Methods and apparatus for monitoring and controlling PH phosphate and sodium to phosphate ratio in boiler systems operating with captive alkalinity

Methods and apparatus for monitoring and controlling pH, phosphate, and sodium to phosphate ratio in boiler systems operating with captive alkalinity chemistry are described. The methods for monitoring and controlling these parameters include the steps of determining the phosphate concentration via FIA, determining the pH, and using these values to determine the sodium to phosphate ratio. These values are then compared to the optimum values for the boiler system being treated; on the basis of this comparison, adjustments to the feed rate of water treatment chemicals being added to the system are then made. The apparatus includes a device for determining phosphate concentration via FIA, a device for determining pH, a means for determining sodium to phosphate ratio and a device for adjusting the feed rate of water treatment chemicals being added to the system. The device for adjusting the chemical feed rate is computer controlled.

FIELD OF THE INVENTION 
The present invention relates to methods and apparatuses generally useful 
for monitoring and/or controlling three related chemical parameters in a 
boiler system operating with captive alkalinity chemistry control--pH, 
phosphate and sodium to phosphate ratio. More particularly, this invention 
relates to methods and apparatuses useful for maintaining these three 
chemical parameters within predetermined, desired ranges through the 
addition of one or more water treatment chemicals containing sodium and/or 
phosphate. 
BACKGROUND OF THE INVENTION 
Treatment of water used for steam generation, such as in a boiler system, 
generally involves, among other things, maintaining various water 
chemistry parameters in accordance with often rigid specifications. 
Typically, these specifications set forth acceptable ranges into which the 
chemistry parameters being maintained should fall. Parameters maintained 
by system operators include, inter alia, pH, sodium concentration, 
phosphate concentration and the ratio of sodium to phosphate. In 
maintaining the system, the operator must first determine the level of 
each parameter and then determine how to adjust each parameter so as to 
maintain each parameter within its acceptable range. The present invention 
is directed to automated means for making these determinations. 
The boiler system environment can be generally described as harsh, having 
an alkaline pH and operating at high temperatures and pressures (i.e. 
temperatures above about 100.degree. C. and pressures above about 900 
psig). This environment is intolerant to deviations from the chemistry 
specifications or contaminants in the system. Contaminants can affect 
various water chemistry parameters in the system, and can therefore 
greatly increase the difficulty of keeping these parameters within the 
specified acceptable range. For example, contaminants can cause 
fluctuation in the pH of the system. Contaminants can also cause carryover 
of chemicals from the boiler to the steam side of the system, and lead to 
deposits and corrosion throughout the system. 
Deposits are formed when the concentration of a particular contaminate 
exceeds its solubility and therefore precipitates out of solution. 
Typically in the form of scale or sludge, deposits can form on any of the 
boiler equipment, although the boiler tubes are particularly susceptible. 
Scale on the boiler tubes reduces the heat transfer ability of the tubes 
which in turn reduces the efficiency of the boiler unit as a whole. Also, 
scale and other deposits can increase the potential for boiler tube 
failure. The problem becomes more severe in systems with high heat 
transfer rates and in high pressure boilers. 
Corrosion is another problem which negatively impacts operation of a 
boiler, and is most typically exemplified by the attack of steel by 
oxygen. This attack can occur in any portion of the boiler system in which 
oxygen is present. High temperatures and low pH conditions generally 
accelerate oxygen attack. Corrosion can also result from alkali or acid 
attack, which is most typically seen in high pressure boilers when caustic 
concentrates in local areas. Failure to maintain water chemistry within 
specifications is also believed to contribute to a corrosive environment. 
Corrosion, like deposits, generally decreases the efficiency of the boiler 
unit. Corroded boiler tubes which cannot conduct water must be taken out 
of service; each tube taken out of service reduces the available heat 
transfer surface in the unit. Often times, the problem of corroded boiler 
tubes may be severe enough to mandate replacement of the tubes, or even 
the boiler itself. Such replacements are costly and require shutdown of 
the system. In addition, the settling of corrosion products can lead to 
sludge accumulation in the boiler system, which may contribute to further 
problems with corrosion and heat transfer efficiency. 
One way to combat potential problems such as deposits and/or corrosion in 
the boiler system is through internal treatment with corrective chemicals 
of the boiler feedwater, the boiler water itself, the steam or the 
condensate. One type of chemical internal treatment of boiler water is 
known in the art as coordinated phosphate/pH or captive alkalinity 
treatment. The present invention is directed to methods and apparatuses 
for use in boiler systems which employ this type of chemical treatment. 
Captive alkalinity is typically recommended for boiler systems which use 
demineralized quality make-up water and in which the internal treatment 
program must contribute little solids to the system, such as, for example, 
those boilers with high heat transfer rate. 
It is desirable to have some alkalinity in the water for the system to 
achieve its optimum pH and to help prevent corrosion of the boiler 
internals. Alkalinity generally promotes formation of a protective iron 
oxide film on the boiler tubes which deters corrodants. If the alkalinity 
is too high, however, it can lead to corrosion. For example, high 
concentrations of caustic, such as sodium hydroxide (NaOH), can form a 
concentrating film on boiler tubes which results in caustic attack or 
caustic gouging usually characterized by pits or grooves in the boiler 
tube. This film attacks the protective oxide layer and provides a fresh 
site for steel oxidation and further caustic attack. 
Captive alkalinity control is designed to prevent the formation of free 
caustic in the system; "free caustic" as used herein generally describes 
any unbonded, strongly alkaline material. Proper control of boiler water 
pH and phosphate through captive alkalinity control is believed to ensure 
the elimination, or at least the reduction, of free caustic. This is 
particularly important in a boiler system operating at high temperatures, 
since the potential for caustic attack increases with temperature. By 
reducing free caustic, the occurrence of boiler tube failure due to 
concentrating film attack may also be reduced. 
