Patent Publication Number: US-6655322-B1

Title: Boiler water blowdown control system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application does not claim the benefit of any previously filed and copending nonprovisional applications (or international applications designating the United States of America) under 35 U.S.C. §§ 120, 121 or 365(c). 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     The present invention was not developed either wholly or partially under any federally sponsored research and development program or grant. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an improved blowdown control method for preventing the formation of scale within a boiler system. 
     2. Description of the Related Art 
     Successful and efficient operation of industrial steam boiler systems requires that the various chemical constituents of boiler water be controlled in a manner that will avoid or at least minimize both the formation of mineral scale or deposits on heat exchange surfaces and the corrosion of metal surfaces within the steam boiler system. While deposition tends to involve corrosion products generated within the boiler system, scale is more commonly results from the precipitation of scale forming minerals, typically salts of calcium and/or magnesium introduced into the boiler systems with the makeup water. 
     Even with the best makeup water pretreatment, scale and deposition control chemistry is required to control residual amounts traces of scale forming minerals and any corrosion products formed within the boiler system. 
     Scale control products may be broadly classified as precipitating or non-precipitating depending on the manner in which they act to prevent scale formation. Precipitating products commonly rely on carbonate or phosphate chemistry to precipitate the hardness minerals and form a fine “mud.” These fine precipitates are typically maintained in suspension through the use of one or more polymer dispersants and are removed along with a portion of the boiler water during the boiler blowdown process. Non-precipitating products, however, are more commonly based on one or more sequestrants or chelant chemistries that react with scale forming minerals to form a soluble compound. As with the precipitating products, a portion of the dissolved compound are removed along with a portion of the boiler water during the boiler blowdown process, thus preventing scale formation. 
     A fundamental requirement of boiler system operation is the need to maintain the concentration of all hardness or scale forming minerals and their related compounds at levels below their solubility limits to prevent scale formation. As noted above, a variety of industrial boiler water scale and deposit inhibitors are helpful in this process. However, while boiler water treatment chemicals can be used to increase boiler system tolerance to scale forming minerals, chemical treatments alone are inadequate to prevent scale formation at higher boiler water concentration levels. Therefore, in addition to boiler water treatment chemicals, a volume of the concentrated boiler water must be periodically removed from the boiler and replaced with a generally corresponding volume of less concentrated feed water during operation with a blowdown process. This is referred to as boiler blowdown. 
     Blowdown processes can be broadly classified as either a manual blowdown process or an automatic blowdown process. As the term implies, manual blowdown is the periodic removal of boiler water by a person, typically the boiler operator, who opens one or more valves on the boiler system to initiate the blowdown process and then closes the valve(s) to terminate the blowdown process. In connection with the manual blowdown process, the boiler operator may also run chemical or other tests on boiler water samples to determine when to initiate the blowdown and/or to determine if a sufficient quantity of the concentrated boiler water was removed. Alternatively, the boiler operator may conduct the manual blowdown according to a schedule providing prescribed intervals and/or durations for the blowdown process. This may include intermittent ‘bottom blowdowns’ or a blowdown rate control valve setting (continuous blowdown) to try to maintain a set point for a desired boiler water dissolved solids level. 
