The disorder of casting process because of nozzle clogging is a long-standing issue ever since the continuous casting process has been introduced. Nozzle clogging is the build-up of solid or semi-solid material on a refractory surface which can become problematic during steel pouring, as it can affect the stream dynamics, reduce the pouring rate, and cause large agglomerated particles to be intermittently released into the liquid steel stream in severe cases. Because of the nozzle clogging, the casting speed is frequently decreased, and even an entire cast is to be abandoned. Further, nozzle clogging can give rise to both quality and productivity problems.
During the continuous casting of liquid steel, steel flows from the tundish to the mould through a submerged entry nozzle, (SEN) as shown in Fig 1. This protects the liquid steel from reoxidizing in contact with the atmosphere. The flow rate is controlled with a gate or stopper rod to maintain optimum casting conditions. If the nozzle clogs and the flow control cannot make up for the reduced flux, the nozzle has to be replaced which means the production is interrupted. The clog build-up can also result in decreased steel quality as oxide particles can loosen from it, giving rise to rather large inclusions.
Fig 1 Schematic of tundish-mould arrangement during continuous casting
Since the steelmaking processes occur at temperatures as high as 1,600 deg C, the interaction between the refractory materials of the SEN and the liquid steel is unavoidable. Hence, the SEN is required to have proper inertness, besides its moderate mechanical properties such as high temperature mechanical strength and thermal resistance. In general, the interaction between the SEN and the liquid steel can be categorized into three different mechanisms namely (i) the chemical reactions between refractory base materials and impurities in the nozzle and the liquid steel, (ii) the attachment of immersed non-metallic inclusions in the liquid steel, to the surface of the nozzle, and (iii) the erosion of the nozzle refractory materials. The first two mechanisms can cause clogging during casting process, which can limit the productivity by interruption of the casting process, restricting the number of charges per tundish, affecting the quality of the produced steels, and consequently increasing the cast product customer rejections.
The nozzle clogging phenomenon has been one of the most disruptive problems for the continuous casting process as long as the casting machines have been operating. This phenomenon produces an inconsistent flow and temperature variations, steel level fluctuations in the mould, impairment of steel quality, and the abrupt interruption of the steel casting. Clogging starts when solid compounds, mainly steel skull, and non-metallic inclusions, are non-uniformly deposited at the inner wall of the SEN at some typical preferential zones characterized for neighbouring dead flow conditions. The primary sources of these inclusions are (i) the reaction between the dissolved oxygen with the deoxidizers, (ii) re-oxidation in the tundish or the nozzle, and (iii) the entrainment of slag or refractory particles.
Tundish nozzle clogging problems take several different forms, and can occur anywhere inside the nozzle, including the upper well, bore, and ports. They are classified into four different types according to their formation mechanism namely (i) the transport of oxides which are present in the steel to the nozzle wall, (ii) air aspiration into the nozzle, (iii) chemical reaction between the nozzle refractory, and the steel, and (iv) steel solidified in the nozzle. In practice, a given nozzle clog is frequently a combination of two or more of these types, and its exact cause(s) can be difficult to identify.
Clogging of the tundish nozzle is a major castability issue in continuous casting of steel for several reasons. Firstly, clogging increases the frequency of operation disruptions to change nozzles or tundishes or even to stop casting. These extra transitions increase operating cost, decrease productivity, and lower quality. Secondly, clogging can lead directly to a variety of quality issues. Clogs change the nozzle flow pattern and jet characteristics leaving the ports, which can disrupt flow in the mould, leading to surface defects in the cast steel product and even breakouts. Dislodged clogs also disturb the flow and either become trapped in the steel or change the flux composition, leading to defects in either case. Quality problems also arise from the mould level transients which occur as the flow control device compensates for the clogging. The solid materials which are normally found in the clogged tundish or SENs are as given below.
Al2O3 – It is always found during the casting of aluminum killed steels. It can be caused by inclusion agglomeration, precipitation, reaction between the steel and the nozzle, or air aspiration through the nozzle.
FeO-Al2O3 – The formation of Hercynite is normally due to reoxidation of the clog after it is removed from the casting machine. The solidified iron oxidizes and reacts with alumina. This phase is not stable in aluminum-killed steels unless in very oxidizing transitory conditions
Al2O3 and Ti2O3 – These are found in the casting of aluminum-killed steels which are subsequently treated by titanium. The presence of titania normally indicates reoxidation of the steel after the titanium has been added.
MgO-Al2O3 (spinel) – With the arrival of ladle metallurgy practices, soluble magnesium contents in steels are increasing and have led to the formation of the magnesium aluminate spinel phase in the liquid steel.
TiN – It is a precipitation product during casting which takes place if the equilibrium condition is violated. It is normally seen during the casting of certain grades of stainless steel.
TiN and MgO-Al2O3 (spinel) – These can be seen in certain 400-series stainless steels.
CaO-Al2O3 (solid or semi-solid) – It is found during calcium treatment of aluminum killed steels if insufficient calcium is added or there is reoxidation after calcium addition.
CaO-Al2O3-CaS (solid or semi-solid) – It is typical build-up in higher sulphur-containing calcium-treated aluminum-killed steels.
CaO-Al2O3-MgO.Al2O3 (solid or semi-solid) – It is typical build-up in calcium-treated steels when soluble magnesium levels in the steel are too high.
Solid steel – It is because of the result of excessive heat transfer through the nozzle.
Nozzle clogging is a natural phenomenon when casting of the liquid steels which contain or can precipitate solid or semi-solid inclusions. All the solid materials found in clogged nozzles, with the exception of the solidified steel, are the result of precipitation of solid material within the liquid steel at the steelmaking temperatures. The presence of solidified steel is clearly a problem of heat transfer.
There are two types of clogs namely (i) thermal, and (ii) inclusional. There are two mechanisms for inclusional clogging namely (i) agglomeration of an already present inclusion, or (ii) precipitation of inclusions because of local supersaturation at the nozzle / steel interface. Both are very sensitive to fluid flow conditions. However, agglomeration clogs can be minimized by ladle metallurgy practices while precipitation clogs are very sensitive to local thermal conditions and compatibility with the nozzle.
Build-ups where the majority of the clog is solidified steel are to be referred to as thermal clogs. Build-ups because of the precipitation and / or agglomeration of solids on refractories at the steelmaking temperatures are to be referred to as inclusional clogs. The combination of thermal and inclusional clogging is also possible, and several clogs contain solidified metal. The most commonly observed clogging material is alumina because of the high percentage of steel which is only aluminum-killed. The consequences of clogging include the following.
