Abstract:
The present disclosure describes a system for predicting explosions in a dissolving tank. The system includes acoustic emission sensors placed in or around the dissolving tank. By filtering the recorded frequencies to the range which is most sensitive for desired explosions “fingerprints,” it is possible to predict a smelt influx before the smelt influx occurs as well as program response actions to prevent compromising explosions.

Description:
CROSS-RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Pat. App. No. 62/252,221 filed on Nov. 6, 2015, the entirety of which in incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates generally to chemical pulping and particularly to recovery boilers and dissolving tanks used in the pulp and paper industry. 
         [0004]    2. Related Art 
         [0005]    Chemical pulping converts lignocellulosic biomass to pulp fibers of various lengths. In the pulp and paper industry, the lignocellulosic biomass often comprises wood chips; but lignocellulosic material may include other plant-based biomass in which the protein lignin is closely associated with cellulosic sugar molecules. With processing, operators can isolate cellulosic pulp fibers for use in a variety of commercial applications, including paper manufacturing. 
         [0006]    When wood is the primary lignocellulosic material for example, production may begin with a log. A debarker removes the bark from (or “debarks”) logs, and a chipper comminutes the logs into small chips. Depending on the particular process and application, operators may pretreat these chips with steam and chemicals to expand pores in the lignocellulosic biomass, or operators may send dried chips directly into a chemical digester. Continuous chemical digesters are generally cylindrical and may be several stories high. 
         [0007]    In the digester, operators typically introduce white liquor and steam into the digester&#39;s upper section. In the Kraft process, the “white liquor” often consists of a sodium hydroxide and sodium sulfide solution. Over the course of several hours, the steamed biomass moves through the digester as white liquor dissolves the lignin. Lignin is a protein that binds the cellulose and hemicellulose in the biomass together. Removal of lignin permits operators to isolate fibers comprising mainly cellulose and hemicellulose. As the lignin and other ancillary biomass compounds dissolve into the liquor, the liquor darkens and becomes “black liquor”. 
         [0008]    After the black liquor solution exits the digester, equipment isolates the cellulosic pulp fibers from the remaining black liquor. Whereas white liquor contains sodium hydroxide and sodium sulfide, the black liquor contains sodium carbonate and sodium sulfate respectively. Sodium carbonate and sodium sulfate are the products of the white liquor&#39;s chemical reaction with the lignin and other compounds in the digester. The products, sodium carbonate and sodium sulfate, are generally less useful for digesting lignin. 
         [0009]    While sodium hydroxide and sodium sulfide are generally inexpensive chemicals, purchasing new solutions of sodium hydroxide and sodium sulfide for every new batch of lignocellulosic biomass is generally cost prohibitive. For this reason, many chemical pulp mills use pyrolytic chemical recovery systems to convert at least a portion of the sodium carbonate and sodium sulfate back into useful sodium hydroxide and sodium sulfide. 
         [0010]    New black liquor from a chemical digester is generally dilute and non-combustible. Therefore, to prepare black liquor for pyrolysis, operators generally funnel the black liquor through flash tanks or other evaporation steps to increase the amount of solid particles concentrated in the black liquor. Operators then heat the concentrated black liquor before injecting the concentrated black liquor through spray nozzles into a chemical recovery boiler. The spray nozzles create coarse droplets. The recovery boiler evaporates the remaining water from the droplets and the solid compounds in the black liquor undergo partial pyrolysis. The inorganic compounds that remain fall to the bottom the furnace and accumulate in a char bed. Some of the carbon and carbon monoxide in the char bed can act as catalysts to convert sodium sulfate into sodium sulfide, which can then be collected from flue gas near the top of the furnace. 
         [0011]    The remaining inorganic compounds in the char bed eventually melt and flow as a smelt through one or more smelt spouts at the bottom of the recovery boiler. Coolant, usually water, may cool the smelt spouts. Coolant tubes may either be integrated into the spout itself, or an ancillary cooling system. The ancillary cooling system is often called a “water jacket” and may surround the outside of the spout. The smelt flowing from the spout falls into a dissolving tank and contacts water or weak white liquor to produce soda lye. The resulting soda lye solution is commonly known as “green liquor.” 
         [0012]    In a sulfate chemical process, such as the Kraft process, the main component of the green liquor is typically sodium sulfide and sodium carbonate. However, different chemical processes produce green liquor with different inorganic compounds. Operators typically collect the green liquor and transport the green liquor to a causticizing plant to further isolate and concentrate the sodium sulfide and sodium carbonate and thereby reproduce white liquor. 
         [0013]    As the smelt contacts the green liquor in the dissolving tank, the smelt explodes and emits a series of audible noises. This is generally known as “banging” by those in the industry. The smelt flowing from the spout is typically between 750 degrees Celsius (° C.) to 820° C., while the average temperature of the green liquor is about 70° C. to 100° C. Moreover, the smelt generally contains reactive alkali metals such as sodium, which reacts explosively with water. Without being bounded by theory, the large temperature differential may increase the reactivity of the smelt and green liquor and thereby cause or contribute to banging. If left unregulated, a sudden influx of smelt may blow up the dissolving tank and recovery boiler, which poses grave risks to nearby operating personnel. 
         [0014]    To manage banging, conventional dissolving tanks generally disrupt the smelt as the smelt falls from the spout. Disruptors may be one or more shatter jets, which blast the falling smelt with steam or other fluid at high pressure to create smelt droplets. These droplets have a smaller volume than the overall flow of smelt and therefore, the explosions are generally less intense than they would be if the smelt contacted the green liquor as a continuous, uninterrupted, undisrupted flow. Typically, the end of the smelt spout is elevated above the level of green liquor and these shatter jets disrupt falling smelt as the smelt falls from the spout end. 
         [0015]    Occasionally, smelt may cool prematurely in the recovery boiler or spout and decrease or eliminate the smelt flow rate. In this antediluvian state, liquid smelt tends to accumulate behind the obstruction. If the obstruction becomes dislodged, the sudden smelt influx may overwhelm the shatter jet&#39;s ability to disrupt the smelt into sufficiently small droplets. Moreover, if the deluge is particularly substantial, the smelt may flow over the sides of the spout and bypass the shatter jets entirely. In other scenarios, a shatter jet may fail. In these situations, the increased volume of smelt contacting the green liquor drastically increases the banging&#39;s explosive intensity and risk of explosion. 
