EP4731827A1

EP4731827A1 - Rejection prevention in a machine for producing a fibrous web

Info

Publication number: EP4731827A1
Application number: EP24736045.6A
Authority: EP
Inventors: Iiris Joensuu; Marjatta Piironen
Original assignee: Kemira Oyj
Current assignee: Kemira Oyj
Priority date: 2023-06-22
Filing date: 2024-06-14
Publication date: 2026-04-29
Also published as: FI20235735A1; WO2024261395A1; CN121285667A

Abstract

A method for preventing rejections or for decreasing downtime of a paper, board, or tissue machine, comprising monitoring at least conductivity and a second parameter in wet end of the machine, determining whether an increased risk for rejections or machine downtime exists based on said monitoring, and taking action in the event an increased risk exists based on the determination.

Description

REJECTION PREVENTION IN A MACHINE FOR PRODUCING A FIBROUS WEB

TECHNICAL FIELD

The present disclosure generally relates to preventing rejections or to decreasing downtime of a machine for producing a fibrous web.

BACKGROUND

This section illustrates useful background information without admission of any technique described herein representative of the state of the art.

It is generally known to adjust process performance in machines for producing a fibrous web, such as paper, board, or tissue machines, based on measurements. However, there is an ongoing need for further development of such methods to even better meet end product quality requirements on all occasions.

SUMMARY

The appended claims define the scope of protection. Any examples and technical descriptions of apparatuses, products and/or methods in the description and/or drawings not covered by the claims are presented not as embodiments of the invention but as background art or examples useful for understanding the invention.

It has now been observed that the chemistry status during startup of a machine for producing a fibrous web, such as a paper, board, or tissue machine, may be quite challenging leading to end product rejections or machine downtime during e.g. the first day or two after the machine has been started after a service outage. Similar chemical conditions may occur after an amount of fresh water has been added to the fibrous web (such as paper, board, or tissue) manufacturing process.

It is an object of certain embodiments of the invention to provide an improved method for preventing rejections or for decreasing downtime of a machine for producing a fibrous web or at least to provide an alternative solution to existing technology. It is a particular object of certain embodiments of the to provide the above especially during machine startup or upon addition of fresh water to the process.

According to a first example aspect of the invention there is provided a method for preventing rejections or for decreasing downtime of a machine for producing a fibrous web (such as a paper, board, or tissue machine), comprising: a) monitoring at least conductivity and a second parameter in wet end of the machine; determining whether an increased risk for rejections or machine downtime exists based on said monitoring; and taking action in the event an increased risk exists based on the determination, or b) monitoring conductivity and at least a second parameter in wet end of the machine; predicting a risk for rejections or machine downtime based on said monitoring; and taking action based on the predicting.

In certain embodiments, the method comprises: monitoring conductivity and pH in wet end of the machine; predicting a risk for rejections or machine downtime based on the monitoring of conductivity and pH; and taking action based on the predicting a risk for rejections or machine downtime based on the monitoring of conductivity and pH.

Accordingly, in certain embodiments, the predicting is or can be implemented based on conductivity and pH alone (or using these parameters as essential parameters).

In certain embodiments, the method comprises: monitoring conductivity and a parameter indicative of an amount of colloidal particles in wire water, and a parameter indicative of the extent of agglomeration in the wire water; predicting a risk for rejections or machine downtime based on the monitoring of conductivity, a parameter indicative of an amount of colloidal particles in wire water, and a parameter indicative of the extent of agglomeration in the wire water; and taking action based on the predicting a risk for rejections or machine downtime based on the monitoring of conductivity, a parameter indicative of an amount of colloidal particles in wire water, and a parameter indicative of the extent of agglomeration in the wire water. Accordingly, in certain embodiments, the predicting is or can be implemented based on conductivity and the parameter indicative of an amount of colloidal particles in wire water, and the parameter indicative of the extent of agglomeration in the wire water alone (or using these parameters as essential parameters).