Decreased productivity – To compensate for clogging, the flow control device (e.g., slide gate) is required to be further opened. If the clogging becomes sufficiently severe, the flow control device is no longer be able to compensate and then either a decrease in casting speed or replacement of the nozzle results. These happenings reduce the net casting throughput and hence reduce the productivity.
Increased cost – Depending on the continuous casting shop, some portions of the clogged nozzles (e.g., SEN) can be independently replaced during casting. Other clogged portions (e.g., tundish nozzle) can only be replaced by changing tundishes. Several studies have reported that nozzle clogging, in lieu of tundish lining lifetime, limits the allowable tundish lifetime. For example, one of the studies have reported that clogging reduces the number of heats cast from twelve to six. Hence, nozzle clogging results in additional costs for tundish refurbishment as well as for nozzle replacement
Decreased quality – Non-metallic particles can become dislodged from the clog build-up and result in unacceptable cleanliness defects in the steel product, especially in deep drawn applications needing oxides be smaller than fifty micrometres in diameter. The restriction of the flow passage can also cause undesirable flow patterns in the mould which cause quality problems (e.g., mould flux ingestion, shell thinning). Also, the mould level transients occurring when a tundish is replaced because of the tundish nozzle clogging, can cause reduced quality.
Nozzle build-ups which lead to nozzle clogging are a completely natural phenomenon when steels are cast containing solid inclusions. Almost all solid inclusions (with the exception of titania) are not wet by liquid steel and hence agglomerate if they come in contact with other similar solid inclusions or another refractory surface. Liquid inclusions, if they contact a refractory surface, spread on that surface, as all slags wet oxide surfaces. In addition, if the liquid inclusion is not saturated with respect to the refractory which it contacts, it reacts with the refractory (leading to perfect wetting). Liquid inclusion films on refractory surfaces also begin to cool due to heat transfer to the refractory. This leads either to erosion of the nozzle if the liquid film remains fluid and is transported downward by the steel flow, or to build-up if the liquid inclusion begins to precipitate solid material because of the decrease in temperature when it contacts the refractory surface.
If it is assumed that the inclusions precipitate away from the refractory surface, then the issue is the transport to the surface. Fluid dynamics then becomes important, and the details of nozzle geometry can be manipulated to slow the build-up rate of the transported and agglomerated material. Turbulence in fluid flow can also be manipulated to attempt to remove build-up, and the build-up rate is the sum of the inclusions transported to the interface minus those removed from the interface. Techniques to increase fluid turbulence can be effective in moving the point at which clogging occurs.
However, it is to be noted that these techniques minimize build-up rate by increasing the frequency of larger agglomerated inclusions within the product and can cause a problem of similar magnitude to the one which is being ameliorated. If it is assumed that the inclusions precipitate on the refractory surface, then the issue is one of inclusional growth and is a function of heat transfer, fluid flow, steel chemistry, and the heterogeneous nucleating ability of the surface. If the refractory material is reacting with the liquid steel, then another mechanism for precipitation is possible because of the local steel chemistry change. In addition, if the refractory is not gastight, air aspiration is possible, which can cause local chemistry change and the potential for precipitation on or close to the refractory surface.
The phenomenon of material build-up on refractory surfaces is a function of heat transfer through the nozzle, steel temperature, steel chemistry, and the details of nozzle chemistry, condition and design. The condition for precipitation of a solid or semi-solid inclusion is to exist within the liquid steel for clogging to be possible. As all steels are deoxidized in some fashion, the potential for clogging always exists. If the primary inclusion is solid, some build-up of material always occurs. If the primary inclusion is liquid, build-up occurs if the liquid inclusion precipitates solid material during processing either by thermal or chemical change. In addition to the thermodynamic conditions necessary for precipitation, it is necessary to consider the issues of inclusion transport to the interface, i.e., where inclusions precipitate and how they arrive at the refractory interface.
The solution to inclusional nozzle clogging is to modify the solid inclusion to a liquid inclusion which remains liquid during casting. Calcium treatment of aluminum-killed steels and development of nozzle ceramics which react with alumina to form a liquid inclusion are the best examples of this view point.
If modification is not possible, clog initiation is to be delayed and the clog growth rate minimized. Clogging is very sensitive to transient operation, and any instances of increased levels of clogging material or increased potential for super-saturation can lead to substantial clog growth rates. All transients in operation, such as those which lead to reoxidation, can cause an increased growth rate and are to be avoided in order to gain casting repeatability. The details of the rate of clog build-up are very sensitive to the details of refractory nozzle design and turbulence in the liquid. All clogs grow at a rate defined by the precipitation or agglomeration rate minus the rate at which the clog spontaneously fragments. Hence, bulk fluid-flow patterns and the details of nozzle chemistry have a major effect in the clogging performance of a given nozzle design. If a liquid steel is to be cast which contains or precipitates solid inclusions, build-up is inevitable. Casting practices which improve casting life normally result in irregular quality issues because of the release of agglomerated inclusions from the build-up over time.
There are several viewpoints concerning deposition and agglomeration of indigenous alumina inclusions. Several studies have reported that the inclusions deposited at the nozzle orifice do not form in situ but are formed elsewhere and deposited at the orifice. A study realized as early as 1949 that the depositing particles are similar to the deoxidation products found in steel. The view point of precipitation of alumina on the refractory surface suggests that the growth of the alumina deposit is by the precipitation of the dissolved aluminum and oxygen. These precipitating types are suggested to have four possible origins as given earlier in the article.
Several studies have concluded that diffusion of air through the refractory pores (or a leakage through the gas injection ports and from between the refractory joints) cause oxidation of the dissolved aluminum in the steel. Alumina is formed as a product and subsequently grow as a deposit on the refractory wall.
When the liquid steel flows past the refractory, dissolved aluminum in the steel can reduce solid or gaseous oxides to form alumina, which gradually grows or is deposited on the nozzle wall as the casting proceeds. A study has proposed that the oxygen from the refractory is transported to the refractory / steel interface as CO (carbon mono-oxide is produced when carbon in the refractory reacts with binders and impurities), which oxidizes aluminum. The study has concluded that the deposition takes place by in situ nucleation of alumina from the steel / refractory reaction and hence, even if the steel is perfectly clean, clogging still occurs.
