Fluid handling – Processes – Affecting flow by the addition of material or energy
Reexamination Certificate
2000-01-14
2001-11-27
Rivell, John (Department: 3753)
Fluid handling
Processes
Affecting flow by the addition of material or energy
C137S251100, C137S487500, C137S827000, C164S147100, C073S861110, C073S861150
Reexamination Certificate
active
06321766
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
This invention relates to the electromagnetic control of the flow of and particularly to the measurement of the flow of an electrically conducting liquid.
2) Description of the Prior Art
Molten metal moves often from one vessel to another during industrial processes. Whether it is from a melting or holding furnace to multiple molds in a batch casting process or from a ladle to a tundish to a mold in a continuous caster, in both the ferrous and non-ferrous industries, the control over the flow of the metal is important or key to the process.
The growth of continuous casting in the United States, the emphasis on “clean steel”, the rise of ladle and tundish metallurgy, the trend to higher production machines and the need for precise control in innovative casting processes have all increased the importance of flow control in molten steel pouring in particular. According to the magazine, 33
Metal Producing,
80% of the steel melted in American furnaces (87 million tons in 1991) passes through a slide gate or valve. An engineering manager of Sumitomo Metal America, Inc. was quoted in 33
Metal Producing
as stating that if a magnetic field controlled the rate of flow, one could eliminate conventional metering or throttling systems and reduce costs. Inland Steel has recently cited the possibility of the use of electromagnetic force to reduce the alumina clogging problem in slide gates. In the steel ingot casting and non-ferrous metal industry, a similar need is felt. Although smaller than the steel production cited above, the combined aluminum and copper production in the United States in 1991 was still approximately six million tons.
Today, the state-of-the-art of molten metal flow control in industrial processes continues to be by mechanical devices. Three major types of conventional flow control devices are used at the discharge of a furnace, holding vessel or tundish: a metering nozzle, a stopper rod or a slide gate. A metering nozzle is a specially contoured hole through a ceramic block. For a gravity driven flow, the flow rate is simply proportional to the square root of the head of the molten metal above the nozzle and to the square of the nozzle throat diameter. The stopper rod is basically a blunt ended rod suspended above a nozzle and configured with a manual or automatic mechanical means for raising and lowering. The flow rate can be varied from fully open to fully closed using a stopper rod. The slide gate is primarily a hydraulically operated mechanism that basically consists of several stacked ceramic plates, each with a central hole therethrough. The holes may be aligned to allow the flow or misaligned to stop the flow. Both linear and rotary versions are available. Slide gates are predominately used on furnaces and ladles because of their ability to hold high heads for long periods of time. However, none of these mechanical means by themselves can fulfill the additional role of measuring the metal flow.
A particularly critical flow control location of great practical significance is from the tundish to the mold in a continuous casting machine for making steel. As schematically shown in
FIG. 1A
, a tundish
4
is an intermediate, shallow vessel that provides several functions in a continuous casting machine. Receiving molten metal from a transfer ladle from the furnace, the tundish
4
distributes the molten metal through multiple bottom openings to individual molds. Multiple ladles may be sequenced using the capacity of the tundish as a reservoir. Also, the tundish provides a residence time to allow metal inclusions to float out. According to a 1986 survey, the metering or free flow nozzle is used on approximately half of the total tundishes in the overall United States steel industry; while stopper rods and slide gates are each used on about one-quarter of the steel tundishes, respectively.
One of the prime functions of a tundish is to provide a controlled, uniform flow. A rough stream has a higher surface to volume ratio and, hence, a higher propensity to reoxidize by direct contact with the air. Further, a rough stream will entrain more air and carry it into the mold resulting in disadvantageous turbulence, foaming and sloshing. With a turbulent pool, new steel is continuously brought to the surface for further contact with air. Very little time is left for proper separation of impurities. Also, oxides tend to be thrown to the outside of the mold where they can be trapped in the surface of the strand. Excessive turbulence in the molten crater of the strand can also be a potential cause of a breakout through the shell.
Both stopper rods and slide gates tend to produce rough streams. Also, slide gates and stopper rods are both subject to clogging when casting aluminum killed steels. Toward the end of a sequence cast, the accuracy of flow control gets worse especially with a stopper rod as the flow area between the rod and the nozzle block becomes fouled. Stream flaring occurs in a slide gate 95% of the time during a heat sequence because the slide gate needs to be in a semi-open position to regulate the flow. The stream exiting the top portion of the slide gate at an angle translates into a circular motion through and exiting the slide gate.
Metering nozzles also suffer from operational problems. The only way to control flow with a metering nozzle is to control the tundish level height, but this is slow and insensitive being a square root function of the head. Other considerations, such as inclusion float time or vortexing, tend to make changes in tundish level undesirable from a quality standpoint. Generally, for a billet caster, nozzle life limits the sequence length. Depending of the specific casting conditions, the nozzle either erodes to the point that the flow rate increases over the allowable limit for the machine or clogs with alumina which also limits casting. Clogging, importantly, now limits the types of steel that may be cast. Further, the only way to stop flow through a metering nozzle is to manually insert a chill plug to freeze the flow. Typically, in the steel industry, this plug must be burned out with an oxygen lance to restart the flow, often damaging the nozzle.
A continuous casting operator is market driven to meet one or more of the following needs: (1) to meet the quality specifications of the grades being cast; (2) to diversify by moving into casting improved grades of steel; (3) to reduce the current cost of casting a given grade of steel; (4) to increase the yield of prime billets, i.e., to reduce waste; and/or (5) to upgrade machinery as it ages to continue to compete in the market.
An electromagnetic flow control device in lieu of the conventional flow control devices has direct bearing on all of these market drivers. The ability to additionally measure the flow rate while controlling it is a distinct advantage to the operation of the caster. As part of an overall caster control system, it does so in a number of important ways to improve process control, improve quality, increase productivity and reduce cost.
When compared to metering nozzles, an electromagnetic flow control device (1) offers the operator of a billet caster the opportunity to now control the flow through the caster rather than react to it; (2) provides independent control over the casting rate to meet tight specifications on the heat removal rates in all commercial grades; (3) provides a greater degree of control over that of changing the tundish level height which is slow and insensitive; (4) offers independent flow control on each nozzle to compensate for uneven nozzle wear or clogging among the multiple strands in a caster fed from the same tundish; and (5) gives the operator the capability to adjust flow independent of strand motion changes to maintain a constant mold level height which is so important to good quality.
Since 95% of the flow control in the minimill market is by metering nozzle, these advantages are particularly important. The minimill sector is no longer just the low cost producer of rebar. Higher
Rivell John
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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