FIELD OF APPLICATION
The present invention relates to an apparatus for online measurement of liquid level and temperature in liquid metal refining and/or casting vessel hazardous environmental conditions. The present invention further relates to a process of deploying the apparatus in the said vessel under normal operating circumstance.
BACKGROUND OF THE INVENION
Continuous casting of metal is currently one of the most popular and important steps in converting the refined molten metal to semi-finished long or flat product. The process is almost universally accepted in the steel making industry. The process itself consists of progressively solidifying the melt first inside an open ended water cooled mould and subsequently by various stages of water cooling. While the heat extraction and solidification is a continuous process, the actual smelting and/or refining of metal is carried out in batches called ladles. Consequently a secondary vessel called a tundish must be used between the ladle and the continuous casting machine, which would maintain a reserve of molten metal. During normal casting, the melt in the ladle slowly empties in the tundish, and then it flows from the tundish to the caster. To prevent oxidation of the high temperature liquid metal and excessive heat loss in the tundish, a layer of slag is floated upon the liquid bath. Under normal circumstances, the rate of influx of melt in and out of the tundish remains equal and its level in the tundish is maintained close to its full capacity. As the ladle becomes empty, the supply of melt in to the tundish ceases, while the outflow to the caster must be maintained. This result in a drop in tundish levels, but usually the tundish has enough capacity to keep feeding the caster with metal till a new ladle is opened and starts filling the tundish with liquid again. Towards the end of the casting sequence, the tundish is drained and the casting machine is stopped for maintenance.

Optimum tundish operation in part constitutes maintaining a safe minimum tundish level. If it is drained below this level during casting, there exists a high probability of tundish slag being carried into the mould which can result in casting defects and even lead to a breakout. The probability of slag entrainment into the mould is indirectly related to the liquid level at the outlet. So during a ladle change operation, it becomes essential to maintain tundish levels substantially above the safe minimum, while at the end of the sequence, the tundish needs to be drained as close to the safe minimum as possible to minimize skull losses.
Due to the high temperatures encountered in the tundish, and the erosive nature of molten metal, there exists no easy means of directly measuring liquid levels in a tundish. Estimations of liquid levels are done my measuring tundish weight, which is less than ideal owing to various uncertainties in total weight of slag added over the length of the sequence, and the amount of solidified metal sticking to the tundish walls, among others. It is not uncommon to have uncertainties in the order of 3 to 4 tons during sequence end. In modern times where maximization of caster yield forces operators to drain tundishes below 10 tons, this error in tundish weight can mean breaching the safe minimum tundish level. Since continuous casting involves heat extraction from the molten metal, changes in temperatures of the liquid entering the mould would have to be accommodated by changing the rate of heat transfer. This can be achieved by controlling the rate of water flow into the caster or the casting speed. Thus measurement of bath temperature at the tundish is necessary for the smooth running of the caster. This is known as superheat control. Most contact type of temperature sensors cannot survive being directly dipped in the liquid melt. Some sensors exist which sheath the sensing element in an attempt to improve its survivability. But the sheath often add errors to the reading and increase the response time, and even then, they are mostly of disposable types, which cannot

provide continuous readings. Pyrometers commonly used for non-contact temperature readings are not useful in taking bath temperatures as the top surface is covered with a layer of insulating slag, and even if the slag can be removed from on spot, it would yield the bath surface temperature, which may be different from that in the interior close to the outlet.
SUMMARY OF THE INVENTION
Accordingly, there is provided in a first aspect of the invention an apparatus for online measurement of liquid level and temperature in a continuous casting tundish under hazardous temperature. In a second aspect of the invention there is provided a process of deploying the apparatus in a tundish under normal operating circumstance.
The present invention combines two different sensors in a single unit, which can provide online measurements of both the bath temperature as well as the tundish liquid level. A refractory tube of suitable bore is gradually dipped into the tundish bath at a location determined not to be detrimental to normal casting operation. A positive pressure of an inert gas is fed into the tube from the top. This has the effect of displacing any slag through the bottom of the tube and exposing a small surface of liquid metal inside the tube, the temperature of which can be measured by any industrial pyrometer placed at the top. Additionally the pressure of gas in the tube measured by a gauge positioned in the gas line would indicate the height of fluid in the tundish above the open end of the tube
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
Figure 1- shows an apparatus for online measurement of liquid level and temperature in a continuous casting tundish under hazardous temperature.
Figure 2 – shows a process of deploying the apparatus in a tundish under normal operating circumstance.
Figure 3 – Simulated motion of slag particles emerging from the device as predicted by CFD