In captive alkalinity control, the reduction of concentrating caustic 
films, as well as the maintenance of pH, phosphate concentration and 
sodium to phosphate ratio, is accomplished with a phosphate 
buffer--typically disodium phosphate and either monosodium phosphate or 
trisodium phosphate. All of these compounds contribute both sodium and 
phosphate to the system being treated. In addition, sodium hydroxide and 
phosphoric acid themselves can also be used to adjust the sodium to 
phosphate ratio, although they typically aren't preferred. Maintaining the 
sodium to phosphate molar ratio between 2:1 and 3:1 typically will keep 
the pH and phosphate concentration of the system within acceptable ranges. 
Captive alkalinity treatment generally operates under the theory that if 
the boiler water pH is maintained at or below that pH which exists when 
the sodium to phosphate ratio is about 3:1, then no free caustic will be 
present in the bulk boiler water. The ideal sodium to phosphate ratio 
(Na:PO.sub.4) will vary from system to system, but in general the optimum 
ratio will be between about 2.2:1 and 2.8:1. 
The pH and phosphate concentration are used to determine the sodium to 
phosphate ratio. Standardized captive alkalinity curves, which will be 
familiar to one having ordinary skill in the art, represent the sodium to 
phosphate ratios which correspond to various pH and phosphate 
measurements. These captive alkalinity curves are best described in terms 
of a graph, with pH on the X-axis and phosphate on the Y-axis. The optimum 
sodium to phosphate range will be depicted on this graph in terms of a 
"target box" which corresponds to an optimum pH range and an optimum 
phosphate range. The target box will be different for every system, 
depending primarily on the pressure at which the system operates. For 
example, a pH of 9.0 and a phosphate concentration of 7 parts per million 
(ppm) would typically be within the target box for a unit operating 
between approximately 1500 and 2000 psig of pressure, but would be outside 
the target box for units operating at less than 1500 psig or more than 
2000 psig. Ideally, the apparatus of the present invention will be 
programmed to maintain the sodium to phosphate ratio in the center of the 
target box, as the center represents the optimum sodium to phosphate 
ratio. 
In addition to the role they play in determining the sodium to phosphate 
ratio, the pH and phosphate concentrations are important for other 
reasons. Out of specification pH may lead to corrosion of boiler 
internals. Caustic attack, discussed above, is an example of a corrosion 
problem related to pH. Concentrating films can also be formed when acidic 
compounds containing such ions as chloride, sulfate, or phosphates are 
present in the boiler environment. Although specifications will vary from 
system to system, the boiler pH should generally be maintained in an 
alkaline range, preferably a range of about 8 to 11. 
Monitoring and controlling phosphate concentration is also important in 
boiler systems. In addition to being used as a buffer to maintain pH in 
captive alkalinity treatment, phosphates are used in aqueous systems such 
as boilers to prevent calcium scales and steel corrosion. Another purpose 
for measuring phosphate concentration is to avoid high total phosphate 
concentrations which may result in the formation of insoluble phosphate 
salts. All of the phosphate which exists in a boiler system will be in the 
form of inorganic orthophosphate. This is because the temperatures and 
pressures of the boiler systems are so high that any other forms of 
phosphate which are introduced to the boiler system will be converted to 
inorganic orthophosphate. 
Other parameters evaluated in determining the sodium to phosphate ratio may 
include, inter alia, the pressure at which the boiler operates, the 
temperature at which the boiler operates, the quality of water which is 
being used in the system, and the ability of the operator to exclude 
contaminants from the system. For example, a system operating at a higher 
pressure will require lower solids--that is lower phosphates--to maintain 
the sodium to phosphate ratio in the desired range; the same is true for 
systems using a higher purity water. 
Currently, the calculation and maintenance of the sodium to phosphate ratio 
is done manually. Typically, this ratio is determined only once a day, 
with no subsequent determination of sodium to phosphate ratio made until 
the following day. The sodium phosphates or other chemicals added to 
maintain the system within an acceptable sodium to phosphate ratio are 
prepared and fed daily based on this one sodium to phosphate ratio 
product. This method does not allow for real time analysis, and results in 
wide fluctuations in control. Further, adjusting the sodium to phosphate 
ratio usually requires the supplemental feed of NaOH along with the 
phosphate product, or frequent manual pump adjustment. In short, manual 
control of the sodium to phosphate ratio requires considerable manpower 
with often imprecise results. 
U.S. Pat. Nos. 5,252,486 and 5,240,681 disclose methods and apparatuses, 
respectively, for monitoring the inorganic phosphate content in aqueous 
systems using flow injection analysis (FIA) apparatus. Neither of these 
patents, however, disclose the simultaneous monitoring of pH or the 
automated control of any of these parameters. 
Pederson et al, Anal. Chim. Acta, 238, 101-199 (1990) disclose a system 
wherein on/off switching control of a municipal pilotscale wastewater 
treatment aeration tank is based on a flow injection analysis of ammonium 
content. 
Steele et al, SPE-Enchanced Coordinated PO.sub.4 /pH Control Improves 
Boiler Operating. Reliability, Off. Proc. Intl. Water Conf., 53rd, 409-14 
(1992) discuss the use of a coordinated phosphate/pH control program along 
with a process control package to enhance operational control and readily 
detect upsets. The reference does not discuss, however, the use of 
automated and/or on-line analysis of either phosphate or pH; nor does the 
reference discuss the use of automated chemical feed to maintain system 
control, as is claimed in the present invention. 
