The present invention generally relates to the charging process of a shaft furnace, in particular a blast furnace. More specifically, the present invention relates to a method and a system for adjusting the flow rate of charge material from a top hopper into the furnace using a flow control valve.
It is well known that, besides proper burdening of materials, the geometrical distribution of charge material in a blast furnace has a decisive influence on the hot metal production process since it determines among others the gas distribution. In order to achieve a desired distribution profile in view of an optimal process, two basic aspects are of importance. Firstly, material is to be directed to the appropriate geometric locus on the stock-line for achieving a desired pattern, typically a series of closed concentric rings or a spiral. Secondly, the appropriate amount of charge material per unit surface is to be charged over the pattern.
Regarding the first aspect, geometrically well-targeted distribution can be achieved using a top charging installation equipped with a distribution chute that is rotatable about the furnace axis and pivotable about an axis perpendicular to the rotational axis. During the last decades, this type of charging installation commonly referred to as BELL LESS TOP™ has found widespread use throughout the industry among others because it allows directing charge material accurately to any point of the stock-line by appropriate adjustment of the chute rotation and pivoting angles. An early example of such a charging installation is disclosed in U.S. Pat. No. 3,693,812 assigned to PAUL WURTH. In practice, this kind of installation is used to discharge cyclically recurring sequences of charge material batches into the furnace by means of the distribution chute. The distribution chute is typically fed from one or more top hoppers (also called material hoppers) arranged at the furnace top upstream of the chute, which provide intermediate storage for each batch and serve as a furnace gas sluice.
In view of the second aspect, i.e. controlling the amount of material charged per unit surface area, the above-mentioned type of charging installation is commonly equipped with a respective flow control valve (also called material gate) for each top hopper, e.g. according to U.S. Pat. No. 4,074,835. The flow control valve is used for adjusting the flow rate of charge material discharged from the respective hopper into the furnace via the distribution chute to obtain the appropriate amount of charge material per unit surface by means of a variable valve opening.
Flow rate adjustment usually aims at obtaining a diametrically symmetrical and circumferentially uniform weight distribution over the desired pattern, which typically requires a constant flow rate. Another important aim is to synchronize the end of a batch discharge with respect to the end of the pattern described by the distribution chute. Otherwise, the hopper may be emptied before the chute reaches the end of the pattern (“undershoot”) or there may remain material to be discharged after the pattern has been fully described by the chute (“overshoot”).
In a known approach, the flow control valve is initially set to a predetermined “average” position i.e. “average” valve opening corresponding to an average flow rate. In practice, the average flow rate is determined in function of the initial volume of the batch stored in the respective top hopper and the time required by the distribution chute for completely describing the desired pattern. The corresponding valve opening is normally derived from one of a set of pre-determined theoretical valve characteristics for different types of material, especially from curves plotting flow rate against valve opening for different types of material. As discussed e.g. in European patent no. EP 0 204 935 a valve characteristic for a given type of material and a given valve may be obtained by experiment. EP 0 204 935 proposes regulating the flow rate by means of “on-line” feedback control during the discharge of a batch in function of the monitored residual weight or weight change of charge material in the discharging top hopper. In contrast to earlier U.S. Pat. Nos. 4,074,816 and 3,929,240, EP 0 204 935 proposes a method which, starting with a predetermined average valve opening, increases the valve opening in case of insufficient flow rate but does not reduce the valve opening in case of excessive flow rate. EP 0 204 935 also proposes updating data indicating the valve position required to ensure a certain output of a particular type of material, i.e. the valve characteristic for a particular type of material, in the light of results obtained from previous charging.
European patent EP 0 488 318, discloses another method of flow rate regulation by means of real time control of the degree of opening of the flow control valve and also suggests the use of tables that represent the relationship between the degree of opening and the flow rate according to different kinds of material akin to the above-mentioned valve characteristic. EP 0 488 318 proposes a method aiming at obtaining a constant ratio of flow rate to (average) grain diameter during the discharge in view of achieving a more uniform gas flow distribution. Because obtaining accurate valve characteristics for different material types from theoretical formula is difficult, EP 0 488 318 further proposes a statistical correction of the material-type based tables in a least square method using the flow rates actually achieved at a given valve opening during subsequent batch discharges.
