) having the same principal axis, these electrodes (1, 2) being separated by a chamber (3) for receiving a plasma gas, and at least one means for generating a magnetic field (7) placed locally to said at least one electrode, the purpose of which is to monitor the wear, in which the arc root is swept over a portion of the surface of the electrode starting from an initial position up to the point where said arc root reaches a defined final position on said portion, the longitudinal progression of said arc root being determined by a function dependent on at least time, function f(t), which is fixed. According to the invention, at least the electrical energy consumed by said torch is measured as a function of time from the commissioning of said electrode (1, 2), said measurements are recorded in a storage unit and, based on the temporal variation in at least said consumed electrical energy on at least some of said measurements, a variable ?(t) for adjusting the function f(t) determined over a time period T determined by the state of wear of said electrode (1, 2).

Description

 Method for controlling the wear of at least one of the electrodes of a plasma torch

The present invention belongs to the field of plasma torches. More specifically, the invention relates to a method for controlling the wear of at least one of the electrodes of a non-transferred arc plasma torch.

It also relates to a non-transferred arc plasma torch for the implementation of this method.

A plasma torch is a system for transforming electrical energy into high density thermal energy. An electric arc caused between two electrodes is typically implemented to provide the energy necessary for the ionization of a plasma gas. Plasma torches are used in industry, for example, to make metal deposits or for welding, or to destroy certain products such as hazardous waste.

The non-transferred arc torches, also called blown arc torches, comprise two electrodes between which is generated an electric arc that is maintained. These electrodes being contained in the plasma torch, the electric arc is confined inside thereof. In contact with this electric arc, the gas flow injected into the torch is heated to very high temperature and is ionized.

The gas thus heated flows through the open end of one of the electrodes, called the downstream electrode. Only gas ejected at high temperature, or plasma dart, is therefore visible outside the torch. While the plasma dart temperature is of the order of 5000 ^° C., the temperature of the electric arc and, in particular, that of the arc feet, is typically of the order of 20,000 ^° C.

This temperature being higher than the melting temperature of the electrodes, and whatever the material used to make these electrodes, the vaporization of the electrodes at the level of the arc feet is inevitable.

Although the electrodes are typically cooled, they are consumables that must be replaced after a shorter or shorter service time.

The longevity of the cooled electrodes can vary from a hundred hours for relatively low power torches to a thousand hours for high power plasma torches.

For many years, research has therefore been carried out to improve the life of the electrodes of plasma torches in order to make them compatible with industrial requirements.

It was first observed that the lifetime of the electrodes depended on several parameters. It is thus possible to play on the shape of the electrodes and on the choice of their constituent material. Nevertheless, the plasma plummet being inoculated with metal particles resulting from the wear of the electrodes, the selected material (s) must be compatible with the envisaged applications for the plasma torch. In order to limit the average surface temperature of the electrodes, they can also be cooled, for example by putting in place a circulation of water, in general, demineralized.

It is also possible to limit the arc current for a given torch operating power. Indeed, at the level of the arc feet, the flow of calories to be evacuated increases with the current fixed across the electrodes.

Thus, for a given torch power, the possibility of achieving this power by a voltage / current pair favoring the voltage relative to the current is a major element because the wear of the electrodes is reduced. However, all of these parameters are inherent to the design of a plasma torch. These parameters are therefore no longer modifiable once the commissioning of this torch operated.

It has also been proposed to distribute the induced erosion at the foot of the electric arc over the largest possible electrode area. By such a method of controlling the position of the electric arc foot, it is sought to prevent the foot of the arc remains attached to a single point on the surface of the electrode causing a very rapid erosion thereof.

This control of the position of the arc foot on the surface of the electrode can be achieved by injecting a variable flow of plasma gas.

Advantageously, such a control is then performed by the sole management of the regulator valve of arrival of the plasma gas. This management does not change the servitudes of the plasma torch.