Makela et al, Interact. Iron-Based Mater. Water Steam, Proc. Int Conf., 
issue TR-102101, 11/1-11/21, (1993) discuss the importance of on-line pH 
measurements and a device for making this measurement, as well as the 
influence of phosphates on pH. The reference does not discuss on-line 
phosphate analysis, coordinated phosphate/pH control or an automated means 
for controlling chemical feed as is claimed in the present invention. 
Mooney, E. F., Instrumentation In The Power Industry Proceedings, 34, 
425-50 (June, 1991) discusses photometric measurement of copper, silica, 
phosphate and sulfate by using a fiber optics probe photometer. The 
reference does not discuss use of these measurements in the control of 
chemical feed, as is claimed in the present invention. 
Boyette et al, An Automated Coordinated Phosphate/pH Controller For 
Industrial Boilers, NACE Conference, p 624/1-624/10 (1995) disclose a 
means for controlling phosphate and pH in boiler systems. The reference 
does not disclose the use of FIA to determine the phosphate content of the 
system, as is claimed in the present invention. In addition, the control 
mechanism disclosed by Boyette operates via an on/off pumping mechanism 
which pumps only one feed product at a time. In contrast, the present 
invention claims methods and apparatus which can proportionally feed two 
products simultaneously. 
None of these references disclose methods or apparatus for the automated 
monitor and control of pH, phosphate and sodium to phosphate ratio. 
Accordingly, there is a need for methods and apparatus which allow for 
such monitor and control. 
SUMMARY OF THE INVENTION 
The present invention generally meets the above described need by providing 
methods and apparatus for monitoring and/or controlling pH, phosphate and 
the sodium to phosphate ratio of a boiler system operating on captive 
alkalinity chemistry control. The methods of the present invention 
comprise the steps of: a) determining the phosphate content of the boiler 
system by using, flow injection analysis (FIA); b) determining the pH of 
the boiler system; c) calculating the sodium to phosphate ratio; and d) 
controlling the feed rate of at least one water treatment chemical being 
added to said system so as to maintain the pH, phosphate concentration and 
sodium to phosphate ratio within desired ranges. The computer can be set 
to respond to a variety of different conditions. For example, it can be 
set to feed phosphate(s) if the phosphate content is the only parameter 
out of range, if the pH is the only parameter out of range, or if both 
phosphate out of range. Typically, if pH or phosphate are out of range, 
the sodium to phosphate ratio will also be out of range. 
FIA is a simple and reliable technique based on continuous flow of a sample 
solution which is introduced directly into an unsegmented carrier stream 
of a reagent solution, thereby forming a well-defined sample zone. While 
it is being transported to a detector device further downstream, the 
sample has an opportunity to react with the reagent and form a new 
chemical species which can be quantitatively measured by the detector. The 
reaction is usually a color-forming one and the detector a colorimeter 
(spectrophotometer), an electrode, or the like. FIA lends itself to the 
automated, rapid and reliable analysis of various samples, and offers many 
advantages over the older technique of air-segmented continuous flow 
analysis. 
The present invention also provides an apparatus for monitoring and/or 
controlling of pH, phosphate, and sodium to phosphate ratio. The apparatus 
comprises an in-line phosphate monitor, preferable an FIA apparatus such 
as that disclosed in U.S. Pat. No. 5,240,681, and an in-line pH meter. 
Both the phosphate monitor and pH meter are attached to a controller, 
preferable a computer. The computer receives output signals which 
represent the pH and phosphate content. If an FIA apparatus is used, it 
will typically have its own computer, so a separate controller will not be 
needed; in this embodiment, the signal from the pH meter will be sent 
directly to the FIA apparatus. 
In yet another embodiment, the FIA apparatus itself is equipped with a pH 
meter which is directly read by the FIA computer. 
The controller, which is programed to calculate the sodium to phosphate 
ratio from the pH and phosphate values, also controls one or more means 
for feeding chemicals, preferably chemical feed pumps. These pumps, in 
turn, control the amount of phosphate being fed to the boiler system. 
Based on the signal received from the controller, the pumps will control 
the rate of chemical feed so that the amount of phosphate(s) necessary to 
maintain the parameters within the desired ranges are added to the system.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to methods and apparatuses for the 
automated monitoring and/or controlling of pH, phosphate, and sodium to 
phosphate ratio (Na:PO.sub.4) in boiler systems operating with captive 
alkalinity chemistry control. Specifically, the present invention is 
directed to a method for monitoring and/or controlling pH, phosphate 
concentration and sodium to phosphate ratio in a boiler system using 
captive alkalinity, comprising the steps of: a) determining the phosphate 
concentration of said system by using flow injection analysis (FIA); b) 
determining the pH of said system; c) calculating the sodium to phosphate 
ratio; and d) controlling the feed rate of at least one water treatment 
chemical being added to said system so as to maintain the pH, phosphate 
concentration and sodium to phosphate ratio within desired ranges. The 
present invention is further directed to an apparatus for monitoring 
and/or controlling pH, phosphate concentration and sodium to phosphate 
ratio in a boiler system operating on captive alkalinity, comprising: a) 
an FIA means for determining the phosphate concentration of said system; 
b) a means for determining the pH of said system; c) a means for 
determining the sodium to phosphate ratio of said system; and d) a means 
for controlling, feed rate of at least one water treatment chemical being 
added to said system so as to maintain pH, phosphate concentration and 
sodium to phosphate ratio within desired ranges. 
