WO2001083832A1

WO2001083832A1 - Purge plug

Info

Publication number: WO2001083832A1
Application number: PCT/SE2001/000915
Authority: WO
Inventors: Jan Sahlin
Original assignee: Sahlin Gjutteknik Ab
Priority date: 2000-05-02
Filing date: 2001-04-27
Publication date: 2001-11-08
Also published as: AU2001252832A1; SE0001592D0; SE515802C2; SE0001592L

Abstract

A purge plug which is adapted to be replaceably attached to the bottom (3) of a metallurgical vessel (1) for purging gas (21) through a heat (2) in the vessel. The purge plug comprises an external, essentially sleeve-shaped body portion (9) and an internal, truncated conical body portion (11) of ceramic material. Between these a gap (12) is arranged which extends between the end faces (14, 15 and 16, 17) of the body and a casing (7) encloses the body. The internal body portion (11) is movable in relation to the external body portion (9) and biased by a resilient bottom plate (18) in a direction towards the heat (2) to close the gap (12). The gas is fed to the external end face (14, 15) of the body in order to, at a predetermined pressure, flow through the gap (12) to the internal end face (16, 17) of the body and into the heat (2). The bottom plate (18) is formed in one piece and comprises an essentially flat, central portion (23) which is attached to the internal body portion (11), a peripheral portion (24) which is attached to the casing (7) and an intermediate, bent portion (25). The central portion (23) and the peripheral portion (24) are essentially parallel to the external end face (14, 15). The intermediate portion (25) is defined by an internal border area (26) and an external border area (27) which smoothly pass into the central portion (23) and into the peripheral portion (24), respectively.

Description

PURGE PLUG

The present invention relates to a purge plug which is adapted to replaceably being attached to the bottom of a ladle, an arc furnace, a converter or another metallurgical vessel for purging gas through a heat in the ves- sel . The purge plug comprises a ceramic body with at least one through gap which extends between the end faces of the body, and a casing which encloses at least the external end face (16, 17) of the body. The gas is fed to the external end face of the body in order to flow, at a predetermined pressure, through said gap to the internal end face of the body and into the heat . The ceramic body comprises an external, essentially sleeve-shaped body portion and an internal, truncated conical body portion between which the gap is arranged. The internal body por- tion is movable relative to the external body portion and is biased by means of a resilient bottom plate in a direction towards the heat for sealing the gap. A gas supply pipe is connected to a gastight space between the external end face and the bottom plate in order to open the gap. The resilient bottom plate is attached in a gastight manner to the casing at a distance from a larger end face of the body. The space is defined by the casing, the larger end faces and the bottom plate . The internal body portion is attached to the bottom plate. Purge plugs are generally used for purging gas through a heat of metal in a metallurgical vessel, for example a converter, an arc furnace or a ladle, for stirring a heat in the vessel, and, where appropriate, also for modifying the heat, and serve to let gas under high pressure, for instance 6-10 bar, into the vessel. A purge plug traditionally consists of a gas-permeable ceramic cone having the shape of a truncated cone of about 10 kg which is wholly or partly enclosed by a sheet-metal casing. The conventional purge plug is made of pressed mag- nesite and usually has a rectangular cross-section. The purge plug is generally supplied in so-called sets, i.e. the cone is mounted in a hollow block which weighs about 50 kg. The purge plug has a key role when manufacturing pure steel and is used by practically all steel plants in the world. One or more purge plugs are replaceably mounted in the bottom of a unit which is found in all steel plants and is called a ladle, i.e. a container lined with refractory ceramic material and adapted to contain melted steel. In the ladle, various high- temperature chemical processes are carried out in which the purge plug is crucial. Gas, generally argon, is purged through the purge plug. The purge plug is worn heavily during the process in its uppermost portion, which is directed towards the heat, and regularly has to be replaced by a new purge plug when its height has shrunk to a minimum permissible level.

The first purge plugs that were made were porous, i.e. they were gas-permeable but did not let steel through. The disadvantage of these purge plugs was that high pressure gas was required to obtain a sufficient flow, and owing to their porosity the wear was considerable . The next step in the development of purge plugs was so-called directional porosity, i.e. a number of ducts with a diameter of about 0.5 mm were formed by casting and gas was purged through the ducts. The ducts generally consisted of steel pipes. The advantage of this purge plug was that it was easy to achieve a gas flow through the plug and that the plug could be made compact, which meant less wear. The disadvantage was a greater risk of infiltration of steel in the ducts, which obstructed the purge plug. In order to avoid the risk of infiltration, a minimum gas flow through the ducts must be maintained.

