Abstract:
Methods and apparatus are provided for using tracers to monitor industrial process equipment to which liquid is added and from which liquid is removed. In preferred embodiments, the methods are directed to detecting water leakage in a boiler in which an aqueous temperature control liquid is supplemented with feedwater at a known or unknown rate and is removed as blowdown at a known rate. The methods include determining the blowdown removal rate, adding a tracer to the temperature control liquid at a rate that is directly proportional to the blowdown removal rate, analyzing the blowdown to determine the measured concentration of tracer contained therein, deriving an expected concentration of tracer in the blowdown, and comparing the measured and expected concentrations to detect excessive variance between them.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to methods and apparatus for monitoring industrial water process equipment. More particularly, the invention is directed to the detection of leaks in water process equipment such as black liquor recovery boilers. 
     BACKGROUND OF THE INVENTION 
     A boiler is an apparatus in which water or some other aqueous temperature control liquid to which makeup water is added and from which blowdown is removed is vaporized into steam by the application of heat from a furnace or heat-generating process system. In most instances, the temperature control liquid is brought into close, indirect contact with the process system to facilitate heat transfer. Leakage in a boiler can result not only in contamination and fouling of the temperature control liquid and the process system, but also in undesired chemical reactions. This is particularly true for the black liquor recovery boilers used in many paper mills. In black liquor recovery boilers, the escape or leakage of aqueous temperature control liquid from the so-called &#34;water side&#34; of the boiler into the hot, highly caustic &#34;fire side&#34; can result in violent explosions. 
     The prior art provides numerous techniques for monitoring and controlling leaks in black liquor recovery boilers and other boiler systems. For example, U.S. Pat. No. 5,320,967 (Avallone, et al.) discloses a boiler system leak detection method that involves introducing an inert tracer to the boiler in a known and uniform proportion to the feedwater, sensing a characteristic of the tracer in the boiler at steady state, converting the sensed characteristic to a value equivalent to the concentration of the tracer in the temperature control liquid, and activating a signal when there is excessive variance in the concentration of the tracer. However, the method disclosed by Avallone, et al. is limited by its requirement that the tracer be detected (sensed) when the boiler is at steady state, which is said to occur only when there is no significant change in any of five process parameters: the concentration of the tracer in the boiler; the blowdown rate; the feedwater rate; the rate of feeding tracer to the boiler; and the steaming rate in the absence of boiler leakage. 
     Consequently, there remains a need in the art for more flexible leak detection methods which can be employed in boiler systems that are not at steady state, that is, where one or more process parameters is subject to change. 
     OBJECTS OF THE INVENTION 
     It is one object of the present invention to provide methods and apparatus for leak detection in industrial boilers. 
     It is a further object of the invention to provide methods and apparatus for detecting leaks in black liquor recovery boilers. 
     It is yet another object of the invention to provide methods and apparatus that can be used in boiler systems other than those operating at steady state. 
     BRIEF DESCRIPTION OF THE INVENTION 
     These and other objects are satisfied by the present invention, which provides methods and apparatus for the detection of leaks in boilers to which liquid is added and from which liquid is removed. In preferred embodiments, an aqueous temperature control liquid is supplemented over time with feedwater at a known or unknown rate and is removed as blowdown at a known rate. The methods of the invention include adding a non-volatile tracer to a liquid containment means in the boiler at a rate that is directly proportional to the liquid removal rate. Following addition of the tracer, the liquid exiting the containment means is analyzed to determine the measured concentration of tracer contained therein. An expected concentration of tracer in the removed liquid then is calculated based on the rate at which tracer has been added to the containment means. The expected tracer concentration and measured tracer concentration then are compared, preferably by subtraction. A positive remainder (over and above a statistically-derived baseline noise level) indicates a possibility that tracer is exiting the boiler other than in the removed liquid. If the remainder is lies outside an acceptable range, an operator initiates an investigation of the boiler for leaks. 
     The present invention also provides apparatus suitable for indicating leaks in boilers. Apparatus according to the invention comprise: flow monitoring means for monitoring a rate associated with liquid removal; addition means for adding tracer to the liquid containment means at a rate that is proportional to the rate at which liquid is removed therefrom; measuring means in communication with the addition means for measuring the concentration of the tracer in the removed liquid; derivation means in communication with the measuring means for deriving an expected concentration of tracer in the removed liquid based on the rate of tracer addition; and comparison means in communication with the measuring means for comparing the measured and expected tracer concentrations. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The numerous objects and advantages of the present invention may be better understood by those skilled in the art by reference to the accompanying FIG. 1, a schematic representation of a monitoring system according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The methods and apparatus of the present invention can be used to monitor virtually any type of equipment to which liquid is added and from which liquid is removed. The methods and apparatus of the invention preferably are used to monitor boilers, especially black liquor recovery boilers. Representative boilers are disclosed by U.S. Pat. Nos. 3,447,895 (Nelson, et al.), 4,462,319 (Larson), 4,498,333 (Parthasarathy), and 4,502,322 (Tero), the contents of which are incorporated herein by reference. 