Generally, the pH, phosphate, and sodium to phosphate parameters can be 
controlled by varying, the rate of chemicals fed, i.e. the amount of water 
treatment chemical(s) added to the boiler system. In the most preferred 
embodiment of the present invention, a combination of disodium phosphate 
and trisodium phosphate should be used to maintain these parameters within 
their desired range. The automated control provided for in the methods of 
the present invention allows for the real time analysis of these critical 
boiler water parameters and prompt response to out of specification 
conditions. The user of the present invention will also minimize blowdown 
loss attributable to control problems. The methods and apparatus of the 
present invention further provide improved reliability, and safe and 
direct measurement and chemistry control, thereby offering several 
advantages over current methods and apparatus. In addition, the methods 
and apparatus of the present invention can be adapted to suit the needs of 
each individual user; this is important because control parameters 
generally vary from one boiler system to the next. 
The methods of the present invention require a continuously flowing sample 
stream running from the boiler system. Two take-off valves are established 
in the sample stream to allow a portion of the sample stream to be drawn 
off for phosphate analysis and a separate portion to be drawn off for pH 
analysis. The first take-off valve is attached to a conduit which runs to 
an in-line phosphate monitor. The sample runs continuously; when not 
being, monitored, or after monitoring, the sample stream runs to waste. 
Step a) of the present invention requires that the phosphate monitor be a 
flow injection analysis (FIA) apparatus. An FIA apparatus suitable for use 
in the methods of the present invention is disclosed in U.S. Pat. No. 
5,240,681. The method of using an FIA apparatus for determining total 
inorganic phosphate content is disclosed in U.S. Pat. No. 5,252,486. 
To carry out step b), the second take-off valve from the sample stream is 
generally attached to a conduit which runs to an in-line pH meter. Any 
standard pH meter can be used, such as those available from Fischer 
Scientific, Inc. or the Orion Co. Again, the sample runs continuously; 
when not being monitored or after passing, through the pH meter the sample 
stream runs to waste. Since the pH meter is capable of monitoring pH on a 
continuous basis, continuous signal representing the pH of the sample will 
therefore be emitted. 
Signals from both the FIA monitor and the pH meter are sent to a 
controller, preferably a computer which typically will be part of the FIA 
apparatus. This can be accomplished by any suitable wiring means known in 
the art. Step c) is then carried out by the computer, which utilizes the 
signals from the pH meter and phosphate monitor and, together with 
pre-programmed data, calculates the sodium to phosphate ratio. The 
computer is pre-programmed to compare pH and phosphate values it receives 
from the pH meter and phosphate monitor with captive alkalinity curves, 
which are also pre-programmed into the computer. From this comparison, the 
computer determines the sodium to phosphate ratio of the system. If the 
sodium to phosphate ratio is in the center of the target box, then no 
adjustments to the chemical feed rate will be made. If, however, the 
sodium to phosphate ratio is not in the center of the target box, or not 
in the target box at all, the computer will determine the amount of sodium 
phosphate, if any, needed to put the sodium to phosphate ratio back into 
the target box. 
Relative to step d), controlling the feed rate of at least one water 
treatment chemical, the computer is programmed to make adjustments to the 
chemical feed rate based on: only the phosphate reading; only the pH 
reading; either the pH or the phosphate reading; or on both the phosphate 
and pH readings. For example, the computer can be programmed to change the 
feed rate if only the phosphate is out of its target range. In this 
scenario, the feed rate of phosphate will not be changed if the phosphate 
concentration is within its target range, even if the pH is outside of its 
target range. 
In a preferred embodiment, the computer is programmed as follows: if the 
phosphate content is at the upper end of its target range, or even if it 
exceeds its target range, chemical will not be fed, because the blowdown 
of the system will eliminate the excess phosphate; if phosphate is 
directly on target, the system will feed phosphate at a rate so as to 
maintain this optimum value; and if the phosphate content is below the 
optimum amount the feed rate will be increased. When the phosphate content 
is below the optimum amount, the computer will be further programmed to 
determine the amount of each phosphate specie present in the boiler 
system. For example, if disodium phosphate and trisodium phosphate are the 
two phosphate species being, used to control the water chemistry, the 
computer will determine how much disodium phosphate is present and how 
much trisodium phosphate is present. The computer will compare the actual 
amount of each phosphate specie with the target amount for each phosphate 
specie, and feed only the specie which is deficient to bring the total 
phosphate content within range. In some cases both species will be 
deficient and both will be fed. 
Similarly, the computer can be programmed to alter the phosphate feed rate 
only if the pH is off target, if either pH or phosphate is off target, or 
only if both the pH and the phosphate are off target. 
The computer or other controller adjusts the feed rate of chemicals into 
the system by controlling one or more chemical feed pumps. Signals are 
sent from the computer to the feed pump(s) by any suitable wiring means 
known in the art. Each feed pump is attached to a chemical feed tank, and 
controls the rate at which the water treatment chemical housed in the feed 
tank is introduced to the boiler based upon the signal received from the 
computer. Although the preferred embodiment of the present invention 
contemplates the use of two chemical feed tanks and two feed pumps which 
operate independently of each other, it is equally within the scope of the 
invention to employ any number of pumps and tanks. Any suitable type of 
feed pump can be used. For example, one could use an AC driven pump in 
which the stroke amount, i.e., the amount of chemical discharged over a 
given time, can be altered. Alternatively, a DC driven pump having, a 
constant stroke value but a variable speed motor can be used; it is 
believed that the DC pump may provide for a more accurate feeding of the 
chemical(s). 
There are several other parameters which may be taken into account in 
monitoring and controlling a boiler system; these parameters are also 
pre-programmed into the computer, based upon the individual 
characteristics of the boiler system being treated. In addition to the 
target box for sodium to phosphate ratio, other operating parameters 
include, inter alia, the types of chemicals being, fed into the system, 
the types of feed pumps being used, how the pumps control the rate of the 
chemical feed, how often pump rates are adjusted, the volume of the boiler 
system being monitored and/or controlled, feedwater flow rate variations, 
boiler blowdown, steaming rate, boiler volume and residence time, and the 
desired phosphate and pH ranges. 
