| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | August 20, 1991 |
| PATENT TITLE |
Concentration cycles, percent life holding time and continuous treatment concentration monitoring in boiler systems by inert tracers |
| PATENT ABSTRACT | Concentration cycles, percent life holding time for a component in the boiler and continuous treatment concentrations are monitored or determined in a boiler system by adding to the feedwater an inert tracer in a predetermined concentration C.sub.I, which reaches a final concentration C.sub.F at steady state in the boiler and which exhibits a blowdown concentration C.sub.t at different points in time. The component is an inert tracer having no significant carryover in the steam, nor significant degradation during boiler cycles. The tracer is monitored by continuously converting a characteristic of its concentration to an analog which may be recorded as a function of time |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | December 19, 1988 |
| PATENT CLAIMS |
We claim: 1. A method of determining blowdown:feedwater concentration cycles in a boiler water system where steam is generated in a boiler from fresh feedwater fed thereto, and wherein the concentration of impurities in the boiler water is reduced by withdrawing boiler water as blowdown while admitting additional feedwater as makeup, said concentration cycles being the value of the concentration (C.sub.F) of a component in the blowdown at steady state divided by the concentration (C.sub.I) of that component in the feedwater, said component likewise having no appreciable carryover into the steam, said method comprising the steps of: employing as the component an inert tracer added to the feedwater in a known concentration (C.sub.I), next, sensing a characteristic of the tracer in the blowdown at steady state equivalent to its blowdown concentration (C.sub.F), converting the sensed characteristic to (C.sub.F), and then recording the concentration cycles value of C.sub.F /C.sub.I for the boiler, said characteristic being one selected from the group consisting of emissivity, absorbance and ion activity. 2. Method according to claim 1 in which a treating agent is added to the feedwater in a predetermined concentration to oppose the tendency of impurities to settle as solids on the boiler surfaces, in which the calculated cycles value of C.sub.F /C.sub.I is compared to a cycles value deemed standard for operation of the boiler, and in which the blowdown rate or dosage of treating agent is changed to establish the standard operating cycles value if the calculated value is not standard. 3. Method according to claim 1 in which the tracer is fluorescent, in which the sensed characteristic of the tracer is emissivity, and including the steps of: converting the sensed emissivity characteristic of the tracer to a voltage analog, and continuously monitoring and recording said analog. 4. Method according to claim 2 including the steps of: converting the sensed characteristic of the tracer to a voltage analog, and continuously monitoring and recording said analog. 5. In a boiler system where a boiler charged with feedwater generates steam therefrom, wherein metal ions detrimental to boiler efficiency are present in the feedwater as impurities and wherein a treating agent in a predetermined concentration is added to the feedwater having the role of removing or neutralizing said impurities, a method of correcting the dosage of treating agent if there is a variance from the amount deemed optimum for the role, including the steps of: adding to the feedwater an inert tracer in a concentration proportioned to the treating agent concentration, measuring a characteristic of the tracer equivalent to its blowdown concentration in the feedwater, said characteristic being one selected from the group consisting of emissivity, absorbance and ion activity, measuring the concentration of metal ions in the feedwater, comparing the two measurements to determine if the concentration of treating agent varies from optimum, and changing the dosage of treating agent if said determination shows a variance. 6. A method according to claim 5 including the steps of converting the sensed characteristic to a voltage analog, and using the voltage analog for comparison to the measurement of metal ion concentration in the feedwater. 7. Method according to claim 5 in which a sample of steam condensate is taken and analyzed for tracer presence. 8. Method according to claim 6 in which a sample of steam condensate is taken and analyzed for tracer presence. 9. In a boiler system where a boiler charged with feedwater of mass M, which may be an unknown mass, generates steam therefrom at a particular temperature, wherein the concentration of impurities in the boiler water is reduced by withdrawing boiler water as blowdown at a particular rate B (mass per unit of time) which may also be an unknown, a method of determining the boiler constant K=M/.sub.B including the steps of: adding to the feedwater an inert tracer in a predetermined concentration C.sub.I which eventually reaches a final state of steady concentration C.sub.F in the boiler; determining at different times the concentration C.sub.t of the tracer in the blowdown and determining C.sub.F of the tracer at steady state; and plotting the straight line slope of 1n(1-C.sub.t /C.sub.F) versus time which slope gives the value of the reciprocal of K. 10. Method according to claim 9 including the step of continuously sensing in the blowdown a characteristic of the tracer equivalent to its blowdown concentration C.sub.t ; said characteristic being one selected from the group consisting of emissivity, absorbance and ion activity; continuously converting said equivalent to an analog and recording the concentration analog as a function of time during the time period required for the tracer to reach its steady-state concentration C.sub.F in the boiler; determining C.sub.F from said recording, calculating the values of C.sub.t /C.sub.F for different times according to said recording, and determining K therefrom according to claim 9. 11. A method of determining it there is mechanical carryover of water droplets into a body of steam generated in a water boiler charged with feedwater, comprising the steps of adding an inert tracer to the feedwater, taking a sample of steam condensate and analyzing the sample for tracer presence. |
