METHOD AND SYSTEM FOR CONTROL OF FLOWS TECHNICAL FIELD [0001] The present invention relates to a method and system for control of flows. In particular, but not exclusively, the present invention relates to control of flows in a pulp mill. In particular, but not exclusively, the present invention relates to control of flows in pulp drying. BACKGROUND [0002] In a modern pulp mill, it is increasingly important that the water consumption of the mill is as low as possible and at the same time the mill produces as little effluent as possible. Such operation is beneficial for environmental reasons and often required by laws or guidelines as well. Accordingly, there is a need for control of flows of water, filtrate and effluent in a mill. [0003] During normal operation a pulp mill is well balanced also with respect to water and effluent flows, as excess water from a certain production phase can be used elsewhere in the mill. For example, excess water produced in pulp drying can be recycled back to the fiber line. However, situations arise in which the fiber line cannot receive all or some of the excess water from pulp drying and in such a case the excess water has typically been diverted to effluent thus increasing water consumption and effluent handling need. [0004] Similar situations have been previously addressed in publication WO2011/095686 in pulp washing by storing the extra filtrate in a pulp tower. However, the solution of said publication changes the consistency in the tower by mixing the filtrate and the stored pulp and accordingly is not suitable for most situation, e.g. in pulp drying. [0005] It is the object of the present invention to provide a method for control of flows that decreases water usage and amount of effluent thus mitigating the problems of prior art. SUMMARY [0006] Various aspects of examples of the invention are set out in the claims. [0007] According to a first example aspect of the present invention, there is provided a method for control of flows of in a pulp mill, comprising storing pulp from fiberline in a first pulp tower; directing the pulp from the first pulp tower to pulp drying; storing white water from the pulp drying into a first water tower and directing the water therefrom back to the fiberline; and when the fiberline is not able to receive the white water from the first water tower, directing the white water to second pulp tower comprising pulp with such consistency that the white water and the pulp remain substantially separated in different layers in the second pulp tower. [0008] The method may further comprise when the fiberline is again able to receive the white water from the first water tower, evacuating the white water from the second pulp tower and directing it to the first pulp tower. [0009] The method may further comprise circulating a part of the white water being directed from the second pulp tower to the first pulp tower past a measurement element for measuring the consistency of the white water from the second pulp tower. [0010] The method may further comprise determining that the white water has been substantially emptied from the second pulp tower when the consistency measured with the measurement element is above a predetermined threshold. [0011] The predetermined threshold may be about 0,5%. [0012] The consistency of the pulp in the second pulp tower may be about 3,5 to 12%. [0013] The method may further comprise directing pulp from the second pulp tower to pulp drying. [0014] According to a second example aspect of the present invention, there is provided a system for control of flows in a pulp mill, comprising a first pulp tower; a second pulp tower; a first water tower; and a measurement element; wherein the system further comprises means for causing carrying out a method according to the second example aspect. [0015] The means may comprise a control system [0016] Different non-binding example aspects and embodiments of the present invention have been illustrated in the foregoing. The embodiments in the foregoing are used merely to explain selected aspects or steps that may be utilized in implementations of the present invention. Some embodiments may be presented only with reference to certain example aspects of the invention. It should be appreciated that corresponding embodiments may apply to other example aspects as well. BRIEF DESCRIPTION OF THE DRAWINGS [0017] For a more complete understanding of example embodiments of the present invention, reference is now made to the following descriptions taken in connection with the accompanying drawings in which: Fig. 1 shows a schematic block view of a system according to an example embodiment of the invention; Fig. 2 shows a further schematic block view of a system according to an example embodiment of the invention; Fig. 3 shows a further schematic block view of a system according to an example embodiment of the invention; Fig. 4 shows a further schematic block view of a system according to an example embodiment of the invention; and Fig.5 shows a flow chart of a method according to an example embodiment of the invention. DETAILED DESCRIPTION OF THE DRAWINGS [0018] Fig. 1 shows a schematic block view of a system according to an example embodiment of the invention. Fig.1 shows a part of the elements of a pulp mill pulp and flow structures. It is to be noted that Fig. 1 is a schematic representation of elements for explaining embodiments of the present invention and accordingly only a part of the elements of the pulp mill are depicted, i.e. it is clear to a skilled person that the majority of the elements and structures