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”).
Japanese patent applications JP 04 198412, JP 56 047506 and JP 59 229407 propose methods that aim at avoiding undershoot or overshoot. In each of these methods, the valve opening of the flow control valve is fixed during the discharge of a given batch but readjusted for a subsequent discharge in case overshoot or undershoot has occurred. As an alternative to readjusting the valve opening, JP 56 047506 also suggests varying the rotational sped of the distribution chute while maintaining an unchanged valve opening. As will be understood, whilst addressing the problem of undershoot or overshoot, the methods proposed in JP 04 198412, JP 56 047506 and JP 59 229407 do not warrant a constant flow rate required for circumferentially uniform weight distribution over the desired pattern. In fact, with a valve opening that remains constant during the discharge of a given batch, the flow rate inevitably varies during the discharge among others because of the decreasing residual mass that remains in the hopper.
In other known approaches, the valve opening is therefore varied during the time of discharge of a given batch. In a typical approach of this kind, 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.
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. According to JP 2005 206848, the valve opening is readjusted by means of “dynamic control”, which uses integral and proportional control action, in discrete steps or intervals. Each interval corresponds to a full revolution of the rotating distribution chute during the discharge. This on-line “dynamic control” readjusts the valve opening for a subsequent interval during the discharge in function of residual weight to be discharged and remaining discharge time. In addition, JP 2005 206848 proposes applying two calculations, a “feed forward” correction and a “feed back” correction, to determine more accurately the required initial valve opening for the first discharge interval i.e. the first chute revolution.
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.
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. For instance, it is deemed not sufficiently adaptive to a wider variety of batch properties, e.g. to batches consisting of a mixture of different charge materials, or to a wider variety of operating conditions of the top charging installation. Moreover, known approaches of “on-line” feedback control, e.g. according to EP 0 204 935 or JP 2005 206848, require accurate selection and tuning of the control parameters to achieve good results.
The invention provides both a simplified method and simplified system for adjusting the flow rate of charge material in shaft furnace charging.
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 and are discharged into the furnace from a top hopper using a flow control valve. 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.
According to the proposed method, a respective set of plural valve settings is stored for each batch. As will be understood, plural settings in the present context means more than one setting and typically multiple settings. Each valve setting of a set is associated to a different stage of the discharge of the respective batch for which the set is stored. Preferably, each batch discharge process is divided into subsequent stages or periods so that each stage corresponds to different operating status of a distribution device used for distributing the discharged batch. In particular, each stage preferably corresponds to a different pivoting position of a distribution chute of the distribution device.
According to the proposed method, a given batch of a charging-cycle is discharged with the flow control valve being set for each stage in accordance with the valve setting associated to the stage in question. Hence, the valve opening remains constant during each stage of the discharge respectively while it can change from stage to stage. Furthermore, at each different stage an actual average flow rate at which charge material is discharged is determined.
According to the proposed method, a main aspect of adjusting the flow rate lies in correcting each of the plural valve settings used for operating the flow control valve. More specifically, each valve setting for a given batch is corrected in offline manner, e.g. immediately after the given stage of a discharge is completed or after the batch is completely discharged or even just before a subsequent discharge of the given batch. For each valve setting, correction is made in function of the actual average flow rate determined for the stage to which the valve setting is associated.
It will be appreciated that flow rate adjustment is simplified and rendered more robust by virtue of the “offline” nature of the valve setting correction according to the invention. Among others, the need for selecting and fine-tuning control parameters, as required with prior art “on-line” feedback control methods, is eliminated. The proposed method is not subject to instability and unsatisfactory results due to improper control parameters or changes in the batch properties. Furthermore, while “on-line” regulation according to the principles of EP 0 204 935 or JP 2005 206848 involves the need for properly determining the initial valve opening for starting a discharge, this need is eliminated by the proposed method. In addition, the proposed approach of flow rate adjustment adapts automatically to changes in the operating conditions of the top charging installation from stage to stage during a discharge, e.g. closure of the flow control valve, and also in between batches.
In accordance with the invention, the system mainly comprises memory means storing the respective set of plural valve settings for each batch and a suitable programmable computing means (e.g. a computer or PLC) programmed to perform the key steps of the proposed method as summarized above.
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
This section describes, 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 obtaining and correcting a batch-specific valve characteristics for 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
In the valve characteristic correction mode, 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:
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 setpoint 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).
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, ρang being the average batch density (e.g. obtained from the data record according to arrow 35) and treal being 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.
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. In the valve characteristic correction mode, such correction is the main function of the module 32 and 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:
T
1
·{dot over (V)}
S
>|Δ{dot over (V)}|>T
2
·{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 valve characteristic correction mode as described above 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 preferably considered to be different batches whenever they are stored in different hoppers of a multiple-hopper installation.
According to the preferred embodiment, the valve characteristic correction mode described above is initially executed during several charging cycles to provide reliable specific valve characteristics. Afterwards, these characteristics are used in adjusting the flow rate according to a subsequent second mode of operation which will be detailed below. Other approaches of obtaining valve characteristics for use in the second mode of operation, e.g. using predetermined valve characteristics without correction, are also within the scope of the invention.
