Patent Publication Number: US-2023149996-A1

Title: Detection Of Faulty Cooling Units Configured To Provide Coolant To Rolling Mills

Description:
TECHNICAL FIELD 
     The present invention relates to method for detecting a faulty cooling unit in a set of cooling units configured to provide a coolant to work rolls arranged to process a work item therebetween. The invention further relates to a corresponding control unit, to a system, and to a rolling mill. 
     BACKGROUND 
     Metal rolling generally relates to producing a metal work piece with reduced and uniform thickness by rolling the metal work piece between two work rolls. In cold rolling the metal work piece is processed at relatively low temperatures, below the crystallization temperature of the metal. Thermal control in cold-rolling mills is important because the temperature difference across the transverse of the work roll affects the flatness of the produced product. 
     Thermal control may be provided by both heating and cooling systems. Cooling system often include nozzles arranged to spray a cooling fluid onto the work rolls. A malfunctioning nozzle may affect the flatness and thus the quality of the final product. 
     Traditionally, offline cooling test are performed in order to inspect the cooling system. For this, the nozzles are controlled to spray a cooling liquid on the work rolls, whereby visual inspection of the spray pattern is performed. However, this is both time consuming and requires manual labor. 
     Accordingly, there is room for improvement with regards to evaluating cooling system operation in rolling mills. 
     SUMMARY 
     In view of the above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide a method for on-line diagnostics of cooling unit operation in rolling mills. 
     According to a first aspect of the invention, there is provided a method for detecting a faulty cooling unit in a set of cooling units configured to provide a coolant to work rolls arranged to process a work item therebetween, the method comprising: varying the flow rate of the coolant ejected from at least one cooling unit; in response to varying the flow rate, determining a flatness variation value of the work item for at least the at least one cooling unit, the flatness variation value being indicative of the work item flatness variation downstream of the work rolls; and detecting a faulty cooling unit based on comparing the flatness variation values to a reference flatness variation value. 
     The present invention is at least partly based on the realization that by varying the flow rates of the cooling units, the flatness of the work item will be altered, and that the flatness alteration will be different for a fully functional cooling unit compared to a faulty cooling unit. Thus, the variation in flow rate is used as an excitation on the work rolls and the measured flatness variation is a response of the excitation. Accordingly, a flatness response as caused by the varied flow rate is evaluated for cooling unit diagnostics. 
     To this end, a flatness variation value is determined. A flatness value is indicative of the flatness of the work item downstream of a cooling unit. The flatness value may be derived from a transversal strain profile of the work item calculated from a measured work item tension profile. Thus, the flatness value may be indicative of a measured stress in the work item. From this transversal strain profile, separate portions may be associated with respective cooling units by spatial correlation. Based on each of the portions may a respective flatness variation value be determined, one for each cooling unit. A flatness variation value may be the difference between two flatness values. Techniques for flatness measurements are known per se including techniques based measuring work item tension profiles. 
     A flatness variation value indicates the variation in flatness that is caused by varying the flow rate from one level to another level. In other words, if the flatness of the work item is continuously measured it is possible to detect the effect that the variation in flow rate has on the flatness of the work item. Thus, a variation in flatness value in response to varying the flow rate of coolant applied onto the work rolls when the work item is being fed in between the work rolls. 
     A cooling unit may include various components for spraying a coolant onto the work rolls. A cooling unit may comprise a nozzle and a valve, where the valve opens and closes a supply of coolant to the nozzle. Pressurized coolant flows through the open valve to the nozzle from where the coolant is sprayed onto the work rolls. A fault may occur anywhere in the cooling unit, but most commonly faults occur in the valve or in the nozzle. 
     A fault in a cooling unit may be caused by many conceivable conditions and may be of different nature. For example, a fault may be a changed spray angle from a nozzle, the width of the spray has changed, a fully or partly blocked nozzle, or a malfunctioning valve, etc. 
     A variation in flow rate means that the flow rate is varied from one flow rate level to another flow rate level. 
