GB2242844A

GB2242844A - A pneumatic flow-control column for molten metal

Info

Publication number: GB2242844A
Application number: GB9007618A
Authority: GB
Inventors: James Herbert Monks
Original assignee: Individual
Current assignee: Individual
Priority date: 1990-04-04
Filing date: 1990-04-04
Publication date: 1991-10-16
Also published as: GB9007618D0; ZA912457B; GB9027661D0; KR920700815A; GB2242636A

Abstract

A tubular ceramic column (1) locates over an outlet nozzle (2) in a melt-containing vessel. Lower part of column (1) comprises two galleries, separated by weir (3). Up-feed gallery (4) has inlet orifice (5) near vessel floor for liquid to enter. Down-feed gallery (6) passes directly into outlet nozzle (2). Pressurised gas is fed into upper chamber (7) via supply hose (10) located in fittings (8) sealed into top of column. Chamber (7) may also be vented to atmosphere as desired. Interaction between outflow and input upon gas confined in chamber (7) results in a self-regulating condition so that column height in down-feed gallery (6) remains stable at any given height. Outflow rate is instantly altered by adjusting column height in down-feed gallery (6); this is achieved by increasing or decreasing system pressure. The column completely drains in halting outflow; this makes for ease of re-start after long halt times. <IMAGE>

Description

lk 2 A PNEUMATIC FLOW-CONTROL COLUMN for MOLTEN METAL - I.- A PNEUMATIC

FLOW-CONTROL COLUMN FOR MOLTEN METAL

TECHNICAL FIELD

This invention relates to an entirely new approach to the flow-control of molten metals in that there are no moving refractory components, hence there is no requirement for a mechanical actuation device.

Hitherto, especially in the field of continuous-casting, regulating the flow of molten steel from tundish to mould has been accomplished by the stopper, slide-gate or rotary-valve, where the integrity of refractory components moving together is relied upon. The actuation devices can be expensive to acquire and maintain, and time-consuming to fit and remove.

The only currently used flow-control method having no moving refractory parts, and where no actuator is needed, is the metering-nozzle system. Its limitation is that the throughput cannot be altered during casting. Any significant nozzle-bore erosion or blockage means that casting has to be terminated.

It is the object of the present invention to provide a reliable and sensitive flow-control system in melt-containing vessels, requiring little capital outlay, capable of being quickly assembled by non- technical personnel, with requirement neither for vessel modifications nor mechanical actuation mechanisms.

SPECIFICATION

According to the present invention, there is provided a tubular ceramic column which is located over an outlet nozzle in a melt-containing vessel, in particular a tundish used in continuous-casting of steel. The column can be located in a seating-block in the floor of the vessel, to be positioned directly above an outlet nozzle.

In the preferred embodiment, the column comprises an upper tubular chamber. the lower part being divided into two galleries separated by a central weir. A gas-supply hose is attached to a fitment sealed into the top of the column so that the pressure inside can be controlled. The internal chamber is either vented to atmosphere or pressurised in order to influence the flow of molten metal through the column.

In one of these galleries, the up-feed, an inlet orifice close to the vessel floor allows molten metal to enter therein. The other gallery, the down-feed, connects directly into the outlet nozzle.

i i The weir height and the outlet nozzle bore-size are chosen to suit a given application. Large diameter nozzl-bores, for example, would favour steels having aluminium levels M15%, where alumina clogging is to be expected.

A specific embodiment of the invention is now described, referring to the accompanying semi-scaled drawing. A tubular ceramic column (1) locates over an outlet nozzle (2) set in the base of a melt-containing vessel; the drawing outlines a preferred arrangement. The tapered base of the column cements into a suitable seating-block Into which the outlet-nozzle is fitted in a separate lower part of the block.

It is important that adequate pre-heating be given to the lower outlet nozzle, and to that part of the column which will be immersed in the molten metal up to operating level (9) in the vessel.