In certain embodiments, predicting a risk for rejections or machine downtime based on said monitoring comprises determining whether an increased risk for rejections or machine downtime exists based on said monitoring, and taking action based on the predicting comprises taking action in the event an increased risk exists based on the determination.

In certain embodiments, monitoring conductivity and at least a second parameter in wet end of the machine comprises:

(i) monitoring buffer capacity of water in wet end through monitoring conductivity and pH, or

(ii) monitoring flocculation in wire water through monitoring conductivity, a parameter indicative of an amount of colloidal particles in the wire water, and a parameter indicative of the extent of agglomeration in the wire water.

In certain embodiments, monitoring buffer capacity of water in wet end comprises monitoring buffer capacity of wire water.

In certain embodiments, buffer capacity of water in wet end or buffer capacity of wire water is monitored through monitoring conductivity and pH in water of a circulation water system of the machine, such as from short circulation, from long circulation, or from feedstock of the machine.

In certain embodiments, conductivity or the second parameter is monitored in water of a circulation water system of the machine (either short or long circulation, preferably in wire water). Accordingly, in the context of wet end the circulation water system may be understood as the long circulation at the wet end, or as the short circulation (preferably monitored from wire water sample(s)).

In certain embodiments, conductivity or the second parameter is monitored in feedstock (such as pulp) of the machine. The pulp may be in the form of a suspension for example in a mixing chest, in a machine chest, or in a headbox of the machine.

In certain embodiments, conductivity is monitored in water of a circulation water system of the machine or in feedstock of the machine, and the second parameter is monitored in water of a circulation water system of the machine or in feedstock of the machine.

In certain embodiments, conductivity is monitored in water of a circulation water system of the machine, preferably in wire water, and the second parameter is monitored in water of a circulation water system of the machine, preferably in wire water.

In certain embodiments, conductivity is monitored in water of a circulation water system of the machine, preferably in wire water, and the second parameter is monitored in feedstock of the machine.

In certain embodiments, conductivity is monitored in feedstock of the machine, and the second parameter is monitored in feedstock of the machine.

In certain embodiments, conductivity is monitored in feedstock of the machine, and the second parameter is monitored in water of a circulation water system of the machine, preferably in wire water.

In certain embodiments, the second (and optionally a third) parameter is monitored in water of a circulation water system of the machine, preferably wire water, and conductivity is monitored either in water of a circulation water system of the machine, preferably in wire water, or in feedstock of the machine.

In certain embodiments, conductivity and the second parameter are monitored at mutually different positions.

In certain embodiments, the second parameter is pH.

In certain embodiments, the method comprises determining a risk indicator for rejections or machine downtime based on monitoring conductivity and pH.

Accordingly, in certain embodiments, the method enables determining whether an increased risk for rejections or machine downtime exists based on monitoring buffer capacity of wire water through monitoring conductivity and pH.

In certain embodiments, the method comprises determining whether an increased risk for rejections or machine downtime exists through monitoring flocculation in wire water.

In certain embodiments, the method comprises monitoring flocculation in wire water through monitoring conductivity, a parameter indicative of an amount of colloidal particles in the wire water, and a parameter indicative of the extent of agglomeration in the wire water.

In certain embodiments, the method comprises determining a risk indicator for rejections or machine downtime based on monitoring conductivity, a parameter indicative of an amount of colloidal particles in the wire water, and a parameter indicative of the extent of agglomeration in the wire water. In these embodiments, the parameter indicative of an amount of colloidal particles in the wire water, and the parameter indicative of the extent of agglomeration in the wire water are considered as the second parameter (or the second and a third parameter).

In certain embodiments, the parameter indicative of an amount of colloidal particles in wire water is obtained by detecting a settled turbidity in a local sedimentation (or settling) experiment. In such an experiment, settling properties of a sample of water may be analyzed in a settling vessel, and once the turbidity settles or saturates to a certain value which does not considerably change anymore this value can be considered as the (final) settled turbidity. The monitoring may be based on light absorption and/or light scattering measurements. The amount of colloidal particles is dependent on the settled turbidity. Accordingly, the settled turbidity can be used in indicating the amount of colloidal particles. Alternatively, the parameter indicative of an amount of colloidal particles in the wire water may be obtained by measuring suspended solids in a water sample. The parameter indicative of an amount of colloidal particles may be presented as an indexed value.