| PATENT DESCRIPTION |
INTRODUCTION This invention relates to boiler water systems and in particular to a method and means for determining cycles, percent life holding time and monitoring treating agents added to the boiler feedwater. Deposits, particularly scale, can form on boiler tubes. Each contaminant constituting the source of scale has an established solubility in water and will precipitate when it has been exceeded. If the water is in contact with a hot surface and the solubility of the contaminant is lower at higher temperatures, the precipitate will form on the surface, causing scale. The most common components of boiler deposits are calcium phosphate, calcium carbonate (in low-pressure boilers), magnesium hydroxide, magnesium silicate, various forms of iron oxide, silica adsorbed on the previously mentioned precipitates, and alumina. At the high temperatures found in a boiler, deposits are a serious problem causing poor heat transfer and a potential for boiler tube failure. In low-pressure boilers with low heat transfer rates, deposits may build up to a point where they completely occlude the boiler tube. In modern intermediate and higher pressure boilers with heat transfer rates in excess of 200,000 Btu/ft.sup.2 hr (5000 cal/m.sup.2 hr), the presence of even extremely thin deposits will cause a serious elevation in the temperature of tube metal. The deposit retards flow of heat from the furnace gases into the boiler water. This heat resistance results in a rapid rise in metal temperature to the point at which failure can occur. Deposits may be scale, precipitated in situ on a heated surface, or previously precipitated chemicals, often in the form of sludge. These collect in low-velocity areas, compacting to form a dense agglomerate similar to scale. In the operation of most industrial boilers, it is seldom possible to avoid formation of some type of precipitate at some time. There are almost always some particulates in the circulating boiler water which can deposit in low-velocity sections. Boiler feedwater, charged to the boiler, regardless of the type of treatment used to process this source of makeup, still contains measurable concentrations of impurities. In some plants, contaminated condensate contributes to feedwater impurities. When steam is generated from the boiler water, water vapor is discharged from the boiler, with the possibility that impurities introduced in the feed water will remain in the boiler circuits. The net result of impurities being continuously added and pure water vapor being withdrawn is a steady increase in the level of dissolved solids in the boiler water. There is a limit to the concentration of each component of the boiler water. To prevent exceeding these concentration limits, boiler water is withdrawn as blowdown and discharged to waste. FIG. 1 illustrates a material balance for a boiler, showing that the blowdown must be adjusted so that impurities leaving the boiler equal those entering and the concentration maintained at predetermined limits. Substantial heat energy in the blowdown represents a major factor detracting from the thermal efficiency of the boiler, so minimizing blowdown is a goal in every steam plant. One way of looking at boiler blowdown is to consider it a process of diluting boiler water impurities by withdrawing boiler water from the system at a rate that induces a flow of feed water into the boiler in excess of steam demand. Blowdown used for adjusting the concentration of dissolved solids (impurities) in the boiler water may be either intermittent or continuous. If intermittent, the boiler is allowed to concentrate to a level acceptable for the particular boiler design and pressure. When this concentration level is reached, the blowdown valve is opened for a short period of time to reduce the concentration of impurities, and the boiler is then allowed to reconcentrate until the control limits are again reached. In continuous blowdown, on the other hand, which is characteristic of all high pressure boiler systems, virtually the norm in the industry, the blowdown valve is kept open at a fixed setting to remove water at a steady rate, maintaining a relatively constant boiler water concentration. SUMMARY AND OBJECTIVES OF THE INVENTION Under the present invention, boiler cycles may be readily calculated by adding an inert tracer to the feedwater being charged to the boiler in a known concentration and then determining an analog of its concentration in the blowdown. Resultantly, if the cycles value does not compare to standard, then the blowdown rate is altered or the dosage of treating agent is changed, or both. The change in concentration of the tracer during the time required for it to attain its final, steady state concentration in the boiler water may also be determined by monitoring the concentration of the tracer in the blowdown, as a function of time. Once the final steady state concentration of the tracer is known, the percent life holding time of the boiler can be calculated, enabling a judicious choice of a