     A flatness variation value may be the absolute value of the difference between a first flatness value and a second flatness variation value of the work item determined for an associated cooling unit. 
     Evaluation of the flatness variation values in view of the reference flatness variation value may be performed in various ways. For example, when the flatness variation value for one of the cooling units deviates by more than a threshold value from the reference flatness variation value for the same flow rate variation, an indication may be provided that the respective cooling unit is faulty. A deviation may be the absolute value of the difference between the flatness variation value and the reference flatness variation value. 
     In one embodiment, the reference flatness variation value may be based on the flatness variation value(s) determined in response to varying the flow rate of at least another one of the cooling units. Accordingly, the evaluation of a faulty cooling unit may be based on comparing flatness variation values to each other. Further, if this is performed for a set of cooling units, a cooling unit with abnormal flatness response, e.g. a flatness variation value that deviates from at least some of the flatness values of the other cooling units, or preferably the median the flatness variation values for all or some of the other cooling units, by more than a threshold value, may be concluded to be a faulty cooling unit. This advantageously provides for improved automation of the faulty cooling unit detection, that at least does not require extensive prior reference measurements. 
     In embodiments, the reference flatness variation value is based on a statistical value determined based on a set of flatness variation values determined in response to varying the flow rates of a plurality of the set of the cooling units. Preferably, the reference flatness variation value may be the median of the flatness variation values for all or some of the cooling units in the set of cooling units. This provides for a more accurate determination of whether a cooling unit is faulty. 
     In some embodiments, the flow rates for a sub-set of cooling units are maintained constant. For example, each cooling unit in the sub-set of cooling units has closest neighboring cooling units which flow rates are maintained substantially constant when the flow rates of the at least one cooling unit is/are varied. This advantageously provides for detecting deviating spray angles or sprays widths for the cooling units. Thus, it is possible to determine if a cooling unit is spraying coolant on a neighboring zone of the work roll where it shouldn’t be spraying coolant. That the flow rates are substantially constant should be interpreted broadly that a small variation in the flow rates may be allowed but that such variation should be small enough to not significantly affect the flatness variation measurement. 
     In embodiments, the at least one cooling unit may comprise cooling units that are interspaced with cooling units which flowrates’ are maintained substantially constant when the flow rates of the at least one cooling unit are varied. In possible implementations, the flowrates of e.g. every second, every third, every forth, or every fifth cooling unit is varied. 
     In some embodiments, when the flow rates of at least one cooling unit is increased, the flow rates of at least one other cooling unit is decreased. This advantageously provides for maintaining the total flow of coolant constant. 
     In embodiments, a time duration between consecutive flow rate variations may be longer than a predetermined time duration. The predetermined time duration is advantageously long enough for the cooling effect on the work roll caused by the varied flow rates is detectable and at least partly settled in the flatness measurements. Thus, the cooling effect on the flatness of the work item may be somewhat delayed and once the effect occurs the flatness of the work item drifts for a time. Therefore, the flow rate is maintained constant for at least the predetermined time duration after the flow rate has been varied from one level to another level. This allows for most of the flatness change to have occurred before the flatness is measured. 
     Further, the time durations between flow rate variations, are preferably equal for each consecutive flow rate variations. In other words, consecutive time durations are equal. Further, the time duration is equal for each of the cooling units. 
     The flatness may be measured in various conceivable ways and with various devices. One such device is generally known as a shape meter which measures the transverse shape of the work item and based on this an indication of the flatness of the work item may be derived. Further, a so-called stressometer may be used for performing flatness measurements. As is known to the skilled person, a stressometer measures the transverse distribution of stress e.g. in terms of N/mm 2 , in the work item, and based on this an indication of the flatness of the work item may be derived. 
     In some embodiments, the flatness variation values may be determined for cooling units for which the flow rates are maintained substantially constant. The allows for detecting fault related to a cooling unit spraying when it shouldn’t, or that neighboring cooling units are spraying in the wrong zones not associated with the cooling unit. 