Since there is a heat loss from the nozzle to the atmosphere, the temperature of the steel drops when it passes through the nozzle. This drop in temperature supersaturates the steel with respect to aluminum and oxygen, resulting in the precipitation of alumina. In addition, because of the lower local temperature, steel can solidify in the inter-particulate spaces and reinforce the clog. This nozzle clogging mechanism, although apparently feasible, cannot account for considerable rates of build-up because of the small mass of material precipitated because of the temperature changes in aluminum deoxidized steels. It, however, determines the smallest possible build-up rate.
In the mechanism consisting of supersaturation of oxygen in liquid steel because of excessive reoxidation, the reoxidation rate is higher than the alumina growth rate, leading to a transient supersaturation which heterogeneously dissipates on the refractory or clog surface.
All the mechanisms described above can possibly contribute to clogging, although they are given different significance in different studies. The majority of studies on alumina deposition suggests that transport and agglomeration of the indigenous inclusion is the major reason of rapid build-up rates.
Hence, although the effect is similar, the cause and subsequent cure is quite different. It is clear that build-ups are related to the precipitation and transport of solids or liquids which can precipitate solids in contact with a refractory surface. It is also clear that build-up of solids on a refractory surface is a natural phenomenon because of either to heat transfer or the nature of the surface chemistry of the inclusion. The major issues in clogging are, ‘why it takes place’ and ‘ the reason which affects the rate of build-up’.
There are four general types of nozzle-clogging during continuous casting of steels. Each is having a different origin. In practice, clogging within a single nozzle can be because of a combination of two or more types. This classification distinguishes between clogs consisting of (i) build-up de-oxidation products such as Al2O3, (ii) solidified steel build up, (iii) build-up of complex oxides such as spinels, and (iv) build-up of reaction products such as CaS.
While some causes are more detrimental than others, all are a problem. Different steels yield a different potential nozzle clogging cause, for example, a resulphurized free machining steel is going to have much more of an issue with the formation of calcium sulphides than spinels. No matter what is the cause, all nozzle cloggings can be detrimental to a continuous casting process. The deposit of clogging material on the side walls of the nozzle can cause irregular flow from the tundish into the mould. Irregular flow through a tundish nozzle increases the probability of generating a number of quality defects such as re-oxidation of the steel and slag entrapment. Nozzle clogging also affects productivity in that less steel is able to be cast because of the blockage in the nozzle. Another thing to consider is the life of the tundish which is frequently limited to the life of the nozzle because of the clogging. If nozzle clogging can be controlled enough so as to extend the nozzle life by even one or two heats longer, that results in substantial process cost savings.
Agglomeration of deoxidation products which have been have been observed in the nozzles, are build-ups consisting of deoxidation products (e.g., alumina, titania, zirconia). These deoxidation products are of the same composition and size (typically 1 micrometre to 20 micrometres) as is found in the mould. The deoxidation products sinter together to form a network. This sintered matrix can have or does not encompass steel. Steel has been found within the matrix for heats with low residual deoxidation product content, as can be found when using vacuum degassing or for high carbon concentrations. No steel is found within the matrix when the deoxidation product concentration is high, as can be found when using argon bubbling during secondary refining and low carbon concentrations (e.g., less than 0.1 % C).
The solid steel build-up takes place when the steel freezes within the nozzle since the superheat is low, and the heat transfer from the stream is high. This is especially true at the start of casting if the nozzle preheating is inadequate. Agglomeration of complex oxides happen when clogs containing non-metallic materials not resulting from deoxidation have also been observed. Clogs have been observed in the SEN port area which have a chemistry indicative of a combination of mould flux and deoxidation particles. Here, it is believed that the mould flux is drawn into the top of the ports because of the recirculation flow pattern in the upper part of the mould and because of the tendency of the flux to coat the nozzle. Once inside the nozzle, the flux assimilates deoxidation particles, thereby increasing the clog volume. Clogs containing calcium aluminates or calcium sulphides have also been observed on calcium treated heats.
In several cases, the clogging with the composition of deoxidation products but deposited in the form of a film instead of a sintered network of particles has been observed. The source for these build-ups has been attributed to reactions between the deoxidant and (i) air drawn into the nozzle because of the negative gauge pressure and the porosity of the nozzle, (ii) oxygen evolved from the steel because of the lower steel temperature adjacent to the nozzle, and (iii) oxygen generated by silica refractory decomposition. These mechanisms are consistent with the reported observations of increased clogging as soluble aluminum concentration is increased.
For those clogs consisting of solidified steel or reaction products, the transport and attachment mechanism are straight-forward since the clogging phenomenon takes place at the nozzle wall. But for clogs containing deoxidation products, the process of their transport and attachment is more complicated. Several theories have been proposed concerning flow patterns and geometries which improve transport of deoxidation products to the nozzle wall.
The turbulent recirculation zones theory says that within a recirculation zone, turbulent velocity fluctuations oriented in all directions are present. Those fluctuations toward the wall enable deposition. Turbulent flow theory says that turbulent eddies, even in the absence of a recirculation zone, transport deoxidation products to the nozzle wall. Rough nozzle walls theory says that as the roughness of the nozzle wall is increased (e.g., because of the irregular build-up or erosion) the probability of interception of entrained deoxidation particles increases. External corners theory says that because the density of alumina is less than steel, alumina tends to be driven toward the wall for flow around an external corner (e.g., tundish nozzle entry). This driving force is expected to be considerable only for large alumina particles (e.g., 35 micrometres to 40 micrometres).
Attachment of deoxidation products to the nozzle wall theory says that the deoxidation particles are attached to the nozzle wall by surface tension and, after sufficient time, by sintered bonds. The surface tension of the steel creates a void and, consequently, an attractive force between the deoxidation product and the wall (or another deoxidation product). The magnitude of this force for the case of a 2.5 micrometres deoxidation product attaching to a ceramic filter has been calculated to be around an order of magnitude higher than the drag and buoyant forces on the particle. The sintered bond between the particle and wall (or another particle) forms relatively rapidly at these temperatures (e.g., only 0.03 seconds is needed for two particles of ten micrometres to develop a sufficient neck between them to withstand drag and buoyant forces.
