HK1119350A1 - Arc furnace pressure ring assembly - Google Patents

Abstract

The invention provides for a pressure ring assembly [10] suitable for use in an electric arc furnace, wherein the pressure ring assembly [10] comprises at least two pressure ring segments [22] engaging each other, at least one contact shoe [50] arranged between the pressure ring [20] and an electrode, and a piston arrangement [40] between the pressure ring segment [22] and the contact shoe [50] for forcing the contact shoe [50] into electrical contact with the electrode. The pressure ring assembly [10] is characterised therein that it is made of a metal alloy wherein a first metal of the alloy is copper, and a second metal is selected from a group comprising of chrome and silver. The pressure ring assembly [10] is further characterised therein that each pressure ring segment [22] includes two tapered engagement formations [27] defined within the pressure ring segment [22] and being adapted to be engaged by complimentarily tapered connecting means [60] such that the pressure ring segments [22] are drawn towards each other during installation of the pressure ring assembly. The pressure ring assembly [10] further provides for a sealing arrangement between the pressure ring and the contact shoe [50].

Description

Pressure ring assembly for electric arc furnace

The present invention is a divisional application of invention patent application No.200580006485.4 entitled "pressure ring assembly for electric arc furnace" filed on 19.1.2005.

Technical Field

The present invention relates to a pressure ring assembly suitable for use in an electric arc furnace and in particular, but not exclusively, to a pressure ring assembly suitable for use on the lower portion of an electrode column.

Background

Electric arc furnaces are commonly used in the steel alloy production industry during pyrometallurgical smelting operations. The electric arc furnace includes one or more electrodes extending into the electric arc furnace and arranged, in use, proximate a furnace load for supplying power in a smelting operation. A transformer is usually placed outside the electric arc furnace, from which the power is transferred to the electrodes by means of contact shoes arranged circumferentially around and releasably engaging the electrodes.

It will be appreciated that in order to achieve optimum electrical conductivity, the contact shoes must maintain proper electrical contact with the electrodes for all relevant times. To maintain proper electrical contact between the contact shoes and the electrode under typically harsh operating conditions, a pressure ring is typically circumferentially disposed around the contact shoes and is sized to maintain a plurality of contact shoes in electrical contact with the electrode.

Various pressure ring arrangements are encountered in the industry, including continuous pressure rings (commonly referred to as solid rings) and segmented pressure rings comprising a plurality of arcuate segments interconnected to form a circular ring. Segmented pressure rings are generally preferred because of the maintenance difficulties associated with the replacement of contact shoe elements when solid rings are used.

One of the difficulties encountered with segmented pressure rings relates to the connection mechanism with which adjacent ring segments are connected to each other. A first difficulty is that the available connection mechanisms are always difficult to connect and disconnect, and once disconnected, even more difficult to reconnect. This is particularly problematic in the case of a hinge and pin type connection. Another problem associated with known connection mechanisms is that adjacent ring segments of the pressure ring need to be perfectly aligned for use of such mechanisms, and these mechanisms are not configured to assist in aligning the individual ring segments when they are installed. Moreover, known connection mechanisms (including hinge and pin type and parallel pin type connection mechanisms) are only designed to prevent the segments from shifting relative to each other in use, but do not pull adjacent segments closer to each other during installation.

Another important requirement concerning the contact shoes is that they must preferably be placed relatively low below the electrodes. However, this means that the contact shoes and thus the pressure ring will be very close to the high temperature part of the arc furnace. The pressure ring is therefore exposed to harsh environmental conditions. In contrast to electrodes, pressure rings are not classified as consumables and therefore must be able to withstand the harsh furnace operating conditions to extend life.

In such operating conditions of the pressure ring, the most important design parameters that determine the life of the pressure ring are typically effective heat dissipation through the pressure ring and sufficient mechanical strength. Various solutions have been proposed to extend the life of the pressure ring by optimizing and/or controlling the above design criteria.

A first solution, which is implemented in the so-called "split design", proposes to provide a heat shield around the outer periphery and the bottom surface of the pressure ring, which are most exposed to the severe operating conditions. The heat shield is made of a material having good thermal conductivity characteristics, such as copper. The heat shield may have limited mechanical strength due to the pressure ring instead of the heat shield absorbing the forces exerted on the contact shoes. The heat shield will thus protect the pressure ring from being exposed to too high furnace temperatures and the thermal conductivity of the pressure ring becomes less critical. However, due to the complex dimensions, such a solution is complex, not necessarily cost effective and often not feasible.