         [0016]    In many mills, operators commonly move in and amongst the processing equipment to monitor process conditions and output. An explosion in the dissolving tank or recovery boiler poses a serious safety risk to personnel in the immediate vicinity, and the resulting fire poses a serious risk to personnel in the rest of the mill. Such explosions also cause an unregulated amount of pollutants to enter the air and groundwater and predicate significant production loss. Explosions of this scale can inactivate a mill for weeks to months. 
       SUMMARY OF THE INVENTION 
       [0017]    Applicant conceived a system in which acoustic emission sensors are placed in or around the dissolving tank. Applicant has discovered that the acoustic emissions filtered to a programmed frequency range of greater than 20 KHz tend to exhibit a distinctive pattern or “pre-influx fingerprint” closely before a smelt influx occurs. By isolating the recorded frequencies of acoustic emissions to detect a pre-influx fingerprint, it is possible to predict a smelt influx before the smelt influx occurs. Upon detection of a pre-influx fingerprint, an exemplary system disclosed herein may modify a process condition or contain the smelt influx and thereby prevent or mitigate upset conditions, which can contribute to compromising explosions. In other exemplary embodiments, operators may program a smelt control mechanism such as the disruptors or the restrictor plate disclosed in U.S. Pat. No. 9,206,548 to control smelt flow. Measuring acoustic emission events from smelt banging in a dissolving tank may be further used to regulate conditions inside the recovery boiler to thereby control the amount of smelt entering the dissolving tank. 
         [0018]    A smelt influx detected by one of more acoustic emission sensors disposed near the disruptor and smelt spout can be corroborated by a “pre-influx fingerprint” comprising an increased rate of acoustic emissions having amplitudes substantially exceeding a first set of processed waveforms by more than 200% and having a frequency of greater than 20 KHz. The acoustic emission system may further comprise a response configured to adjust smelt flow when the acoustic emission system detects a pre-influx fingerprint. The response may comprise restricting smelt flow, changing process conditions within the recovery boiler, or a combination thereof. 
         [0019]    The problem of boiler explosions is mitigated by using exemplary embodiments of the system and method disclosed herein. In an exemplary embodiment, the acoustic emission system may comprise acoustic emission sensors configured to detect acoustic emissions. An acoustic emission sensor may comprise a transducer having a resonant frequency, wherein the transducer is configured to convert an acoustic wave into an electric signal. A preamplifier may communicate with the transducer. The pre-amplifier is typically configured to amplify an electric signal. The pre-amplifier generates an amplified signal in turn, and transfers the amplified signal to a data processor. The data processor can be configured to filter the amplified signal to a programmed frequency range above 20 KHz. The data processor may further evaluate frequencies in the programmed frequency range to detect the pre-influx fingerprint. Once the data processor detects the pre-influx fingerprint, the data processor may initiate a response, which may include changing one or more operating conditions in the recovery boiler or activating safety devices to reduce or prevent the smelt influx from contacting the green liquor in the dissolving tank. 
         [0020]    In an exemplary system, multiple acoustic emission sensors may be disposed in and around the dissolving tank. For example, acoustic emission sensors comprising a wave guide may be disposed in the wall of the dissolving tank. The acoustic sensor may have a reading end at the end of the wave guide and a second end, opposite the reading end, disposed outside of the dissolving tank. The reading end may be disposed within the dissolving tank. In certain exemplary embodiments, two or more acoustic emission sensors may have wave guides extending into the green liquor. In other exemplary embodiments, an acoustic emission sensor may have a wave guide disposed within the fluid emitted from the disruptor. In still other exemplary embodiments, an acoustic emission sensor may have a wave guide disposed within the dissolving tank above the green liquor level and outside of the disruptor fluid. In other exemplary embodiments, an acoustic emission sensor may be disposed adjacent to the dissolving tank. 
         [0021]    Under normal operating conditions the disruptors disperse the smelt flowing off of the smelt spout into smelt droplets. The smelt droplets then contact the green liquor and emit a small “bang.” The “bang” comprises both audible acoustic emissions and as acoustic emissions above and below the range of human hearing. Under these normal operating conditions, hundreds of small bangs may occur every second. In an exemplary embodiment of the process, the acoustic emission sensor detects the acoustic emissions and transduces the acoustic emission waves into an electric analog signal. The signal may proceed to a series of pre-amplification stages followed by one or more high pass, low pass or bandpass filter stages to isolate desirable frequencies in a frequency range above 20 KHz. The signal may be further refined before being converted into a digital signal. An analog-to-digital (“A/D”) converter may convert the analog signal to a digital signal. The digital signal may then be sent to a data processor such as a field-programmable gate array (“FPGA”), which may utilize either the continuous count method or conduct Fourier Transformation to process and thereby simplify the digital signal. The Fourier Transform may be a Fast Fourier Transform (“FFT”), or other Fourier Transform. In other exemplary embodiments, the FPGA may utilize other signal processing or transformation methods to show maximum correlations on each individual process part e.g. by using the root mean square (“RMS”) method, standard deviation method, skewness method, kurtosis method, mean method, variance method, or by utilizing fuzzy logic, neural networks, and other signal processing methods. In still other exemplary embodiments, the data processor may be an application-specific integrated circuit (“ASIC”). Furthermore, an exemplary system may analyze signals produced by the multiple acoustic emission sensors. 
         [0022]    An exemplary system may continuously monitor the dissolving tank for smelt influx above a baseline level of smelt flow. 
         [0023]    An exemplary system may process and analyze the signals derived from acoustic emissions in the dissolving tank to predict a smelt influx and initiate a response to prevent smelt influx. 
         [0024]    A further exemplary system may regulate the operating conditions in the recovery boiler based upon signals derived from acoustic emissions in the dissolving tank. 
         [0025]    Yet another exemplary system and method may comprise a computer-based system having software configured to monitor the dissolving tank based on signal input from the acoustic emission sensors. The computer-based system may have defined condition alerts to indicate when a signal exceeds a predetermined signal amplitude threshold. 
         [0026]    The problem of upset conditions in dissolving tanks is solved by using a method of monitoring the dissolving tanks comprising: inserting one or more acoustic emission sensors through a wall or roof in the dissolving tank; continuously listening to the amount and intensity (“aggressivity”) of banging in the dissolving tank, relaying this banging to a computer system, analyzing the data, comparing with dissolving tank process status and returning an output once the data meets programmed conditions. 
         [0027]    A characteristic feature of the arrangement according to an embodiment of the present disclosure is that the present disclosure comprises: a sensor for measuring the acoustic emission caused by the smelt contacting the green liquor. The sensor may comprise a wave guide having a first end and a second end, wherein the first end is disposed at a distance inside the dissolving tank and the second end is located outside of the dissolving tank. The second end may be provided with a piezoelectric sensor configured to convert a received acoustic emission into an analog electric signal. The wave guide may comprise an uninsulated portion for receiving the acoustic emission and an insulated portion disposed downstream of the uninsulated portion. The acoustic emission sensor may further comprise pre-processors for processing the received analog electric signal. 