     Automatic blowdown processes, on the other hand, utilize some form of instrumentation for measuring one or more properties of the boiler system that generally correspond to the concentration of scale forming minerals in the boiler water. When the measured parameter(s), such as electrical conductivity, reach a predetermined level, the automatic blowdown system will open one or more valves to initiate the blowdown process and/or introduce feed water into the boiler system. The automatic system will typically continue the blowdown process until the measured parameter(s) indicate that the scale forming mineral concentration within the boiler water has been reduced to an acceptable level and then terminate the blowdown process. 
     Although the automatic blowdown processes tend to represent an improvement over manual blowdown processes, the use of such processes still tend to result in periods of the boiler operating cycle during which the boiler water is either over-concentrated, increasing the likelihood of scale formation, or under-concentrated, reducing boiler efficiency and increasing the consumption of expensive boiler treatment chemicals. Further, the sensors used for measuring the selected parameters typically require increased maintenance and cost associated with sensor replacement, calibration, and/or cleaning. Some types of sensors, in particular those used to measure conductivity, are themselves sensitive to thermal shock and eventually can lead to an underreporting of the boiler water conductivity, further delaying the blowdown cycle and increasing the potential for scale formation. 
     BRIEF SUMMARY OF INVENTION 
     The present invention provides a method for controlling boiler water concentration in boiler systems that utilize bimodal feedwater pumps that have only an ON mode and an OFF mode capable of providing feedwater to the boiler at a substantially constant flow rate in the ON mode. The present method is generally not applicable, however, to boiler systems that utilize boiler feedwater pumps that incorporate proportional feedwater flow control valves that regulate the feedwater flow rate within a given range. 
     In those boiler systems that utilize a bimodal boiler feedwater pump, the present invention utilizes the same signal or signals that control the boiler feedwater pump (i.e., turning it ON or OFF) to open or close a blowdown valve in synchronization with the boiler feedwater pump. When the boiler feedwater pump is turned ON, the blowdown valve is opened and boiler water is directed to a rate control valve that fixes the flow rate of the blowdown water. Using the pump curve data for the boiler feedwater pump, the flow capacity of the rate control valve is set to a predetermined percentage of the feedwater flow rate produced when the boiler feedwater pump is ON under the prevailing boiler conditions. This synchronized operation of the boiler feedwater pump and the blowdown valve assures that a fixed, proportional volume of concentrated boiler water is removed from the boiler whenever feedwater is entering the boiler and thereby maintain the scale forming minerals within a desired concentration range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing illustrating the basic mechanical and electrical components of a first embodiment of the present invention. 
     FIG. 2 is a schematic drawing illustrating the basic mechanical and electrical components of a second embodiment of the present invention. 
     FIG. 3 is a schematic drawing illustrating the basic mechanical and electrical components of a third embodiment of the present invention. 
     FIG. 4 is graph representing a boiler feedwater pump curve. 
     FIG. 5 is a representative a flow control valve chart. 
     FIG. 6 is a graph demonstrating the superior control achieved by the present. invention over a prior art manual blowdown method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a method for automatically maintaining the concentration of scale forming minerals in boiler water for boiler systems that utilize bimodal feedwater pumps that deliver a substantially constant feedwater flow when ON. In order to achieve control, such boiler systems require variable feedwater flow regulation in which the feedwater flow is adjusted in response to changes in the steam demand and /or boiler steam drum water level. The present invention provides an accurate means for providing automated control of the concentration of dissolved solids in boiler water that is responsive to changing steam loads and/or intermittent boiler operation in which boiler water is removed at a predetermined rate equal to a selected percentage of the feedwater flow rate. The present invention provides a simple and accurate means for controlling boiler blowdown in certain types of boiler systems without the control deficiencies resulting from manually controlled blowdown, changing feedwater quality, or the concerns regarding the reliability of conductivity controller probes, the need for temperature compensation or the need for complicated instrumentation calibration procedures. 