A second solution, usually implemented in so-called "hybrid designs", proposes to manufacture the pressure ring from a material with good thermal conductivity characteristics, such as pure copper. However, pure copper lacks mechanical strength to withstand the forces exerted on the pressure ring by the contact shoes and is particularly susceptible to creep. The occurrence of creep is furthermore proportional to the temperature of the pressure ring, which renders pure copper generally unsuitable for use. To overcome the disadvantages in terms of mechanical strength, a pressure ring made of pure copper would have to be of considerable dimensions, allowing the maximum stresses in the pressure ring to be reduced to such an extent that creep development is below acceptable limits. However, this results in the pressure ring becoming heavy and bulky and thus expensive to manufacture and difficult to operate. Such pressure rings are also incompatible with standard furnace configurations due to insufficient space for the larger pressure ring.

One way to compromise is to consider using a material that has better mechanical strength than pure copper, but which has lower thermal conductivity than pure copper as a compromise. A pressure ring made of such a material will thus be able to cope with the stresses caused by the contact shoes, while still being well adapted to severe temperature conditions. Materials such as carbon steel, stainless steel and aluminium bronze are widely used in industry, but these materials all have the common disadvantage of sub-optimal thermal conductivity and therefore adversely affect the life of the pressure ring.

A further aspect of pressure ring assemblies that is often problematic is the interface between the pressure ring and the contact shoes. The contact shoes are urged from the pressure ring toward the electrode to engage therewith. This can generally be achieved using a hydraulic piston arrangement or some other mechanical transmission system.

A variety of hydraulic piston devices are known in the industry. In most devices, the piston is disposed within a plenum defined by the concave surface of the pressure ring segment. Fluid is introduced under pressure into the plenum chamber through flow passages embedded in the pressure ring segments and exerts an outward hydraulic force on the piston which in turn transmits a force to the adjacent contact shoes to force the contact shoes into engagement with the electrodes. It will be appreciated that such an arrangement will only work optimally when the pressurisation cavity between the pressure ring segment and the piston is properly sealed, as otherwise it will not be possible to increase the pressure in the pressurisation cavity to fully actuate the piston. In addition, the sealing means between the pressure ring segment and the piston should be able to accommodate relative displacement between the pressure ring and the piston.

Various seals have been previously used in this application. For example, rubber membranes have been used because of their inherent elasticity, but have proven to be prone to premature failure because they are not compatible with high temperature conditions.

Another type of resilient seal is a metal bellows, which has been used with success in pressure ring assemblies. The prior art provides two types of metal bellows in a pressure ring, namely, formed bellows and plate or diaphragm bellows. Shaped bellows are produced by shaping a uniform metal tube or sleeve into a continuously wound bellows, while diaphragm bellows are formed from a single sheet that deforms under pressure.

However, several problems have been encountered with the use of both formed bellows and diaphragm bellows. First, the shaped bellows occupy a considerable space, thus resulting in the piston having to be of a smaller diameter in order to fit inside the pressurisation cavity in the pressure ring segment. The smaller the diameter of the piston, the higher the hydraulic pressure required to apply the same force to the contact shoes.

A second problem associated with both formed bellows and diaphragm bellows is that only a limited number of convolutions can be accommodated in the small space between the piston and the pressurisation chamber, and this relatively small number of convolutions must be sufficient for the required displacement of the piston. This causes the convolutions to be beyond optimal design criteria which in turn results in high stresses and metal fatigue in the bellows and requires high hydraulic pressures to achieve adequate piston movement. Furthermore, diaphragm bellows suffer from low displacement potential and high stress, leading to metal fatigue and premature failure.

As an alternative to hydraulics, mechanical drive systems have also been used to push the contact shoes towards the electrodes. However, these systems have the disadvantage of being complex and prone to failure, and therefore cannot be used as a suitable replacement for hydraulic devices.

Those skilled in the art will appreciate that electric arc furnaces operate at sub-atmospheric pressure, which means that there is a tendency to draw atmospheric air into the furnace. In the vicinity of the electrode (i.e. where the electrode protrudes through the roof of the furnace) this can cause unwanted combustion of CO due to pyrometallurgical reaction processes within the furnace, thereby increasing the temperature in the vicinity of the pressure ring. This can cause the pressure ring, contact shoes and piston to overheat and break.