DETAILED DESCRIPTION OF THE INVENTION
As per the invention, the apparatus is best suited to be installed inside a tundish (1), close to the outlet / stopper rod (2) as shown in figure 1. It essentially consists of a refractory tube of internal radius (R) and of length (L)which is higher than the maximum fluid level (H) in the tundish to be encountered. Since the present disclosure encompasses an apparatus applicable for a potentially large number of tundishes or other similar vessels, a rigorous quantification of the tube dimensions would naturally depend on a large number of unknown variables. With a view to retaingenerality of the possible applications of the apparatus, amethodology has been proposed to configure the refractory tube.
The value of R should be selected such that the surface area to volume of liquid trapped within the tube is small enough to ensure that the liquid does not freeze once inside the tube bylosing heat to the latter. The wall thickness of the tube would depend on attaining adequate strength to sustain the forces exerted on it by the liquid flowing around it and also to contain the excess pressure encountered inside the tube. The scope of the present invention encompasses any such methodology, experimental / numerical or otherwise adopted to achieve the above mentioned design requirements of the refractory tube, with the aim towards its utilization as mentioned in the disclosure.
The lower part of the tube is open ended and is dipped into the molten bath, while the upper end is connected to an instrumentation / gas manifold via a detachable but sealable linkage. This arrangement is utilized to ensure easy replacement of the damaged refractory tube while retaining other components of the apparatus. The top part of the tube is connected to a large bore valve (8 / 10) which, when kept open, permits equalization of pressure within the tube and the ambient during tube deployment. A pressure gauge (5) either analogue or digital is installed open to the tube interior to measure the positive pressure. Additionally, there is a pressurized inert gas line feeding into the tube,

which can deliver the gas at a pressure in excess of that of the hydrostatic pressure of the liquid. The flowrate of gas can be controlled via a needle valve or similar throttling arrangement, to introduce inert gas into the tube. To the top end of the tube is attached a pyrometer (6) whose sensing aperture is aimed vertically downward so that it can measure the melt temperature at the end of the tube. While in this particular disclosure, a pyrometer has been mentioned as a means of measuring melt temperature. The invention also encompasses installation of any relevant noncontact type sensors requiring line of sight access to the exposed molten metal surface. The entire arrangement can be translated vertically along guide rails (7), actuated by a position controller (9). This facilitates lifting the apparatus and away from the tundish during the latter’s removal for maintenance and subsequently lowering it to its operational position.
Figure 2 shows the sequence to be followed to deploy the apparatus when the tundish has reached its working capacity. First,a first ball valve(10) is opened to atmosphere and slowly dipped into the bath to a submergence depth of approximately H/2. This ensures that a plug of slag and molten metal is entrained into the tube and it attains the same height both inside and outside. Having hot liquid in contact to both sides of the tube ensures that it is rapidly heated up to the working temperatures. Once the tube has been properly preheated, the said valve is closed and a slow flow of gas (12) started, to push the fluid trapped in the tube out the bottom. The liquid metalcoming out of the tube mixes with the bath, whiles the slag particles (13) emerging, rises to the top due to its lower density. The tube is then further inserted to its full submergence depth in the bath till its closest proximity with the tundish walls is around 5 cm. At this position the apparatus may be operated in place for the entire length of the sequence subject to the survivability of the refractory tube during this time.
The pressure gauge showing a pressure reading of p in Nt/m2