     The standard terminology in the boiler and steam generation industry defines the rate of boiler blowdown as a percentage of the feedwater flow. For example, a five percent blowdown means that boiler water is systematically removed at a rate equal to five percent of the feedwater flow rate into the boiler in order to maintain an acceptable concentration of dissolved solids in the boiler water. Industry guidelines prepared and distributed by the ASME. (American Society of Mechanical Engineers) and the ABMA (American Boiler Manufacturers Association) provide recommended concentration limits for the total dissolved solids (TDS) present in boiler water for various ranges of boiler water parameters. 
     The ASME and ABMA guidelines, among others, are widely used by engineers and boiler operators to determine the amount of boiler water that must be periodically removed from the boiler system to maintain the TDS concentrations within the desired range. Depending on the condition of the feedwater and the boiler operating conditions, the amount of boiler water that must be removed can vary widely, typically comprising between 2 to 20 percent of the feedwater flow. The boiler water is typically removed using a blowdown process in which a valve is opened, either manually or automatically, thereby allowing a portion of the boiler water to be removed from the operating boiler. 
     In order for manual blowdown processes to provide acceptable TDS control a number of conditions must exist simultaneously within the boiler system. These conditions are 1) the boiler steam load is constant; 2) the feedwater quality is constant; 3) the percent condensate return is constant; and 4) the boiler operators perform regular tests on the boiler water, typically at least once per shift, to determine the proper blowdown rate settings. In the real world, however, this combination of conditions rarely, if ever, exists. 
     Indeed, most industrial and commercial boiler systems will typically experience variations in the steam load, feedwater quality and/or the percentage of condensate return. These variations may be the result of, for instance, ambient temperature changes that affect heating requirements, plant production rate changes, and/or different production equipment coming online or going offline. When faced with such continually changing conditions, boiler operators are constantly “chasing” the proper manual settings on blowdown control valves in their efforts to maintain the desired boiler water chemistry. 
     The most basic automatic blowdown systems provide for a series of automatic blowdowns conducted according to a preset schedule, with the boiler operator adjusting the schedule based on changes in the boiler system operation timed and executed. Again, however, the reliance on testing and subsequent adjustment of the blowdown schedule render such systems vulnerable to the same “chasing” problems associated with fully manual blowdown processes. More commonly, automatic blowdown systems incorporate a control module that will modulate the frequency, duration and/or volume of the blowdown process based on input from one or more detectors. 
     The detectors employed in such systems typically monitor the TDS in the boiler system by measuring, either continuously or intermittently, the conductivity of the boiler water. When the measured conductivity exceeds a predetermined set point, the control module will open a blowdown valve to remove a portion of the boiler water from the boiler drum. For such systems to operate properly, however, the boiler water should be cooled and flow continuously over the detector, thus increasing the complexity of the boiler system in order to provide a blowdown circulation loop and cool the blowdown before it reaches the detector. Boiler systems that do not or cannot incorporate the additional piping and equipment necessary to provide a continuous flow of cooled blowdown across the detector typically experience problems with flashing of the blowdown on the conductivity probe, affecting measurement accuracy, and thermal shocks that degrade detector integrity. Further, a boiler system utilizing such detectors must also provide for regular detector calibration intervals, increasing the maintenance expenses. 