     The work item may preferably be a metal strip. 
     According to a second aspect of the invention, there is provided a control unit configured to detect a faulty cooling unit in a set of cooling units configured to provide a coolant to work rolls arranged to process a work item therebetween, the control unit is configured to: in response to flow rates being varied of the coolant ejected from at least one of the cooling units, acquire flatness data indicating a flatness variation value of the work item for each of the at least one of the cooling units, the flatness variation value being indicative of the work item flatness variation downstream of the work rolls; and detect a faulty cooling unit based on comparing the flatness variation values to a reference flatness variation value. 
     The control unit may be configured to: control a valve for varying the flow rates of coolant provided to a nozzle of the at least one cooling unit. Thus, the control unit may be communicatively connected to control circuitry or a control mechanism of the valve such that the control unit may send control signals to the valve to cause the flow rates to vary. 
     A magnitude of the flow rate variation may for example depend on the material of the metal strip and on a temperature difference between the coolant and the work roll. The flow rate variation should be large enough such that a sufficiently large flatness variation is caused and that is detectable. As an example, the flow rate variation may be between 0% and 100% flow rate, or between 10% and 100%, or between 10% and 90% flow rate, between 20% and 80% flow rate, between 30% and 70% flow rate. However, other possible flow rate variations are conceivable as long as a flatness response is detectable. 
     Further effects and features of the second aspect of the invention are largely analogous to those described above in connection with the first aspect of the invention. 
     According to a third aspect of the invention, there is provided a system for detecting a faulty cooling unit in a set of cooling units configured to provide a coolant to work rolls arranged to process a work item therebetween, the system comprising: a sensing arrangement for measuring a flatness value of the work item downstream of the work rolls, and a control unit configured to: receive flatness data from the sensing arrangement indicative of the flatness measured in response to the varied flow rates, determine a flatness variation value of the work item for each of the at least one cooling unit based on the flatness data, the flatness variation value being indicative of the work item flatness variation downstream of the work rolls; and compare the flatness variation values to a reference flatness variation value, for detecting a faulty cooling unit. 
     Further effects and features of the third aspect of the invention are largely analogous to those described above in connection with the first aspect and the second aspect of the invention. 
     There is further provided a rolling mill comprising at least two working rolls configured to process a work item therebetween, and a system according to embodiments of the third aspect of the invention. 
     The rolling mill may be a cold rolling mill. 
     Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein: 
         FIG.  1 A  conceptually illustrates work rolls processing a work item, and spray patterns of coolant on the work rolls according to an embodiment of the invention; 
         FIG.  1 B  is a conceptual view of a set of cooling units applying coolant to a work roll; 
         FIG.  1 C  is a side view of the work rolls in  FIG.  1 A , and a conceptual illustration of a system according to an embodiment of the invention; 
         FIG.  1 D  is a diagram conceptually exemplifying a flatness measurement; 
         FIG.  1 E  is a diagram conceptually exemplifying absolute flatness variation values; 
         FIG.  2    is a flow-chart of method steps according to embodiments of the invention; and 
         FIG.  3    is a graph illustrating oppositely varying cooling unit flow rates according to embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the present detailed description, various embodiments of the present invention are herein described with reference to specific implementations. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. While specific exemplary embodiments are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations can be used without parting from the scope of the invention. 
       FIG.  1 A  conceptually illustrates work rolls  100   a  and  100   b  arranged to process a work item  102  therebetween. The work rolls  100   a  and  100   b  may be comprised in a rolling mill process plant  500 , such as a cold rolling mill, configured to process work items in the form of a metal strip  102 . 
     During processing of the metal strips, as is known in the art, the flatness of the final product depends to some degree on the temperature variations across the width of the work rolls  100   a  and  100   b . It is therefore of importance to have efficient thermal control of the work rolls  100   a - b . Cooling of the work rolls  100   a - b  is provided by applying a coolant to each the work rolls  100   a - b  through cooling units arranged near the work rolls  100   a - b . In  FIG.  1 A , the spray patterns or zones  104  of individual cooling units are conceptually illustrated. Note that not all the spray patterns or zones are numbered. 