The gas was supplied through a pressure box which is in- tegrated with the sheet-metal casing and which is connected to the ducts.

The third step in the development of purge plugs was to form slots or gaps by casting, having a thickness of about 0.2 mm and a length of about 20 mm, through the purge plug instead of ducts. The advantage of this was that it was easy to achieve a high gas flow, without infiltration of steel in the slots. However, there is always a risk of infiltration of steel as long as there are open gaps in the purge plug.

A purge plug must fulfil three requirements: a high degree of durability, satisfactory gas permeability and avoid infiltration of steel in the gaps of the purge plug. - Satisfactorily durability is obtained by the purge plug being made of a high-quality, compact refractory material . - A high degree of gas permeability is obtained by the gaps having a sufficient sectional area. - Minimum infiltration is obtained by the gaps being sufficiently narrow.

These three requirements counteract one another and a purge plug is a compromise of these three properties. German patent DE 196 10578 discloses a prior-art purge plug which comprises an external body having a truncated cone-shaped cavity in its upper portion, a cylindrical, enlarged cavity in an intermediate portion and a cylindrical duct in its lower portion. The cavities communicate with one another to allow gas with a specific pressure to flow from one end face to another. In the cavities, in the above-mentioned order, a truncated cone, a piston and a tubular piston rod that is provided with openings are movably arranged as a unit. The cone is biased to a closing position by means of an external pres- sure spring that engages with the free piston rod end protruding from the purge plug. When gas with a sufficient pressure (in order to counteract the force of the spring and the metallurgical pressure) is supplied through a gas supply pipe and via the piston rod to the enlarged cavity, the piston raises the cone and the gap between the cone-shaped cavity and the cone is opened. Then the gas can flow around the piston and through the gap into the heat .

The disadvantages of this construction are as follows :

- The construction is very difficult to manufacture since purge plugs are generally cast in a refractory material . It is technically difficult to make an intermediate, enlarged cavity (16) by casting since the template used in casting is very difficult to remove. The construction has complicated protruding parts (the spring suspension beneath the purge plug) , which are inconvenient in a steelworks environment since they may easily break by being subjected to impacts or other rough handling. The spring suspension is expensive to manufacture. - The purge plug is worn gradually. When it is worn down to the enlarged cavity (16) , a breaking through of the plug is obtained and steel flows through the bottom of the ladle with disastrous results. It is thus very important that the plug not be worn down to this level. Advanced equipment is required in order to determine how far down the purge plug has been worn.

Another patent that is based on a purge plug having a movable and a fixed body, where the gas flows through an annular gap, is US-A 4 470 582. What mainly makes this construction different from that in the above-mentioned German patent is that the pressure spring has been replaced by a lever mechanism which is preloaded by means of a hydraulic cylinder.

The disadvantages of this construction are as fol- lows:

Because of its complicated shape, it is very expensive to manufacture . The gas flow is controlled by an external valve which opens and closes the annular gap by guiding the movable body upwards and downwards, respectively. The disadvantage is that an external controlling device is required, which takes up space and is inconvenient in a steelworks environment . It can easily be damaged by external forces . - As in the above-mentioned German patent, the purge plug is worn with the same result as indicated above. One object of the present invention is to provide a purge plug having a gap, the width of which is a function of the applied gas pressure, and which is easy and inexpensive to manufacture.

Another object of the invention is to provide a purge plug having an adjustable gap which does not require complicated mechanisms and does not exhibit any protruding components.

Yet another object is to provide a purge plug which can withstand a considerable (ferrostatic) pressure from the heat in the metallurgical vessel without the gap being opened but which requires a much lower gas pressure in order to open the gap.

A further object of the invention is to provide a reliable purge plug that can be worn almost completely before there is a breaking through and which has an always easily adjustable gap.

According to the invention, these objects are achieved by a purge plug as mentioned by way of introduction, which is characterised in that the bottom plate is formed in one piece and comprises an essentially flat, central portion which is attached to the internal body portion, a peripheral portion which is attached to the casing and an intermediate, bent portion, that the central portion and the peripheral portion are essentially parallel to the external end face, and that the intermediate portion is defined by an internal border area and an external border area, which smoothly pass into the central portion and the peripheral portion, respectively.