In certain embodiments, the parameter indicative of the extent of agglomeration in wire water is obtained by detecting a settling velocity in a local sedimentation (or settling) experiment. In such an experiment, settling properties of a sample of water may be analyzed in a settling vessel. Once the settling experiment has commenced, the settling will soon enter a rapid settling period until the settling slows down. By measuring the change in turbidity during the rapid settling period through the use of e.g. light absorption or light scattering measurements, a settling velocity is obtained. The settling velocity is dependent on the extent of agglomeration. Accordingly, the settling velocity can be used in indicating the extent of agglomeration. The parameter indicative of the extent of agglomeration may be presented as an indexed value.

In certain embodiments, historical data of the machine is used in predicting a risk for rejections or machine downtime or in determining whether an increased risk for rejections or machine downtime exists. For example, historical data during preceding weeks or several months may be taken into account.

In certain embodiments, said taking action comprises recommending preventive action to an operator.

In certain embodiments, said taking action comprises providing a control system of the machine for producing a fibrous web with a control signal.

In certain embodiments, the method is used in machine start-up or in an occasion in which fresh water is added into the machine. In certain embodiments, the method is used in response to machine start-up or in response to an occasion in which fresh water is added into the machine.

According to a second example aspect of the invention there is provided an apparatus for preventing rejections or for decreasing downtime of machine for producing a fibrous web, comprising: a processor; and a memory including computer program code, the memory and the computer program code being configured, with the processor, to cause the apparatus to perform the method of the first aspect or any of its embodiments.

Accordingly, an apparatus of the second example aspect may comprise: a processor; and a memory including computer program code, the memory and the computer program code being configured, with the processor, to cause the apparatus to: (a) monitor at least conductivity and a second parameter in wet end of the machine; determine whether an increased risk for rejections or machine downtime exists based on said monitoring; and take action in the event an increased risk exists based on the determination; or

(b) monitor conductivity and at least a second parameter in wet end of the machine; predict a risk for rejections or machine downtime based on said monitoring; and take action based on the predicting.

According to a third example aspect of the invention there is provided a computer program comprising computer executable program code which when executed by a processor causes an apparatus to perform the method of the first aspect or any of its embodiments.

Accordingly, the computer program of the third example aspect may comprise computer executable program code which when executed by a processor causes an apparatus to:

(a) monitor at least conductivity and a second parameter in wet end of a machine for producing a fibrous web; determine whether an increased risk for rejections or machine downtime exists based on said monitoring; and take action in the event an increased risk exists based on the determination; or

According to a fourth example aspect there is provided a computer program product comprising a non-transitory computer readable memory medium having the computer program code of the third example aspect stored thereon. Any foregoing memory medium may comprise a digital data storage such as a data disc or diskette, optical storage, magnetic storage, holographic storage, opto- magnetic storage, phase-change memory, resistive random access memory, magnetic random access memory, solid-electrolyte memory, ferroelectric random access memory, organic memory or polymer memory. The memory medium may be formed into a device without other substantial functions than storing memory or it may be formed as part of a device with other functions, including but not limited to a memory of a computer, a chip set, and a sub assembly of an electronic device.

Different non-binding example aspects and embodiments 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 different implementations. Some embodiments may be presented only with reference to certain example aspects. It should be appreciated that corresponding embodiments apply to other example aspects as well.

BRIEF DESCRIPTION OF THE FIGURES

Some example embodiments will be described with reference to the accompanying figures, in which:

Fig. 1 a shows real-time chemistry measurements at a startup period of a paper or board machine according to certain example embodiments;

Fig. 1 b shows determined disturbance levels of pH according to certain example embodiments;

Fig. 1 c shows determined disturbance levels of conductivity according to certain example embodiments;

Fig. 2 shows scaled values for predicting a risk for end product quality according to certain example embodiments;

Fig. 3 shows actual end product quality during the time period shown in Fig. 2;

Fig. 4 shows real-time or near real-time determination of the amount of colloidal particles in wire water and the extent of agglomeration according to certain example embodiments;

Fig. 5 shows the extent of agglomeration as determined and measurements on conductivity in certain embodiments; Fig. 6 shows certain parts of a wet end of a machine for producing a fibrous web; and

Fig. 7 shows an apparatus in accordance with certain embodiments.