Nevertheless, this method is not very flexible since it is then imperative to limit the ranges of variations of the flow rate in order to prevent any exit of the electric arc foot from the working zone to the surface of the corresponding electrode. In addition, excessive variations in flow rate prevent good arc stability within the plasma torch. The control of the position of the arc foot on the surface of the electrode can also be achieved by the application of a fixed magnetic field with a mechanical mobility of the permanent magnet generating this magnetic field.

Such a control allows a distribution of wear on the surface of the electrode over a range of lengths related to the displacement amplitude of the permanent magnet.

However, the displacement of this permanent magnet is completely independent of the operating points of the plasma torch, and when it reaches the end of the stroke, the wear is greatly accelerated on the fixing location of the arch foot because the latter then describes a simple rotation. Moreover, the speed of movement of this magnet is generally constant over a defined period of time.

The control of the position of the arc foot on the surface of the electrode can still be achieved by the application of a variable magnetic field.

Document FR 2 609 358 discloses a non-transferred arc plasma torch comprising a field coil surrounding the upstream electrode of the torch and an electrical circuit for supplying variable DC current to this coil so as to describe at the foot of the arc in contact with the upstream electrode a longitudinal stroke which is superimposed oscillation of the arc foot during this race. This method increases the number of degrees of freedom for controlling the position of the electric arcing feet.

However, this oscillation of the arch foot remains however limited in excursion of the surface of the electrode. This limitation of the surface seen by the foot of arc does not make it possible to optimize the erosion of the electrode. In addition, this solution is expensive and induces an electrical consumption that is added to the power consumption of the plasma torch. The interest of such a technology disappears for plasma torches having a power of less than 1 MW.

Moreover, this field coil technology in the form of slab is bulky (weight and dimensions), which makes it difficult to implement this type of torch in a constrained environment.

If these control systems of the displacement of the electric arcing foot have made it possible to extend the operational life of the electrodes, they can be further improved in order to better distribute the wear of the electrode. The short life of the electrodes is indeed a significant disadvantage for some industrial applications.

The objective of the present invention is therefore to provide a method for controlling the wear of at least one of the electrodes of a plasma torch which is simple in its design and in its operating mode, to optimize the position of the foot electric arc on the surface of this electrode and, therefore, the longevity of these electrodes.

For this purpose, the invention relates to a method for controlling the wear of at least one of the electrodes of a plasma torch, this torch comprising two electrodes having the same main axis between which an arc is established, these electrodes being separated by a chamber intended to receive a plasmagene gas, and at least one means for generating a magnetic field placed locally at the said at least one electrode whose wear is to be controlled, in which the arch foot is longitudinally scanned on a portion of the surface of this electrode from an initial position until said arch foot reaches a determined end position of said portion involving the change of this electrode, the longitudinal progression of this arch foot being determined by a function dependent at least the time, f (t) which is fixed.

According to the invention, at least the electrical energy consumed by this torch is measured as a function of time since the commissioning of the electrode, these measurements are recorded in a storage unit and determined from the temporal evolution. at least this electrical energy consumed on at least a part of these measurements, an adjustment variable ξ (t) of the function f (t) over a period of time τ determined by the state of wear of this electrode .

The term "since the commissioning of the electrode" that these measurements are performed in real time or at regular intervals from a new electrode or not. In the latter case, the initial and intermediate positions are, however, determinable in order to be able to take over the wear of the electrode at the position where it had stopped in the event of maintenance on the plasma torch for example.

The term "electrodes having a same main axis" that these electrodes are coaxial or that the upstream electrode, marked with respect to the flow direction of the plasma, has the same main axis as the downstream electrode.

"Locally" means that the means for generating a magnetic field creates a magnetic field at the level of the electrode whose wear is to be limited, in order to cause the displacement of the arc foot on the surface of the electrode. considered. As a purely illustrative example, the plasma torch comprises a field coil surrounding the upstream electrode to generate a local magnetic field at this electrode, this coil being further fixed in position but powered with a variable DC current i (t) (= f (t)).