of the pulp mill are not shown which does not in any way define the presence or absence of the same. Hereto it is further noted that the actuators, such as piping, pumps and valves, of the system used in carrying out the method according to embodiments of the invention have not been represented and a skilled person appreciates that they comprise actuators and further elements of conventional construction. [0019] The system 100 according to an embodiment of the invention comprises a first pulp tower 10. In an embodiment the first pulp tower 10 comprises a bleached pulp tower configured to receive and store bleached pulp arriving from the fiberline A prior to the pulp being dried at B. The system 100 further comprises a second pulp tower 20. In an embodiment, the second pulp tower 20 comprises a transition pulp tower configured to receive and store pulp from the fiberline at A in transition situations in which the pulp type is changed, for example from hardwood to softwood or vice versa. In a further embodiment, the second pulp tower 20 comprises a pulp tower with similar functionality and purpose as the first pulp tower 10, for example in a mill having two fiberlines or a fiberline with two pulp towers even if producing single pulp quality. In this case. in an embodiment, during regular operation of the mill, the pulp from the second pulp tower 20 is dosed in small amounts into production together with the pulp from the first pulp tower 10. [0020] The system 100 further comprises a first water tower 30, i.e. a white water tower configured to receive white water from pulp drying at C. In regular operation of the mill, the water tower 30 is configured to forward the white water received from pulp drying back to the fiberline at D. In an embodiment, the water tower 30 is further configured to forward white water into the first pulp tower for dilution. Fig. 1 shows the regular operating situation of the system 100. The pulp is delivered from the fiberline at A to the first pulp tower 10 in which it is stored at a consistency in the range of 3,5-12%, for example at a consistency of about 10%. In an embodiment, the volume of the first pulp tower 10 is for example between about 1000 and 27500 cubic meters. From the first pulp tower 10 the pulp is then forwarded to pulp drying at B. Pulp drying produces white water which is delivered to the water tower 30, stored therein and forwarded back to the fiberline at D. In an embodiment, white water is further forwarded into the first pulp tower for dilution. In the regular operating situation, the fiberline is able to receive substantially all white water from the water tower 30. [0021] Fig. 2 shows a further schematic block view of a system according to an example embodiment of the invention. Fig. 2 shows the elements of the system 100 as described hereinbefore with respect to Fig. 1. Fig. 2 shows an operating situation in which the fiberline is not able to receive white water from the water tower 30 for some reason, for example because the fiberline is undergoing a service break. In such an operating situation the water tower 30 would start to overfill relatively quickly and accordingly, according to the present invention, the white water from the water tower 30 is forwarded into the second pulp tower 20 to be stored therein. [0022] The white water is pumped into the second pulp tower 20 from the lower part thereof. As the consistency of the pulp in the second pulp tower 20 is from about 3,5% to 10%, the white water and the pulp substantially remain in separate layers with the white water layer below and a pulp cake above it. It is to be noted that remaining substantially in separate layers does not mean that in an embodiment, there could not exist an intermediate layer in which the pulp and the white water start, at least given time, to mix to a certain extent. Since the excess white water from the water tower 30 can be diverted to the second pulp tower 20, the pulp drying can remain operational even in the situation in which the fiberline can not receive the excess water without causing a large amount of effluent. [0023] Fig. 3 shows a further schematic block view of a system according to an example embodiment of the invention. Fig. 3 shows the elements of the system 100 as described hereinbefore with respect to Fig. 1. Fig. 3 shows an operating situation in which the fiberline is again able to receive white water from the water tower 30 after the operating situation of Fig. 2 in which white water has been forwarded to the second pulp tower 20. In such an operating situation, the white water temporarily stored at the second pulp tower 20 is removed therefrom. [0024] In order to empty the white water from the second pulp tower 20, the white water is evacuated from the bottom part thereof and forwarded to the first pulp tower 10 and used to dilute the pulp therein. Simultaneously, a part of the white water being evacuated from the second pulp tower is circulated past a measurement element 40 in order to measure the consistency of the flow leaving the second pulp tower 20. When the consistency measured by the measurement element 40 raises above a certain predefined threshold, it is determined that the white water stored in the second pulp tower 20 has been successfully emptied therefrom and substantially only the pulp layer remains and/or the intermediate layer in which the white water and pulp have mixed or started to mix to a certain extent is being evacuated. In an embodiment, the value of the predefined threshold is chosen in accordance with the situation, for example by an operator. In