       FIG.  1 B  conceptually illustrates a top view of a set of cooling units  106   a - n  that are arranged to provide a coolant  101  to the work roll  100   a . The flow rates from the cooling units are individually controllable for varying the flow rate of coolant provided from each of the cooling units  106   a - n  to the work roll  100   a . 
       FIG.  1 B  conceptually illustrates a flatness profile  105  representing the flatness, e.g. a conceptual tension profile” of the work item  102  downstream of the work rolls  100   a - b . A set of flatness variation values Va-Vn, may be determined from the flatness profile  105 . Here, the flatness variation value Vf is illustrated as a difference between a flatness value of the flatness profile  105  and a flatness value from a prior determined flatness reference profile  107 . Thus, the flatness profiles  107  and  105  are the profiles across the width of the work item  102  at two difference time instances, before the flow rate is varied or at least before the effect of the varied flow rate is detected, flatness profile  107  and in response to the varied flowrate, flatness profile  105 . The flatness profile of the work item  102  may be continuously monitored, whereby a flatness variation may be determined from comparing flatness profiles to each other. Note that the “flatness profile” may refer to a transversal strain profile that is indicative of a flatness profile of the work item. 
       FIG.  1 C  is a side view of the work rolls  100   a - b  processing the work item  102 . The work item  102  is fed in between the rotating work rolls  100   a - b , whereby the separation between the work rolls  100   a - b  provides for reducing the thickness and flattening of the work item  102  between the work rolls  100   a - b . Cooling units  106   a  and  108  are arranged to provide a spray of coolant  101  to a respective work roll  100   a , and  100   b . Each cooling unit comprises a valve  302  and a nozzle  308 . It is understood that the components shown in  FIG.  1 C  are not to scale, but is a conceptual drawing shown for understanding of embodiments presented herein. 
     As mentioned above, the coolant is used for thermal control of the work rolls  100   a - b . The temperature variation across the transverse of the work rolls  100   a - b  will affect the flatness of the work item downstream of the work rolls  100   a - b , e.g. at the location of the sensing arrangement  202  configured to measure the flatness of the work item  102 . Such as sensing arrangement may be provided as a shape meter, for example a so-called “stressometer”, which may be configured to measure pressure changes against a roll caused by deviating flatness of the work item. Shape meters are generally known per se. 
       FIG.  1 D  conceptually illustrates a flatness measurement of a work item. The measurement provides a set of flatness values, each represented by a bar in the diagram  600  where one bar is numbered  602 . A flatness variation value is the variation in height of a bar in response to varying the flow rate of coolant. For example, in response to varying the flow rate from one level to another level from one cooling unit, the height of bar  602 , i.e. the flatness value represented by the bar  602 , may change to a second flatness value represented by the second bar  604  show as a dashed line. The difference between the two flatness values is the flatness variation value V. 
       FIG.  1 E  is a diagram  700  of measured absolute flatness variation values, of which one is denoted  702 , for flow rate variation in each cooling unit versus position along width of the work item. The diagram  700  illustrates a reference flatness variation value, which is preferred embodiments is the median of the absolute flatness variation values. The diagram  700  further conceptually illustrates two different threshold levels that are applicable for embodiments described herein. An upper threshold and a lower threshold. Thus, if an absolute flatness variation value exceeds the upper threshold, or if an absolute flatness variation value is below the lower threshold, the corresponding cooling unit may be concluded to be faulty. 
       FIG.  1 E  exemplifies a deviation D between an absolute flatness variation value  702  and the reference flatness variation value. If this deviation D is larger than a threshold value given by the difference between the reference flatness variation value and e.g. the upper threshold level, the corresponding cooling unit may be concluded to be faulty. Note that the difference between the reference flatness variation value and the upper threshold may be different from the difference between the reference flatness variation value and the lower threshold. Thus, there may be an upper and a lower threshold value, used depending on if the absolute flatness variation value is larger or smaller than the reference flatness variation value. 