The most obvious means to reduce clogging is to decrease the concentration of deoxidation products and the formation of reoxidation products. Means to achieve this is increasing of steel cleanliness. The important aspects of clean steelmaking include (i) ladle refining practice (a vacuum degassing treatment yields better cleanliness than does argon bubbling), (ii) reoxidation prevention (submerged ladle-to-tundish pouring, shielded tundish surface, and leak-tight refractory joints reduce exposure of the steel to oxygen and hence improve its cleanliness, (iii) deoxidation product removal (optimal tundish flow patterns as well as filtration and electromagnetic techniques can remove deoxidation products from the melt, and (iv) flux entrainment prevention (submerged ladle-to-tundish pouring and avoidance of ladle slag carryover reduce the quantity of exogenous inclusions in the melt).
It is unlikely that steel cleanliness improvements completely eliminate nozzle clogging. A study has calculated that for typical casting conditions, nozzle blockage can occur if as little as one in every 1,500 non-metallic inclusions are deposited on the nozzle. For reducing the deposition of the entrained deoxidation products, several techniques have been utilized as given below.
Argon Injection – Argon injected through the nozzle wall or stopper rod into the steel stream is widely used to reduce nozzle clogging. A typical injection rate is 5 litres/minute at standard temperature and pressure (STP).
Several reasons have been suggested for the improved clogging resistance which include (i) a film of argon is formed on the nozzle wall which prevents the deoxidation product from contacting the wall, (ii) the argon bubbles flush the deoxidation products off the nozzle, (iii) the argon bubbles promote the flotation of deoxidation products, (iv) argon injection increases the turbulence and hence causes the deposit to be flushed off (this mechanism contradicts a previously mentioned hypothesis which states that turbulence enhances deposition), (v) the pressure inside the nozzle is increased which thereby reduces air aspiration through the nozzle (in the absence of argon injection, negative gauge pressure has been measured in water models near the slide gate and the stopper rod seating surface), and (vi) the argon prevents a chemical reaction between the steel and the refractory.
The argon can be injected through the pores in the refractory material or through machined or laser cut holes in the refractory. Tailoring the argon flow to be greater in areas of high deposition and to be locally uniform has been shown to reduce clogging. Disadvantages of argon injection include increased quality defects and nozzle slag line erosion because of the increased mould level fluctuations, bubble entrapment by the shell, and nozzle cracking because of high back pressure, or decreased nozzle thermal shock resistance. It is also suspected that argon injection tends to move the clogging problem to a different location.
Calcium treatment – Alumina clogging can be reduced by adding calcium to the steel to prevent the formation of solid alumina. As shown in Fig 2, for a typical liquid steel temperature of 1,550 deg C, liquid is the equilibrium phase for calcium oxide-alumina mixtures containing 40 % to 60 % alumina. Also, it is believed that under steelmaking conditions, mixtures containing a higher fraction of alumina are also be liquid. This is based on the observation that when CaO·2Al2O3 inclusions (79 % alumina) are found in the final cast product, these inclusions take a spherical shape and the nozzle experiences much less clogging.
Fig 2 Calcium oxide-alumina phase diagram
The disadvantages of calcium treatment include (i) increased clogging relative to the non-treated condition if insufficient calcium is added, because of the formation of CaO·6Al2O3, and (ii) erosion of refractories. Also, calcium treatment does not work for high sulphur steels since calcium reacts with sulphur to form solid calcium sulphide instead of liquefying the alumina (e.g., sulphur is to be less than 0.007 % for a typical total aluminum concentration of 0.04 %). However, it has been proposed that calcium treatment is still be successful if the sulphur is added after calcium treatment.
Nozzle material modifications – A variety of nozzle compositions have been investigated. CaO additions to the nozzle have yielded decreased clogging by liquefying the inclusions. The effectiveness of this method is limited by the diffusion of the CaO to the refractory surface. Other compositions and coatings have also been attempted, but the cause for the decreased clogging is uncertain. For example, the addition of boron nitride has shown to markedly reduce clogging. However, it is not known whether the beneficial effect of boron nitride is because of the formation of a liquid boron oxide film, decreased surface roughness, or another cause. Other possible explanations for the observed clogging reduction of the several materials studied are decreased thermal conductivity, decreased contact angle with steel, reduced reactivity with steel, and decreased air aspiration.
Nozzle geometry modifications – In an effort to reduce the effect of clogging, oversized nozzle bores and replaceable SEN are widely employed. To reduce the degree of clogging, the subjects which have been studied are (i) improvement in joint sealing by strengthening the steelwork which holds the nozzle in place since it has been found to reduce air aspiration and hence reduce clogging, (ii) rounded nozzle entrance, since incorporating a rounded entrance (instead of a sharp corner) to the tundish nozzle and ensuring proper vertical alignment can reduce clogging at the nozzle entrance by eliminating separated flow, (iii) internal step e.g., a five millimetres annular step incorporated at the mid-height of the submerged entry nozzle has been found to decrease alumina build-up in the lower part of the nozzle as well as decreasing flow impingement on the mould wide face, (iv) varying nozzle internal diameter as increasing the nozzle internal diameter just below the stopper rod seating surface has reduced clogging, (v) flat bottomed nozzle since decreased port clogging has been observed when the elevation of the nozzle internal bottom and port bottom are coincident (i.e., no nozzle well), and (vi) insulation around nozzle as insulation, as well as preheat and heating, around the clogging location can reduce clogging.
The details of the morphologies of various types of clogs and specific conditions for their formation are discussed below.
Aluminum-killed carbon steels – In the case of aluminum killed steel, the matrix of oxide deposit has been identified in 1971 as a three-dimensional network of small alumina particles sintered to each other. In all build-ups a three-dimensional network is seen. The bulk of the deposit is composed of oxide particles (normally alumina) of micron size. The white powdery alumina deposit has a porosity of 80 % or higher. The voids can be empty or filled with steel. In the case of absence of steel, the deposit is very friable.
The bulk of the deposit is not of uniform thickness along the length, and it is also different in the different sections at the same height. A slide gate system shows the most prominent dependence on the flow. The deposit is preferentially present at the sites where flow eddies are present. In general, a bulk deposit is heavily present in the areas submerged inside the melt pool in the mould. The straight part of the SEN is essentially clean. In the case of tundish well nozzles, the deposit is heavy near the entry of metal from the tundish into the nozzle. A schematic of the deposit structure along the length of the nozzle shows the difference in the shape of the clog at these two different positions. In all clogged nozzle, a distinct relation of the deposit to the meniscus is normally seen.