         [0028]    An exemplary method according to the present disclosure comprises: receiving acoustic emission caused by the chemical and thermal reactions of smelt and green liquor in an interior of a dissolving tank through an acoustic emission sensor extending into the interior of the dissolving tank. The method may further comprise converting the acoustic emission into a digital signal, transmitting the digital signal to a computer, and graphing the digital signal on a frequency spectrum to create a graph frequency spectrum. One may then compare the graphed frequency spectrum to a stored frequency spectrum indicative of a normal operating condition, and generate a response when the graphed frequency spectrum exceeds the stored frequency spectrum by more than 200%. 
         [0029]    In another exemplary embodiment, the computer may produce a digital output signal that deploys a restrictor plate such as the one described in U.S. Pat. No. 9,206,548. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    The foregoing will be apparent from the following more particular description of exemplary embodiments of the disclosure, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, with emphasis instead being placed upon illustrating the disclosed embodiments. 
           [0031]      FIG. 1  is a cross sectional side view multiple acoustic emission sensors disposed around a dissolving tank. 
           [0032]      FIG. 2  is detailed cross sectional view of acoustic emission sensor. 
           [0033]      FIG. 3  is a flow chart depicting an exemplary embodiment of the acoustic emission system. 
           [0034]      FIG. 4  is a graph schematically representing an exemplary pre-influx fingerprint. 
           [0035]      FIG. 5A  is depicts an FFT output on a display, wherein the output is a first set of processed waveforms representing a baseline level of activity. 
           [0036]      FIG. 5B  depicts an FFT output on a display, wherein the output has a second set of processed waveforms exceeding the baseline by more than 200%. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    The following detailed description of the preferred embodiments is presented only for illustrative and descriptive purposes and is not intended to be exhaustive or to limit the scope and spirit of the invention. The embodiments were selected and described to best explain the principles of the invention and its practical application. One of ordinary skill in the art will recognize that many variations can be made to the invention disclosed in this specification without departing from the scope and spirit of the invention. 
         [0038]    Corresponding reference characters indicate corresponding parts throughout the several views. Although the drawings represent embodiments of various features and components according to the present disclosure, the drawings are not necessarily to scale and certain features may be exaggerated in order to better illustrate embodiments of the present disclosure, and such exemplifications are not to be construed as limiting the scope of the present disclosure in any manner. 
         [0039]    References in the specification to “one embodiment”, “an embodiment”, “an exemplary embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
         [0040]    Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiment selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. 
         [0041]    The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Numerical values should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the states value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value. 
         [0042]    All ranges disclosed herein are inclusive of the recited endpoint and are independently combinable (for example, the range “40 decibels (‘dB’) to 60 dB” is inclusive of the endpoints, 40 dB and 60 dB, and all intermediate values. 
         [0043]    As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially,” may not be limited to the precise values specified. The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example the expression “from about 2 to about 4” also discloses the range “from  2  to  4 .” 
         [0044]    It should be noted that many of the terms used herein are relative terms. For example, the terms “upper” and “lower” are relative to each other in location, i.e. an upper component is located at a higher elevation than a lower component in a given orientation, but these terms can change if the device is flipped. The terms “inlet” and “outlet” are relative to a fluid flowing through them with respect to a given structure, e.g. a fluid flows through the inlet into the structure and flows through the outlet out of the structure. The terms “upstream” and “downstream” are relative to the direction in which a fluid flows or a signal moves through various components, i.e. the signal encounters an upstream component prior to encountering the downstream component. 
         [0045]    The terms “top” and “bottom” or “base” are used to refer to locations/surfaces where the top is always higher than the bottom/base relative to an absolute reference, i.e. the surface of the Earth. The terms “upwards” and “downwards” are also relative to an absolute reference; an upwards flow is always against the gravity of the Earth. 
         [0046]      FIG. 1  is a schematic diagram depicting a dissolving tank  160  having acoustic emission sensors  150  extending through walls  162  and a top  164  of the dissolving tank  160 . Although  FIG. 1  depicts a single smelt spout  110  and a single dissolving tank  160 , it will be understood that multiple smelt spouts  110  and dissolving tanks  160  may extend around the recovery boiler  100 . The acoustic emission sensor  150  has a reading end ( 222 ,  FIG. 2 ) and a second end ( 224 ,  FIG. 2 ), opposite the reading end  222  disposed outside of the dissolving tank  160 . The reading end  222  is disposed within the dissolving tank  160 . In other exemplary embodiments, an acoustic emission sensor  150  may be disposed entirely within the dissolving tank  160  such that both the reading end  222  and the second end  224  are disposed within the dissolving tank  160 . 
         [0047]    In still other exemplary embodiments, an acoustic emission sensor  150  may be disposed entirely outside of the dissolving tank  160  such that both the reading end  222  and the second end  224  are disposed outside of the dissolving tank  160 .  FIG. 1  depicts multiple acoustic emission sensors  150  disposed through the dissolving tank  160 . Multiple acoustic emission sensors  150  can be used to provide additional detailed signal data. An acoustic emission sensor  150  may be glued, fastened, or otherwise attached to the top  164  or walls  162  of the dissolving tank  160 . In other exemplary embodiments, an acoustic emission sensor  150  may be engaged to pipes proximate to or communicating with the dissolving tank  160 . In still other exemplary embodiments, magnets may engage the acoustic emission sensors  150  to the dissolving tank  160  or to pipes. 
         [0048]    As the smelt droplets  130  contact the green liquor  165 , the smelt droplets  130  emit acoustic emissions  167 . A passerby may hear some of these acoustic emissions  167  as an audible bang. An example acoustic emission sensor  150  may detect the acoustic emissions  167 , transduce the acoustic emissions  167  into an electric analog signal  307  ( FIG. 3 ), and pre-amplify the signal  307  before transmitting the amplified signal  311  for further processing. The acoustic emissions  167  may be sound waves or other energetic waves transmitted through the dissolving tank  160 . 
         [0049]    The acoustic emission sensors  150  may comprise a piezoelectric sensor, a micro-electro-mechanical system (“MEMS”) sensor, or other acoustic sensors configured to detect acoustic emissions  167  and transduce the acoustic emissions  167  into an electric signal  307 . Furthermore, an acoustic emission sensor  150  may comprise a filter  316  ( FIG. 3 ) such as a broad band acoustic emission filter. In other exemplary embodiments, an acoustic emission sensor  150  may comprise a narrow band acoustic emission filter. 