This section describes, with reference to
As illustrated by means of a tabular representation of a data structure 42 in
As will be understood, the discharge process of any given batch can be subdivided into different successive stages, each corresponding to a different operating status of the distribution device that controls the distribution of charge material during discharge according to a desired distribution pattern. In particular and most preferably, each stage corresponds to a different pivoting i.e. tilting position of a distribution chute of the charging device. Alternatively, the discharge process may be subdivided into successive stages that correspond to a full revolution of the distribution chute respectively or any other suitable parameter related to the desired discharge pattern. The different stages for which a set, e.g. “VAD set 1”, includes an associated valve setting can be determined using the top charging parameters (column “BLT”) provided in the data structure 24 for the respective batch.
A set “VAD set 1” . . . “VAD set 4” hence represents a temporal sequence of variable valve settings to be used in succession during a discharge of a given batch for operating flow control valve 10 in synchronization with the operating states of the distribution device. Even though illustrated by a separate data structure 42, the sets “VAD set 1” . . . “VAD set 4” may be stored in any suitable form, separately or as part of another data structure, e.g. data structure 24, in a data memory e.g. in non-retentive memory of a PC type computer system implementing the HMI of a PLC of the process control system 26.
In case of a flow control valve 10 of the type as illustrated in
For the discharge of a given batch in VAD mode, the control system 26 uses the respective set of valve settings “VAD set 1” . . . “VAD set 4” to operate the flow control valve 10, as illustrated by arrow 29 in
In fact, in VAD mode, the control system 26 determines the actual average flow rate at which charge material is discharged during each stage respectively, e.g. using the hopper weighing equipment connected to the control system 26, for a subsequent offline correction of the associated valve settings as will be set out below.
Data processing in VAD mode includes the following main aspects:
According to the embodiment illustrated in
Step i) is performed typically prior to discharge of a given batch, and necessary only initially or in case the flow rate setpoint changes. Step ii) is performed typically after discharge of the given batch. Preferred embodiments of the above steps i) to ii) are as follows:
In VAD mode, before discharging a given batch of the charging cycle, the following data is provided, e.g. to the module 32:
Prior to the first discharge of a given batch in VAD mode, its respective set of valve settings (e.g. “VAD set 1”) is initialized. To this effect, a valve setting is defined for each stage in the discharge, e.g. for each used pivoting position as derived from data structure 24. These valve settings are then all initialized to the valve setting that corresponds the requested flow rate setpoint {dot over (V)}SP in accordance with the valve characteristic (e.g. “specific VC 1”) specific to the given batch, preferably obtained according to the according valve characteristic correction mode described hereinabove.
At subsequent discharges in VAD mode, any significant change of the currently requested flow rate setpoint {dot over (V)}SP with respect to the previous flow rate setpoint {dot over (V)}LAST preferably triggers an update of each of the valve settings of the set stored for the given batch. To this effect, the absolute difference between the previous flow rate setpoint and the setpoint for the next discharge is calculated and compared to a predetermined variation tolerance according to:
|{dot over (V)}LAST−{dot over (V)}SP|>T3 (12)
where T3 is a typically user-defined variation tolerance, that is predetermined e.g. using the HMI.
If inequality (12) is satisfied, an updated value for each valve setting of the set stored for the given batch is calculated as follows:
where
The value of the updating term Δαt is determined to correspond to the flow rate variation Δ{dot over (V)} based on the specific valve characteristic, which is associated to the given batch to be discharged. As illustrated in
The valve characteristic is used to determine a first flow rate {dot over (V)}1;t that corresponds to the previous stored flow rate setpoint α0;t by linear interpolation according to:
where i identifies the sequence index of the valve characteristic such that αi≦α0;t≦αi+1 as illustrated in
A second flow rate {dot over (V)}2;t is then determined as the sum of the first flow rate {dot over (V)}1;t and the setpoint variation Δ{dot over (V)} according to:
{dot over (V)}
2;t
={dot over (V)}
1;t
+Δ{dot over (V)} (15)
where the setpoint variation Δ{dot over (V)} may be positive or negative,
A second valve setting α2;t that corresponds to this second flow rate {dot over (V)}2;t is then also determined by linear interpolation, according to:
where j identifies the sequence index of the valve characteristic such that {dot over (V)}j≦{dot over (V)}2;j<{dot over (V)}j+1, as illustrated in
The updating term Δαt for the valve setting in question is then determined using the second valve setting α2;t according to
In other words, considering equations (13) and (17), the valve setting αt is updated to be equal to the second valve setting α2;t. As will be appreciated, updating the valve setting in function of the requested flow rate setpoint is thus preferably implemented by modifying the previous opening angle α0;t for each stage by the local variation Δαt that corresponds to the flow rate setpoint variation of Δ{dot over (V)} according to the specific valve characteristic see
Updated valve settings are preferably limited according to:
with αmin and αmax being the minimum and maximum allowable valve settings respectively, and further preferably according to:
with
Initializing and updating the valve settings for each stage as set out above is a preferred but auxiliary aspect of adjusting the flow rate according to the invention, since it may not be required in case an invariable flow rate setpoint is associated to each batch of the charging cycle. The key aspect of adjusting the flow rate according to the invention corresponds to the above step ii) of correcting the valve settings, which is preferably executed as follows:
In VAD mode, each of the valve settings of the set stored for a given batch is corrected in offline manner respectively. The correction of a valve setting depends mainly on an actual average flow rate determined for the associated stage. A preferred mode of correction is implemented as follows:
For correcting the valve settings after discharge, the following data is provided:
The actual average flow rate {dot over (V)}Act;t for each stage t is determined after the given batch has been completely discharged, or after a given stage of the discharge is completed. These actual average flow rates are determined in any suitable manner, e.g. analogous to the flow rate calculation described hereinabove for step b), i.e. by the process control system 26 using connected weighing equipment.