It will also be appreciated that the electrodes and their components are pressurised by means of a fan which forces the blown air down the electrodes to create an air seal over the furnace to prevent the escape of furnace gases and thereby the formation of a gas chimney up the electrodes. In the event of an improper seal around the pressure ring assembly, unwanted escape of furnace gases can degrade the gas seal and can also cause damage to the electrode components.

The prior art makes little contribution to the sealing between the pressure ring and the contact shoe, and at best suggests only heat resistant cotton or soft refractory clay to fill the gap between the pressure ring and the contact shoe. Such expedient seals are generally of low integrity and are often broken out of the piston in the event of a sudden pressure increase in the furnace (as in the case of an explosion in the furnace).

Objects of the invention

It is therefore an object of the present invention to provide a pressure ring assembly that will at least partially obviate the disadvantages associated with existing pressure ring assemblies and/or provide a new and useful alternative to existing pressure ring assemblies.

Disclosure of Invention

According to the present invention there is provided a pressure ring assembly suitable for use in an electric arc furnace, the pressure ring assembly comprising a pressure ring, characterised in that the pressure ring is made of a metal alloy, wherein a first metal of the alloy is copper and a second metal is selected from the group consisting of chromium and silver.

The alloy may include at least 97% copper. Where the second metal is chromium, the alloy may comprise between 0.5% and 3.0% chromium, especially 1.5% chromium. Where the second metal is silver, the alloy may comprise between 0.05% and 0.5% silver, especially 0.15% silver.

Another feature of the invention is to provide the use of a metal alloy in which the first metal is copper and the second metal is selected from the group consisting of chromium and silver for the manufacture of pressure rings suitable for use in electric arc furnaces.

In accordance with a second aspect of the present invention, there is provided a pressure ring assembly suitable for use in an electric arc furnace, the pressure ring assembly comprising at least two pressure ring segments dimensioned to engage with one another to form a pressure ring around an electrode, each pressure ring segment having a top end, a bottom end, two opposed side ends, an inner surface facing the electrode and an opposed outer surface; the pressure ring assembly is characterised in that each pressure ring segment comprises two engagement formations located adjacent the opposite lateral ends and extending at least partially between the top and bottom ends, each engagement formation being further characterised in that it defines an at least partially tapered formation in at least one surface of the pressure ring segment, the engagement formations being adapted to be engaged by a connecting means whereby the pressure ring segments can be drawn together towards each other during installation of the pressure ring assembly.

Each engagement formation may be a substantially continuous elongate formation and may be defined by a groove embedded in a surface of the pressure ring segment or a protrusion projecting from a surface of the pressure ring segment. The engagement formation may comprise a lip dimensioned to cooperate with a complementary dimensioned lip engagement formation defined in the connection device so as to produce a secure engagement between the engagement formation and the connection device.

In a preferred embodiment of the invention, the engagement formation comprises two elongate grooves defined in the inner surface of the pressure ring segment adjacent the opposite side ends and tapering downwardly between the top and bottom ends. In particular, the elongated grooves taper away from the lateral ends from the top end down to the bottom end.

According to a further aspect of the present invention, there is provided a connecting device for interconnecting adjacent pressure ring segments of a pressure ring assembly suitable for use in an electric arc furnace, the connecting device being characterized in that it comprises two legs interconnected by means of an intermediate bridging portion, wherein the legs are at least partially tapered with respect to each other and with respect to an elongate axis of the connecting device, the connecting device being further adapted to engage with the pressure ring segments such that the pressure ring segments are drawn together towards each other during installation of the pressure ring assembly.

The attachment means may be an elongate attachment bracket adapted to engage with the pressure ring segment. In particular, the connection means may be an elongate sliding bracket adapted to slide into or over a complementary tapered engagement formation defined in the pressure ring segment.

The bridging portion may span substantially the length of the connection means. Alternatively, the bridging portion may comprise a plurality of spaced apart cross braces extending between the legs of the connection means.

According to one embodiment of the invention, the connecting means comprises two separate legs and one separate intermediate bridging portion. In an alternative embodiment of the invention, the legs may be defined by end regions of the arcuate sheet material. The transverse cross-sectional profile of the connection means may be substantially C-shaped, U-shaped or V-shaped.