Japanese patent application JP 2005 206848 discloses another method of “on-line” feedback control of the valve opening during the time of discharge of a batch. In addition to readjusting the valve opening during a discharge by means of a “dynamic control”, JP 2005 206848 proposes applying two calculations, a “feed forward” correction and a “feed back” correction to a valve opening derived from a standard opening function, which approximates a valve characteristic based on values of flow rate and valve opening stored for different material types. In similar manner, patent application JP 59 229407 proposes a control device that stores relationships of valve opening to discharge time (akin to characteristics) for different material types and applies a correction term to the valve opening derived from the stored relationships. Contrary to EP 0 488 318 however, JP 2005 206848 and JP 59 229407 do not suggest correction of the stored values.
The practice of “on-line” flow regulation according to EP 0 204 935 is currently widespread. Despite its obvious benefits regarding circumferentially uniform weight distribution, this approach leaves room for improvement, among others because it requires a rather complex control system. Moreover, it has been found that known approaches are not sufficiently adaptive and, under certain circumstances, may lead to unsatisfactory results, especially in case of variations in batch properties and in case of batches consisting of a mixture of different charge materials.
The invention provides both a simplified method and simplified system for adjusting the flow rate of charge material, which reliably adapt to a variety of batch properties and variations thereof during the charging procedure.
The present invention relates to a method of adjusting the flow rate of charge material in a charging process of a shaft furnace, in particular of a blast furnace. Such charging process typically involves a cyclic succession of batches of charge material, which form a charging-cycle. As will be understood, a batch thus represents a given quantity or lot of charge material, e.g. one hopper filling or load, to be charged into the furnace in one of the several operations that constitute a charging-cycle. The batches are discharged into the furnace from a top hopper using a flow control valve. The latter valve is associated to the top hopper for controlling the flow rate of charge material. Pre-determined valve characteristics are preferably provided for certain types of material. Each pre-determined characteristic indicates the relation between flow rate and valve setting of the considered flow control valve as pertaining to a certain material type.
The proposed method provides a specific valve characteristic for each batch of charge material respectively as well as for each flow control valve in case of a multiple-hopper charging installation. Each such specific valve characteristic is bijectively associated to a different batch of the charging-cycle. Hence, each of the latter characteristics is specific to a particular batch according to a one-by-one relationship. Each of them thus indicates the relation between flow rate and valve setting of the considered flow control valve for the associated batch. In order to initially obtain such specific characteristics, the specific valve characteristic are preferably initialized to reflect one of the aforesaid pre-determined valve characteristics, which is for instance chosen in accordance with a predominant type of material contained in the associated batch. The method further comprises in relation to discharging a given batch of the charging-cycle from the top hopper:
In other words, a valve characteristic specific to each batch (and each control valve) is provided and corrected as often as required in function of the actual flow rate at which an instance of the batch in question was discharged. These specific valve characteristics are thus caused to match more and more closely the true valve characteristic that applies to the discharge of the batch in question. They thereby adapt automatically to any batch-inherent properties that influence the flow rate (material mixtures, granularity, total weight, humidity, . . . ) during discharge. Using valve settings derived from the progressively corrected specific valve characteristics will thus gradually adjust the flow rate to the desired flow rate setpoint. Moreover, as opposed to known adjustment methods, in which flow rate control for different batches of the same material type in a charging cycle relies on one and the same predetermined valve characteristic for this material type, the proposed method automatically adapts to differences in the top charging parameters of different batches of the same type, for instance to closure of the flow control valve between different chute pivoting positions. As will be appreciated, compared to the known approach of providing only a limited number of characteristics for each different type of material (e.g. agglomerated fines, coke, pellets, or ore) respectively, the presently proposed solution is particularly beneficial when charging one or more batches that comprise a mixture of different material types.
A corresponding system for adjusting the flow rate is proposed in claim 7. In accordance with the invention, the system mainly comprises memory means storing the specific valve characteristics and a suitable programmable computing means (e.g. a computer or PLC) programmed to perform the key aspects of the proposed method as itemized above.
Preferred features of the proposed method and system are defined in dependent claims 2-6 and 8-12 respectively.