Because of the presence of steel up to the meniscus on the outside of the nozzle, the steel flow inside is expected to be different from that in the upper region of the SEN. It can be thought that the presence of steel in the form of a pool at the bottom of the SEN causes recirculation patterns in the steel, which accelerates the inclusion deposition several-fold. This reasoning can be used to explain the more-than-noticeable difference in the deposit below and above the meniscus levels.
There can be several different types of clogging materials and numerous potential causes of clogging. However, there are three generic issues which are (i) thermal problems, either because of the heat transfer to the nozzle or low steel pouring temperatures, (ii) transport and agglomeration of solid or semi-solid inclusions to a refractory interface, and (iii) precipitation of solid or semi-solid materials on the refractory surface. An understanding of these issues leads to the potential solutions to clogging.
The solution to clogging is simple, that is, not to precipitate a solid inclusion and precipitate only a liquid inclusion which always remains liquid during the pouring of the liquid steel. Calcium treatment of aluminum-killed steels or the use of manganese and silicon as a deoxidant instead of aluminum is very effective in stopping build-up, as long as the inclusion remains liquid during processing. In fact, in these systems, when the inclusion is fully liquid, nozzle erosion is common, and the issue becomes the erosion rate rather than build-up rate. Problems in calcium treatment or in the casting of manganese silicon-killed steels are always related to the transformation of the liquid inclusion to a semi-solid inclusion or failure to attain an inclusion chemistry which ensures that the inclusion remains liquid throughout the processing.
Another approach to the problem of casting steels containing solid inclusions is to develop a reactive nozzle where the solid inclusional material in the steel reacts with the nozzle to form a liquid at the steel / nozzle interface. This approach can be quite successful as long as the inclusion flux to the nozzle is less than the capacity of the nozzle to transform the solid inclusion to a fully liquid inclusion. Hence, thermal conditions at the nozzle / steel interface and steel cleanliness are to be controlled for success of this technology.
If solid inclusions are to be cast, then the issue is to minimize the level of inclusions and to optimize the dynamics of fluid flow to control the build-up rate. Hence, clean steel practices to minimize inclusion production, insulation and nozzle preheat to minimize the thermal effect of the nozzle, and proper nozzle design to control fluid flow all act together in developing a proper strategy to minimize build-up rates. The solution to clogging in steels containing solid or semi-solid inclusions is based upon detailed knowledge of the cause of clogging and determination of which of the above three generic issues is responsible for the build-up. Hence, the first issue in determining a solution is to study in detail the clog. Once the chemical nature, its position, its frequency, and the extend is determined, a strategy to minimize or eliminate the build-up can be developed.
Build-ups which contain only steel or an inclusion which precipitates because of the reduced temperature near a refractory interface frequently are thermally induced or affected. These are solved by introducing heat through increased preheating or superheating or minimizing heat loss by insulation.
The first issue in clogs which are primarily caused by transport and agglomeration is to determine whether the clogging is caused by a transient condition or a continuous problem. Hence, a determination of the build-up rate or the frequency of clogging is to be conducted. For example, if the majority of casting situations do not cause a problem and the issue is one of a few irregular but catastrophic clogs then one can assume that there is an irregular local increase in the production rate of inclusions because of a processing issue. Most common irregular problems are related to reoxidation in the tundish, aspiration through a slide gate, variable ladle metallurgy practices leading to variable incoming steel cleanliness, and problems with refractory integrity.
Irregular problems with reoxidation are frequently seen in the casting of calcium-treated, aluminum killed steels. A change in the gradient of the stopper rod position as a function of time, from negative to positive, is a very sensitive indicator of the change from a fully liquid inclusion to one which is semi-solid. Hence, causes of irregular clogging can be easily determined when casting calcium treated grades.
Once transient issues have been solved, the next issue is to decrease the build-up rate. Majority of the casting operators have seen that very dirty steels clog quickly and have spent a lot of efforts on clean steel practices in the ladle to provide a constant low inclusional mass in the steel from the ladle. This type of practice is very important when the maximum casting time is short because of excessive build-up (e.g., in thin-slab casting because of the small nozzle size). However, the reduction of the indigenous inclusion mass cannot be a complete solution to the problem of clogging. It can only decrease the rate of build-up of the clog and increase the casting time before clogging becomes problematic. Hence, as one increases sequence length, clogging eventually becomes a problem, even in very clean steels.
Clean steel practices, while very important, are a necessary condition only for long casting times, but are not a sufficient condition. If one assumes that clean steel with a low mass of inclusions is constant from the ladle, the mechanism for extending casting time is to be related to the techniques to minimize the rate of build-up. There are two issues in minimizing the rate of build-up namely (i) extending the time before initiation of the build-up, and (ii) decreasing the clog growth rate. Initiation of clogging can occur by direct reaction between the steel and the refractory, precipitation on the refractory, or transport of inclusions to the refractory. Hence, there are a number of issues in clog initiation. However, the importance of fluid-flow control cannot be understated, and the details of the complete design and orientation of the flow control system is required to be considered.
Refractory compatibility is important. The details of nozzle manufacture and design tend to be an area of secrecy. However, it is clear that the nozzle which is in contact with the steel is to be inert. Frequently reaction between the nozzle and the glazes applied to nozzle surfaces can initiate build-up in aluminum-killed steels. Refractory integrity is also important. Tundish nozzles and SENs are to survive severe thermal conditions without failure. All refractories are to be gas tight, and all joints between different types of refractories are to be gas tight. Cracking because of the thermal shock is to be avoided to avoid external reoxidation.
Fluid flow at the steel / refractory interface is important since the transport of inclusions from the bulk steel to the interface can lead to initiation of clogging. Hence, a refractory design which promotes smooth fluid flow and a defined laminar flow near the interface can cause clog initiation times to be extended. Details of fluid-flow design to avoid bulk eddies and the development of a very smooth refractory / steel interface to promote a laminar layer next to the interface are keys to extending the initiation time of clogging.
The clog growth rate is because of the transport of inclusions from the bulk. Once the clog is initiated, the interfaces immediately roughen and allow the continued contact of turbulent eddies and an increased transport rate of inclusions. Following initiation, the growth rate can be considerable. There are a number of issues in controlling clog growth rate as described below.
The first is the minimization of inclusions If the mechanism of growth is transport of existing inclusions, then the level of existing inclusions is a factor in the growth rate, and clean steel practices are to be incorporated.