According to a further aspect of the present invention there is provided a pressure ring assembly suitable for use in an electric arc furnace, the pressure ring assembly comprising at least one pressure ring segment, at least one contact shoe and piston means, wherein the contact shoe is located radially inwardly from the pressure ring segment, the piston means comprising a piston push plate located between the pressure ring segment and the contact shoe to force the contact shoe into electrical contact with an electrode, the piston means comprising a pressurisation cavity in flow communication with a source of high pressure fluid, the piston means comprising a seal within the pressurisation cavity to seal the pressurisation cavity, wherein the seal is characterised in that it comprises a plurality of gasket-shaped sealing discs located side by side and welded together to form a resilient accordion-shaped bellows.

In particular, the seal comprises a plurality of generally parallel, thin, annular metal discs welded circumferentially to one another to form an accordion-shaped bellows. Alternatively, the seal may comprise a plurality of shims disposed between metal discs, in which case the metal discs are welded to the shims.

The thickness of the annular disc may be between 0.1mm and 2 mm.

Another feature is the provision of a plenum chamber disposed in flow communication with the liquid supply channel and the liquid return channel. In one form of the invention, the liquid supply channel and the liquid return channel may be embedded within the pressure ring segment and may together with the pressurisation cavity define a first flow channel for conveying fluid through the pressure ring segment and the pressurisation cavity for actuating the piston arrangement and simultaneously cooling the pressure ring segment. In an alternative form of the invention, the pressure ring segment may comprise a separate second flow channel embedded in the pressure ring segment for conveying a fluid through the pressure ring segment for cooling thereof.

In another alternative form of the invention, the liquid supply and return passages may be provided externally of the pressure ring segment.

The plenum chamber may be defined by a housing, characterized in that the housing is made of a material having a thermal conductivity of at least 100 watts per meter kelvin.

The piston device is further characterized in that it comprises a second seal disposed between the piston push plate and the housing. In particular, the second seal may be disposed in an annular gap formed between the piston push plate and the sleeve-shaped housing. In particular, the piston push plate may include a circumferential groove for receiving the second seal. The second seal may be in the form of a metal ring.

According to a further aspect of the present invention there is provided a pressure ring assembly suitable for use in an electric arc furnace, the pressure ring assembly comprising a pressure ring disposed around an electrode and a contact shoe disposed between the pressure ring and the electrode such that an annular gap is formed between the pressure ring and the contact shoe, the pressure ring assembly being characterised in that it comprises a sealing arrangement between the pressure ring and the contact shoe, the sealing arrangement comprising a groove in one or both of the pressure ring and the contact shoe and a seal trapped in the groove to seal the annular gap.

The seal may be a resilient seal and in a preferred form of the invention, the seal is also biased so as to constantly seal the annular gap during displacement of the contact shoe. The seal may be of an insulating material, in particular ceramic or alternatively silicon carbide.

The seal may include a plurality of seal segments arranged in an end-to-end fashion in the groove to form a substantially continuous annular seal.

Drawings

Embodiments of the invention are described below, by way of non-limiting example, with reference to the accompanying drawings, in which:

figure 1 is a perspective view of a pressure ring assembly according to the present invention;

figure 2 is a top view of the pressure ring assembly shown in figure 1;

figure 3 is a cross-sectional side view of the pressure ring assembly shown in figure 1;

figure 4 is a cross-sectional top view of the pressure ring assembly shown in figure 1;

FIG. 5 is a cross-sectional top view of a connection device according to the present invention;

FIG. 6 is a cross-sectional side view of a sealing device according to the present invention;

FIG. 7 is a cross-sectional view of a bellows for use with the pressure ring assembly of FIG. 1;

FIG. 8 is a front view of a pressure ring segment in accordance with the present invention;

FIG. 9 is two perspective views of a connection device according to the present invention; and

FIG. 10 is a graph showing the relationship between creep rate and temperature for various materials.

Detailed Description

Referring to the drawings, a non-limiting embodiment of a pressure ring assembly according to the present invention is generally indicated by reference numeral 10. The pressure ring assembly 10 comprises a pressure ring 20, a plurality of contact shoes 50 and a plurality of piston arrangements 40, wherein the pressure ring 20 comprises a plurality of interconnected pressure ring segments 22, the plurality of contact shoes 50 are adjacent to the effective inner surface 36 of the pressure ring 20, and the plurality of piston arrangements 40 are interposed between each contact shoe 50 and the pressure ring 20. In use, the pressure ring 20 and contact shoes 50 surround an electrode (not shown) for use in an electric arc furnace (not shown).