The vertical height(h) of fluid from the bath surface to the open end of the tube in meters is given by the relation
Where g = 9.8 m/s2
And ρ = density of molten at the temperature measured by pyrometer. If a suitable relation does not exist, between melt density and temperature a suitable constant value may be assumed, as an example for practical purposes the density of liquid steel can be taken as 7200 kg/m3.
With the tube being placed at its desired position, and having an internal gas pressure sufficient to counter the hydrostatic head, the supply of gas can be theoretically closed, while conserving the internal pressure. However to accommodate slight leakage of gas from the high pressure system inside the tube to the atmosphere and any variation in gas volume in the tube due to changing pressure and temperature, a tickle flow of gas should be maintained, whose sufficiency can be confirmed by emergence of a few bubbles of gas at the tundish surface every minute. This can be further validated by ensuring as the constant pressure gauge readings, coincide with a steady state tundish operation where the tundish weight (hence liquid height) remains constant. The apparatus under its normally deployed condition is inherently safe against infiltration of the working fluid by air. This is because the entire tube along with the small area of exposed molten metal within it, remains shrouded in an inert gas which is at a positive pressure with respect to the environment. This ensures that in the event of a leak, internal gas will escape out instead of air entering the apparatus. Loss of pressure due to small gas leakages can be effectively counteracted by adding more gas, and the apparatus would continue to function as designed. In the event of a larger leak which cannot be effectively accommodated by adding more gas, the apparatus would cease to provide correct measurement. In this case, a total loss in pressure would not be detrimental to safety and still guard against air infiltration. The liquid level would only rise inside the tube where it equalizes with the outside level, and the remaining flow of gas would still continue to shroud the exposed metal in the tube.

A credible concern regarding utilization of the proposed apparatus is that gas bubbles as well as slag pieces emerging from the lower end of said tube can get entrained into the molten metal stream flowing into the mould. Molten metal being opaque and not amenable to rigorous experimentation, numerical models were adopted to study feasibility of the proposed invention, to see if any detrimental slag / gas entrainment can occur. A case has been presented where a generic 35 ton single strand trapezoidal tundish (figure 3) was numerically modelled with a throughput of 3.5 tons per minute. A 25 mm diameter simulated viewing tube (15) was incorporated to study how particles of different density and size would interact with the flow fields inside the tundish. The results revealed that particles of slag (16) as small as 1 mm would comfortably take up a buoyancy driven path to the surface. Identical results should be obtained with gas bubbles of similar size. This is as expected as size bubbles being less dense than slag, would experience higher buoyancy forces. Furthermore, the tube in question would certainly be in excess 10 mm in diameter. Due to the relatively calm fluid flow conditions within the tundish, and very slow flowrate of gas, there exists very low turbulence in the region near the device. This precludes any substantial shearing forces acting on the slag globules emerging from the tube, which would ordinarily break the former into smaller particles. Thus it is reasonable to surmise that the slag pieces would be well in excess of the 1 mm sized particles simulated. The larger pieces would experience higher buoyancy forces and would quickly float to the surface.
Although particular embodiments of the invention has been shown and described in details here, there is no intention to thereby limit the invention to the variants of possible embodiments falling under the concept of the invention. On the contrary, the intention is to cover all modifications, alternatives, embodiments, usages and equivalents which fall within the scope of the present invention as disclosed herein.

WE CLAIM
1. An apparatus for online measurement of liquid metal level in a continuous casting
tundish, the apparatus comprising:
a refractory tube having a lower open end being dipped into the metal bath;
a valve (10,11) coupled to the refractory tube, the valve being configured for permitting equalization of pressure within the refractory tube and the atmosphere;
an inert gas line coupled to the upper part of the refractory tube to purge inert gas into the refractory tube at controlled measureable pressure and flow rate after closing the first valve for sustaining the static pressure in the refractory tube and exposing a molten metal devoid of slag layer at the lower open end of the tube;
a pressure gauge (5) installed to the refractory tube for measuring the pressure inside the refractory tube due to the supply of inert gas, the measured pressure being a function of molten metal height above the bottom lower end of the refractory tube; and
a pyrometer type sensor disposed across the said refractory tube for measuring the temperature of the exposed melt surface at the lower and of the refractory tube.
2. The apparatus as claimed in claim 1, wherein the vertically reciprocating motion
of the apparatus being guided by a positional controller, the positional controller
comprising
a vertical static guide rail;

a pneumatic/hydraulic piston being coupled to the vertical static guide rail by means of a slide bearings over the vertical static guide rail for vertically reciprocating motion; and the pneumatic/hydraulic piston being further coupled to the apparatus imparting the vertically reciprocating motion.
3. The apparatus as claimed in claim 1, wherein the inert gas is argon.
4. The apparatus as claimed in claim 1, wherein molten metal is steel.