The second is the fluid-flow control. Normally, it is in areas of low liquid steel flow rate where build-ups grow very fast. Nozzle design is important to avoid this problem. However, it is impossible to completely design out the problem, as there is always fluid flow and transport to the interface after build-up initiation. In SENs, the bottom of the nozzle is always problematic, as there is no simple fluid-flow solution to ensure that the turbulence level is low. Proper nozzle design can, however, considerably reduce build-up rates by eliminating local issues. Bulk turbulence or flow instability can cause the clogging material to break up and be re-entrained into the flowing stream. Increased bulk turbulence, as induced by gas injection, can lead to an increase in the clog break-up rate and decrease the overall build-up rate. This, of course, leads to longer casting times, but with the penalty of increased potential for irregular quality and casting machine operational problems. Hence, argon injection, while allowing longer cast lengths, carries the penalty of increased levels of irregular, large inclusions.
The third is the minimizing of the thermal problems. Nozzle preheating and insulation practices are extremely important. Frequently build-ups are stabilized by solidifying the steel which is trapped within the build-up. Hence, insulation and preheating can prevent initiation of the build-up and also lead to a decrease in the build-up rate. In the same manner, increased superheat also helps. The effect of decreasing thermal gradients on reducing build-up rates was one of the first issues discovered by early casting operators.
Precipitation build-ups are those build-ups which are caused by precipitation. For such build-ups, it is necessary to affect either the temperature field or change the steel chemistry. Increased superheat and improved nozzle preheat and insulation are always helpful. Calculation of the exact inclusion precipitation thermodynamics allows one to tailor heat chemistries in order to avoid clogging.
Frequently thermal conditions at cast start initiate precipitating build-ups and fluid-flow conditions within the nozzle, then control the transport of material to the interface. The growth rate is then a function of the local temperature at the build-up / steel interface and turbulence within the nozzle. For example, frequently in titanium-treated steels, titanium nitride precipitates immediately on the nozzle during nozzle fill. However, after that transient in temperature is overcome, the thermodynamic condition for precipitation is not met, and the layer starts to dissolve until thermodynamic equilibrium is reached. If the pouring temperature is too low, and the condition for precipitation is always satisfied at the build-up / steel interface, the build-up continues to grow at the rate at which the elements are transported from the bulk liquid.
Frequently in casting there are multiple reasons for clogging. For example, the build-up can be initiated by reaction between the nozzle and the steel and then grow by transport and agglomeration of already present inclusions. Under this condition, changes in nozzle design and chemistry can have a remarkable effect on the time to clogging. Sometimes there can be precipitation and agglomeration occurring simultaneously. The most common reason for clogging is irregular reoxidation. Hence, slow-growing clogs can accelerate during these times.
Steels which are chemically designed to precipitate liquid inclusions are not supposed to clog. However, clogging phenomenon is quite common and is because of the inadvertent additions of reactive elements which change the liquid inclusion to a solid or semi-solid inclusion. For example, in case manganese silicon steels, it is necessary to avoid the inadvertent addition of aluminum to the steel either from the ferroalloys or the slag modifiers. Recent trends in clean steel practices (slag modification and limitation) have led to steel chemistries becoming more sensitive to residual reactive elements such as aluminum, magnesium, and calcium. Additions of ppm (parts per million) levels of magnesium and more than 50 ppm of aluminum make the liquid manganese silicates precipitate alumina or magnesium aluminate spinel inclusions and become semi-solid or fully solid. Hence, a key in clogging prevention is to carefully limit the inadvertent addition of reactive elements.
In case of calcium-treated, aluminum-killed steels, insufficient calcium treatment, reoxidation or the inadvertent additions of magnesium through ferroalloy additions, extensive ladle reheating, degassing, or long holding times in the ladle at too high a temperature, can lead to the build-up of soluble magnesium in the steel and problems with material build-up during casting. These build-ups are frequently semi-solid rather than fully solid, and they are easily released from the nozzle interface, if a new heat of steel is poured with a higher level of residual calcium. This extra calcium fluidizes the liquid holding the semi-solid mass to the nozzle and leads to potentially catastrophic release of the clogging material, which can then lead to mould overflows during casting.
Some stainless steels treated with titania at a level below which precipitation of TiN is possible are susceptible to thermal clogging. This is because of the reaction of titanium with the nozzle refractory and the precipitation of titania on the nozzle surface. Titania is wet by steel and leads to no contact resistance between the ceramic and the steel. It also leads to an increased heat transfer rate between the nozzle and the steel. This leads to an increased build-up growth rate. The reaction of titanium also increases penetration of steel into the refractory and causes a larger area of heat dissipation. Hence, thermal clogging can be seen in titanium-treated steels where little thermally-induced clogging is measured if the same grade is cast with no titanium.
In case of high phosphorus-containing steels, there is considerable evidence that aluminum-killed, low-carbon steels which are rephosphorized have accelerated build-ups compared to the identical grade which is not rephosphorized. The build-up is found to be solely alumina. In these steels, either the phosphorus addition leads to increased inclusional mass which is not removed before casting or the phosphorus content leads to an interfacial effect similar to that of titanium.
The most important cause of nozzle clogging is the deposition of solid inclusions already present in the steel entering the nozzle. These can arise from several sources namely (i) deoxidation products from steelmaking and refining processes, (ii) reoxidation products from exposure of the liquid steel to the air, (iii) slag entrapment, (iv) exogenous inclusions from other sources, and (v) chemical reactions such as the products of inclusion modification.
One of the studies has calculated that a typical clogged nozzle contains 16 % of the oxide inclusions which pass through the nozzle. Hence, it is beneficial both to reduce the number of inclusions, as well as to limit their transport and attachment to the nozzle walls. The transport of inclusions to the nozzle walls can be lessened by streamlining the flow pattern within the nozzle to minimize the frequency of contact of inclusions with the walls. In particular, slight misalignment, separation points in the flow pattern, turbulence, and fluctuations in casting speed are all very detrimental and are to be avoided.
Nozzle walls are to be smooth to increase the thickness of the laminar boundary layer and discourage contact. Once oxide particles touch the nozzle wall, they attach because of the surface tension forces, and eventually sinter to form a strong bond. Nozzle wall coatings can help to reduce attachment. The best way to avoid this source of clogging is to minimize the number of solid inclusions passing through the nozzle. Inclusions making up a clog otherwise ends up in the final product, where they frequently have the same composition and structure.