DETAILED DESCRIPTION

In the following description, like reference signs denote like elements or steps.

The following description discloses a method in which rejections are prevented or (related) downtime of a machine for producing a fibrous web (such as a paper, board, or tissue machine) is decreased. The method comprises monitoring at least conductivity and a second parameter in wet end of the machine. The method, in certain embodiments, further comprises determining whether an increased risk for rejections or machine downtime exists based on said monitoring, and taking action in the event an increased risk exists based on the determination.

Fig. 1 a shows real-time chemistry measurements at a startup period of the paper or board machine after a service break or shutdown. For the purpose of preventing rejections or decreasing downtime during the startup period, the measurements in this embodiment comprise measurements on conductivity (presented in scaled values) and measurements on pH as the second parameter. The measurements indicate as an observed trend that conductivity of the water at the wet end tends to be rather low initially, but to converge towards typical values. Further, the pH of the wire water tends to increase initially, but then to settle towards a typical value (here: approximately pH value 8).

It has been observed that an increase in pH together with low conductivity may have serious consequences on the end product quality. In order to find out whether preventive or corrective actions need to be taken, disturbance levels for the measured parameters (pH and conductivity) and risk indicators for predicting end product quality are determined.

Fig. 1 b and Fig. 1 c shows, separately, determined disturbance levels of pH (Fig. 1 b) and conductivity (Fig. 1 c). The values shown in Figs. 1 b and 1 c are scaled values in which the values ranging from -100 to 0 represent acceptable values, the values above 0 up to 50 represent increased values (over target), and values above 50 represent (high) non-regular values.

Fig. 2 shows scaled values for predicting a risk for end product quality. The values shown in Fig. 2 are determined by combining (e.g. by using mathematical operation(s)) the disturbance level values of Figs. 1 b and 1 c. The values ranging from -100 to 0 represent an acceptable estimated end product quality. The values above 0 up to 50 represent an increased risk for poor end product quality, and values above 50 represent a high risk for product rejection.

As is shown in Fig. 2, the predicted risk indicates, in the presented embodiment, already from the start that there is a risk for poor end product quality (the reference numeral 1 indicates a time frame during which the predicted risk stays within a level which may be considered as a “warning level”, here: between values 0 and 50). Further, the reference numeral 2 indicates the point in time at which the predicted risk exceeded an “alerting level”. As to the determination of the warning and alerting level(s), historical data of the measured parameters of the paper or board machine in question may be used.

Fig. 3 shows the actual corresponding end product quality (with the aid of scaled values) during the time period shown in Fig. 2 and without using preventive actions. The values ranging from -100 to 0 represent an acceptable (or good) end product quality. The values above 0 up to 50 represent an end product quality below target, but still acceptable, and values above 50 represent a product rejection. It is observed that product rejections occurred during the time at which the predicted risk for end product quality exceeded the alerting level.

By the disclosed method, the product rejections can be (or in the shown embodiment, could have been) prevented.

The method preferably operates such that when the predicted risk for end product quality raises into the warning level, a first set of preventive action is taken. The first set may include, depending on the embodiment, alerting an operator, recommending a preventive action, or producing a direct control signal to effect said preventive action (e.g., providing a control system of the paper or board machine with a control signal). In certain embodiments, said preventive action includes recommending the operator to adjust a chemical dose (e.g., a small increase of PAC (poly aluminum chloride) and/or acid and/or sizing agent and/or retention polymer) into the paper or board manufacturing process. In other embodiments, the chemical dose is adjusted through the use of the direct control signal.