     The present invention, however, is completely mechanical and thus does not depend on conductivity detectors or other instrumentation that requires regular calibration, are subject to intermittent failures, experience drifts in accuracy, or require frequent, regular operator input. As a result, the present invention reduces maintenance costs and provides energy savings by maintaining blowdown rates as a fixed percentage of feedwater flow. Further, because the blowdown stream flows only when the feedwater pump is ON, the blowdown may be passed through a feedwater heat exchanger to provide a generally consistent level of feedwater heating. As a result, the boiler system operation is both improved and simplified, providing a blowdown rate and boiler system energy balance that are substantially constant and are not subject to the fluctuations and complexity associated with automatic conductivity-controlled blowdown systems. 
     As illustrated in FIG. 1, a boiler system according to the present invention comprises a feedwater source  3 , typically comprising both condensate return  1  and makeup water  2 , containing feedwater  7  characterized by a relatively low concentration of TDS. The feedwater flows out of the feedwater source  3  through a line  4  that is connected to a feedwater pump  5 . When the feedwater pump is turned ON, a substantially constant flow of feedwater is delivered through line  6  to the boiler drum  9  where the feedwater is incorporated into the boiler water  10 . The boiler water  10  is then heated to generate steam which exits the boiler through line  11  for use in other equipment or operations. 
     The boiler drum is also provided with a sensor  15  that monitors one or more parameters within the boiler drum, preferably at least the boiler water level. When the sensor indicates that additional water is required in the boiler drum, e.g., if the water level reaches a predetermined minimum level, it generates a first control signal  16  that turns the feedwater pump ON and opens blowdown valve  13 . As the feedwater pump begins to deliver feedwater to the boiler drum, a portion of the boiler water  10  flows through line  12 , the blowdown valve and a rate control valve  14  that limits the blowdown flow to within a narrow predetermined percentage range of the feedwater flow. Once the boiler water level has returned to a satisfactory level, the sensor generates a second control signal that turns the feedwater pump OFF and simultaneously closes the blowdown valve. As will be appreciated, the second control signal may simply be the cessation of the first control signal. 
     Another embodiment of the present invention that also provides for treatment chemical feed is illustrated in FIG.  2 . In addition to the basic components illustrated in FIG. 1, the second embodiment of the invention includes a treatment chemical source  17  from which a solution comprising one or more treatment chemicals is fed through a line  18  to a chemical feed pump  19 . As with the first embodiment, the sensor  15  will generate a control signal  16  in response to a boiler parameter. 
     In the second embodiment, however, in addition to turning the feedwater pump ON and opening the blowdown valve the same control signal  16  will turn chemical feed pump  19  ON to deliver treatment chemicals to the feedwater line  6  (or directly into the boiler drum) at a predetermined rate to maintain an acceptable level of treatment chemicals in the boiler water  10 . Once the boiler water level has returned to an acceptable level, the sensor will generate a control signal that simultaneously turns the feedwater pump OFF, closes the blowdown valve, and turns the chemical feed pump OFF. 
     A third embodiment of the invention is illustrated in FIG.  3 . In addition to incorporating the basic components illustrated in FIGS. 1 and 2, the third embodiment of the invention includes a heat exchanger  22 . This heat exchanger allows recovery of a portion of the heat energy from the blowdown stream entering through line  21  and uses that heat energy to increase the temperature of the feedwater entering through line  6  and increase the energy efficiency of the boiler system. 
     For example, the amount of blowdown required may be calculated by a boiler operator using the following variables: 
     FW, feedwater, pounds per hour (kilograms/hour); 
     BD, blowdown, expressed as a percentage of the feedwater; 
     E, evaporation, expressed as a percentage of the feedwater; 
     R fw , cycles of concentration (also referred to as CoC or simply cycles) (based on the acceptable ratio of feedwater and boiler water constituents); and 
     Steam, steam production, expressed in pounds per hour (kilograms/hr); and the following formulas: 
                       R   fw     =       boiler                 water                 constituent       feedwater                 constituent              
             [   1   ]               BD   =       100      %       R   fw               [   2   ]                         E =100%− BD   [3] 
     