particular treating agent to be made. The concentration of the treating agent in the feedwater and elsewhere may itself be monitored by proportioning the treating agent and tracer. The primary objects of the present invention are to employ an inert tracer, preferably a fluorescent tracer, to simplify the determination of cycles [impurity (contaminant) concentrations] in boiler waters, especially on a continuous basis; to employ an inert tracer to calculate the percent life holding time (e.g. half-life time); and to employ an inert tracer as a reference standard monitor to determine the concentration of a treating agent (e.g. dispersant polymer) used to resist (oppose) the tendency of impurities to settle on the boiler surfaces. The inert tracer may be used for all or any single determination. BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram showing how boiler water solids (scales) are controlled by blowdown; FIG. 2 is a curve showing the variation of a concentration ratio as a function of time; FIG. 3 is a logarithmic plot based on FIG. 2 also showing how the concentration ratio varies with time; FIG. 4 is a schematic view of instrumentation; FIG. 5 is a plot showing how closely tracer and treating agent concentration analogs compare at a ratio of 900/1; FIG. 6 is a diagram of combined instruments to measure cycles; FIG. 7 is a diagram showing use of combined instruments in a feedback control system to maintain treating agent/metal ion feed ratio at a preset value; FIG. 8 depicts graphically continuous monitoring values; FIG. 9 is a schematic diagram for colorimetry monitoring; FIGS. 10 and 11 illustrate the use of an ion selective electrode as a monitor transducer. DETAILED DESCRIPTION A: Boiler Cycles Boiler cycles is defined herein as the concentration ratio of a particular impurity (or component) in the blowdown C.sub.F and the feedwater C.sub.I, that is, ##EQU1## and the value (which is an equilibrium value) will always be greater than one since the impurity in the blowdown is always more concentrated than in the feedwater due to water removed as steam. For high pressure boiler systems determination of cycles by this method is very difficult since feedwater purity is very high and therefore concentration of feedwater contaminants is very low. Monitoring cycles in boiler systems is quite important since suspended solids can concentrate in the boiler water up to the point which exceeds their solubility limit as discussed in more detail above. If the cycles value is too low, there is wastage of water, heat and any treating agent which may be present. If the value is too high, there is likelihood of dissolved solids settling out. Inert tracers, such as fluorescent tracers, offer a particular advantage for cycles determination since they do not appreciably carry over into the steam and can be selectively detected at very low levels (0.005 ppm or less). The tracer will have a characteristic which can be sensed and converted to a concentration equivalent. For example, fluorescent emissivity, measured by a fluorometer, is proportional to concentration; emissivity can be converted to an electrical analog. Their concentration in the boiler water does not contribute significantly to conductivity, which is of advantage. B: Percent Life Holding Time (% HT) Any time there is a change in addition of a treating agent added to the feedwater, it takes time for the boiler to reach steady state where the concentration of the component is at equilibrium. This time lapse is the holding time for the boiler. If percent life holding time is known, it may be used for judicious or efficient treating agent dosage. It may indicate a need to adopt a different cycles value. In any event, the life holding time, that is, the percent time for a component to reach its final concentration in the boiler, is a diagnostic tool for the boiler; each boiler is as unique as a fingerprint and the present invention permits the boiler to be fingerprinted easily and quickly. Knowledge of the cycles value does not take into account all the specifics of the boiler. Different boilers, though of similar construction, can operate at the same number of cycles but, depending on the operating boiler volume and blowdown rate, they can have quite different percent life holding times. Steady state is defined herein as the circumstance where a stable or inert component (e.g. the inert tracer) in the feedwater reaches its final concentration (C.sub.F) in the boiler without any appreciable or significant changes in the system except generation of steam. The concentration of the component inside the boiler and in the blowdown will be the same (C.sub.t) at any particular point in time so that a measurement of one measures the other. The rate at which a stable component will reach steady state in the boiler water is determined by the boiler characteristics M (mass of boiler water, in lbs) and B (blowdown rate, in The time required to reach steady state can be an important factor for application of the treating agent. In terms of its differential equation, this time value is expressed as t=-K1n(1-C.sub.t /C.sub.F) (1) where C.sub.F =final steady-state boiler water concentration of the component K=boiler constant=M/B C.sub.t =concentration of component in the blowdown at any time t. Equation 1 can be rearranged: 1n (1-C.sub.t /C.sub.F)=-(1/K)t (2) and a plot of 1n(1-C.sub.t /C.sub.F) versus time gives the slope of 1/.sub.K. Using these equations, it is possible to calculate percent life holding time (%HT) of the boiler. %HT(P)=-K1n [1-(P/100)] (3) where (P) symbolizes percent life of component C and P=C.sub.P /C.sub.F x100 where C.sub.P =concentration of component C at the desired %HT and where C.sub.F =steady state boiler concentration of component C. Thus, at the half life of the boiler for example [%HT(50)], P=50 and equation (3) becomes % HT(50)=0.693K. If K and C.sub.F are known, %HT(P) can be calculated for an assumed value of C.sub.P ; or if %HT(P) is assumed, then C.sub.P can be calculated in equation (3). The boiler constant K is rarely known in the field, since very often neither the operating boiler volume nor the blowdown rate is exactly known. It is very important for the application of internal boiler treatments, by a treating agent meant to prevent or inhibit scaling, to know the boiler percent life holding time. One reason is that different treating agents perform differently over prolonged periods at a given temperature, or at different temperatures for the same time, and cost may be a factor. To be on the safe side, the recommendation may be that the treating agent be held in the boiler no more than ninety percent, or even fifty percent, of the holding time of the boiler. In other words, thermal stability or sustained potency of internal boiler treatment at high temperature (e.g. up to 300.degree. C.) is affected by the time required to reach steady state, calculated for example by the boiler percent life holding time especially in high pressure boilers in which the pressure may be 2000 pounds. It is possible that in some high pressure systems the blowdown rate has to be increased in order to decrease the percent life holding time and still maintain acceptable treating agent concentration in the boiler water. In other words, if the percent life holding time is inordinately long so that scarcely any treating agent at reasonable cost can withstand the rigors of time-temperature-pressure inside the boiler, then the blowdown rate should be increased since that will bring in more (cold) feedwater. Besides, the treating agent then has less residence time in the boiler. Inert tracers such as fluorescent tracers can be used very effectively to measure the boiler constant K=M/B and the percent life holding time by determining how tracer concentration varies as a function of time. Thus, the tracer becomes the "component" in the above equations by which cycles and percent life holding time may be calculated under the present invention. C: Tracer Monitoring The concentration of the treating agent is very often difficult to monitor due to complicated, tedious analytical methods or difficulty in proper operator training. The addition of an inert tracer can help solve this problem and allows continuous monitoring to be undertaken. If the treating agent/tracer ratio is known, any variation in tracer concentration will be directly related to the concentration of the treating agent which can therefore be easily controlled by continuous monitoring of the tracer. The use of an inert tracer also makes it possible to identify improper treating agent feed due to mechanical problems (such as feed pumps) and changes in boiler operation due to general malfunctions (such as a plugged blowdown valve). Naphthalene Sulfonic acid (2-NSA) is an inert fluorescent compound which may be employed under the present invention. The concentration of the fluorescent tracer is preferably measured by excitation at 277 nm and emission observed at 334 nm. The emission results are referenced to a standard solution of 0.5 ppm 2-NSA (as acid actives). A Gilford Fluoro IV dual-monochromator spectrofluorometer was used for fluorometric determinations. By "inert" we mean the tracer is not appreciably or significantly affected by any other chemistry in the system, or by the other system parameters such as metallurgical composition, heat changes or heat content. There is invariably some background interferences, such as natural fluorescence in the feedwater, and in such circumstances the tracer dosage should be increased to overcome background interference which, under classical analytical chemistry definitions, shall be less than 10%. FIG. 1 is an aid to the description to follow. It shows a typical material balance for a boiler. Blowdown (BD) needs to be adjusted so that impurities ("solids") leaving the boiler equal those entering; the boiler concentration of impurities is maintained at predetermined limits. The balance may be: boiler water containing an equivalent of 1000 mg/l of potential solids; feedwater (FW) at one million lb/day; solids equal to 100 mg/l; solids added/day equals 100 lb; blowdown: 100000 lb/day; solids content 1000 mg/l; solids removed, 100 lb/day; steam at 900,000 lb/day; solids essentially zero. The cycles value is 1000/100=10. The boiler solids concentration can be decreased by opening (moreso) the blowdown valve 10; feedback controller 12B also opens (moreso) the feedwater valve 14. The concentration of the tracer component in the feedwater may be monitored and controlled (12F) as will be explained. A. Determination of Boiler Concentration Cycles Dependability, reliability and accuracy of the present invention was determined in a laboratory where the M and B values for K could be measured ("mechanical mode") exactly, and where chloride and sodium analyses could be conducted without incurring corrosion of equipment and deposition of solids on the equipment. The inert tracer was 2-NSA. A determination of boiler concentration cycles was made by measuring 2-NSA concentration in both feedwater (C.sub.I) and blowdown (C.sub.F). The instrumentation to be described is shown in FIG. 4. The results were compared with cycles determined by other different methods: as mechanical, conductivity, and chloride (or sodium) ions. |
| PATENT EXAMPLES | available on request |
| PATENT PHOTOCOPY | available on request |
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