The pressure ring 20 according to the specific embodiment is a circular ring when seen in top view, two adjacent pressure ring segments 22 of the pressure ring being shown in figures 1 and 2. Each pressure ring segment is arcuate in plan view and includes an outer surface 35 facing away from the electrode and an inner surface 36 facing toward the electrode (not shown). Each pressure ring segment has a top end 24 and a bottom end 25, which in this embodiment terminates in a substantially L-shaped portion. The circumferential groove 33 is arranged towards the bottom end 25 of the pressure ring segment 22 and in this particular configuration the groove 33 is located on the surface of the L-shaped portion facing the contact shoe 50. The seal 54 is disposed in the groove and is described in more detail below. A suspension 23 extends from the top end 24 and is used to suspend the pressure ring segments 22 and thus the pressure ring 20 from a suspension 51 connected to the contact shoes 50.

Each pressure ring segment 22 also has opposite parallel side ends 26, with the side ends 26 being generally perpendicular relative to the top and bottom ends 24, 25. The engagement structure 27 is located adjacent the side end 26 and extends at least partially between the top end 24 and the bottom end 25 of the pressure ring segment 22. In this particular embodiment, the engagement formations 27 are in the form of recesses in the inner surface 36 of the pressure ring segment 22, but it will be appreciated that the engagement formations may be in the form of elongate projections rather than recesses, and that the engagement formations may be in the outer surface 35 of the pressure ring segment 22 rather than the inner surface 36. The groove 27 is tapered relative to the side end 26 and in particular tapers away from the side end 26 from the top end 24 down to the bottom end 25. In the particular embodiment, the grooves 27 are tapered along their entire length, but it will be appreciated that only a portion of each groove need be tapered, and the remainder may be parallel, for example, with respect to the side ends 26.

As can be seen from fig. 1 and 5, the groove 27 is also undercut towards the side end 26 of the pressure ring segment 22 so as to form a lip 28, which lip 28 contributes to fixedly connecting adjacent pressure ring segments 22.

Adjacent pressure ring segments 22 are releasably connected by means of outwardly tapering or diverting connecting means 60. In this particular embodiment, the attachment means is in the form of an elongated sliding bracket, as shown in the perspective view of fig. 9. Each sliding bracket 60 comprises two elongated legs 61, which legs 61 are interconnected by means of an intermediate bridging portion 62. The two elongate legs 61 are tapered relative to each other and to the elongate axis 64 of the sliding bracket 60, and in particular the two legs 61 are bifurcated so as to mate with complementary tapered grooves 27 in adjacent pressure ring segments 22. The bridging portion 62 shown in this embodiment is a substantially solid plate-like portion extending between the legs 61 along substantially the entire length of the legs 61, however, the bridging portion may also include a plurality of spaced cross braces (not shown). It will be appreciated that the leg and bridging portions may be integrally formed, for example from sheet material bent into the appropriate shape, or by cast steel. The sliding bracket 60 also includes holes 63 for use during installation and removal of the sliding bracket 60. As can be seen from fig. 5, the sliding bracket 60 is substantially C-shaped when viewed in a cross-sectional top view. Similarly, the bracket may also be U-shaped or V-shaped.

In use, two pressure ring segments 22 are disposed adjacent one another, so that adjacent recesses 27 form two diverging channels or engaging formations. Proper alignment is facilitated by providing alignment holes 29 and alignment bosses 34 in the side ends 26 of the pressure ring segments. The attachment means 60 is then inserted from the top end 24 of the pressure ring segment 22 so that the elongate leg 61 enters the recess 27 from above. When the connecting means 60 is pushed down, the legs 61 engage with the groove 27 and the lip 28, thus forcing the two pressure ring segments 22 against each other due to the tapered configuration of the sliding bracket 60 and the groove 27. It will be appreciated that the groove 27, and therefore the leg 61 of the sliding bracket 60, need not be bifurcated or tapered along its entire length. The gripping device 61 portion and the groove 27 may be generally parallel, so long as at least some portions (whether upper or lower) are tapered.