Another factor to be determined by the user is the frequency with which the 
sodium to phosphate ratio will be calculated. If both the phosphate and 
the pH readings are constantly monitored, the sodium to phosphate ratio 
could be determined at almost any time. It is more typical, and preferred, 
however, to program the computer to calculate sodium to phosphate ratio 
and provide for pump adjustments at designated intervals, rather than on a 
continous basis. Ideally, the computer will be programmed to allow for 
some lag time between chemical addition--that is, some time for the boiler 
system to respond to the water treatment chemicals added in response to a 
previous sodium to phosphate ratio calculation--before an additional 
calculation/addition is performed. 
As stated above, the phosphate concentration of the system is determined 
via FIA. FIA methods and apparatus require the continuous flow of a sample 
stream which mixes with a continously flowing reagent stream(s) to form a 
color reaction mixture product which is read on a colorimeter. The FIA 
methodology is typically carried out in an apparatus comprising a closed 
system in which the sample and reagent stream(s) are carried in conduit 
means consisting of tubing of suitable dimensions and materials. The 
sample stream is propelled to the FIA apparatus by the pressure of the 
boiler system. Within the FIA apparatus, the continuous movement of the 
reagent stream(s), as well as the combined sample/reagent stream, or 
reaction mixture, is produced by a positive pressure accomplished by any 
suitable means, for example pumping means, such as a peristaltic pump, or 
a pressurized system in which compressed air or an inert gas such as 
nitrogen is used to propel the sample/reagent stream through the tubing 
and other apparatus means used to carry out the method. A pressurized 
system using, compressed air is preferred. Pressure is also maintained by 
the use of pressure regulators, restrictor coils with reduced internal 
diameters, back pressure loops and/or semi-permeable membranes through 
which the reaction mixture passes to remove entrained air, in combination 
with the pressurized gas. The pressure in the system should be between 2 
and 10 psig, preferably between 4 and 6 psig. 
The flow injection analysis methods generally involve the steps of: a) 
establishing a filtered sample stream from said system from which sample 
units may be selected at designated intervals; b) bringing together and 
admixing on a continuous basis two reagent composition streams so as to 
form a basic flow injection analysis stream, the two reagent composition 
streams comprising a color-forming reagent, said color-forming reagent 
comprising an inorganic acid and molybdenum (V and VI), and a reducing 
agent and preservative composition; c) interrupting the flow of the 
reducing, agent and preservative composition reagent stream and 
substituting therefor the filtered sample stream of step (a) for 
sufficient time to select a sample unit, thereby allowing mixing, with the 
color-forming reagent to form a reaction mixture; d) restoring the flow of 
reducing agent and preservative composition stream; e) heating the 
reaction mixture to approximately 40.degree. C. for a sufficient time to 
effect the reaction of substantially all of the phosphate in the sample 
with the molybdenum V and VI to form a color complex, and thereafter 
allowing, the reducing agent to partially reduce the molybdenum V and VI 
so that it has an average oxidation state between 5 and 6; f) passing the 
reaction mixture containing the color complex through a colorimeter 
having, a 600-850 nanometer (nm) filter and reading a signal produced 
thereby; and g) calculating the concentration of phosphate in the sample 
from the signal and previously available standardized data; wherein all of 
the above steps are carried out under a pressure of from 2-10 psig. 
Alternatively, steps b), c) and d) above can be substituted with the steps 
of: at one said designated interval, selecting a sample unit and injecting 
it as a discrete unit into a continuously flowing reducing agent stream 
comprising a reducing, agent and preservative composition, so that the 
reducing agent stream is present in front of and behind said sample unit; 
and continuously injecting a reducing agent and a color-forming, reagent 
stream comprising an inorganic acid and molybdenum V and VI into the 
sample unit in such a manner that the sample unit and color-forming 
reagent are thoroughly admixed while bounded in front and behind by said 
reducing agent stream forming a reaction mixture. 
The tubing which is used to carry the sample stream, as well as the reagent 
composition streams and reaction mixture stream, must be composed of a 
material which is able to withstand the rather harsh conditions to which 
it is continually subjected, such as elevated temperatures and pressures 
and strong reagents, while maintaining dimensional uniformity within very 
strict tolerances, which is essential for assuring consistency and 
reproducibility of the analytical results over a long period of time. Any 
suitable inert material can be used, preferably a polymer material such as 
polypropylene, polytetrafluoroethylene (PTFE), or polyetheretherketone 
(PEEK); PEEK is preferred for use in the apparatus of the present 
invention. 
The size of the tubing is selected so as to accomplish a desired flow rate 
with respect to a sample size within a desired range, which makes economic 
use of the required reagents and affords an adequate reaction time. In the 
methods and apparatus of the present invention, it has been found useful 
to employ tubing having, an internal diameter of from 0.0125 to 0.1000 
centimeters (cm), with an internal diameter of 0.0500 cm (=0.02 inches) 
being, preferred. By using tubing, having the preferred 0.0500 cm internal 
diameter, a flow rate throughout the flow injection system of between 0.13 
and 0.18 milliliters per minute (mL/min), preferably 0.15 mL/min, is 
maintained. With such a flow rate, the sample unit size may vary between 
10 and 150 microliters (.mu.L), preferably between 20 and 125 .mu.L, and 
most preferably 20 .mu.L. 