       FIG.  2    is a flow-chart of method steps according to embodiments of the invention. The method steps in  FIG.  2    will be described in conjunction with  FIGS.   1 A-D . 
     The method is for detecting a faulty cooling unit in a set of cooling units  106  configured to provide a coolant to work rolls  100   a - b  arranged to process a work item  102  therebetween. 
     In step S 102  of the method, varying the flow rates of the coolant ejected from at least one  106   f  of the cooling units  106   a - n . In response to varying the flow rate(s), determining, in step S 104 , a flatness variation value Vf, see also V in  FIG.  1 D , of the work item  102  for each of the at least one cooling units. The flatness variation value is indicative of the work item  102  flatness variation downstream of the work rolls  100   a - b . The flatness is determined by a sensing arrangement  202  downstream of the work rolls  100   a - b . 
     In step S 106 , detecting a faulty cooling unit based on comparing the flatness variation value(s) Vf to a reference flatness variation value. 
     In one possible implementation, when the flatness variation value for one of the cooling units deviates by more than a threshold value from the reference flatness variation value, providing an indication the respective cooling unit is faulty. 
     For example, if the flatness variation value Vf exceeds the reference flatness variation value more than acceptable, i.e. more than a threshold value, it may be concluded that the cooling unit  106   f  is a faulty cooling unit. 
     As mentioned above in relation to  FIG.  1 B , the flatness variation value reflects a variation of the flatness of the work item. The flatness variation value is the difference between flatness values determined at different times as the work item is being fed in between the work item. Thus, the flatness values being used for determining the flatness variation value reflects the flatness at two different times and therefore locations on the work item. At least one of the flatness values may be determined in response to the varying flow rate while the work item is processed. The difference between the flatness values may be the absolute value of the difference. In other words, the flatness variation value determined for a cooling unit is based on the evolution of the flatness in response to varying the flow rate. Turning to  FIG.   1 C  and  FIG.  1 D , as the work item is fed through the work rolls  100   a - b , the sensing arrangement  202  continuously measures the flatness of the work item  102 . Thus, a flatness value  602  may be obtained before the flow rate is varied from one level to another level or at least before the effect of the varied flow rate occurs in the work item, and this flatness value may be compared to a flatness value  604  obtained after the flow rate is varied at a time when the cooling effect on the flatness has occurred, see also discussion with reference to  FIG.  3   . The difference between the flatness values reflects the flatness response as caused by the flow rate variation. 
     The reference flatness variation value may be based on the flatness variation value determined in response to varying the flow rate of at least another one of the cooling units. In other possible implementations, the reference flatness variation value may be based on statistical value determined based on a set of flatness variation values determined in response to varying the flow rates of a plurality of the set of the cooling units. Preferably, the reference flatness variation value may be a median of the flatness variation values for the cooling units  106   a - n . With further reference to  FIG.  1 D , a flatness variation value V is determined for each of the flatness values, e.g. each bar position shown in the diagram. A flatness variation value is determined for each of the cooling units, in other words, each bar  602  in the diagram  600  may be associated with a respective cooling unit. Subsequently, the median of all the flatness variation values is determined and used as a reference flatness variation value. If a flatness variation values deviates too much, i.e. more than the threshold value, from the median of the flatness variation values, then the respective cooling unit is concluded to be faulty. Other statistical values may be applied such as, standard deviations e.g. determining if a flatness variation value is within a predetermined number of standard deviations to the reference flatness value, percentiles, etc. 
     For example, now turning to  FIG.  1 B , if the median of the flatness variation values Va-Vn, or the median of the flatness variation values for at least a sub-set of the cooling units, e.g. the flatness variation values Vb-Vm, then the flatness variation value Vf will deviate significantly from the median of the flatness variation values Va-Vn, and if only a sub-set is considered, the median of the subset Vb-Vm. In other words, the flatness variation value Vf may be considered abnormal in relation to the other flatness variation values, whereby the cooling unit  106   f  may be concluded to be faulty. Preferably, the median of all the flatness variation values is used, however, subsets of flatness variation values may be conceivable to use. In case of a subset, the end values for the edges of the work item, e.g. Va and Vn may be considered a subset. 