A flexible steel supply hose from a pneumatic control panel connects to the fitting (8) sealed into the top of the column. The control panel can be a simple manually operated arrangement consisting basically of two valves, one to pressurise the column from a regulated inert gas supply, the other to vent the column to atmosphere. The panel would also incorporate pressure monitoring displays. The component parts used for making up the panel are standard; it is not therefore necessary to describe its layout.

As molten metal is introduced into the vessel, it enters the inlet orifice (5) to rise in the up-feed gallery (4). As the outside metal level (9) in the vessel exceeds the internal weir height, the liquid in the upfeed column (4) rises over the top of the weir (3) and feeds over into the down-feed column (6) to initiate flow through the lower nozzle (2). The column must be vented to atmosphere If this start procedure Is adopted.

A preferred technique for starting, however, safeguards against any possibility of freezing during this critical stage by preventing the first input of steel into the vessel from entering the up-feed gallery. A ceramic pad, or board, pressed up against the outlet nozzle enables the chamber to be pressurised when molten metal In the vessel covers the Inlet orifice (5). By admitting small amounts of gas into the upper chamber (7) ingress of liquid into the up-feed gallery (4) may be prevented altogether.

One notable feature of the Invention is that molten metal may be prevented from entering the column for as long as desired (with the vessel filled or partially filled) without jeopardising the start process.

The following start-procedure, aimed at preventing chilled liquid entering the system may be adopted:- 1. As the vessel is filling, or has been filled, the prevailing system pressure (which may be natural back-pressure or deliberately applied) is relieved In a controlled manner to allow molten metal into the up-feed gallery (4). The pressure indication enables the tundish depth to be - A_ - assessed. If this is below the weir height, then the system pressure can be vented to zero with no molten metal passing over the weir.

2. In order to thoroughly heat the up-feed gallery (4) and weir (3) before allowing liquid into the down-feed (6) the liquid In the up-feed column can be changed several times. After the venting procedure described, pressure is applied, which causes the level in the up-feed gallery to retreat back down again to the intake orifice at the base of the column. The procedure can be repeated, alternately venting and pressurising in a controlled manner, raising and lowering the up-feed column without allowing any liquid to pass over into the down-feed gallery (6). In this way, the refractory material comprising the column becomes thoroughly heated.

An explanatory note is necessary at this point, to explain how the height of the liquid column in the up-feed gallery is assessed during the start period:- Considering the situation with a vessel filled to normal operating level (9), as long as internal system pressure is greater than the pressure created by the head of liquid above weir height, then the column in the upfeed gallery will remain below the weir. As an example, the weir (3) in the scaled drawing is 20Omm. below tundish operating level (9). In the venting procedure described, therefore, the system pressure may be relieved down to - 2.5 psi; the liquid in the up-feed column would then have risen to within 5Omm. of the weir top. (As an approximation, in particular for steel, it is here assumed that each 10Omm. steel depth in the vessel exerts 1 psi. ferrostatic pressure).

When the up-feed column (4) Is lowered during the venting procedure described, the exact depth of liquid in the vessel is known by the pressure at which gas issues from the orifice at the base of the up-feed column. The instrumentation clearly indicates when this point is reached.

The alternate venting and pressurising at the start can be carried out either during the filling of the vessel, or when it is full, and is accomplished quickly and easily. Trouble-free starting is assured when molten metal is finally allowed over the weir (3).

3. When it is desired to allow molten metal over the weir into the downfeed column and thence into the lower nozzle, the system pressure is vented down as described (in this specific case to 2.5.psi). The ceramic pad is then removed from the lower nozzle which immediately causes system pressure to fall to normal atmospheric value.

There is an immediate influx of liquid over the weir (3) Into the downfeed gallery (6), so that liquid issues from the lower nozzle. A small column builds rapidly in the down-feed gallery; the input of liquid initially exceeds output from the lower nozzle, so that the gas occupying the upper chamber cavity (7) again becomes confined, being entrapped between the up-feed and the down-feed columns.