Although boiler water generally should be relatively free from fine solids 
or other particulate matter, sample filtration may be desired to separate 
suspended material and to prevent plugging of the FIA instrument. If 
filtration is used, it should be established after the take-off valve and 
before the sample stream enters the FIA apparatus. Bypass membrane 
filtration is preferred, with tangential entry of bypass being desirable 
for on-line sample filtration because membrane fouling is slowed by the 
cleaning action of the sample stream. Commercial filtering systems which 
are suitable include the Minitan-S filter assembly from Millipore Corp., 
Bedford, Mass., and the Collins Swirlclean Bypass Filter from Collins 
Products Company, Livingston, Tex. Any filter material or device which 
will remove the suspended fine solids from the sample stream is suitable; 
it has been found that good results are achieved when particles of 0.45 
microns and larger are removed. 
A conduit running from the first take-off valve and optionally through a 
filtering system is attached to the FIA apparatus via a three-way valve or 
some other standard valve means known in the art. Use of a three-way valve 
allows for a continuous flow of fresh sample to run from the take-off 
valve either to waste or, by switching the three-way valve, to the 
remainder of the FIA apparatus. 
Because the sample stream coming from the boiler is continuously flowing, 
however, it is necessary to establish a way by which sample units may be 
selected at designated intervals. This is suitably carried out using a 
selector valve together with an injection valve, either alternatively or 
in addition to the three-way valve. Both the selector valve and injection 
valve are of known design and allow the sample stream to flow in a 
continuous manner through the selector and injection valves to waste, but 
not through any other part of the flow injection analysis system. In 
addition to assuring that a fresh sample unit is provided whenever a 
sample is to be analyzed, the selector valve also functions to permit the 
introduction of standards and distilled water into the basic flow 
injection analysis stream. It will be appreciated that other devices may 
be substituted for the selector valve. 
Sample units for evaluation by the flow injection system are selected at 
designated intervals as frequently or infrequently as the operator 
desires. The designated intervals are predetermined based on the number of 
samples that it is desired to test within a given period of time, and are 
usually pre-programmed into the computer or similar device which controls 
the operation of the entire flow injection analysis system. During 
conventional operation, the selector valve will be set so that the sample 
stream enters the selector valve and then goes on to the injection valve, 
and from there to waste. On command from the computer or other control 
device, or even manually, the injection valve directs the sample stream 
through a sample loop of tubing which is of the appropriate dimensions to 
give the desired sample size, most preferably 20 .mu.L. 
The sample loop is preferably in the separate device termed the injection 
valve, which has as its function the injection of the sample unit into the 
continuously flowing, reagent stream. The injection of the sample unit 
into a reagent stream may take place in at least two different ways which, 
while accomplished by different means, are conceptually the same. One such 
means is a mixing valve, which has two or more inlet ports and a single 
outlet port. Within the valve assembly, means controlled by the operation 
of a solenoid allow measured quantities of the contents of a tube leading 
to one of the inlet ports to pass through the valve assembly and out the 
outlet port. The solenoid then closes that inlet port and opens a second 
inlet port, where again a measured quantity of the contents of a tube 
leading to the second inlet port are allowed to pass through the valve 
assembly and out the outlet port. By alternating the opening and closing 
of these inlet ports, e.g., once a second, a thorough mixing of the 
contents of the two tubes entering the inlet ports is achieved. A solenoid 
operated mixing, valve of the type sold by Bio-Chem Valve Corporation or 
General Valve Company has the advantages of efficiently, reliability, and 
economy. Such mixing, valves feature low power consumption, isolated 
solenoids, high cycle life, low internal volume, fast response time, 
Teflon wetted parts, and valve seat travels adjusted for accurate fluid 
sampling. The mixing valve can mix together the proper ratio of reagents 
and samples by switching from one stream to another rapid succession, 
resulting in a well mixed solution with faster reaction times and sharper 
peak shape from the colorimeter. 
When a mixing valve is used for the FIA determination of orthophosphate 
content, it functions as follows. The two reagent composition streams 
which form the basic flow injection analysis stream are brought together 
and admixed at the mixing valve. These two reagent composition streams 
are: (a) the color-forming, reagent comprising an inorganic acid and 
molybdenum (V and VI); and (b) the reducing agent which optionally 
contains a preservative composition. Either the timing of the solenoid 
which controls the amount of each reagent stream leaving the outlet port, 
or the concentrations of the reagent compositions themselves, may be 
adjusted so as to predetermine the ratio of the reagent concentrations in 
the basic flow injection analysis stream. These can be set as desired, 
depending on the makeup and stoichiometry of the reagent composition 
streams. For example, where concentrated sulfuric acid is used in the 
color-forming, reagent and ascorbic acid is used as the reducing agent, 
the time and/or concentrations are adjusted to provide a 1:1 molar ratio 
of the reagents. 
When a sample unit is to be analyzed, the selector and injection valves are 
set and activated so that a sample unit travels through a tube to a third 
inlet port of the mixing valve described above, where it enters the mixing 
valve. At the same time, however, the inlet port for the reducing agent 
and preservative composition is closed, so that the sample unit is, in 
effect, substituted therefor, and as a consequence, the sample unit 
becomes admixed with the color-forming reagent which is still entering the 
mixing, valve. After the sample unit has completely passed through the 
mixing valve, its inlet port is closed and that for the reducing agent and 
preservative is reopened. As a consequence of the above actions, it will 
also be seen that the reducing agent and preservative reagent composition 
is present in front of and behind the sample unit in the basic flow 
injection analysis stream. 
Alternatively, a three-way valve connected by tubing directly to the mixing 
valve, through which sample continuously flows to waste through one of the 
ports of the three-way valve, can be used in place of the selector and 
injector valves. By means of such a valve, it is possible to have a 
continuous flow of fresh sample, and then by switching the three-way 
valve, provide for direct flow of a sample unit to the mixing, valve, the 
unit size being determined by the length of time that the three-way valve 
remains open for passage of sample. 