       
         
           
             
               
                 
                   FW 
                   = 
                   
                     Steam 
                     
                       E 
                       / 
                       100 
                     
                   
                 
               
               
                 
                   [ 
                   4 
                   ] 
                 
               
             
           
         
         
         
             
         
       
     
     EXAMPLE 
     To configure a boiler system to achieve steam production of 11,885 pounds per hour (5391 kg/hr) in a 0-300 psig (0-2.07 MPa) boiler using feedwater having 7.5 ppm silica, a boiler operator may refer to ASME guidelines for 0-300 psig (0-2.07 MPa) boilers that provide for a maximum silica concentration in the boiler water of 150 ppm. Based on the feedwater analysis and the ASME guidelines, the boiler operator can calculate the allowable cycles of concentration using formula [1] as:          R   fw     =         150                 ppm       7.5                 ppm       =     20                 cycles                 of                 concentration                       
     Once the cycles of concentration have been determined, the boiler operator can determine the amount of blowdown required to maintain this level of concentration using formula [2], 
     
       
           BD =100/R fw =100/20=5.00% blowdown required 
       
     
     as well as the evaporation rate and the quantity of feedwater required to maintain the boiler system using formulas [3] and [4] 
     
       
           E =100−5.00=95% evaporation 
       
     
     
       
           FW =11,885/0.95=12,510 lbs/hr (5674 kg/hr) feedwater 
       
     
     Thus, in order to operate the boiler system to produce 11,885 lbs/hr (5391 kg/hr) of steam, the boiler operator must input feedwater into the boiler drum at a rate of 12,510 lbs/hr (5674 kg/hr) and remove the concentrated boiler water at a rate of 625 lbs/hr (283 kg/hr) through a blowdown process. 
     To implement the boiler system described in the Example, if the boiler system is configured so that the nominal head at the feedwater pump is 250 feet (76.2 meters), the boiler operator or engineer will consult the pump performance date for the feedwater pump, i.e., the pump curve illustrated in FIG. 4, to determine the feedwater pump output under those operating conditions, i.e., 50 gallons/minute (189.2 liters/minute). In order to achieve the desired 5% blowdown, the rate control valve will, in turn, need to be configured, using a flow control chart as illustrated in FIG. 5, to pass 2.5 gallons/minute (9.46 liters/minute) at its nominal operating pressure. 
     Accordingly, once the boiler operator has determined the necessary blowdown rate, the blowdown rate flow control valve, preferably an in-line orifice, is sized to provide a maximum upper limit on flow that will produce the desired blowdown rate. It is preferred that the actual rate flow control valve be selected so that the desired maximum upper limit on blowdown flow is approximately 50% of the control valve capacity at the operating pressure. The selection of the rate control valve is commonly guided by a valve selection chart, e.g., FIG. 5, that a boiler operator or engineer can use to determine the necessary valve sizing based on the boiler system pressure and the maximum flowrate desired. Although not preferred, it is certainly possible to use a fixed geometry orifice as the rate flow control valve to produce the blowdown rate, but any flow rate adjustments would require an adjustment of the operating pressure and may be more difficult to implement. Once set, the blowdown rate control valve will provide a fixed, repetitive blowdown flow as long as the feedwater pump is running. By setting the blowdown flow rate at the desired percentage of the feedwater feed rate, a boiler system according to the present invention maintains the desired cycles of concentration in the boiler system. Once the boiler system start up has been completed, the feedwater and boiler water chemical constituents may be checked again to confirm and fine-tune the blowdown valve rate setting. 
     As a result of the parallel connections made between the feedwater pump electrical circuit and the blowdown valve, a boiler system according to the present invention will operate automatically and provide acceptable blowdown control over a range of boiler system variations. As will be appreciated, the selection of the particular pumps, valves, solenoids, sensors, and control modules necessary to configure a particular boiler system to operate in accord with the present invention will be within the ability of boiler operators and other plant maintenance personnel. A four-v port TASCO® bronze Flocontrol® valve has been found to be particularly well suited for use in implementing the present invention in existing boiler systems. 
     An experimental two-month trial of a blowdown control system according to the present invention was conducted on an industrial gas-fired boiler system to confirm the ability of the present invention to provide effective control of scale forming minerals. As reflected in the un-neutralized conductivity data provided in FIG. 6, the present invention afforded significantly improved control of the boiler water conductivity, reducing instances of excessive blowdown, i.e., conductivity less than 2500 μmhos, and insufficient blowdown, i.e., conductivity greater than 3500 μmhos, thereby reducing the risk of scale formation while simultaneously improving boiler efficiency, by coordinating the feedwater and blowdown processes. 
     The improved control of the boiler water concentration will also allow the boiler system to be operated at higher cycles of concentration, increasing energy savings, reducing expenses associated with makeup water and disposal of blowdown water. As an example, for a gas-fired boiler system operating configured to produce steam at a rate of 5175 lbs/hr (2347 kg/hr) with 60% condensate return at a boiler pressure of 125 psig (0.86 MPa) and a fuel cost of $8.50/10 6  btu, ($8.06/1000 Mjoule) shifting from 15 cycles of concentration to 35 cycles of concentration with blowdown control according to the present invention could produce annual savings on the order of $6000 in fuel costs alone. When the decreased maintenance and expense of the present invention compared to a conventional conductivity-based automated system are considered, the boiler operation savings will only increase. 
     The description and illustrations of the present invention provided above are merely exemplary in nature and it is anticipated that those of ordinary skill in the art will appreciate that many variations or rearrangements of the specific apparatus described are possible without departing from the spirit and scope of the invention in which the blowdown and feedwater flows are synchronized and proportional regardless of changes in other system variables.