     Generally, a time duration between consecutive flow rate variations is longer than a predetermined time duration. For example, a first flow rate variation is performed to a first flow rate level which is maintained for a time duration being equal to the predetermined time duration. In this way, the effect on the flatness caused by the varied flow rate can occur and be detectable in the flatness measurement. After the measurement, the flow rate may be varied to a second flow rate level, and so on. 
       FIG.  3    is a graph schematically describing a flow rate variation scheme according to embodiments of the invention. The y-axis represents the flow rate, and the x-axis represents time. In the graph, the dashed line represents the coolant flow rate versus time for a first cooling unit  106   b  and the solid line represents the coolant flow rate versus time for a second cooling unit  106   e  indicated in  FIG.  1 B . The dashed line may also represent a first sub-set of cooling units, and the solid line may equally well represent a second sub-set of cooling units. Thus, the flow-rate for sub-sets of cooling units may be varied according to the scheme in  FIG.  3   . 
       FIG.  3    illustrates that the flow rates for two cooling units,  106   b  and  106   e , are varied, and that when the flow rates of a first cooling unit  106   b  is increased, the flow rate of the second cooling unit  106   e  is decreased, and vice versa. For example, at time t1, the flow rate of the first cooling unit  106   b  is increased from F1 to F2, whereas at the same time the flow rate for the second cooling unit  106   e  is decreased from F2 to F1. At time t2, the flow rate of the first cooling unit  106   b  is decreased from F2 to F1, whereas at the same time the flow rate for the second cooling unit  106   e  is increased from F1 to F2. 
     A time duration T for maintaining the flow rates, before switching to another flow rate lapses from t1 and t2 for allowing the effect of the flow rate variations on the flatness of the work item to occur and be measurable. The flatness is measured before the end of the time duration, but after a predetermined time duration Tth has lapsed to ensure that the flatness effects are measurable. 
     The time duration T, that the flow rates are maintained constant, i.e. the time durations between flow rate variations, are preferably equal for each consecutive flow rate variation, in other words consecutive time durations are equal. Further, the time durations for each of the cooling units are preferably equal. As illustrated in  FIG.  3   , the time duration T is the same for consecutive time durations between flow rate variations. Further, it is preferred that the flatness measurements used for determining the flatness variation values are performed after equal times have lapsed from the corresponding flow rate variation, for example at the predetermined time duration, Tth. 
     Varying the flow rate in opposite directions for two cooling units, or two sub-sets of cooling units advantageously maintains the total flow of coolant closer to constant than if the flow rate for all the cooling units were varied equally. This provides for better overall cooling during cooling unit testing. 
     To improve the ability to detect overlapping coolant spray flows it is advantageous to leave at least one or two cooling units unchanged between the varying flow rate cooling units. 
     In other words, in order to further improve the ability to detect the effects of overlapping spray flows, the flow rate for a sub-set of cooling units are maintained constant when the flow rates of selected cooling units are varied. For example, the flow rates for the cooling units 106c-d arranged between the cooling units  106   b  and  106   e  may be maintained substantially constant. 
     In advantageous embodiments, each of the at least one cooling unit has closest neighboring cooling units, e.g. cooling units 106c-d mentioned above which flow rates are maintained constant are closest neighbors to the respective cooling units  106   b  and  106   e  which flow rates are varied. 
     The at least one cooling unit  106   a - n  may comprise cooling units that are interspaced with cooling units, e.g. cooling unit 106c-d which flowrates are maintained substantially constant when the flow rates of the at least one cooling unit is/are varied. 