Further developments of the invention are evident from the features indicated in the subclaims. A preferred embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which

Fig. 1 is a schematic vertical cross-section of the bottom of a ladle with applied purge plugs, Fig. 2 is a vertical cross-section of a purge plug according to the invention with a closed gas-flow gap,

Fig. 3 shows the purge plug of Fig. 2 with an open gas-flow gap, and

Figs 4, 5 and 6 are schematic vertical cross- sections of different embodiments of the bottom plate of the purge plug.

With reference first to Fig. 1 the bottom portion of a metallurgical vessel 1, for example a converter, an arc furnace, a ladle or the like, is schematically illus- trated, as regards a heat 2, usually a heat of metal, such as steel. In the bottom 3 of the vessel 1, a number of replaceable purge plugs 4 are in a conventional manner detachably attached by means of attaching means known per se, as indicated by the reference numeral 5. A gas supply pipe 6 is connected to the respective purge plugs 4. All this is prior-art technique and thus requires no further explanation.

Reference is now made to Figs 2 and 3, in which the construction and function of the purge plug according to the invention are illustrated. The purge plug 4 comprises a casing 7 that extends from the lowest portion of the purge plug in Fig. 1 and towards the uppermost portion of the purge plug. The casing 7 preferably extends to the uppermost portion of the purge plug, as shown in Fig. 2, but can also end on a level with the reference numeral "22" in said figure. Since the purge plug is to be used for a bloom of steel 2, the casing 7 is preferably made of steel . The casing has been illustrated as a truncated cone having the thin end in contact with the heat, see Fig. 1, but it can also be inverted or cylindrical (not shown) . A ceramic body 8 is enclosed by the casing 7 and is preferably attached to the casing 7 along its entire length.

The body 8 is, in its turn, divided into an external, concentric, essentially sleeve-shaped body portion 9 with a truncated conically shaped internal cavity and an internal, truncated conically shaped body portion 11 that entirely fills the cavity. The internal body portion 11 is movable, i.e. displaceable longitudinally in relation to the external body portion 9 having a gap 12 between them. When manufacturing the body portions 9 and 11, the internal body portion 11 can be easily formed, well adapted to the external body portion 9, by placing a plastic foil or a plastic sheet which has been pre-formed to a truncated cone-shaped casing element, concentrically in the casing 7 at the occasion of casting and then re- moving it when the refractory material (the ceramic compound) has solidified. Further, in connection with the casting, threaded blind holes 13 can be formed or threaded sleeve inserts be cast into the larger end face 14 of the internal body portion 11. In Figs 2 and 3, the gap 12 is shown to be linear and of uniform thickness along its entire length. However, the gap can be somewhat bent (convex) longitudinally and/or slightly divergent from the upper or smaller end face 16, 17 in Figs 2 and 3 in a direction towards the lower or larger end face 14, 15. It is essential that the gap 12 can be sealed at the upper or internal (see Fig. 1) end face. Owing to said shape of the gap, the frictional forces between the internal body portion 11 and the external body portion 9 are reduced, which im- plies that a lower pressure on the purging gas is required in order to displace the body portions 11 and 9 in relation to one another than if the gap 12 were of uniform thickness.

Preferably, but not necessarily, the larger end face 15 of the external body portion 9 is on a level with the above-mentioned end face 14. The same goes for the smaller end faces 16 and 17, respectively, of the body portions 9 and 11.

The internal body portion 11 is displaceable downwards relative to the fixed external body portion 9 (see Fig. 2) and is biased upwards to close the gap 12 by means of a resilient or elastic means that acts against the end face 14. This means has the shape of a resilient bottom plate 18 which is attached in a gastight manner to the casing 7 at a distance from the external end face of the body 8, in this case the end faces 14 and 15, a space or chamber 19, which can be pressurised, being defined between them. This distance can be small and even nonexistent within limited areas of the end faces 14 and 15 - the only condition being that a chamber 19 is formed when pressurised fluid (purging gas) is supplied. A gas supply pipe 6 is connected to the bottom plate 18 and is connected to the chamber 19 for pressurisation of the same with the purging gas 21. The internal body portion 11 is fixedly attached to the bottom plate 18, for in- stance by means of bolts 22 or some other suitable attaching means . The gap 12 is kept closed by the internal body portion 11 being pressed upwards in Fig. 2 into abutment against the internal peripheral surface of the external body portion 9 by means of the biasing of the bottom plate 18.