A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings in which:
In the embodiment illustrated in
Hereinafter, the flow rate adjustment according to the present invention will be described with reference to
As illustrated in
As illustrated in
As will be understood, instead of pertaining to a certain type of material, each of the valve characteristics “specific VC1” . . . “specific VC4” is specific to one batch i.e. it expresses the aforesaid relationship for the one particular batch to which it is associated. This bijection can be implemented in simple manner by storing a specific valve characteristic as a data item of the respective data record “batch #1” . . . “batch #4” existing for the associated batch in an embodiment as illustrated in
As further seen in
In a step illustrated by arrow 31, relevant data required for process control is derived from a data record e.g. “batch #1” of the temporary data structure 24 as illustrated in
In relation to flow rate adjustment on the basis of a specific valve characteristic and for discharging a given batch, e.g. in accordance with data record “batch #1” as illustrated in
The above step d) is preferably performed by a software module 32 implemented on the computer system that provides the HMI. The above steps a) to c) are preferably implemented on an existing process control system 26 as illustrated in
The module 32 operates in particular on the specific valve characteristic of the given batch to be discharged. To this effect, the specific valve characteristics “specific VC1” . . . “specific VC4” may have any appropriate format in terms of data structure. They may be stored in the form of an ordered e.g. array-type collection of pairs of flow rate values and valve setting values ({dot over (V)}i;αi) representing a discretization that approximates a true characteristic curve. In even simpler form, instead of storing both values of a pair, it may suffice to store a singleton sequence (ordered list) of valve setting values αi (right hand column of tabular representation in
Preferred embodiments of the above steps a) to d) are as follows:
a) Determining the Flow Rate Setpoint
Before discharging a given batch, a flow rate setpoint {dot over (V)}S is calculated, typically by dividing the net weight of the batch by the targeted total batch discharging time, the result multiplied by the average density of this batch (for volumetric flow rates). The net weight is typically determined using suitable hopper weighing equipment, e.g. as disclosed in U.S. Pat. No. 4,071,166 and U.S. Pat. No. 4,074,816. The process control system 26, to which the weighing equipment is connected, inputs the weighing results or the calculated flow rate set point to the module 32 as illustrated by arrow 33. The targeted discharging time corresponds to the time required by the distribution device to complete the desired charging pattern. This time is pre-determined by calculation, e.g. in function of the length of the desired charging pattern and the chute motion speed. Targeted discharging time and average density are included as a data item in the respective record, e.g. “batch #1”, of the temporary data structure 24, and input to the control system 26 according to arrow 31 or to the module 32 according to arrow 35 depending on where step a) is implemented.
b) Deriving the Requested Valve Setting from the Specific Valve Characteristic
For discharging a given batch, the associated specific valve characteristic, e.g. “specific VC1” for “batch #1” in
More specifically, the adjacent flow rate values {dot over (V)}i;{dot over (V)}i+1 in the specific valve characteristic between which the flow rate setpoint {dot over (V)}S is comprised are determined according to inequality:
{dot over (V)}i≦{dot over (V)}S<{dot over (V)}i+1 (1)
and used, in conjunction with their associated valve setting values αi;αi+1 for interpolation of the requested valve setting value α according to equation:
with i determined such that αi≦α<αi+1.
For example, with the values in as illustrated in
Before starting the discharge of the given batch, the module 32 outputs the requested valve setting α determined according to equation (2) to the process control system 26 as illustrated by arrow 37. The process control system 32 in turn outputs the requested valve setting α in form of a suitable signal as manipulation input (valve control setpoint) to the controller 28 to operate the control valve 10 (see arrow 29).
c) Deriving the Actual Average Flow Rate
After the given batch has been discharged, the actual time required for the discharge is known (e.g. by means of the weighing equipment or other suitable sensors such as vibration transmitters) so that, similar to determining the flow rate setpoint, the actual average flow rate at which the given batch was discharged can be determined according to:
with {dot over (V)}real being the actual average flow rate, W being the total net batch weight, e.g. as obtained from the weighing equipment connected to the process control system 26, ρavg being the average batch density (e.g. obtained from the data record according to arrow 35) and treal the time that discharging the given batch actually took. The result {dot over (V)}real is input to the module 32 according to arrow 33 if step c) is implemented on the process control system.
d) Correcting the Specific Valve Characteristic Associated to the Given Batch
After the batch has been completely discharged, the actual average flow rate {dot over (V)}real is compared with the flow rate setpoint {dot over (V)}S. In case of a stipulated deviation (control variance) between them, a correction of the specific valve characteristic is considered necessary in order to gradually minimize such deviation over subsequent discharges of identical batches, e.g. according to data record batch #1. In other words, such correction causes gradual adjustment of the flow rate to the desired setpoint. Such correction is the main function of the module 32 and is preferably carried out as follows:
The difference between the flow rate setpoint and the actual flow rate is calculated according to:
Δ{dot over (V)}={dot over (V)}S−{dot over (V)}real (4)
A stipulated deviation is considered to have occurred in case the absolute value of the resulting difference according to (4) satisfies the inequality:
T1·{dot over (V)}S>|Δ{dot over (V)}|>T2·{dot over (V)}S (5)
with T1 being a maximum tolerance factor used to set the maximum deviation beyond which no correction is performed and T2 being a minimum tolerance factor used to set the minimal deviation required to perform a correction of the specific valve characteristic. In case of a deviation |Δ{dot over (V)}|>T1·{dot over (V)}S an alarm is preferably generated by the HMI to indicate abnormal conditions. Suitable values may be e.g. T1=0.2 and T2=0.02.