It is known that a set value of the current supplying the field coil corresponds to a given position of the arc foot on the upstream electrode. Moreover, since this torch has a given configuration (geometry of the upstream electrode, electromagnetic characteristics of the field coil, etc.), it is possible to determine experimentally by methods known to those skilled in the art the representative curve of the position of the arc foot on the upstream electrode according to the intensity of the current applied to the field coil. Thus, by having determined the equation of this curve and knowing the law i (t) (regular or non-regular alternating sweep, pulsating wavy, ...) governing the longitudinal progression of the arc foot on the surface of the electrode, the person skilled in the art knows how to check the wear of the electrode the longitudinal scan of the arc foot on the surface of the upstream electrode between two positions.

However, the operating speed of the torch may vary over time, the torch not working, for example, at full speed continuously. On the contrary, the plasma torch can experience periods of standby or power variations over time depending on the applications envisaged for this torch.

The wear of the electrode for a set value of the arc current is then slowed down or, on the contrary, accelerated.

The adjustment variable ξ (t) then makes it possible to take into account either the "supposed state" of the electrode as defined by the function f (t), but its actual state which depends on the actual stresses of the flare. plasma.

If the wear of the electrode is, for example, low for a set value of i (t) at time t ₀ because the torch is in standby, we will try to maintain the arch foot longer in the position of the surface of the corresponding electrode so as to extend the life of this electrode. For this, we will apply an adjustment variable, or corrective, ξ (t) = i _∞ r (t) such that the variable DC current applied to the field coil to control the position of the foot of the arc is i _B ( t) = i (t) - i _cor (t) for a duration T determined not only by the time during which the plasma torch remains in standby state but also by the time necessary to reach a state of wear requiring the passage at another point on the surface of the electrode whose wear is to be controlled.

In the case where this same field coil is further mechanically displaced in translation, then the adjustment variable ξ (t) is a function of the form F (i (t), z (t)).

Of course, the operations of determining the adjustment variable ξ (t) can be performed by a computer that controls the control means of the position of the arch foot. In the case of a field coil, for example, this computer controls the supply current of this coil. In various particular embodiments of this method, each having its particular advantages and susceptible to many possible technical combinations:

the arc current is also measured as a function of time since the commissioning of the electrode,

This measurement of the arc current advantageously allows a more precise determination of the adjustment variable ξ (t) of the function f (t). In fact, for the same electrical power P _arc consumed by the torch, it is possible to have arc currents that are different. - oscillating on itself, during the scanning, the foot of arc around an average position defined by the function f (t),

this adjustment variable ξ (t) is determined from the determination of the temporal evolution of the electrical energy consumed on the one hand, on the whole of the measurements and on the other hand, on the measurements obtained since a determined time interval T corresponding to a different operating regime of said torch,

said at least one means for generating a magnetic field is chosen from the group comprising a field coil, a permanent magnet and combinations of these elements. When this means for generating a magnetic field is a field coil, it will preferably be slab type for the upstream electrode. According to different variants, this coil may consist of:

a metal winding coaxial with the electrode. The conducting wire may be solid or hollow, of square, rectangular or round section, of a single electrical conductor wire,

- Several electrical conductors not physically linked together permanently, that is to say the coil can also be segmented,

a number of layers N> = 2. Each layer is then constituted by a number of turns S> = 8. S is not necessarily identical for the N layers.

The coil may locally surround the electrode but the center of the coil is not necessarily bound to the center of the electrode along the axis of the torch. Alternatively, the coil can be connected either in series with the electrode, or in parallel, that is to say without any electrical contact with the electrode. The coil may be longer than the electrode, shorter or the same size as the electrode.

For reasons of compactness, the diameter of this field coil may be reduced (radial field loss). This radial field loss can then be partially compensated by the addition, over all or part of the length of the field coil, of one or more permanent magnets. If this or these permanent magnets are cylindrical, they will be coaxial with one of the electrodes.