Careful refining practices can minimize the quantity of deoxidation products. For example, vacuum degassing greatly lowers average inclusion levels, relative to conventional argon bubbling. In addition, ladle and tundish slag compositions are to be designed to have a low enough oxygen potential to absorb inclusions, while not being so reactive that steel composition is altered. Late aluminum additions are dangerous since the small inclusions which form do not have sufficient time to agglomerate and be removed.
One study has suggested that aluminum is only to be added at tap when the oxygen content is high and the inclusion morphology enables easy flotation. After the last alloy additions, it is suggested to first stir vigorously for a brief time in order to encourage mixing and collisions for the inclusions to agglomerate. Argon bubbles are better than electro-magnetic stirring since they contribute greatly to the attachment, agglomeration, and flotation removal of the inclusions. Then, a long period of gentle stirring or simple natural convection is to follow, to allow time for the inclusions transport to the slag or wall surfaces and be removed. Without enough of this gentle ‘rinse’ time, further collisions generate more detrimental large clusters to be sent into the tundish. Finally, an optimized tundish flow pattern with a basic slag is helpful as the final refining step prior to entering the tundish nozzle.
Reoxidation products are caused by the exposure of the liquid steel to air. Reoxidation during ladle treatment can be avoided by providing an adequate slag composition and thickness and then avoiding excessive stirring which opens up ‘eyes’ in that slag cover. Reoxidation during steady tundish operation is easy to avoid with a non-porous slag cover and with ladle nozzles and baffles to avoid excessive surface turbulence. Reoxidation during ladle opening and tundish filling is a much bigger problem which needs high operational care.
In particular, it is important to use a submerged ladle shroud (preferably bell shaped) throughout, maintain minimal turbulence during tundish filling, add a tundish slag which quickly forms a continuous liquid layer, use a tight sealing tundish cover, and even purge the tundish with argon prior to filling.
Increasing stirring intensity (indicated by turbulence dissipation rate) encourages faster inclusion removal. Lowering stirring intensity decreases collision rates, so fewer large inclusions form and removal processes can lower their numbers. Slag entrapment is avoided firstly by minimizing slag carryover. A sensor to consistently detect the presence of slag is necessary in this regard. Care is needed during ladle exchanges when slag can become entrapped in the tundish in several ways, including stream impingement on the slag layer and vortexing.
Tundish flow control using baffles and weirs, a pour box or impact pad is important to give any entrained and emulsified slag a chance to float out. Finally, it is important to maintain adequate submergence of the tundish nozzle since mould slag can be drawn into the top of the ports because of the recirculation flow pattern in the upper part of the mould and because of the tendency of the flux to coat the nozzle. Once it is deposited on the nozzle walls, entrapped slag collects other inclusions, hence aggravating clogging. Clogs caused by slag entrapment are easy to identify by matching the average composition of the inclusion particles with either the ladle, tundish, or mould slag compositions.
Exogenous inclusions come from several sources apart from slag entrapment. Loose ceramic material, mortar, and dirt can be picked up when steel first flows over the refractory surfaces. Ladle packing sand can become entrained in the flowing steel. Ladle, nozzle, and tundish wall refractory material, and existing oxide deposits can become dislodged and entrained also. These particles are identifiable from their large size and unusual shapes. Great care is to be given to refractory preparation, assembly, maintenance, and clean-up. Filtration and electromagnetics are also effective solutions, but are costly and catch only a limited number of particles.
Chemical reactions generate solid inclusions in several different ways. For example, ladle slags with high FeO or MnO content frequently have sufficient oxygen potential to react with aluminum in the steel to form alumina. This is correlated with increased clogging. Magnesium residuals in the steel, in the aluminum alloy additions, or in the tundish liner can react to form magnesium aluminate spinels. Titanium reacts to form inclusions which are particularly prone to clogging, perhaps due to their effect on surface tension.
Calcium is frequently added to avoid clogging by keeping the inclusions liquified in the liquid steel. Improper calcium treatment can worsen clogging, however, by producing solid inclusions if the CaO content does not almost match the alumina mass. Too little CaO causes clogs with calcium-aluminates (e.g., CaO·6Al2O3), while too much calcium produces calcium sulphides, even in low S steel. Calcium treatment is best after alumina and especially sulphur have already been minimized. It is also important to control the slag composition (e.g., maintain 2 % FeO) and to rinse stir both before and after calcium addition. Finally, it is important to choose refractory compositions which are compatible with the steel, or they can be eroded to form inclusions.
Air aspiration into the nozzle through cracks and joints leads to reoxidation, which is an important cause of inclusions and clogging. While regulating the liquid steel flow, the flow control device creates a local flow restriction which generates a large pressure drop. This ‘venturi effect’ creates a low-pressure region just below the slide gate or stopper rod. This minimum pressure region can fall below 100 kPa (zero-gauge pressure) according to both water model measurements and calculations. This allows air to be drawn into the nozzle. The rate of air ingress can be huge, approaching that of the steel flow rate for a pressure of -30 kPa. The minimum pressure is affected by argon injection, tundish bath depth, casting speed, gate opening, shape of the surfaces, and clogging.
Clogs caused by air aspiration can be identified in several ways. Firstly, if the inclusions are large and dendritic in structure, this indicates that they are formed in a high oxygen environment, such as found near an air leak in the nozzle. Secondly, an erratic or low argon back pressure during casting likely indicates a crack, leak, or short-circuiting problem which can allow air aspiration. Finally, nitrogen pick-up in the steel between the tundish and the mould indicates exposure to air.
If air enters the nozzle, the oxygen reacts with aluminum in the steel locally to form alumina inclusions. The aspirated oxygen also can create a surface tension gradient in the steel near the wall. This can generate surprisingly large forces attracting particles towards the nozzle walls. This is likely the dominant clogging mechanism in regions of low turbulence and non-recirculating flow. Hence, it is critical to avoid air aspiration. Air aspiration can be addressed through several nozzle design and operating practices.
The nozzle refractory is to maintain a stable non-porous barrier which does not allow air to diffuse through it even after thermal cycling. Tight tolerances are to be used for all nozzle joints. When assembling the nozzle, smoothing and cleaning of all the joint surfaces and employing of non-cracking, non-porous mortar is to be done. Joint movement is to be carried out by holding the nozzle in place with a strong steel support structure. The argon gas line is to be checked for leaks which can entrain air. The oxygen content of the argon is to be monitored. Finally, argon gas injection is to be optimized.