     An exemplary monitoring system according to the invention is shown in FIG. 1, wherein a first (&#34;water side&#34;) containment means or &#34;boiler&#34; 10 containing temperature control liquid 12 is adjacent to and in thermal communication with a second (&#34;fire side&#34;) containment means 14 that typically contains hot vapors and a molten smelt bed. Boiler 10 is in fluid communication with blowdown line 18 for the discharge of blowdown to discharge port 20 and with steam line 22 for the discharge of steam to condensation means 24. The discharge of blowdown is controlled through actuation of blowdown valve 26, which can be operated manually or under the control of an external computer or some other processing means (not shown). It is not necessary that the blowdown valve be under control of or monitored by the system of the invention. Between boiler 10 and valve 26, blowdown line 18 is in fluid communication with monitoring means 34 to provide information on the blowdown flow rate. Downstream of valve 26, part of the blowdown flow is diverted to cooling means 30 for sampling and analysis. Measuring means 32, such as an online analyzer based upon UV/visible spectroscopy, is downstream of cooling means 30 to provide means for determining the concentration of tracer in the blowdown. Measuring means 32 and 34, in turn, are in electrical communication with processing means 28. 
     Blowdown flow monitor 34 can use any number of flowmeters designed for high temperature liquid use including orifice plate meters, vortex-shedding meters, flow nozzle meters, venturi meters, strain gauge meters, doppler (transit time) meters, turbine meters, mag meters, and pitot-type devices. Cooling means 30 can be any number of sample coolers with sufficient cooling water flowing through it to lower the sample water temperature to ambient. The selected measuring means 32 depends on the tracer being used. For example, with a molybdate tracer a FPA 800 analyzer manufactured by Tytronics (Waltham, Mass.) can be used. Alternatively, if a phosphate is used as a combined tracer and treatment chemical, a Hach (Loveland, Colo.) Series 5000 phosphate analyzer can be used. 
     Boiler 10 also is in fluid communication with feedwater source 36 via feed line 38. As shown in FIG. 1, feed line 38 is in fluid communication with tracer source 40 via chemical feed line 42. Alternatively, tracer source 40 is directly coupled with boiler 10 via chemical feed 44. In either embodiment, the absolute amount of tracer added to boiler 10 is controlled and recorded by processing means 28. Tracer source 40 can contain tracer or a mixture of tracer and other treatment chemicals to be fed to the boiler. Downstream of tracer source 40, a flow measurement apparatus (&#34;drawdown assembly&#34;) 46 provides a precise measurement and control of tracer flow being injected into the feedwater line 38 via an electrically driven pump 48. Both drawdown assembly 46 and pump 48 are in electrical communication with processing means 28. Drawdown assembly 46 provides a feedback signal to processing means 28 which, in turn, controls the pumping rate of pump 48 to ensure a verified feed of chemicals to feed lines 42 or 44. Processing means 28 and associated drawdown assembly 46 preferably are constructed in accordance with the teachings of U.S. Pat. No. 4,897,797, assigned to the same assignee as this invention and incorporated by reference herein. 
     During normal operation, the controlled addition of feedwater to boiler 10 compensates for the removal of blowdown and steam, and maintains a desired volume of temperature control liquid 12 within boiler 10. In accordance with the present invention, the temperature control liquid is further supplemented with a known amount of at least one tracer. Tracers according to the invention are organic and/or inorganic compounds that are soluble in the feedwater, temperature control liquid, and blowdown under the operating conditions encountered. Tracers also should be thermally stable and non-volatile. In certain embodiments, the selected tracer is a reactive chemical treatment added to the boiler to, for example, control corrosion or the deposition of scale. In other embodiments, the tracer is substantially nonreactive with (i.e., inert to) the feedwater, temperature control liquid, blowdown, and contacted surfaces of the process equipment. It is preferred that the feedwater introduced to the process equipment upstream of tracer injection be substantially free of tracer, i.e., that it contain less than about 0.002 ppm of the tracer and/or that it have a tracer concentration less than about one percent of the tracer concentration of the temperature control liquid. 