One or more other permanent magnets having fields different from the preceding ones may be positioned outside the field coil either upstream or downstream in order to locally modify the shape of the field.

said at least one means is moved to generate a magnetic field along this main axis so as to vary the position on this electrode of the foot of the electric arc generated between the electrodes, said at least one means is displaced to generate a magnetic field with a variable speed in time,

said at least one means is moved to generate a magnetic field with a speed varying gradually or in stages.

said at least one means is moved to generate a magnetic field on either side of a reference position, a movement of said at least one means for generating a magnetic field along said main axis is carried out, simultaneously or successively; and the application of variable DC current.

These means for controlling the wear of the electrode can therefore act together to combine their effects.

The invention will be described in more detail with reference to the accompanying drawings in which:

- Figure 1 is a sectional view of a non-transferred arc plasma torch in a particular embodiment of the invention; FIG. 2 schematically shows the intensity component I ₂ superimposed on an intensity h of base, I ₂ being an oscillation such that I ₂ <li and I = h + I ₂ being the variable DC current applied to said coil of l upstream electrode of Figure 1; Figure 1 shows a non-transferred arc plasma torch according to a particular embodiment of the invention. This torch comprises two tubular electrodes 1, 2 arranged collinearly along a main axis. These electrodes 1, 2 are cooled by a water cooling device (not shown) known from the state of the art and which will not be described in more detail here.

These electrodes 1, 2 are separated from each other by a chamber 3 for receiving a plasma gas.

A power supply system 4 connected to these two electrodes 1, 2 makes it possible to apply a potential difference between them causing a maintained electric arc.

The arc current arc and the _arc voltage U _ar c are measured to determine the electrical power consumed P _arc = larc x U _arc .

The plasma gas which is supplied by a gas supply source 6 is forced into this chamber 3. This plasmagenic gas is preferably introduced between the electrodes 1, 2 with a swirling motion, or else in a vortex, in order to ensure a sheathing by the gaseous fluid and stabilization of the electric arc.

On the other hand, this swirling movement ensures a natural rotational movement of the upstream and downstream arc feet on the surface of the corresponding electrodes.

The means for generating a magnetic field advantageously comprises a field coil 7 which is fed with a variable DC current 8. By "variable DC current" is meant a DC current whose intensity varies as a function of time.

This field coil 7 is here placed around the upstream electrode 1 to control the position of the upstream arc foot on the surface of this electrode.

Preferably, the intensity I of this variable DC current comprises an intensity I ₂ superimposed on an intensity,, I ₂ being an oscillation such that I ₂ <li, the variation of the intensity h being chosen from the group comprising linear variation , stepwise variation, exponential variation, logarithmic variation, variation according to a polynomial function, or a combination of these elements. For purely illustrative purposes, the plasma torch is fed with a variable DC current whose basic intensity varies in steps, each step having a duration of several hundred hours, the wear of the electrode then being "slices". Alternatively, this intensity can vary linearly or according to a "curved" law such as exponential or polynomial.

Figure 2 shows the shape that can take intensity oscillation I ₂ , which makes it possible to oscillate the foot of arc around an average position and therefore to limit the wear of the upstream electrode. This oscillation may have a sinusoidal shape (Fig. 2a), a square shape (Fig. 2b) or a triangle shape (Fig. 2c).

The amplitude and frequency of this oscillation may vary over time depending on the electrical energy consumed by the plasma torch and the state of wear of the electrode. Typically, the amplitude will be all the more limited as the torch will be in an extreme operating range (low power, nominal power). The frequency of the wave will depend on the enthalpy of operation of the torch.

The shape of the wave will be selected according to the observation of the stability of the operating points of the torch. If the torch power varies discretely from one power to another and in a programmed manner, a square shape will be preferred.