Another means for accomplishing the injection of the sample unit into a 
reagent stream involves the use of a selector valve and an injection valve 
as described above together with a T-connector. As with the mixing valve 
embodiment, during the stage of readiness for receiving a sample unit, the 
two reagent streams are mixed together on a continuous basis, but by means 
of being brought together at the T-connector rather than through a mixing 
valve. When a sample unit is to be analyzed, the injection valve is 
activated and the sample unit is injected into the reducing agent and 
preservative composition reagent stream, which also passes through the 
injection valve on a continuous basis. As a consequence, the reducing, 
agent stream is present in front of and behind said sample unit, viewed as 
a continuously flowing stream, just as with the mixing valve embodiment 
described further above. The reducing agent stream pushes the sample unit 
on ahead of it so that when the sample unit reaches the T-connector, only 
sample and color-forming reagent are admixed at the T-connector, just as 
with the mixing valve embodiment described further above. 
In both embodiments described above, as the sample/color-forming reagent 
mixture passes through the remainder of the flow injection analysis system 
the color reaction mixture product is formed. Specifically, this colored 
product is a result of the reaction between orthophosphate and molybdenum. 
Orthophosphate and molybdenum VI will react to form a heteropoly yellow 
complex. Subsequent reduction of the yellow complex with a reducing agent, 
or the initial reaction of orthophosphate with molybdenum V, results in a 
heteropoly blue complex, which is the color product read by the 
colorimeter. During the course of this passage through the apparatus, the 
molybdate solution and reducing agent completely mix with the 
orthophosphate to form this heteropoly blue complex. 
As stated above, one of the two reagent composition streams is a reducing 
agent stream which comprises a reducing agent and, optionally, a 
preservative composition. The reducing, agent acts to reduce the 
phosphomolybdate yellow complex to the heteropolymolybdate blue form. Any 
suitable reducing, agent known in the art can be used. A commonly employed 
reducing agent recognized for this purpose is ascorbic acid, and this is 
the preferred reducing agent for use in the methods of the present 
invention. 
Decomposition of a reducing agent such as ascorbic acid will occur without 
the use of one or more preservatives. Such decomposition can be caused by 
dissolved oxygen in the boiler system, or by the presence of oxygen 
radicals. The presence of heavy metals may also catalyze such 
decomposition. Preservative agents for use with the reducing agents of the 
present invention, and which act as oxygen scavengers, include those 
recognized in the art as suitable for that purpose, e.g., ketones, such as 
methylethyl ketone or acetone, which is preferred, glycerol and glycol. 
They may be used alone or in combination. 
Chelating agents which bind to heavy metals capable of catalyzing the 
decomposition of the reducing, agents may also be used in the preservative 
composition. Any chelating agent which will chelate metals which cause 
instability of the ascorbic acid, and which is otherwise compatible with 
the other elements present in the methods of the present invention, may be 
used. A preferred chelating agent is ethylenediaminetetraactic acid (EDTA) 
in any of its various salt forms, e.g., tetrasodium EDTA, edetate sodium, 
edetate disodium, edetate trisodium, and edetate calcium disodium. 
Disodium EDTA is preferred. Nitrilotriacetic acid may also be used, for 
example. 
The amount of reducing agent, such as ascorbic acid, employed will be 
between 10 and 30 grams per liter (g/L), preferably between 15 and 20 g/L. 
The amount of preservative such as acetone employed will be between 45 and 
55 milliliters per liter (mL/L), preferably 50 mL/L. 
A preferred reducing agent and preservative composition for use in the 
method of the present invention has the following composition: 16.6 g 
ascorbic acid; 50 mL acetone; 7.6 mg disodium EDTA; in 1 L of deionized 
water. The disodium EDTA is conveniently added as 2 mL of Calgon Reagent 
R-5010 which is 0.001M EDTA and contains sufficient NaOH to solubilize the 
EDTA, as well as a very small quantity of a preservative. 
The second of the two reagent composition streams is a color-forming 
reagent stream which comprises an inorganic acid and molybdenum (V and 
VI). This molybdenum color-forming reagent composition may be prepared in 
accordance with procedures known in the art. Alternatively, in the 
preferred method, a molybdate reagent for use in the methods of the 
present invention may be prepared simply by dissolving from 5 to 15 g, 
preferably 10 g of ammonium molybdate tetrahydrate (NH.sub.4).sub.6 
Mo.sub.7 O.sub.24.4H.sub.2 O! in from 60 to 120 mL, preferably 102 mL of 
concentrated (95%) sulfuric acid (H.sub.2 SO.sub.4). The solution may then 
be diluted to 1 L with deionized water to give the molybdenum (V and VI) 
reagent solution. When this reagent mixes with the reducing agent stream 
containing, e.g., ascorbic acid, the ascorbic acid partially reduces the 
molybdenum so that it has an average oxidation state between 5 and 6. The 
molybdenum blue color complex results. 
The next step involves heating the reaction mixture. Heating the reaction 
mixture to a temperature not exceeding 40.degree. C. catalyzes or 
facilitates the reaction of substantially all of the phosphate with the 
color-forming reagent. The time for this step to be completed will be from 
10 to 25 minutes, usually from 15 to 20 minutes. A typical residence time 
for the completion of this step is 16 minutes. The device most convenient 
for carrying out this step is a simple reaction coil, e.g., one coil of 
tubing 1000 cm in length and 0.0500 cm (=0.02 inches) internal diameter 
encased in an aluminum block heater. Other devices may be substituted for 
the reaction coil; any device known in the art can be used. Because the 
heating coil contributes to the mixing, of the reagents and sample, if 
using a heating, device without a coil an in-line mixer situated before 
the heating device could be employed to ensure adequate mixing of the 
sample and reagents. 