Using the determined actual average flow rates {dot over (V)}Act;t, a flow rate deviation (flow rate error) is determined for each stage t respectively according to:
Δ{dot over (V)}
t
={dot over (V)}
SP
−{dot over (V)}
Act;t (20)
A correction for any given valve setting is performed in case the absolute value of the flow rate deviation Δ{dot over (V)}t for the associated stage exceeds a predetermined deviation tolerance according to inequality:
|Δ{dot over (V)}t|>T4 (21)
where T4 is a typically user-defined deviation tolerance, that is predetermined e.g. using the HMI.
In order to adjust the flow rate during a subsequent discharge of the given batch to the requested flow rate, each valve setting for which inequality (21) holds is corrected offline according to:
where αt′ is the corrected valve setting, αt is the currently stored uncorrected valve setting associated to stage t, and Δαt is a correction term to be determined for each stage t respectively, and K2 is a typically user-defined predetermined constant to prevent overcorrection, with K2 preferably such that K2≧2.
The correction term Δαt for each stage t is preferably determined using linear interpolation on the valve characteristic specific to the given bath in similar manner to the updating term as described above. However, the value of flow rate deviation Δ{dot over (V)}t will normally be different for each stage t. By reference to
The valve characteristic is used to determine a first flow rate {dot over (V)}1;t that corresponds to the stored uncorrected flow rate setpoint αt by linear interpolation according to:
where i identifies the sequence index of the valve characteristic such that αi≦αt<αi+1 as illustrated in
A second flow rate {dot over (V)}2;t is then determined as the sum of the first flow rate {dot over (V)}1;t and the flow rate deviation Δ{dot over (V)}t for the associated stage t according to:
{dot over (V)}
2;t
={dot over (V)}
1;t
+Δ{dot over (V)}
t (24)
where the setpoint variation Δ{dot over (V)}t may be positive or negative.
A second valve setting α2;t that corresponds to this second flow rate {dot over (V)}2;t is then also determined by linear interpolation, according to:
where j identifies the sequence index of the valve characteristic such that {dot over (V)}j≦{dot over (V)}2;j<{dot over (V)}j+1 and αj≦α2;t<αj+1, as illustrated in
The offline correction term Δαt for the valve setting in question i.e. for stage t is then determined using the second valve setting α2;t of (16) according to
For each stage t at which a significant flow rate deviation occurred, i.e. for which inequality (21) is satisfied, the associated uncorrected valve setting αt is then corrected by applying the corresponding correction term Δαt according to equation (22).
Similar to equations (18) and (19), correction of the valve settings is preferably so that each corrected valve setting αt′ is limited according to:
with αmin and αmax being the minimum and maximum allowable valve settings respectively, and further preferably according to:
An exemplary program sequence in pseudo-code for performing correction as set out above in section ii) is as follows:
FUNCTION MGAngleLimits (α,
Similarly, an exemplary program sequence in pseudo-code for performing updating as set out above in section i) is as follows:
Although the VAD mode as described above refers to a single set of valve settings per batch, it will be understood that, in case of a multiple-hopper installation, an independent set of valve settings for each flow control valve is stored for each batch respectively.
In summary, adjusting the flow rate according to the above VAD mode varies the valve opening during discharging of a batch without the need for online feedback control. After the batch has been discharged, the actual average flow rate per stage, e.g. per used chute position, is compared with the initially requested flow rate set point. After each discharge, the valve aperture for each stage is gradually corrected, if required, in order to reach the desired flow rate set point for each stage. For each stage during the discharge, the material gate aperture is constant but can vary from stage to stage, e.g. in accordance with different chute positions. In order to provide ideal correction results in VAD mode, several initial batch discharges are preferably carried out in valve characteristic correction mode as described hereinabove.
Number | Date | Country | Kind |
---|---|---|---|
91526 | Feb 2009 | LU | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/EP10/51733 | 2/11/2010 | WO | 00 | 8/10/2011 |