     Tracers according to the invention possess at least one physical property that permits their detection in samples of the blowdown. Preferred tracers absorb and/or emit measurable amounts of light (or form reaction products that absorb and/or emit measurable amounts of light) in proportion to the amount of tracer present. Tracers preferably are detectable by at least one analytical technique selected from electrochemistry, UV/visible spectrophotometry, or fluorescence emission spectroscopy. Representative tracers include the tracers disclosed in U.S. Pat. Nos. 4,783,314 (Hoots, et al.), 4,966,711 (Hoots, et al.), 5,041,386 (Pierce, et al.), 5,200,106 (Hoots, et al.), 5,304,800 (Hoots, et al.), and 5,320,967 (Avallone, et al.), the contents of which are incorporated herein by reference. Preferred tracers are iodide-, sodium-, lithium-, and phosphate-containing chemical treatments and transition metal compounds, including salts, ions, oxy-anions, cations, and complexes of metals belonging to Group VIII of the Periodic Table. Particularly preferred are molybdenum-containing compounds, including molybdenum salts such as sodium molybdate. 
     Tracers can be added to the temperature control liquid in any number of ways. For example, tracer-containing solutions can be added directly to the temperature control liquid, they can be pre-mixed with feedwater that is added to the temperature control liquid, or they can be pre-mixed with treatment chemicals and then added to the feedwater. The particular addition technique is not believed to be critical, so long as the tracer is added to the temperature control liquid as a liquid of known concentration at a rate that is directly proportional to the rate at which blowdown is discharged. In general, the amount of added tracer should be sufficient to establish a tracer concentration of from about 0.2 to about 20 ppm in the temperature control liquid and, hence, in the blowdown. Preferably, sufficient tracer is added to establish a tracer concentration of from about 1.0 to about 5.0 ppm, more preferably from about 2.0 to about 3.0 ppm. 
     A natural consequence of steam generation in a boiler is a concentrating of incoming, non-volatile components. To control this &#34;cycling up&#34; effect, one or more volumes of the relatively-concentrated temperature control liquid typically are removed from the boiler as blowdown and corresponding volumes of relatively-dilute feedwater are added. In accordance with the present invention, the blowdown either is sampled at regular or irregular known intervals or is continuously monitored to determine the measured concentration of tracer contained therein. This determination can be made, for example, by directly or indirectly comparing the amount of light absorbed by the blowdown with the amount of light absorbed by standard solutions containing known concentrations of the tracer and other solutes found in the blowdown. Alternatively, the blowdown is mixed with a reagent that reacts with the tracer and imparts a color to the blowdown in proportion to its tracer concentration. The tracer concentration can be determined by comparing the color with the color of one or more standard solutions prepared by mixing solutions containing known concentrations of the tracer with the reagent. 
     The methods of the invention further involve deriving an expected concentration of tracer in the blowdown. At equilibrium, the rate at which the tracer-containing solution is added to the boiler is directly proportional to the rate at which blowdown is removed. In the absence of leaks or other means for the removal of tracer, the amount of tracer that is added to the boiler per unit time should equal the amount of tracer that is removed from the boiler per unit time. This can be expressed as: 
     
         C.sub.E =C.sub.S ·S/B 
    
     where: 
     C E  is the expected equilibrium tracer concentration in the blowdown; 
     C S  is the concentration of tracer in the tracer-containing solution; 
     S is the rate of addition of tracer-containing solution; and 
     B is the blowdown rate. 
     Boilers do not always operate at equilibrium. For example, non-equilibrium conditions can arise during boiler start-up or when target treatment concentrations associated with the tracer are changed. Therefore, corrections for non-equilibrium conditions should be made by modifying the equation for predicting C E . One preferred modification is: 
     
         C.sub.E(i) =C.sub.T -[(C.sub.T -C.sub.(i-dt))(e.sup.-Bdt/M)] 
    
     where 
     C E (i) is an expected concentration of tracer in the blowdown at time i; 
     C T  is a tracer concentration setpoint, calculated as (C S  ·S/B); 
     dt is an elapsed time interval; 
     M is the mass of the temperature control liquid; and 
     C.sub.(i-dt) is a measured value for tracer concentration in the blowdown at time i-dt. 
     The expected and measured concentrations of tracer in the blowdown are compared, preferably by subtracting the measured concentration from the expected concentration. If all of the tracer that is added to the boiler is removed in the blowdown, the remainder will be zero, plus or minus some statistically-acceptable variance. If however, there is a leak in the boiler or in some other portion of the equipment between the point at which tracer is added and the point at which blowdown is sampled, the measured concentration of tracer in the blowdown will be significantly less than the expected concentrations. Thus, if comparison of the expected and measured concentrations indicates a statistically-significant variance, the boiler operator initiates an investigation into the source of such variance. This typically involves physical and/or acoustical examination of the boiler and, depending on the magnitude of the variance, complete shutdown of the boiler. 
     Those skilled in the art will appreciate that numerous changes and modifications may be made to the preferred embodiments of the invention and that such changes and modifications may be made without departing from the spirit of the invention. It is therefore intended that the appended claims cover all such equivalent variations as fall within the true spirit and scope of the invention.