1 i i 1 The rate of Input over the weir is quickly diminished by a rise In system pressure created by the input surge itself. which depresses the level of the up-feed column (4) until It falls below weir height. Interrupting the supply of liquid to the down-feed column. The outlet continues to flow due to the combined effects of the metallostatic and the system pressure acting upon it; the outflow itself lowers system pressure, which again allows the up-feed level to rise over the weir thereby restoring supply to the down-feed column.

The cycle continues, with interaction between the incoming liquid increasing the pressure of the gas confined in the upper chamber (7) and the outgoing liquid reducing it. A self-regulating condition develops within the column. the input matching output, the height of the level in the down-feed gallery (6) remaining steady. At this point the system pressure will indicate exactly the difference in the height between weir and outside metal level. This 'critical value' remains the same for any height of down-feed column selected.

4. In the specific application of the pneumatic control column in continuous-casting, the start procedure outlined so far achieves a column height in the down-feed gallery (6) sufficiently low to allow for a safe and unhurried mould-fill time, and a slow initial withdrawal rate of the as-cast section. Both these conditions are of paramount importance for trouble-free starts. If desired in another application, the maximum throughput which the combination of nozzle size and vessel pool-depth will allow can be achieved immediately.

OPERATING POINTS:- To increase throughput (hence casting speed), the height of the column in the down-feed gallery is raised. This is achieved by venting the system to atmosphere for an instant. In so doing, the self-regulating process is momentarily interrupted and a little extra input allowed over the weir, so causing the level of the liquid in the down-feed gallery to rise. The self-regulating condition immediately restores itself after each adjustment is made, becoming stable at the new level.

To decrease throughput, the system pressure is this time increased for an instant, slightly depressing the column in the down-feed gallery; the height of the down-feed column (6) falls a little, and as previously there Is a brief interruption of the self-regulating condition, before it automatically restores itself. Height adjustments in the down-feed column occur immediately the appropriate control button is pressed; they may be made in very small increments for fine-tuning the out-flow. Since the response is so positive, automatic operation would maintain constant mould level and/or casting speed.

Should it be necessary to halt the liquid metal flow, system pressure is increased above the critical value ( > 2 psi. in the case of the example illustrated) and this mist he sustained. This maintains the height of the up-feed column (4) below weir height, so that the down-feed column quickly drains off since input over the weir no longer takes place; the ceramic pad may then be re-applied to the outlet nozzle to allow system pressure to be maintained above critical. Re-starting the flow would be a repeat of the start procedure.

Should a tube-changer be in use, again in the specific application of continuous-casting, for example on a large section-size with submergedpour tubes, the blank plate would take the place of the ceramic pad both for the start procedure and for a halt. For whatever reason the flow is halted, it is advantageous, when using a gate of any kind, for example a tube-changer, to completely drain molten metal from the system; this is easily accomplished using the pneumatic column, and re-starts would not be problematic.

At the end of a casting cycle the draining of the vessel, for example a tundish in continuous-casting, relies upon a syphoning effect. Internal system pressure within the closed column reflects the fall in the level (9) of the molten metal outside; by the time this level has fallen to weirheight, system pressure reduces to zero. The self-regulating condition is stIll preserved even when the outer level has fallen below weir height, the difference being that system pressure assumes a negative value.

Under normal conditions, a positive system pressure creates an additional driving force to the metallostatic pressure in the column influencing throughput. Conversely, during the draining of the vessel, as the outer level falls below the weir, the negative pressure has a restraining influence so that flow-rate gradually reduces (as with normal open nozzles) as a result of intensifying negative pressure caused by the decreasing level in the vessel. Column height in the down-feed (6), however, remains constant, the self-regulation continuing until the vessel Is drained.