Although correcting the flow rate values and maintaining valve setting values (as sampling intervals) is theoretically possible, it is considered preferred to perform correction on the valve setting values while maintaining unchanged flow rate values. Furthermore, for maintaining a consistent characteristic, correction is preferably performed by adjusting each and every of the individual valve setting values αi of the sequence by applying a respective correction term to each valve setting values αi. The respective correction term is preferably determined using a function chosen to increase with the actual deviation Δ{dot over (V)} and to decrease with the difference, preferably with the distance in terms of sequence index, between the valve setting value to be corrected and the valve setting value that approximates or is equal to the requested valve setting value. Accordingly, the magnitude of the correction term will vary in accordance with Δ{dot over (V)} while it will be smaller the more “remote” the setting value to be corrected is from the requested valve setting α as determined e.g. by equation (2). In a preferred embodiment this correction term is determined as follows:
For the requested valve setting α, the corrected valve setting value that would have been required to achieve the requested flow rate setpoint is:
using the notations of equations (2) and (4).
Accordingly, a respective correction term Cn for each of the valve setting values αn respectively is determined as follows:
The respective correction term Cn resulting from equation (8) is then applied to each valve setting of the given specific valve characteristic:
αn′=αn+Cn; n=1 . . . N (10)
where αn′ is the corrected valve setting value, αn is the currently considered (uncorrected) valve setting value in the sequence, {dot over (V)}n is the corresponding average flow rate according to the current (uncorrected) characteristic, i identifies the sequence index such that αi≦α<αi+1, N is the total number of values in the specific valve characteristic (sequence length), n is the sequence index (position in the sequence according to the table of
Correction is preferably limited according to:
with αmin and αmax being the minimum and maximum allowable valve settings respectively. As will be understood, other suitable functions may be used for computing a correction term Cn the magnitude of which increases with an increasing actual deviation Δ{dot over (V)} and decreases with an increasing difference between the valve setting to be corrected αn and the requested valve setting α.
In a further step, the module 32 preferably ensures that the sequence of valve setting values is strictly monotonically increasing, e.g. by running a program code sequence as follows (in pseudo-code):
whereby any valve setting value that is less than or equal to the valve setting value that precedes in sequence is incremented until a strict monotonically increase is reached so as to ensure a positive slope of the characteristic curve.
After completion of the computations, the module 32 corrects each of the valve setting values of the specific valve characteristic under consideration by replacing αn with αn′ for n=1 . . . N.
An exemplary program sequence in pseudo-code for performing the above correction calculations is as follows:
After a correction has been made, the module 32 returns the resulting corrected specific valve characteristic as illustrated by arrow 39 in
Although the above description refers to a single specific valve characteristic per batch, it will be understood that, in case of a multiple-hopper installation, a dedicated specific valve characteristic for each flow control valve is stored for each batch respectively and corrected when the respective flow control valve is used. Equivalently, identical material lots, i.e. having identical desired weight, material composition and arrangement as provided from the automated stockhouse, are considered to be different batches whenever they are stored in different hoppers of a multiple-hopper installation.
Although the proposed mode of adjusting the flow rate may be used in combination with other control procedures, in particular with a subsequent flow control that requires accurate valve characteristics, significantly reduced flow rate deviations can be achieved even when using a constant valve opening that is fixed during the entire discharge of a given batch (i.e. no “on-line” feedback control).
Gradually adjusting the flow rate as proposed, i.e. in a manner specific to each batch of a charging-cycle respectively, automatically takes into account recurring properties of the respective batch that have a secondary influence on the flow rate obtained for a given valve setting. Such properties are granulometry, initial batch weight and humidity and, in particular, material mixtures. In fact, as opposed to the conventional approach of using material-type-based characteristics, the proposed approach adapts to mixtures of plural material types within the same batch at any varying proportion without necessitating measurements for establishing a corresponding pre-determined curve.
Number | Date | Country | Kind |
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91525 | Feb 2009 | LU | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2010/051748 | 2/11/2010 | WO | 00 | 8/11/2011 |
Publishing Document | Publishing Date | Country | Kind |
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WO2010/092132 | 8/19/2010 | WO | A |
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Entry |
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International Search Report PCT/EP2010/051748; Dated Jun. 16, 2010. |
Number | Date | Country | |
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20110311926 A1 | Dec 2011 | US |