         [0050]    As shown in  FIG. 1 , a first end of the smelt spout  110  may be disposed in, engaged to, or extend toward the recovery boiler  100  and the second end of the smelt spout  110 , opposite the first end of the smelt spout  110  can be, disposed over, engaged to, or extend toward a dissolving tank  160 . Smelt  115  from a recovery boiler  100  flows down a smelt spout  110  into the dissolving tank  160 . The dissolving tank  160  is generally disposed under a hood  170 . Disruptors  140  emit fluid  145  that disrupts the flow of smelt  115  to create smelt droplets  130 . The fluid  145  is generally steam. The disruptors  140  may be shatter jet nozzles. 
         [0051]    In the depicted embodiment, an acoustic emission sensor  150 ′ extends into the fluid  145  emitted by the disruptor  140 . The area in which the fluid  145  extends may be known as the fluid path of the disruptor  140 . A wave guide  125  of at least one of the acoustic emission sensors  150  desirably extends into the liquid in the dissolving tank  160 . In exemplary embodiments depicted in  FIG. 1 , the liquid is green liquor  165 , but it will be understood that the liquid may be any liquid used in dissolving tanks  160 . A wave guide  125  of at least one other acoustic emission sensor  150 ′ does not contact the green liquor  165 . 
         [0052]    Acoustic emission sensor  150 ″ is configured to detect the first signs of uneven smelt flow. This is an example of using the acoustic emission sensors  150 ,  150 ′ and  150 ″ in a master-slave processing configuration, wherein a master sensor (see  150 ″) is mounted near an area of interest (e.g. the area in which the smelt contacts the fluid) and slave or guard sensors (see  150 ,  150 ′) surround the master sensor (see  150 ″) and eliminate noise generated from outside the area of interest. For example, the acoustic emission sensor  150 ′ having a waveguide  125  disposed in the fluid path of the disruptor  140 , may continuously monitor the acoustic emissions  167  produced in the fluid path of the disruptor  140 . The system may process the signal as described below and generate a signal profile indicative of normal disruptor operating conditions. A data processor  336  ( FIG. 3 ) may then subtract the signal profile of the normal disruptor operating conditions from the signal recorded by the master sensor (see  150 ″) positioned over the area in which the smelt contacts the green liquor  165 . In this manner, one may use the master-slave technique to eliminate irrelevant background noise from the signal generated at the master sensor  150 ″. 
         [0053]    In other exemplary embodiments, the guard sensors (see  150 ,  150 ′) may detect a baseline level of activity  442  ( FIG. 4 ) representative of a first rate of smelt flow at normal operating conditions in a guard sensor&#39;s detection area. For example, the acoustic emission sensor  150 ′ having a waveguide  125  disposed in the fluid path of the disruptor  140 , may continuously monitor the acoustic emissions  167  produced in the fluid path of the disruptor  140 . A data processor  336  communicating with the acoustic sensor  150 ′ may register a first set of processed waveforms  432  ( FIG. 4 ) indicative of a baseline level of activity  442  in the fluid path of the disruptor  140 . Before a smelt influx occurs, the data processor  336  may further register a second set of processed waveforms  433  ( FIG. 4 ) that exceeds the baseline level of activity  442  by more than 200%. The second set of processed waveforms  433  can be representative of a second rate of smelt flow. In certain exemplary embodiments, the data processor  336  may corroborate the first set of processed waveforms  432  produced from a guard sensor (see  150 ,  150 ′) with a first set of processed waveforms  432  produced from a master sensor (see  150 ″) to confirm that the dissolving tank  160  is operating at normal operating conditions. In still further exemplary embodiments, the data processor  336  may corroborate the second set of processed waveforms  433  produced by the guard sensors (see  150 ,  150 ′) with the second set of processed waveforms produced by the master sensor (see  150 ″). By comparing the second sets of processed waveforms  433 , the data processor may confirm the existence of a pre-influx fingerprint  372  ( FIG. 3 ) and thereby initiate a response to prevent or contain the smelt influx. 
         [0054]    Smelt droplets  130  may have an average temperature between 750° C. to 820° C. The average temperature of the green liquor  165  is about 70° C. to 100° C. To withstand the heat within the dissolving tank  160  and exposure to the fluid  145 , the acoustic emission sensors  150  may have a housing  151  made of a material configured to withstand the high temperatures and pressures. Examples include aluminum, duplex stainless steel, or regular stainless steel. Furthermore, example acoustic emission sensors  150  having electronics or transducing elements disposed within the dissolving tank  160  may be configured to operate temperatures up to and above 100° C. or at temperatures up to and above 160° C. depending on the average temperature within the dissolving tank  160 . Acoustic emission sensors  150  having electronics or transducing elements disposed outside of the dissolving tank may be configured to operate at temperatures up to and above 50° C. 
         [0055]    Referring to  FIGS. 1, 3, and 4 , the acoustic emission sensor  150  detects acoustic emissions  167  continuously and the data processor  336  may continuously process or transform the digital signal in preparation for signal analysis (e.g. analyzing the processed signal to detect a pre-smelt influx fingerprint  372 ). In other exemplary systems, the acoustic emission sensor  150  may sample the acoustic emissions  167  at time intervals, such as, for example, at 10 milliseconds (“ms”), one second, or sixty seconds. Once processed, the data processor  336  outputs an output signal O. The output signal O may be transmitted to a computer  338  and a display  339 . The output signal O comprises a first set of processed waveforms  432  representing a first rate of smelt flow (i.e. a baseline level of activity  442 ). Depending on the scale of the display  339 , the first set of processed waveforms  432  may appear to have a substantially constant amplitude. On a display  339  (see  FIG. 4 ,  FIG. 5A , and  FIG. 5B  for display outputs), the first set of waveforms  432  having a substantially constant amplitude may appear to map to a substantially straight line representing the average amplitudes of the first set of waveforms  432 . The display  339  may further output a floating threshold F, which is a threshold having an amplitude established by the time average measure of the signal. In  FIG. 4 , the floating threshold F represents the average amplitudes of the output signal O during a time interval. This first set of processed waveforms  432  represents a baseline level of activity  442  indicative of normal, even smelt flow and disruptor fluid flow based on inputs from one or more acoustic emission sensors  150 . Normal even smelt flow may comprise a first rate of smelt flow. The baseline  442  may further indicate nominal recovery boiler activity. In other exemplary embodiments, acoustic emission sensors  150 ′ placed near the outlet of each smelt spout  110  will detect the first signs of uneven smelt flows, problems with disruptors  140 , and smelt influx. 