If, as the vessel had begun the drain period, the down-feed column height (6) was at a low level (for example if a large diameter nozzle were in use, requiring minimum column height) the intensifying negative pressure finally halts the syphoning effect before completely draining the vessel. The down-feed column height (6) is, however, easily raised by venting to atmosphere as the outer level falls towards weir height, when syphoning will continue until the vessel drains.

The column can be subjected to extra negative pressure, if so desired, if the outside vessel level has been allowed to fall below weir height prior to raising the down-feed column. The level in the column will then rise well above the weir; this very high level will be sustained in the selfregulating mode until the vessel drains.

Throughput alterations are achieved in a rapid and precise manner by adjustments in the height of the metal column over the outlet nozzle, the down-feed column quickly stabilising after each change is made. In the continuous-casting operation, a basic control panel made up from offtheshelf valves, gauges. regulators etc. permits the control column to be operated in a simple manual mode to achieve the throughput desired with no alterations to any existing auto-level system which may be in use. Alternatively, the device would link in with the auto-level signal to maintain constant speed of casting.

I;- i i - 7 1 The start of casting may be delayed as long as desired with the vessel filled, for example if it is necessary td delay casting on one strand of a multi-strand casting machine. At any time, a steady controlled start-up can be initiated. Flow may be halted and easily re-started if necessary, even after extended halt times.

SUMMARY OF ADVANTAGES:-

1. The refractory column is inexpensive and simple in its construction, Its cost being similar to that of a standard stopper.

2. No actuator mechanism is required, since there are no moving refractory parts in the pneumatic column; considerable savings in actuator repairs and maintenance compared with other systems are, therefore, effected.

3. Existing tundishes require no modification.

4. Tundish turn-round time Is rapid since the pneumatic column is easily fitted by unskilled personnel.

5. The start of cast may be delayed as long as desired.

6. Flow is initiated in a steady controlled manner to ensure trouble-free start to casting.

7. Smooth, laminar flow is preserved through the system at all times which reduces erosion or blockage tendencies. In comparison, systems employing throttled orifices create severe turbulence within the nozzle bores.

8. Flow can be arrested for long periods and re-started (the system is drained in the halt process).

9. Reliance is not placed upon the integrity of precision refractory components moving together to retain control of the flow: there are no moving parts.

10. Nozzle erosion which might arise, for example from highly erosive steel grades, is compensated for and lengthy sequence casting with these steels is possible.

11. The pneumatic control column can be used with open-pour billet casters, providing an inexpensive conversion to full flow-control capability, or with larger sections which employ submerged-pouring into the mould. The widely used tube-changing system would complement the pneumatic device.

Claims

CLAIMS:-

1. A flow-control device for molten metals consisting of a refractory column in the form of a tube, the lower part of the tube being divided by a central weir into two separate galleries. The column is intended to locate above an outlet nozzle in a melt-containing vessel.

2. A tubular column as in Claim 1 which is provided with an inlet aperture near the base of one of its two galleries into which molten metal may be admitted. The other gallery is open-ended, leading directly into an outlet nozzle located below in the floor of the vessel.

3. A tubular column as in Claims 1 and 2 with a steel fitment sealed into the top of the upper chamber into which may be screwed a steel gas-supply pipe from a regulated supply of inert gas. Gas pressure in the column may be increased, or maybe vented to atmosphere using standard pneumatic control equipment.

4. A tubular column as in the preceding claims such that, by manipulation and control of the pressure within the internal cavity of the column, molten metal may be admitted into the gallery having the inlet aperture, from a melt-containing vessel in which it stands. By similar alterations in pressure, flow can be initiated through the outlet nozzle when required, and its rate of flow altered at will, or halted if desired.

5. A flow-control device, as in the previous claims, substantially as described herein with reference to the accompanying diagram.

i Published 1991 at The Patent Office. Concept House. Cardiff Road. Newport. Gwent NP9 1RH. Further copies may. be obtained from Sales Branch. Unit

6. Nine Mile Point. Cm-mfelinfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques ltd. St Mary Cray. Kent.