The bottom plate 18 is formed in one piece of, for example, steel sheet and comprises an essentially flat, central portion 23 which is attached to the internal body portion 11 by means of the bolts 22. This portion may also be provided with adjusting bolts (not shown) in order to adjust the biasing of the bottom plate 18. Furthermore, the bottom plate 18 comprises an external por- tion or peripheral portion 24 which in a suitable manner, for example by welding, is attached in a gastight manner to the lowest portion of the casing 7, see Fig. 2, in particular. These portions 23 and 24 which preferably are concentric with one another are essentially parallel to the external end face 14, 15. The portions 23 and 24 are connected to one another by means of an intermediate portion 25 which is bent in an unloaded state, and the internal border area 26 of which smoothly passes into the central portion 23 and the external border area 27 of which smoothly passes into the peripheral portion 24. The bottom plate 18 can thus be said to generally have the shape of an ordinary plate.

When it is desirable to purge gas through the heat 2 in the vessel 1, gas is supplied under high pressure to the space 19 via the gas supply pipe 6. Thus, the intermediate portion 25 of the bottom plate 18 is caused to be straightened out, see Fig. 3, and pulls along the internal body portion 11, whereby the annular gap 12 is opened and the gas can flow into the heat. The flow path of the purging gas has been illustrated by means of arrows 21 in Figs 2 and 3. The more pressure one applies, the more the gap opens until the intermediate portion is completely straightened out. When the gas flow is stopped or the pressure of the gas is decreasing, the bottom plate 18 springs back and pushes the internal body 11 back and closes the gap completely. This prevents infiltration of the heat into the gap 12. The construction of this new purge plug means that it is possible to obtain an almost unlimited gas flow without having any infiltration of steel into the gap in the purge plug. This is achieved by the size of the gap varying with the applied pressure. In Figs 2 and 3, the bottom plate 18 is shown to have a constant thickness. However, it is of course pos- sible to control the function of the pressure of the gas on the gap width by using bottom plates 18 with specific spring properties. Apart from using bottom plates of various thicknesses for this aim, it is possible to achieve the desired spring properties by giving the bottom plate a thickness which is not even, that is having its thickness varying in a direction away from its pe- riphery towards its centre.

In Fig. 2 the intermediate portion 25 has been illustrated comprising a concave area or concavity which is concentric with both the central portion 23 and the peripheral portion 24. "Concavity" here means that the bend bends inwards to the larger end face 14, 15. This inward bending will be evident if Figs 2 and 3 are compared with one another with respect to the portion 25. However, it is also possible, if desired, to give the intermediate portion, a convex shape, i.e. it can be bent outwards (not shown) .

In the embodiment according to Figs 2 and 3 , the bottom plate 18 has been described and illustrated as terminating with a peripheral edge, which is almost perpendicularly welded to the casing 7. Fig. 4 shows an al- ternative embodiment of the bottom plate and this embodiment differs from the previous embodiment by the bottom plate terminating with an annular flange 28 which extends essentially perpendicular to the bottom plate or the peripheral portion 24 and which is attached in a gastight manner to the casing (not shown) . The flange 28, which is preferably formed in one piece with the rest of the bottom plate, facilitates the mounting of the bottom plate and stiffens the peripheral portion 24. Moreover, the concavity discussed above will be evident from this Fig- ure at 29.

Fig. 5 shows yet another alternative embodiment of the bottom plate. The Figure illustrates that the intermediate portion 25 can have two or more concave areas or concavities 29, 29' and intermediate convex areas or con- vexities, as indicated by way of the reference numeral 31. In the embodiments of the bottom plate 18 according to Figs 2-5, it has been shown throughout that the central portion 23 is arranged at a larger distance from the end face 14, 15 than is the peripheral portion 24, see Fig. 2, in particular. Although this is preferred, it is of course possible to arrange the peripheral portion 24 at a larger distance or at the same distance from the end face 14, 15 than or as is the central portion 23. This has been schematically illustrated in Fig. 6. When testing a purge plug having a bottom plate according to Figs 2 and 3, it has surprisingly been found that, although a very high pressure (considerably higher than the expected ferrostatic pressure) was applied to the smaller end face 16 of the internal body portion 11, the gap 12 was not opened, i.e. the internal body portion was not displaced in relation to the external body portion 9. However, when a pressure was applied in the space 19 by means of the purging gas, the pressure being considerably lower than the above-mentioned pressure, the gap 12 was opened. The purge plug according to the invention thus serves as an efficient one-way valve, which is desired as regards metallurgical vessels.