         [0056]    Just before a sudden smelt influx, the amplitude of the processed signal  368  may be substantially lower than the baseline level of activity followed by a second set of processed waveforms  433  having amplitudes that are substantially higher than the baseline level of activity  442 , such as 100% higher, more than 150% higher, at least 200%, more than 200% higher, at least 300%, more than 300% higher, at least 500%, or more than 500% higher than the baseline level of activity  442 . The second set of processed waveforms  433  may be characterized by one or more amplitude peaks  461 . The amplitude peaks  461  of the second set of processed waveforms  433  substantially exceed the average amplitudes of baseline level of activity  442  by at least 200%. The second set of processed waveforms  433  can represent a second rate of smelt flow. The pre-influx fingerprint  372  comprises the second set of waveforms  433  substantially exceeding the baseline level of activity  442  by at least 200%. In still other exemplary embodiments, one or more amplitudes peaks  461  in the second set of waveforms  433  may further comprise the pre-influx fingerprint  372 . In still other exemplary embodiments, the pre-influx fingerprint  372  may comprise three or more amplitude peaks  461  in the second set of waveforms  433  substantially exceeding the baseline level of activity  442 . In still other exemplary embodiments, the pre-influx fingerprint  372  may comprise at least five amplitude peaks  461  in the second set of waveforms  433  substantially exceeding the baseline level of activity  442 . 
         [0057]    In the conditions leading up to a smelt influx, the frequency of acoustic emissions  167  may be lower than the baseline level of activity  442  at one or more of the smelt spouts  110 . That is, once pre-processed and processed, the output signal O may further comprise a third set of processed waveforms  441  having amplitudes below the average amplitudes of the baseline level of activity  442 . In systems comprising a display  339  or user interface, the third set of processed waveforms  441  may not be depicted, or the third set of processed waveforms  441  may be represented as a gap in the first set of processed waveforms  432 . The lower rate of acoustic emissions  167  would be independent of process conditions that would otherwise account for a lower rate of acoustic emissions  167 . An exemplary system  305  may compare processed signals  368  derived from the acoustic emission sensor  150 ′ disposed near the disruptor  140  and smelt spout  110  with processed signals  368  derived from the acoustic emission sensor  150  disposed throughout the dissolving tank  160  to determine whether a lower rate of acoustic emissions  167  is an expected outcome of current dissolving tank or recovery boiler conditions. If the lower rate of acoustic emissions  167  (and resulting third set of processed waveforms  441 ) is not an expected outcome of current dissolving tank or recovery boiler conditions, a low rate of acoustic emissions  167  can be indicative of smelt spout blockage, or indicate fluctuating smelt flow in the recovery boiler  100  and may further comprise the pre-influx fingerprint  372 . 
         [0058]    As seen in  FIG. 4 , the pre-influx fingerprint  372  may comprise an initial pre-influx fingerprint  372   a  characterized by the third set of processed waveforms  441  having a lower rate of acoustic emissions  167  that is not an expected outcome of process conditions and an imminent pre-influx fingerprint  372   b  characterized by a second set of processed waveforms  433  having one or more amplitude peaks  461  exceeding the first set of processed waveforms  432  by more than 200%. In certain exemplary systems, the data processor  336  may initiate a response, such as an alarm, or a change in process condition, or initiate smelt containment upon detection of the initial pre-influx fingerprint  372   a . In other exemplary systems, the data processor  336  may trigger a first alarm in response to detecting an initial pre-influx fingerprint  372   a . 
         [0059]    In other exemplary embodiments, the display  339  may display a first floating threshold  479  defined by the average amplitudes of the first set of processed waveforms  432  and a second floating threshold  480  defined by the second set of processed waveforms  433 . The pre-influx fingerprint  372  may further comprise a transition (see  372   b ) from the first floating threshold  479  to the second floating threshold  480 , wherein the second floating threshold  480  exceeds the first holding threshold  479  by at least 100%. That is, the pre-influx fingerprint  372  may comprise an increase in the floating threshold F by more than 100%. 
         [0060]    It will be appreciated that transforming an acoustic emission signal with any signal processing formula to predict a smelt influx, wherein the signal is above 20 KHz, and emanates from banging in a dissolving tank is considered to be within the scope of this disclosure. The 20 KHz frequency represents the upper limit of human hearing. It will be further appreciated that transforming an acoustic emission signal with any signal processing formula to predict a smelt influx, wherein the signal is above 100 KHz, and emanates from banging in a dissolving tank is considered to be within the scope of this disclosure. 
         [0061]    In other exemplary embodiments, the pre-influx fingerprint  372  may comprise an amplitude decay pre-influx fingerprint  372   c  in which two or more amplitude peaks  461  surpass the threshold  483  within a set unit of time. The threshold  483  may be a voltage threshold, floating threshold, system examination threshold, or other threshold set by the user or instrument against which the pre-influx fingerprint  372  may be compared. For example, when the display  339  displays an output signal O at a one second resolution, the amplitude decay pre-influx fingerprint  372   c  may comprise two or more amplitude peaks  461  surpassing the threshold  483  every second. In embodiments in which the display  339  displays an output signal O at 10 ms, the amplitude decay pre-influx fingerprint  372   c  may comprise two or more amplitude peaks  461  surpassing the threshold  483  every 10 milliseconds. The longer the amplitude peaks  461  surpass the threshold  483 , the more likely the smelt influx will cause the dissolving tank to explode (see  531 ,  FIG. 5B ). 
         [0062]      FIG. 2  depicts acoustic emission sensor  250  wherein the acoustic waves  167  ( FIG. 1 ) vibrate the wave guide  225 . The acoustic emission sensors  250  are configured to detect acoustic emissions  167  continuously. The acoustic emission sensor  250  has a wave guide  225  engaged to a transducer  285 . In the depicted embodiment, the transducer  285  is a piezoelectric crystal, although it will be understood that other transducer known in the art may be used. The wave guide  225  has a length L 1  extending from the transducer  285  to a reading end  222 . 
         [0063]    A protective sleeve  227  may shield a portion of the wave guide length L 3  from smelt splashes, liquor splashes, and the temperature and pressure inside the dissolving tank  260 . An exposed portion of the wave guide L 2  may be directly exposed to the green liquor  165  within a dissolving tank  265 . Operators may insert the wave guide  225  through an inlet sleeve  229  disposed within the wall  262  or top  164  of the dissolving tank  260 . Insulation  228  may seal the opening in the inlet sleeve  229  and isolate the wave guide  225  from dissolving tank walls  262  and sleeve  229  to minimize background signals, not relevant to banging detection. 