When the predicted risk for end product quality raises into the alerting level, a second set of preventive action is taken. The second set may include, depending on the embodiment, alerting an operator, recommending a preventive action, or producing a direct control signal (e.g., providing a control system of the paper or board machine with a control signal). In certain embodiments, said preventive action includes recommending the operator to adjust a chemical dose (e.g., a bigger increase of PAC (poly aluminum chloride) and/or acid and/or sizing agent) into the paper or board manufacturing process. In other embodiments, the chemical dose is adjusted through the use of the direct control signal.

It can be observed from Figs. 1 -3 that if the disclosed method would have been in use, the product rejection occurring between 12:00 and 18:00 could have been prevented. The monitoring of the conductivity and pH, in essence the monitoring of buffer capacity of the wire water through monitoring these parameters, would have produced an indication and proposed preventive action already hours before the actual rejection occurred.

Figs. 4 and 5 concern other embodiments, in which conductivity and a second parameter are monitored. However, instead of monitoring the buffering capacity through monitoring conductivity and pH, the method comprises monitoring flocculation in wire water. Through monitoring flocculation, a risk indicator for predicting end product rejections or machine downtime is determined. The monitoring of flocculation in wire water is performed through monitoring conductivity, and a second parameter. In an embodiment, a parameter indicative of an amount of colloidal particles in the wire water, and a parameter indicative of the extent of agglomeration in the wire water are applied as the second parameter.

The parameter indicative of an amount of colloidal particles in the wire water, and the parameter indicative of the extent of agglomeration in the wire water can be obtained e.g. by taking a side stream from the wire water into a settling vessel and performing a settling experiment known as such. The parameters indicative of an amount of colloidal particles in the wire water and indicative of the extent of agglomeration in the wire water may be presented as indexed values.

Disturbance levels for conductivity, and the second parameter(s) are determined similarly as shown for conductivity and pH in the preceding embodiment, and a risk indicator for rejections or machine downtime based on monitoring the conductivity and the second parameter(s) is determined.

Fig. 4 shows a graph including results on a real-time or near real-time determination of the parameter indicative of an amount of colloidal particles as an indexed value, and the parameter indicative of the extent of agglomeration as an indexed value. Further, Fig. 5 shows determined indexed values of the extent of agglomeration and (real-time) measurements on conductivity (presented in scaled values). The reference numeral 3 shows detected process conditions at which the extent of agglomeration is high, the amount of colloidal particles is low, and the conductivity is (rather) low. These conditions indicate the risk for presence of over-flocculation of particles (or fines). Over-flocculation in general increases the risk of poor end product quality, e.g. weak formation, bright spots or thinner areas appearing in the end product and is thus a risk indicator for poor end product quality.

As mentioned, the risk indicator is determined. The risk indicator in this embodiment practically indicates the predicted risk for over-flocculation and thus the predicted risk for poor end product quality. The manner of taking preventive action corresponds to that described in the preceding embodiment.

Accordingly, when the predicted risk for poor end product quality raises into a warning level, a first set of preventive action is taken. The first set preferably comprises alerting an operator, recommending a preventive action, or producing a direct control signal to effect said preventive action (e.g., providing a control system of the paper, board, or tissue machine (or machine for producing a fibrous web) with a control signal). In certain embodiments, said preventive action includes a reduction of a cationic chemical dose (such as a polymer dose) to prevent overflocculation.

When the predicted risk for end product quality raises into the alerting level, a second set of preventive action is taken (comprising a bigger reduction in the chemical dose, if appropriate).

Similarly as in the preceding embodiment, historical data of the paper or board machine may be used in the determination of the warning and alerting level(s).

Logic and rules for recommendations in different embodiments may be generated beforehand.

In the described embodiments, the status of disturbances and risk indicators are preferably monitored continuously, and action is taken whenever determined necessary.

Fig. 6 shows certain parts of a wet end of a machine for producing a fibrous web in accordance with certain embodiments. A headbox 11 distributes feedstock on a wire section 12. Water removed from the wire section 12 is circulated as a short circulation 10 in a circulation water system of the machine and used to dilute further feedstock entering the headbox 11 .