Some clogs appear as a uniform film, rather than a sintered network of particles. These clogs are attributed to reactions between aluminum in the steel and an oxygen source in the refractory. This oxygen can come from carbon monoxide when carbon in the refractory reacts with binders and impurities or from silica refractory decomposition. Controlling refractory composition (e.g., avoiding Na, K, and Si impurities) or coating the nozzle walls with various materials, such as pure alumina or boron nitride can help to prevent this and other clogging mechanisms. Controlling chemical reactions at the refractory / steel interface has also been suggested as a counter-measure to clogging. Incorporating CaO into the nozzle refractory can prevent clogging by liquifying alumina inclusions at the wall, so long as CaO diffusion to the interface is fast enough and nozzle erosion is not a problem.
Although heat losses from the nozzle refractories are very small, steel can freeze within the nozzle either at the start of cast, if the nozzle preheating is inadequate, or within a clog matrix, where the flow rate is very slow. These problems are more likely if the steel superheat is very low, or the alloy freezing range is very large. Freezing occurs initially since the preheated nozzle wall temperature is considerably below the steel solidus. The nozzle walls heat up within a few minutes to melt this layer away, however, but clogging can start if another mechanism is triggered.
Clog networks can grow more easily when they are supported by a matrix of solidified steel. Some clogs consist solely of dense concentrations of oxides, as surface tension rejects steel from inner spaces. Other clogs consist of a network of small oxide particles which contain steel, especially for high carbon steels. These clogs appear to form by first collecting and sintering together a network of oxides against the nozzle wall. After an initial clog layer of 3 mm to 12 mm thick has built up, the liquid steel trapped within it flows so slowly that it can start to solidify depending on the flow and thermal conditions. This strengthens the otherwise weak inclusion network and allows it grow further into the liquid, filtering inclusions from the steel flowing through it as it grows. Only the inner-most 3 mm to 12 mm of the inclusion network is to be strong enough to withstand the drag of the turbulent steel flowing through it. As the roughness of the clog surface increases, the probability of intercepting and entraining particles increases and clogging can accelerate.
A number of different practices are helpful to minimize clogging. The remedies can be classified as inclusion prevention, inclusion modification (calcium treatment), nozzle material / design improvement, and argon injection. The best remedial action depends on the steel grade and exact cause of the specific clogging problem being considered. Hence, the first step is to identify the clogging cause by monitoring important parameters during casting and by visual, microscopic, and chemical examination of clogged material. A lot can be learned about the cause of clogging from careful visual inspection and analysis of the clog itself.
Clogs above the slide gate are particularly disruptive since they need a tundish change. Clogging can be reduced in the operation by making several practice changes and operating improvements. In particular, the ladle opening practice is to be improved by redesigning the ladle to tundish shroud system and decreasing ladle slag carryover. In addition, the tundish well preheating time is to be increased to avoid cracks in the ceramic because of the thermal shock and perhaps also to reduce initial skulling. This type of analysis is important to identify the cause of a clog so that proper corrective action can be taken in the plant. However, it is also important to identify clogging during casting so that potential quality problems in the cast product can be minimized or at least anticipated. In addition, real time feedback can help in the assessment of clogging counter-measures.
Clogging can best be detected during casting by simultaneous monitoring of several different parameters in real time such as argon back pressure, nitrogen pickup, mould level fluctuations, and flow control position relative to casting speed.
The argon back pressure is a good early indicator of potential air aspiration problems. Abnormally low or sudden drops in back pressure indicates that argon is short circuiting and there can be a crack or leak in the refractory allowing air aspiration somewhere. Increases in back pressure can indicate clogging over the nozzle pores, causing increased resistance to argon injection. Increases in nitrogen content between steel in the tundish and the mould indicate the extent of reoxidation problems in the tundish nozzle. Increased clogging causes an increase in metal level fluctuations in the mould. These are caused by difficulties with the flow control trying to accommodate changes in the pressure drop needed to maintain a constant flow rate into the mould. Subtle changes in the shape of the flow passage caused by clogging or nozzle erosion cause considerable changes in the pressure drop.
There are differences involving the geometry near the slide gate. In each case, turbulent recirculation zones with high gas concentration are found just above and below the slide plate and in its cavity. These recirculation zones and the sharp edges of the slide gate surfaces generate a large pressure drop, need a 52 % gate opening, with no clogging. Slight erosion by the flowing steel can round off the ceramic corners. This lowers the pressure drop and needs the gate gas at STP. Hence, it is not feasible for argon injection to eliminate the vacuum in the nozzle when the tundish bath is deep and the casting speed is high.
Other steps are to be taken to avoid air aspiration, such as choosing nozzle bore diameters according to the steel flow rate in order to avoid linear gate openings near 50 % to 70 % (around 50 % area fraction). Less argon is needed if intermediate casting speeds are avoided so that the gate is either nearly fully open or is less than 50%. To increase gate openings above 70 %, a smaller nozzle bore diameter can be used, but this allows little accommodation for clogging. To decrease gate openings to below 50 %, a larger bore diameter is needed.
Since the nature of steelmaking produces large volumes of liquid containing inclusions, which all channel through a restricted nozzle opening, tundish nozzle clogging is likely to remain a chronic problem of every continuous casting operation. Clogging problems can be solved by first identifying the cause, through analysis of the clog material. Solutions philosophies are based on minimizing inclusions by improved steelmaking practices, optimizing fluid flow and transfer processes, controlling steel alloy additions, slag and refractory compositions, improving nozzle material and design, and avoiding air aspiration.
Air aspiration problems in the tundish nozzle can be detected by monitoring argon back pressure during casting and by checking for nitrogen pickup between the tundish and mould. Clogging and other quality problems are indicated by level fluctuations in the mould, which result from the changes in the nozzle pressure drop and jets exiting the ports. The extent of clogging can be inferred by comparing the measured steel flow rate with the theoretical value for the given geometry, tundish depth, gas flow rate, and percent gate opening. This is not easy since clogging initially increases flow before restricting it. The argon injection rate is to be optimized to prevent air aspiration while fostering good mould flow.
Alternatively, clogging tends to build-up initially in the recirculation regions. Initial clogging of this shape can streamline the flow path and decrease the total pressure drop across the nozzle. Again, the gate is to be closed to accommodate this. Further clogging produces additional streamlining and smaller pressure drops. A comparison of these predictions with the plant measurement suggests that some rounding, initial clogging, or both occurs in practice. Because the changes in flow resistance vary greatly with small changes in the clog shape, the flow control does not always respond appropriately, and the resulting changes in flow rate cause level fluctuations.