Given that the average current that the field coil, when it reaches its maximum, fails to push sufficiently the electric arc downstream of the torch, there is copper not seen by the arch feet . It will therefore be advantageous to move the field coil by moving it. This displacement therefore makes it possible to lower the value of the average current of the field coil and to apply new laws of growth of the average current as well as of waveforms.

For this, the plasma torch comprises means 9 for moving said at least one means for generating a magnetic field 7 along the main axis so as to vary the position on the electrode whose wear is to be controlled. foot of the electric arc generated between these electrodes 1, 2.

These means 9 here comprise a worm rotated by a motor. The field coil 7 is linked to this screw so that the setting rotation of the worm causes a translation of the field coil 7.

In the event that it is desired to move said at least one means for generating a magnetic field on either side of a reference position, this motor may for example be an alternating motor.

Advantageously, it is also possible to modulate the injection of a secondary plasma gas 10 in order to control the position of the arch foot.

Claims

1. A method for controlling the wear of at least one of the electrodes (1, 2) of a plasma torch, said torch comprising two electrodes (1, 2) having the same main axis between which is established an arc (5), said electrodes (1, 2) being separated by a chamber (3) for receiving a plasmagene gas, and at least one means for generating a magnetic field (7) placed locally at said at least one electrode (1, 2) whose wear is sought to be controlled, wherein said arc foot is longitudinally scanned on a portion of the surface of said electrode (1, 2) from an initial position until said foot arc reaches a determined end position of said portion involving the change of said electrode, the longitudinal progression of said arc foot being determined by a time-dependent function, f (t) which is fixed, characterized in that at least the electrical energy consumed by said torc is measured as a function of the time since the putting into service of said electrode (1, 2), said measurements are recorded in a storage unit and the time evolution of at least said electric energy consumed on at least one part of said measurements, an adjustment variable ξ (t) of the function f (t) over a period of time τ determined by the wear state of said electrode. 2. Method according to claim 1, characterized in that said arc current is also measured as a function of time since the commissioning of said electrode (1, 2). 3. Method according to claim 1 or 2, characterized in that it oscillates on itself, during scanning, said arc foot around a mean position defined by the function f (t). 4. Method according to any one of claims 1 to 3, characterized in that realizes said measurements in real time or at regular time intervals. 5. Method according to any one of claims 1 to 4, characterized in that said adjustment variable ξ (t) is determined from the determination of the temporal evolution of said electrical energy consumed on the one hand, on all of those measures and, on the other hand, on the measurements obtained from a determined time interval T corresponding to a different operating regime of said torch. 6. Method according to any one of claims 1 to 5, characterized in that said at least one means for generating a magnetic field (7) is selected from the group comprising a field coil, a permanent magnet and combinations of these elements. 7. Method according to claim 6, characterized in that said means for generating a magnetic field (7) comprising a field coil, said coil is fed with a variable DC current (8). 8. Method according to claim 7, characterized in that the intensity I of said variable DC current comprising an intensity I ₂ superimposed on an intensity is selected from the group consisting of linear variation, stepwise variation, exponential variation, logarithmic variation, variation according to a polynomial function, or a combination of these elements. 9. Method according to any one of claims 1 to 8, characterized in that displacing said at least one means for generating a magnetic field (7) along said main axis so as to vary the position on said foot electrode the electric arc generated between said electrodes (1, 2). 10. The method of claim 9, characterized in that displacing said at least one means for generating a magnetic field (7) with a variable speed in time. 11. The method of claim 10, characterized in that moving said at least one means for generating a magnetic field (7) with a gradually varying speed or increments. 12. The method of claim 10 or 11, characterized in that displacing said at least one means for generating a magnetic field (7) on either side of a reference position. 13. Method according to one of claims 7 to 12, characterized in that it carries out, simultaneously or successively, a displacement of said at least one means for generating a magnetic field (7) along said main axis and the application of the variable DC current. 14. Method according to any one of claims 1 to 13, characterized in that modulates the injection of a secondary plasma gas (10) to control the position of the arc foot.