Wire pit overflow is typically recycled in a long circulation 20 of the circulation water system of the machine (or the whole mill) from which water may be returned to the short circulation at some point.

In certain embodiments, a side stream is taken from the wire water (short circulation) into a settling vessel 15, and the local settling experiment to monitor the parameter indicative of an amount of colloidal particles in the wire water, and the parameter indicative of the extent of agglomeration in the wire water can thus be implemented. Conductivity and pH may be measured from the short circulation, from the long circulation, or from the feedstock of the machine (or in the settling experiment).

Fig. 7 shows an apparatus in accordance with certain embodiments. In certain embodiments, the shown apparatus 70 is e.g. capable of functioning as a monitoring (or control) apparatus implementing preceding method(s). The apparatus 70 is for example a general-purpose computer or some other electronic data processing apparatus.

The apparatus 70 comprises a communication interface 75, at least one processor 71 , a user interface 74, and at least one memory 72.

The communication interface 75 comprises in an embodiment a wired and/or wireless communication circuitry, such as Ethernet, Wireless LAN or WI-FI, Bluetooth, GSM, CDMA, WCDMA, LTE, and/or 5G circuitry. The communication interface 75 can be integrated in the apparatus 70 or provided as a part of an adapter, card or the like, that is attachable to the apparatus 70. The communication interface 75 may support one or more different communication technologies. The apparatus 70 may also or alternatively comprise more than one communication interface 75.

The at least one processor 71 may be a central processing unit (CPU), a microprocessor, a digital signal processor (DSP), a graphics processing unit, an application specific integrated circuit (ASIC), a field programmable gate array, a microcontroller, or the like, or a combination of such elements.

The user interface 74 may comprise a circuitry for receiving input from a user of the apparatus 70, e.g., via a keyboard, graphical user interface shown on the display of the apparatus 70, speech recognition circuitry, microphone, or an accessory device, such as a headset, and for providing output to the user via, e.g., a graphical user interface or a loudspeaker.

The at least one memory 72 comprises a work memory 73 and a persistent (nonvolatile, N/V) memory 76 configured to store computer program code 77 and data 78. The memory 76 may comprise any one or more of: a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), a random-access memory (RAM), a flash memory, a data disk, an optical storage, a magnetic storage, a smart card, a solid state drive (SSD), or the like.

The apparatus 70 may comprise a plurality of memories 76. The memory 76 may be constructed as a part of the apparatus 70 or as an attachment to be inserted into a slot, port, or the like of the apparatus 70 by a user or by another person or by a robot. The memory 76 may serve the sole purpose of storing data, or be constructed as a part of an apparatus 70 serving other purposes, such as processing data.

A skilled person appreciates that, depending on the embodiment, in addition to the elements shown in Fig. 7, the apparatus 70 may comprise other elements, such as further microphones, displays, as well as additional circuitry such as an input/output (I/O) circuitry, memory chips, application-specific integrated circuits (ASIC), a processing circuitry for specific purposes such as a source coding/decoding circuitry, a channel coding/decoding circuitry, a ciphering/deciphering circuitry, and the like. Additionally, the apparatus 70 may comprise a disposable or rechargeable battery (not shown) for powering the apparatus 70 when an external power supply is not available.

As to the disclosed method(s), the at least one processor 71 controls monitoring at least conductivity and a second parameter in wet end of the machine based on the program code 77. The at least one processor 71 further controls determining whether an increased risk for rejections or machine downtime exists based on said monitoring and taking action in the event an increased risk exists based on the determination. The apparatus 70 may receive measurement data via the communication interface 75. The measurement data may be stored as data 78 in the memory 76 and processed e.g. to determine further parameters and risk indicator(s) by the at least one processor 71 based on the program code 77. Said taking action can be implemented by sending a signal via the communication interface to be displayed for an operator or to be directly used by the machine control system.

The described embodiments are well suited in particular to preventing end product rejections and decreasing machine downtime during machine start-up or in an occasion in which fresh water is added into the machine.