     In embodiments, the flatness variation values may be determined for cooling units for which the flow rates are maintained constant. Thus, flatness variation values may be determined even for cooling units which flow rates are not varied in a present flow-rate variation. This improves the ability to detect cooling units spraying coolant when they shouldn’t, or to detect spray zones receiving coolant when they should not be receiving coolant which may indicate that a neighboring cooling unit is faulty. 
     The total number of cooling units or the number of cooling units in the sub-sets are here only shown for example purposes and may include any number of cooling units depending on the specific rolling mill setup at hand. For example, a rolling mill may comprise 10, 15, 20, 25, 30, 35, 40, 50, 60, or any other number of cooling units. 
     The number of cooling units in a sub-set for which the flow rates are varied may be any sub-set of the cooling units arranged to provide coolant to the work roll. The sub-set may even be a single cooling unit, although the preferred embodiment includes to have every third cooling unit in a sub-set. In other words, the flow rates for cooling units  106   c ,  106   f ,  106   i , and  106   l  may be varied simultaneously for determining flatness variation values . The flow rates for the remaining cooling units may be maintained substantially constant. Once the flatness variation values for cooling units  106   c ,  106   f ,  106   i , and  106   l  have been determined, the flow rates for a set of the remaining cooling units are varied, and so on until a flatness variation values for all cooling units have been determined. 
     Turning to  FIG.  1 C  again. In one implementation, a control unit  300  is configured to detect a faulty cooling unit in a set of cooling units. As described above, the cooling units  106   a - n ,  108  being configured to provide a coolant to work rolls  100   a - b  arranged to process a work item  102 . 
     The control unit  300  is configured to, in response to flow rates of the coolant ejected from at least one of the cooling units  106   a - n  being varied, acquire flatness data indicating a flatness variation value of the work item  102  for each of the at least one cooling units. The flatness variation value being indicative of the work item flatness variation downstream of the work rolls. In other words, the control unit  300  is in communication with the sensing arrangement  202  for receiving flatness data from the sensing arrangement  202 . The flatness data may include the flatness variation value itself, or the control unit  300  processes the flatness data to compute the flatness variation value. 
     The control unit  300  is configured to detect a faulty cooling unit based on comparing the flatness variation values to a reference flatness variation value, in ways described above. 
     The control unit  300  may be connected to a valve  302  which controls the flow rate for the cooling units. The control unit  300  may receive a control signal from the valve  302  that the flow rates are being varied, whereby the control unit acquires flatness data, after the predetermined time duration threshold Tth has lapsed. 
     In other possible implementations, the control unit  300  initiates the flatness measurement procedure for detecting a faulty cooling unit. For this, the control unit controls the valve  302  to vary the flow rate of coolant provided to a nozzle  308  of the respective cooling units  106   a - n . Accordingly, the control unit  300  transmits control signals to the valve  302  for instructing it to open or close to vary the flow rates for the cooling units. 
     The valve  302  is configured to control the flow of pressurized coolant from a reservoir to the nozzles by means. 
     Turning again to  FIGS.  1 A-C , and in particular  FIG.  1 C , there is further provided a system  400  for detecting a faulty cooling unit in a set of cooling units configured to provide a coolant to work rolls arranged to process a work item therebetween. The system comprises a sensing arrangement  202  for measuring a flatness value of the work item  102  downstream being processed by the work rolls. The system further comprises the control unit  300  configured to: receive flatness data from the sensing arrangement  202   indicative of the flatness measured in response to the varied flow rates, determine a flatness variation value of the work item for each of at least one cooling unit based on the flatness data, the flatness variation value being indicative of the work item flatness variation downstream of the work rolls  100   a - b ; and compare the flatness variation values to a reference flatness variation value for detecting a faulty cooling unit. 
     The system may comprise at least one valve  302  configured to control the flow of coolant from a coolant reservoir to the cooling units. 
     It should be noted that the drawings are not to scale and provide conceptually illustrations of the embodiments. 
     A control unit may include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. The control unit may also, or instead, include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where the control unit includes a programmable device such as the microprocessor, microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device. 
     Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. 
     Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.