The contact shoes 50 are located adjacent the inner surface 36 of each pressure ring segment 22. In the completed pressure ring assembly 10, the contact shoes 50 are thus located radially inwardly from the pressure ring, so that the pressure ring 20 surrounds the contact shoes 50. The contact shoes 50 are displaceable relative to the pressure ring segment 22 so as to be able to engage with electrodes (not shown) placed inside the pressure ring 20 and the contact shoes 50. When the electrode is engaged, a reaction force is exerted on the contact shoe towards the pressure ring segment 22, which in turn absorbs the reaction force. It should be noted that the weight of the contact shoes 50 is carried by the contact shoe suspension 51, rather than by the pressure ring segments 22.

As shown in fig. 3, a piston arrangement 40 is provided on each pressure ring segment 22 to facilitate movement of the contact shoe 50 relative to the pressure ring segment 22. The piston arrangement 40 comprises a booster cavity 42, which booster cavity 42 is formed by the piston push plate 41, the sleeve-shaped housing 43 and the inner surface 36 of the pressure ring segment 22. The piston push plate 41 is movably connected with respect to the pressure ring segment 22 and the sleeve-shaped housing 43 by means of a sealing element 44 in the form of a bellows, which sealing element 44 also serves to seal the pressure charging chamber 42. The bellows 44 is shown in more detail in fig. 7 and comprises a plurality of washer-like metal discs 46, the metal discs 46 being arranged side by side with a gasket 49 provided therebetween. The outer edges of the metal discs 46 are welded to the spacers 49 and the inner edges 47 are welded to each other to form an accordion bellows. This type of bellows is often referred to as leaf bellows (leaf bellows). In use, the bellows 44 is displaceable in the direction indicated by arrow a. The thickness of the metal disc is typically between 0.1mm and 2 mm.

A first end of bellows 44 is welded to piston push plate 41 and a second end of bellows 44 is welded to the inner surface of sleeve-shaped housing 43. This configuration results in the piston push plate 41 being movable relative to the bellows housing 43, but still sealed relative to the bellows housing 43. The plenum chamber 42 is thus still continuously sealed along the entire stroke of the piston push plate 41. The movement of the piston push plate 41 relative to the pressure ring segment 22 is limited by a stroke limiter 45, which stroke limiter 45 may be of various constructions, but in particular is in the form of a bolt that can be set in a desired position.

The pressure of the pressurizing chamber 42 is increased to transmit the external force to the piston push plate 41. The resilience of the bellows 44 allows the piston push plate 41 to move away from the pressure ring segment 22 so as to urge the contact shoe 50 into abutment with an electrode (not shown) in use. It will be appreciated that the piston arrangement 40 may be a cartridge-type piston assembly, in which case the booster cavity 42 will be defined by a piston push plate 41, a sleeve-shaped housing 43 and an end plate extending from the sleeve-shaped housing 43. Such an assembly will be separate from the pressure ring segment and will only be placed inside the groove provided in the pressure ring segment.

The plenum chamber 42 is pressurized by introducing a high pressure fluid (e.g. water) into the plenum chamber 42 via a first flow passage 31 embedded inside the pressure ring segment 22. In this particular embodiment, each pressure ring segment 22 includes two internal flow passages constructed in parallel. The second flow channel 32 serves only as a cooling channel, wherein the heat of the pressure ring segment 22 is removed by a cooling fluid flowing inside the cooling channel. At the same time, the first flow channel 31 also contributes to cooling, which also transports liquid to the piston chamber 42 or from the piston chamber 42. An increase in the supply pressure to the first flow passage 31 will therefore cause an increase in the pressure in the pressurisation chamber 42 and hence movement of the piston push plate 41.

In use, heat in the pressure ring segment will be removed by the fluid in the first flow passage 31 even if the second flow passage 32 is inactive, and vice versa. This is beneficial in that the pressure ring segment remains effective even after the use of the first 31 or second 32 flow passages has terminated due to, for example, a failure of the bellows 44, since sufficient heat transfer will be provided via the remaining flow passages until the system can be shut down for maintenance. This is a major improvement over other systems that use a single common channel for heat transfer and piston displacement. It will be appreciated that the first and second flow passages 31, 32 may be disposed externally of the pressure ring segment 22 when a cartridge-type piston assembly is used as described above.

A second seal 48 is also provided between the piston push plate 41 and the sleeve-shaped housing 43. The second seal 48 is disposed in a circumferential sealing groove located at the periphery of the piston push plate and is typically in the form of a metal ring. The seal 48 prevents dust and dirt from entering the piston assembly 40, and in particular the gaps between adjacent discs 46 of the bellows 44.