         [0064]    As shown in  FIGS. 2 and 3 , acoustic waves  167  contact the wave guide  225 , the wave guide  225  vibrates and mechanically transfers the acoustic wave  167  to the transducer  285 . The acoustic emission sensor  250  may have a threshold level. The threshold level is a threshold amplitude against which the amplitudes of the acoustic emissions  167  are compared. The acoustic emission sensor  250  may be configured to register acoustic emissions  167  that have amplitudes greater than the threshold level or less than the threshold level. The transducer  285  then transduces the acoustic emissions  167  above the threshold level into an initial electric signal  307 . The transducer  285  and associated electronics are generally protected within a housing  220 . 
         [0065]    A pre-amplifier  221  can then amplify the signal  307 . Subsequent amplifiers (see  313 ) may further amplify the signal  307  before the data processor  336  receives the pre-processed signal  318 . A typical acoustic emission sensor  250  generally has a pre-installed preamplifier  221 , although nothing in this disclosure limits the acoustic emission sensors  250  to having pre-installed preamplifiers  221 . The preamplifier  221  may amplify the signal  307  by generally 40 to 60 decibels (“dB”). A filter  316 , such as a high pass, low pass, or band pass filter may then filter the signal to a programmed frequency range above 20 KHz. In other exemplary embodiments, the filter  316  may filter the signal to a programmed frequency range of above 100 KHz. An A/D converter  326  may then convert the analog signal  307  to a digital signal  312 . The data processor  366  receives the amplified and filtered digital signal  311 ,  317 ,  312  (i.e. the pre-processed signal  318 ) and performs a processing or signal transformation method  356  to generate a processed signal  368 . The data processor  366  may further be configured to detect a pre-influx fingerprint  372 . An acoustic emission sensor  250  may include electronics for complete signal processing, which may include an amplifier  313 , filter  316 , A/D converter  326 , data processor  366 , and display  339 . In other exemplary embodiments in which the acoustic emission sensor  250  does not contain all processing elements, the cable  223  may transmit the signal to the next signal processor. In still other exemplary embodiments, the acoustic emission sensor  250  may transmit the signal wirelessly. 
         [0066]    Although acoustic emission sensors  250  may be configured to detect a range of acoustic emissions, acoustic emission sensors  250  typically have a resonant frequency. That is, the acoustic emission sensor  250  is generally configured to provide a highly defined electric signal at the resonant frequency. While the acoustic emission sensor  250  may detect acoustic waves  167  and transmit signals  307  above or below the resonant frequency, the detail of these non-resonant signals is comparatively less than the detail detected at the resonant frequency. In piezoelectric sensors, the thickness of the piezoelectric crystal defines the resonant frequency of the sensor. In an exemplary embodiment, the acoustic emission sensor  250  may have a resonant frequency above 20 KHz and desirably above 100 KHz. 
         [0067]    Piezoelectric sensors also typically have a temperature at which the piezoelectric crystal loses its piezoelectric properties. In embodiments where the acoustic emission sensor  250  is a piezoelectric sensor it is desirable to select a piezoelectric sensor configured to function at temperatures typical to dissolving tanks  260 . 
         [0068]      FIG. 3  is a flow chat representing an exemplary acoustic emission system  305  for detecting a pattern of banging in a dissolving tank  260 . One or more acoustic emission sensors  250  detect acoustic emissions  367  continuously. The signal transducer  385  transduces the acoustic emissions  367  to an electric analog signal  307 . Pre-processors  335  then pre-process the signal  307 . The order in which the signal  307  undergoes pre-processing prior to the application of the signal transformation method  356  is immaterial. The pre-processors  335  may comprise a filter  316 , an amplifier  313 , an A/D Converter  326 , or a computer  338 . Signal pre-processing may comprise one or more pre-processors  335 , less than all listed pre-processors  335 , or multiple types of select pre-processors  335 . For example, pre-processing may comprise both pre-amplifying the signal by 40 dB to 60 dB in the acoustic emission sensor  250  and further amplifying the signal in an amplifier disposed outside of the acoustic emission sensor  250 ; however, both the pre-amplifier disposed inside the acoustic emission sensor  250  and the amplifier disposed outside of the acoustic emission sensor  250  are considered amplifiers  313  for purposes of pre-processing and pre-processors  335 . 
         [0069]    The filter  316  generates a filtered signal  317 . The filter  316  may be an analog filter, high pass filter, low pass filter, band pass filter, digital filter or other filter used in signal processing. The filter  316  filters out undesirable low frequencies (high pass filter), undesirable high frequencies (low pass filter), or both undesirable high frequencies and low frequencies (band pass filter). Operators may select the desired filter  316  manually. In the exemplary systems disclosed herein, operators may isolate signal frequencies between 100 KHz and 300 KHz. This range is sufficiently high to escape most mechanical noise, but is also low enough to detect acoustic emissions  167  sufficiently far from the source. This can allow the operators to place the acoustic emission sensors  150  in the dissolving tank walls  262  or proximate to the dissolving tank  260 . In other exemplary embodiments, the filter  316  may be set automatically. Undesirable frequencies below 20 KHz (e.g. frequencies irrelevant to predicting sudden smelt influx) can be filtered out in this manner. In other exemplary embodiments, undesirable frequencies below 100 KHz may be filtered out in this manner. 
         [0070]    Without being bounded by theory, a high pass filter may be desirable to filter out hydraulic noise, which may emanate from turbulent flow of fluids, boiling of fluids, and leaks. The high pass filter may further filter out mechanical noise emanating from mechanical parts in contact with the system. Cyclic noise, e.g. repetitive noise from reciprocating or rotary machinery, may also be filtered out with a high pass filter. A low pass filter may be useful for filtering out electro-magnetic noise. Applicant has discovered that the frequency of mechanical noise is usually lower than an acoustic emission burst from the highest frequency range of banging in the dissolving tank  260 . 
         [0071]    The amplifier  313  amplifies the amplitude of the signal to produce an amplified signal  311 . An amplifier  313  may be an analog amplifier, pre-amplifier, digital amplifier, or other amplifier used in signal processing. An amplifier may pre-amplify the signal  307  produced from the signal transducer  385 . The signal  307  may be further amplified after filtering and an A/D converter  326  may then convert the analog signal to a digital signal  312 . Variations in the order of pre-processing are considered to be within the scope of this disclosure. 