an example embodiment, the predefined threshold is about 0,2% to 2%, in an embodiment about 0,5%. In a further embodiment, the water stored in the second pulp tower 20 is forwarded substantially in its entirety to the fiberline, for example in an operating situation in which pulp drying is not operational and cannot supply water to the fiberline. In a further embodiment, the water from the second pulp tower 20 is forwarded to the water tower, for example in a situation in which the water tower 30 needs to be filled up. [0025] In the operating situation of Fig. 3, the water tower 30 receives white water from pulp drying at C and forwards white water back to the fiberline at D and in an embodiment to the first pulp tower 10 for dilution. Furthermore, pulp from the first pulp tower 10 is forwarded to pulp drying at B and pulp arrives at the first pulp tower 10 from the fiberline at A. In an embodiment, the pulp from the fiberline could also be forwarded to the second pulp tower 20 while the pulp from the first pulp tower is forwarded to pulp drying. In a situation in which white water is evacuated form the second pulp tower 20, the amount of white water forwarded to the fiberline increases by the amount of white water taken from the second pulp tower 20 in order to keep the system balanced and to prevent the need to forward white water into effluent. Furthermore, in a further embodiment, the water emptied from the second pulp tower 20 is in some situations forwarded directly, i.e. without being first forwarded to the fiberline or the water tower 30, to pulp drying in order to be used for control of consistency. [0026] Fig. 4 shows a further schematic block view of a system according to an example embodiment of the invention. Fig. 4 shows the elements of the system 100 as described hereinbefore with respect to Fig. 1. Fig. 4 shows an operating situation after the operation situation hereinbefore described with reference to Fig.3 in which the white water has been emptied from the second pulp tower 20. The system 100 operates in the regular operating situation described hereinbefore with reference to Fig.1 and pulp from the second pulp tower 20 is also forwarded to pulp drying mixed with the pulp from the first pulp tower 10.In a further embodiment, only pulp from the second pulp tower 20 is forwarded to pulp drying. Furthermore, the water tower 30 receives white water from pulp drying at C and forwards white water back to the fiberline at D. [0027] Fig. 5 shows a flow chart of a method according to an example embodiment of the invention. In an embodiment, the method according to an example embodiment of the invention is caused to be carried out by a control system. In an embodiment, the control system is a standalone control system configured to control the method. In a further embodiment, the control system is integrated into a mill-wide control system. [0028] At step 510 the system is in a regular operating situation and it is checked whether the fiberline is receiving the excess water of pulp drying, i.e. from the water tower 30. If the fiberline is not able to receive the excess water from pulp drying, the excess water from white water tower 30 is directed to the bottom of the second pulp tower as hereinbefore described with reference to Fig.2 at step 520. As soon as the fiberline is again able to receive the excess water from pulp drying, the excess water is returned to the fiberline at step 530 and the water is evacuated from the second pulp tower by directing it to the first pulp tower at step 540 as described hereinbefore with respect to Fig. 3. Simultaneously the consistency of the water being evacuated from the second pulp tower 20 is monitored with a measurement element 40and it is checked whether the consistency is above a predefined threshold at step 550. If the consistency is above the predefined threshold, normal operation of the system as described hereinbefore with reference to Figs. 1 and 5 is resumed at step 560. [0029] Without in any way limiting the scope, interpretation, or application of the claims appearing below, a technical effect of one or more of the example embodiments disclosed herein is reduction of pulp mill water usage. Another technical effect of one or more of the example embodiments disclosed herein reduction of effluent and need for effluent purification capacity. Another technical effect of one or more of the example embodiments disclosed herein is ensuring uninterrupted operation of pulp drying in irregular operating situations without compromising the water balance. A still further technical effect of one or more of the example embodiments disclosed herein is a more environmentally friendly operation. A still further technical effect of one or more of the example embodiments disclosed herein is energy saving. [0030] If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the before-described functions may be optional or may be combined. [0031] Although various aspects of the invention are set out in the independent claims, other aspects of the invention comprise other combinations of features from the described embodiments and/or the dependent claims with the features of the independent claims, and not solely the combinations explicitly set out in the claims. [0032] It is also noted herein that while the foregoing describes example embodiments of the invention, these descriptions should not be viewed in a limiting sense. Rather, there are several variations and modifications which may be made without departing from the scope of the present invention as defined in the appended claims.