Various embodiments have been presented. It should be appreciated that in this document, words comprise, include and contain are each used as open-ended expressions with no intended exclusivity.

The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented in the foregoing, but that it can be implemented in other embodiments using equivalent means or in different combinations of embodiments without deviating from the characteristics of the invention.

Furthermore, some of the features of the afore-disclosed example embodiments may be used to advantage without the corresponding use of other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.

Claims

1 . A method for preventing rejections or for decreasing downtime of a machine for producing a fibrous web, comprising: monitoring conductivity and at least a second parameter in wet end of the machine; predicting a risk for rejections or machine downtime based on said monitoring; and taking action based on the predicting.

2. The method of claim 1 , comprising: monitoring conductivity and pH in wet end of the machine; predicting a risk for rejections or machine downtime based on the monitoring of conductivity and pH; and taking action based on the predicting a risk for rejections or machine downtime based on the monitoring of conductivity and pH.

3. The method of claim 1 or 2, comprising: monitoring conductivity and a parameter indicative of an amount of colloidal particles in wire water, and a parameter indicative of the extent of agglomeration in the wire water; predicting a risk for rejections or machine downtime based on the monitoring of conductivity, a parameter indicative of an amount of colloidal particles in wire water, and a parameter indicative of the extent of agglomeration in the wire water; and taking action based on the predicting a risk for rejections or machine downtime based on the monitoring of conductivity, a parameter indicative of an amount of colloidal particles in wire water, and a parameter indicative of the extent of agglomeration in the wire water.

4. The method of claim 1 , wherein predicting a risk for rejections or machine downtime based on said monitoring comprises determining whether an increased risk for rejections or machine downtime exists based on said monitoring, and taking action based on the predicting comprises taking action in the event an increased risk exists based on the determination.

5. The method of claim 1 or 4, wherein monitoring conductivity and at least a second parameter in wet end of the machine comprises:

6. The method of claim 5, wherein monitoring buffer capacity of water in wet end comprises monitoring buffer capacity of wire water.

7. The method of claim 5 or 6, wherein buffer capacity of water in wet end or buffer capacity of wire water is monitored through monitoring conductivity and pH in water of a circulation water system of the machine, such as from short circulation, from long circulation, or from feedstock of the machine.

8. The method of any preceding claim, wherein conductivity or the second parameter is monitored in water of a circulation water system of the machine, preferably in wire water.

9. The method of any of claims 1 -7, wherein conductivity or the second parameter is monitored in feedstock of the machine.

10. The method of any of claims 1 -7, wherein conductivity is monitored in water of a circulation water system of the machine or in feedstock of the machine, and the second parameter is monitored in water of a circulation water system of the machine, preferably in wire water, or in feedstock of the machine.

11 . The method claim 1 or 4, wherein the second parameter is pH.

12. The method of any preceding claim, comprising determining a risk indicator for rejections or machine downtime based on monitoring conductivity and pH.

13. The method of any preceding claim, comprising determining a risk indicator for rejections or machine downtime based on monitoring conductivity, a parameter indicative of an amount of colloidal particles in the wire water, and a parameter indicative of the extent of agglomeration in the wire water.

14. The method of any preceding claim, wherein historical data of the machine is used in predicting a risk for rejections or machine downtime or in determining whether an increased risk for rejections or machine downtime exists.

15. The method of any preceding claim, wherein said taking action comprises recommending preventive action to an operator.

16. The method of any preceding claim, wherein said taking action comprises providing a control system of the machine for producing a fibrous web with a control signal.

17. The method of any preceding claim, wherein the method is used in machine start-up or in an occasion in which fresh water is added into the machine.

18. An apparatus for preventing rejections or for decreasing downtime of machine for producing a fibrous web, comprising: a processor; and a memory including computer program code, the memory and the computer program code being configured, with the processor, to cause the apparatus to perform the method of any of claims 1 -17.

19. A computer program comprising computer executable program code which when executed by a processor causes an apparatus to perform the method of any of claims 1 -17.