         [0072]    It will be understood that some or all of the pre-processors  335  may reside in the acoustic emission sensor  250 , (e.g. within a single housing  251 , on a single circuit board, etc.). In other exemplary embodiments pre-processors  335  may reside in the system as separate devices outside of the acoustic emission sensor  250 . 
         [0073]    The pre-processors  335  produce a pre-processed signal  318 . A data processor  366  receives the pre-processed signal and applies a signal transformation method  356  to generate a processed signal  368 . The processed signal  368  may be output from the data processor as the output signal O. The data processor  366  may be a field programmable gate array (“FGPA”). In still other exemplary embodiments, the data processor  366  may be an application-specific integrated circuit (“ASIC”). The data processor  366  receives the processed signal  318  and may perform continuous counting analysis as the signal transformation method  356 . 
         [0074]    In other exemplary embodiments, the data processor  366  may conduct a Fast Fourier Transform (“FFT”) as the signal transformation method  356 . In other exemplary systems, the signal transformation method  356  may comprise the root mean square (“RMS”) method, standard deviation method, skewness method, kurtosis method, mean method, variance method, or the signal transformation method may use fuzzy logic, neural networks, and other signal processing methods to produce a processed signal  368 . The data processor  366  may be further configured to detect a pre-influx fingerprint  372  before outputting an output signal O. 
         [0075]    The output signal O may then be sent to a computer  338 , which may be configured to confirm the pre-influx fingerprint  372  and display the output signal O on a display  339  or other user interface. By way of example, the output signal O may be displayed as a continuous frequency spectrum display, a long-time envelope, or by displaying merely portions of the signal that exceed predetermined thresholds (e.g. the portions that exceed the first set of processed waveforms  432 ). 
         [0076]    In certain exemplary embodiments, the display  339  may display the processed signal in which the processed signal is a rectified, time averaged acoustic emission signal depicted on a linear scale and reported in volts. The display  339  may further display the energy of the processed signal, wherein the energy of the processed signal is evaluated as the integral of the volt-squared function over time. The signal strength may also be displayed in which the signal strength is measured as the areas of the rectified acoustic emission signal in units proportional to volt-seconds. In still other exemplary embodiments, the display  339  may display only processed signals that exceed a threshold. 
         [0077]    The threshold may be user-adjustable, fixed, or a floating threshold. The floating threshold varies with time as a function of noise output. A floating threshold can be used to distinguish between background noise and acoustic emission events in conditions in which the background noise is high and varying. A voltage threshold is a voltage level on an electronic comparator such that signals with amplitudes larger than this level will be recognized. 
         [0078]    The display  339  may display count trend resolutions at 10 milliseconds (“ms”), one second, 60 seconds, or any other time interval selected by the operators. All other trends (Fast Fournier Transform, root mean square, etc. are desirably displayed at a one second resolution. Because the acoustic emission sensors  150  detect acoustic emissions  167  continuously, the total time trend can last for as long as the acoustic emission sensors  150  remain functional, such as for a period of years. 
         [0079]    In further exemplary embodiments, when the computer  338  recognizes the pre-influx fingerprint  372 , the computer  338  may initiate a response  353 . The response  353  may comprise changing a process condition, such as restricting or blocking smelt flow with a restrictor plate such as the one disclosed in U.S. Pat. No. 9,206,548. In other embodiments, the response  353  may comprise adjusting a process condition within the recovery boiler. Changing a process condition within the recovery boiler may include adjusting the combustion rate, rate of black liquor flow, rate of air flow, air flow path, black liquor flow path, temperature, pressure, and concentration of reactants. Changing process condition may include changing a second rate of smelt flow indicative of a smelt influx into a first rate of smelt flow indicative of a baseline level of activity  442 , such as by restricting the rate of smelt flow in the smelt spout  110  or by preventing the smelt  115  in the smelt spout  110  from entering the dissolving tank  160 . Software may be configured to initiate the response  353 . In yet other embodiments, the response  353  may comprise, increasing the rate of fluid exiting the disruptor  140 . In still other exemplary embodiments, the response  353  may comprise triggering one or more alarms. Combinations of the disclosed responses  353  and other common ways to control smelt flow are considered to be within the scope of this disclosure. 
         [0080]    In certain exemplary embodiments, the data processor  366  may reside in the computer  338 . In other exemplary embodiments, a data process disposed outside of the computer  338  may begin processing the pre-processed signal  318  such as by using a signal transformation method  356  to transform the signal and then transmit the transformed signal to the computer  338  for pre-influx fingerprint detection. In still other exemplary embodiments, a computer  338  may comprise a pre-processor  335  and perform some or all of the signal pre-processing. In still other exemplary embodiments, a computer  338  may apply a signal transformation method  356 . 
         [0081]      FIG. 5A  depicts an FFT output signal O that may be seen on a display  339 . The output signal O is a first set of processed waveforms  532  representing a baseline level of activity  442  ( FIG. 4 ) in a dissolving tank  160 . In the depicted embodiment the pre-processed signal  318  has been filtered to above 100 KHz. This is well beyond the range of human hearing and microphones that detect audio waves transmitted through air. A user may set a threshold  583  at for example, at 20. In the depicted scale, the first set of processed waveforms  532  has occasional, randomly distributed, threshold-surpassing peaks  578 . The amount of times these occasional threshold-surpassing peaks  578 ′ generally surpass the threshold  583  depends on where the threshold is set and the specified period of time. For example, when the display  339  is displaying an amplitude peak  578  every second, the occasional threshold-surpassing peaks  578 ′ may not surpass the threshold  583  more than a few times per minute. In the depicted embodiment, the threshold  583  is set at 20 and the display resolution is set to one second. Generally, occasional threshold-surpassing peaks  578 ′ do not surpass the threshold  583  at three consecutive seconds. Because the processed signal  368  ( FIG. 3 ) is a signal above 20 KHZ and the background noise has been filtered out during pre-processing  335 , the occasional threshold-surpassing peaks  578 ′ represent normal smelt banging, or periodic minor smelt influxes that do not jeopardize the structural integrity of the dissolving tank. The first set of processed waveforms  532  and baseline level of activity  442  comprise these occasional threshold-surpassing peaks  578 ′. It will be understood that the occasional threshold-surpassing peak represent normal smelt banging activity. The depiction of these occasional threshold-surpassing peaks will vary depending on a specific dissolving tank environment and the rate and scale at which users choose to display the output O. The display  339  may further display a floating threshold F ( FIG. 4 ), which represents the average amplitudes of the output signal O during a specified time interval. 