The peripheral surface of the internal body portion 11 and the internal peripheral surface of the external body portion 9 define, according to that mentioned above, the shape of the gap 12. Usually, it is preferred that the cross-sectional profile of the gap, i.e. the shape of the gap in a plane perpendicular to the longitudinal axis of the purge plug (a plane which is parallel to the bot- torn plate) is a circle or an oval. However, the cross- sectional profile of the gap is totally optional, such as a triangle, a square or a polygon, or star-shaped. The two body portions 9 and 11 can be cast at the same time, having between them a separating casing element of plas- tic foil or plastic sheet with a chosen cross-sectional profile, which, after the solidification of the compound, can be easily removed. It should be noted that the movable body portion 11 tapers off upwards, i.e. from the external end face 14 towards the internal end face 16 adjacent to the heat 2 in the shown embodiments of the purge plug according to the invention.

The following properties are thus achieved by means of the purge plug according to the invention:

It can easily be manufactured (as inexpensive as the purge plugs which are available on the market today) . - Infiltration is completely avoided.

The ratio pressure/flow may be adapted to the needs of the respective users. - An almost unlimited gas flow can be obtained.

The invention is not limited to that described above or that shown in the drawings but can be changed within the scope of the appended claims.

Claims

1. A purge plug which is adapted to be replaceably attached to the bottom (3) of a ladle, an arc furnace, a converter or some other metallurgical vessel (1) for purging of gas through a heat (2) in the vessel, the purge plug comprising a ceramic body (8) having at least one through gap (12) which extends between the end faces (14, 15 and 16, 17) of the body and a casing (7) which encloses at least the external end face (16, 17) of the body, the gas being fed to the external end face (14, 15) of the body in order to flow, at a predetermined pressure, through said gap (12) to the internal end face (16, 17) of the body and into the heat (2) , the ceramic body (8) comprising an external, essentially sleeve-shaped body portion (9) and an internal, truncated conical body portion (11) , between which said gap (12) is arranged, and the internal body portion (11) being movable in rela- tion to the external body portion (9) and being biased by a resilient bottom plate (18) in a direction towards the heat (2) to close the gap (12) , and a gas supply pipe (6) which is connected to a gastight space (19) between the external end face (14, 15) and the bottom plate (18) to open the gap (12) , the resilient bottom plate (18) being attached in a gastight manner to the casing (7) at a distance from the external end face (14, 15) of the body (8) , the space (19) being defined by the casing (7) , the external end faces (14, 15) and the bottom plate (18) , and the internal body portion (11) being attached to the bottom plate (18) , charac t e r i s e d in that the bottom plate (18) is formed in one piece and comprises an essentially flat, central portion (23) which is attached to the internal body portion (11) , a peripheral portion (24) which is attached to the casing (7) and an intermediate, bent portion (25) , that the central portion (23) and the peripheral portion (24) are essentially parallel to the external end face (14, 15) , and that the intermediate portion (25) is defined by an internal border area (26) and an external border area (27) which smoothly pass into the central portion (23) and into the peripheral portion (24) , respectively.

2. A purge plug as claimed in claim 1, cha r ac t e r i s ed in that the intermediate portion (25) comprises at least one concavity (29) which is concentric with the central portion (23) .

3. A purge plug as claimed in claim 2, char a c t e r i s ed in that the intermediate portion (25) comprises at least two concavities (29, 29') and, between these, formed convexities (31) which are concentric with the central portion (23) .

4. A purge plug as claimed in any one of claims 1-3, chara c t e r i s e d in that the peripheral portion (24) is arranged at a shorter distance from said external end face (14, 15) than is the central portion (23) .

5. A purge plug as claimed in any one of claims 1-3, cha ra c t e r i s e d in that the peripheral portion

(24) is arranged at essentially the same distance from said external end face (14, 15) as is the central portion (23) .

6. A purge plug as claimed in any one of the preced- ing claims, chara c t e r i s e d in that the thickness of the bottom plate (18) varies in a direction away from its central portion (23) towards its peripheral portion (24) .

7. A purge plug as claimed in any one of the preced- ing claims, c harac t e r i s e d in that the peripheral portion (24) comprises an annular flange (28) which extends essentially perpendicularly to the bottom plate (18) and which is attached in a gastight manner to the casing (7) .

8. A purge plug as claimed in any one of the preceding claims, chara c t e r i s e d in that the gas supply pipe (6) is fixedly attached to the centre of the central portion (23) .

9. A purge plug as claimed in any one of the preceding claims, charac t e ri s ed in that the cross- section of the gap (12) is circular, oval, triangular, square or polygonal .

10. A purge plug as claimed in claim 9, char a c t e r i s ed in that the gap (12) diverges in a direction away from the internal end face (16, 17) of the body (8) towards its external end face (14, 15) .