As described above, the seal 54 is disposed within the recess 33 in the bottom end 25 of the pressure ring segment 22. The lower portion 53 of the contact shoe 50 defines opposing sealing surfaces to seal the annular gap between the contact shoe 50 and the pressure ring 20. The sealing device is shown in more detail in fig. 6. The seal may include a plurality of seal segments disposed adjacent to one another to form a substantially continuous circumferential seal. It will be appreciated that various combinations may be used, such as providing a circumferential groove in the contact shoe opposite the pressure ring, or providing a circumferential groove in both the pressure ring and the contact shoe. Various seals 54 may be used in this application. For example, a ceramic seal may be used, in which case the seal may be biased towards a sealing position by providing a spring washer between the seal and the groove. In the alternative, a resilient seal may be used, in which case the use of a spring washer or the like would not be required.

The pressure ring 20 is exposed to high temperatures due to its location in close proximity to the furnace interior. In addition, a generally outward reaction force is exerted on the pressure ring due to the clamping action of the contact shoes 50 on the electrodes. It is therefore important to manufacture the pressure ring 20, and in particular the individual pressure ring segments 22, from a material which firstly has good thermal conductivity characteristics to ensure proper heat removal and secondly has good mechanical strength properties at high temperatures. It is particularly important that the material have a relatively low creep rate under high temperature, high stress conditions.

The inventors have surprisingly found that copper/chromium and copper/silver micro-alloys have proven to be particularly suitable for this application. Experiments were conducted on alloys comprising 0.15% (by weight) silver and 1.5% (by weight) chromium and the improved properties were recorded. For example, the improved performance of copper and silver (represented as CuAg in the figure) is shown in fig. 9, which is a comparison between creep versus stress for different materials at high temperatures per 1000 hours. As is evident from the figure, the copper/silver alloy can withstand higher stresses at the same creep rates experienced by conventionally used materials such as oxygen free copper (CuOF) and high thermal conductivity copper (HCCu). The addition of silver or chromium to copper slightly reduces the thermal conductivity of the material compared to pure copper, but the thermal conductivity is still within acceptable limits, especially it is still up to 6.5 times the thermal conductivity of aluminium bronze and carbon steel, and up to 20 times the thermal conductivity of stainless steel, which materials have previously been used in this application.

It has been found that proper material selection in designing the bellows housing 43 and piston push plate 41 substantially helps to extend the life and increase reliability of the pressure ring assembly. In this regard, it has been found that the use of a material having a thermal conductivity in excess of 100 watts per meter kelvin, such as copper, substantially increases the cooling capacity of the bellows housing and piston push plate, thereby resulting in an extended life expectancy.

It will be understood that the above is but one embodiment of the invention and that various changes in detail may be effected therein without departing from the spirit and scope of the invention as claimed.

Claims (4)

1. A pressure ring assembly (10) suitable for use in an electric arc furnace, the pressure ring assembly (10) comprising a pressure ring (20), characterized in that the pressure ring (20) is made of a metal alloy, said alloy comprising at least 97% by weight of copper and between 0.05% and 0.5% by weight of silver.

2. A pressure ring assembly (10) according to claim 1, wherein the alloy comprises 0.15% silver by weight.

3. Use of a metal alloy for manufacturing a pressure ring (20) suitable for use in an electric arc furnace, wherein the alloy comprises at least 97% by weight of copper and between 0.05% and 0.5% by weight of silver.

4. A pressure ring assembly (10) suitable for use in an electric arc furnace, the pressure ring assembly (10) comprising at least two pressure ring segments (22), said pressure ring segments (22) being dimensioned for mutual engagement so as to form a pressure ring (20) around an electrode, each pressure ring segment (22) having a top end (24), a bottom end (25), two opposite side ends (26), an inner electrode facing surface (36) and an opposite outer surface (35), the pressure ring assembly (10) further comprising at least one contact shoe (50) and a piston arrangement (40), the at least one contact shoe (50) being arranged between the pressure ring (20) and the electrode such that an annular gap is formed between the pressure ring (20) and the contact shoe (50), the piston arrangement (40) comprising a piston push plate (41) between the pressure ring segment (22) and the contact shoe (50) for urging the contact shoe (50) into electrical contact with the electrode, the pressure ring assembly (10) being characterised in that, the pressure ring (20) is made of a metal alloy, wherein said alloy comprises at least 97% by weight of copper and between 0.05% and 0.5% by weight of silver.