         [0082]    In  FIG. 5B , the FFT output signal O comprises a first set of processed waveforms  532  transitioning into a second set of processed waveforms  533 . In the depicted embodiment the pre-processed signal  318  has been filtered to above 100 KHz. The second set of processed waveforms  533  comprises a pattern of amplitude peaks  561  that consistently surpass the threshold  583  over a specified period of time. For example, in  FIG. 5B , the threshold  583  is set at 20 and the display  339  displays an amplitude peak every second. 
         [0083]    The pre-influx-fingerprint  372  may comprise the pattern of amplitude peaks  561 . Furthermore, the pre-influx fingerprint  372  may comprise a cyclic pre-influx fingerprint  372   d  characterized by repeating amplitude decay pre-influx fingerprints (see  372   c ,  FIG. 4 ) over a time interval. In the depicted embodiment, the cyclic pre-influx fingerprint  372   d  comprises at least five amplitude decay pre-influx fingerprints C 1 , C 2 , C 3 , C 4 , and C 5 . In the depicted embodiment, the cyclic pre-influx fingerprint  372   d  occurred over a period of approximately three hours. It will be understood however, that a cyclic pre-influx fingerprint  372   d  may comprise at least two amplitude decay pre-influx fingerprints  372   c . The system described herein may initiate a change in process condition upon detection of any pre-influx fingerprint  372 . In the depicted embodiment, the system may trigger a first alarm or change in process condition upon detection of the cyclic pre-influx fingerprint  372   d  and a second alarm or change in process condition upon detection of a prolonged pre-influx fingerprint  372   e . 
         [0084]    The pre-influx fingerprint  372  may comprise a prolonged pre-influx fingerprint  372   e . A prolonged pre-influx fingerprint  372   e  is shown in  FIG. 5B , over time interval E. A prolonged pre-influx fingerprint  372   e  has multiple amplitude peaks  561  over the resolution interval and may not readily exhibit the amplitude decay pre-influx fingerprint  372   c  or the cyclic pre-influx fingerprint  372   d . It will be understood that the time interval E may vary depending upon the configurations and conditions of a particular dissolving tank  160  and the sampling frequency of the acoustic emission sensor  150 , data processor  366  and resolution of the output signal O. In the depicted output signal O for example, the time interval E occurred over approximately one hour and forty five minutes. Regardless of how the prolonged pre-influx fingerprint  372   e  is depicted or displayed, the prolonged pre-influx fingerprint  372   e  indicates that a smelt influx is imminent or presently occurring. The system, or a computer in the system, may initiate an immediate change in process conditions or contain the smelt in response to detecting a prolonged pre-influx fingerprint  372   e . If smelt flow is not contained upon detection of a prolonged pre-influx fingerprint  372   e  an explosion  531  may be imminent. Upon adjustment of a process condition or containment of the smelt influx, the second set of processed waveforms  533  may transition back into the first set of processed waveforms  532  indicative of a baseline level of activity  442 . 
         [0085]    The pre-influx fingerprint  372  may further comprise a count trend and a Fast Fourier Transform trend, wherein the count trend depicts decreasing banging intensity in the dissolving tank  160  prior to frequency bands in the Fast Fourier Transform trend surpassing the first set of processed waveforms  432  by more than 300%. 
         [0086]    Furthermore, an exemplary method of predicting a smelt influx in a dissolving tank may comprise: detecting acoustic emissions emanating from smelt banging within the dissolving tank with an acoustic sensor; converting the acoustic emissions into an initial electric signal; amplifying the initial electric signal to produce an amplified signal; filtering the amplified signal to a programmed frequency range of greater than 20 KHz; outputting a first output signal in the programmed frequency range, wherein the first output represents a baseline level of activity  442  within the dissolving tank in the absence of a smelt influx; outputting a second output signal substantially exceeding the first output signal by more than 200%, wherein the second output signal comprises signal peaks, and wherein three or more signal peaks in the second signal output comprise the pre-influx fingerprint; reducing smelt flow into the dissolving tank in response to the pre-influx fingerprint. 
         [0087]    An exemplary system may comprise: a dissolving tank adjacent to a recovery boiler, wherein a smelt spout is in fluid communication with the recovery boiler and the dissolving tank; smelt disposed in the smelt spout, wherein the smelt flows from the recovery boiler through the smelt spout into the dissolving tank at a first rate, and wherein the smelt contacts a liquid in the dissolving tank and thereby generates acoustic emissions; an acoustic emission sensor having a reading end oriented to detect the acoustic emissions emanating from the dissolving tank, wherein the acoustic emission sensor has a transducer in signal communication with the reading end, and wherein the transducer is configured to transduce the acoustic emissions into an initial electric signal; a pre-processor configured amplify, filter, and digitize the initial electric signal to produce a pre-processed signal having a frequency of greater than 20 KHz, wherein the pre-processor is disposed downstream of the transducer; a data processor in signal communication with the pre-processor, wherein the data processor is configured to transform the pre-processed signal with a transformation method to produce an output signal, wherein the output signal comprises a first set of processed waveforms representative of the first rate, and a second set of waveforms representative of a second rate of smelt flow, the second set of processed waveforms having amplitude peaks exceeding the first set of processed waveforms by more than 200% to comprise a pre-influx fingerprint. 
         [0088]    A further exemplary system may comprise: a dissolving tank adjacent to a recovery boiler, a smelt spout having a first end proximate a recovery boiler and a second end opposite the first end proximate a dissolving tank, wherein the smelt spout is configured to receive a smelt from the recovery boiler and convey the smelt to the dissolving tank; an acoustic emission sensor having a reading end configured to detect acoustic emissions emanating from the smelt contacting a liquid in the dissolving tank, and wherein the acoustic emission sensor has a transducer in signal communication with the reading end, and wherein the transducer is configured to transduce the acoustic emissions into an initial electric signal; a pre-processor configured amplify, filter, and digitize the initial electric signal to produce a pre-processed signal having a frequency of greater than 20 KHz, wherein the pre-processor is disposed downstream of the transducer; a data processor in signal communication with the pre-processor, wherein the data processor is configured to transform the pre-processed signal with a transformation method to produce an output signal, wherein the output signal comprises a first set of processed waveforms representative of a first rate of smelt flow, and a second set of waveforms representative of a second rate of smelt flow, the second set of processed waveforms having amplitude peaks exceeding the first set of processed waveforms by more than 200% to comprise a pre-influx fingerprint. 
         [0089]    While this invention has been particularly shown and described with references to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.