Even though the reaction temperatures for the step described above are 
below the 100.degree. C. boiling point of water, and it is therefore 
unlikely that significant amounts of dissolved air gases (oxygen and 
nitrogen) will come out of solution, it is preferred to employ an air 
filter which will remove any such bubbles of gas which may unexpectedly 
appear. The evolution of gas bubbles can cause unacceptable detector 
"noise" when the reaction mixture containing the color complex is passed 
through the colorimeter for reading. The air filter is conveniently a 
semipermeable membrane through which the reaction mixture is passed to 
remove any extraneous gases which have formed. Such air filters are well 
known in the art. 
The next step in the flow injection analysis determination of phosphate 
content involves passing the reaction mixture containing the color complex 
through a colorimeter. This is typically a flow-through cell 
spectrophotometer equipped with a filter which permits monitoring of the 
heteropoly blue complex within a wavelength range of from 600 to 850 nm. A 
650 nm filter is usually, and preferably, employed. The path length for 
the flow-through colorimeter cell is from 0.5 to 2 cm, but is preferably 1 
cm in length. 
The last step of the flow injection analysis methods as used in the present 
invention involves taking the information obtained from the colorimeter 
reading in the preceding step and, together with standardized data, 
calculating the concentration of total inorganic phosphate contained in 
the boiler system from which the sample was obtained. It is desirable to 
employ standards and routinely test these so as to obtain and have readily 
available 2-point or 3-point standardization data. It is most convenient 
to employ a computer to process all of this data and calculate the 
phosphate content. The signal from the colorimeter may be sent directly to 
such a computer that permits a very rapid and automatic readout of the 
concentration of total orthophosphate in the boiler system on an ongoing 
and regular basis at the desired intervals. The FIA apparatus of the 
present invention preferably is equipped with its own computer which is 
programmed to both determine phosphate content and to also receive the 
output signal coming from the pH meter. The FIA computer will further be 
programmed to calculate the sodium to phosphate ratio from the pH value 
and phosphate concentration. In addition, the computer will be attached to 
and control the feed rate of the chemical feed pumps. 
In another embodiment of the present invention, the FIA methodology 
includes the step of determining pH. In this embodiment, the FIA apparatus 
itself is equipped with a pH probe. The pH probe is employed at some point 
in the system after filtration of the sample, but before the addition of 
any reagents. Preferably, a solid state pH probe approximately 0.125 
inches in size will be placed within the tubing of the FIA apparatus. 
Other pH probes, such as those used in chromatography, familiar to those 
skilled in the art can also be used. A three-way valve will be employed, 
through which travels two streams--the sample stream and a buffer solution 
used to calibrate the pH meter. Switching the three-way valve determines 
which of the streams will pass through the rest of the FIA apparatus. The 
pH probe is connected directly to the FIA's computer, which will directly 
read the pH probe. Accordingly, in this embodiment, the computer will be 
programmed to determine both the pH value and the phosphate concentration 
as described above, use these values to determine the sodium to phosphate 
ratio, and finally control the feed pumps as needed to achieve or maintain 
this ratio. 
FIG. 1 of the drawings depicts a typical analyzer apparatus for carrying 
out the methods of the present invention. The solid lines depict conduit 
means, i.e. tubing, while the dotted lines represent wires or other 
suitable means by which signals can be transmitted. The depiction is not 
drawn to scale. 
A sample stream 12 runs continuously from a boiler system 10. The sample 
stream 12 flows to waste 14. A first take-off valve 16 is positioned in 
the sample stream 12 to remove a portion of the sample stream 12 for 
determination of orthophosphate content. This portion of the sample stream 
is carried through tubing 18, to an FIA apparatus 20. The FIA apparatus 20 
is equipped with a computer. When a sample is selected, the sample stream 
12 passes through the FIA apparatus 20 to waste 21. 
A second take-off valve 22 is also positioned in the sample stream 12 to 
remove a portion of the sample stream for determination of pH. This 
portion of the sample stream is carried through tubing 24 to a pH meter 
26. The sample stream passes through the pH meter 26 to waste 25. 
Out-put signals from the pH meter 26 are sent via line 62 to the computer 
of the FIA apparatus 20. The computer in the FIA apparatus 20 in turn 
controls two pumps 34 and 36 via line 38, which splits into lines 40 and 
42 which run to pumps 34 and 36 respectively. 
The first pump 34 is connected via conduit 44 to a first chemical storage 
tank 46. The second pump 36 is connected via conduit 50 to a second 
chemical storage tank 48. The first pump 34 controls the flow rate of a 
first water treatment chemical 45 from the first storage tank 46 to a 
T-connector 56 via conduit 52. Likewise, the second pump 36 controls the 
flow rate of a second water treatment chemical 47 from the second storage 
tank 48 to T-connector 56 via conduit 54. The two conduits 52 and 54 
leading from the two pumps 34 and 36 are joined at T-connector 56. The 
first and second water treatment chemicals 45 and 47 are then carried via 
conduit 58 to the boiler system 10. 
FIG. 2 depicts another embodiment of the present invention in which the FIA 
apparatus 20 also contains a pH meter. The remainder of the apparatus and 
its function are as described further above for FIG. 1. 
EXAMPLES 
The following examples are set forth to illustrate the invention and should 
not be construed as limiting the invention in any way. 
Example I 
A simulated boiler system was established with the following conditions and 
control parameters, as if being, operated at a pressure of 900 psig: 
System volume: 35 gallons 
Boiler water phosphate target: 25 ppm total phosphate 
Boiler water pH target: 10.07 
Two chemicals being fed: monosodium phosphate and trisodium phosphate 
Half-life*: between about 7-8 hours 
High purity water 
N.sub.2 purged system 
FNT * Half-life refers to the amount of time it would take for the phosphate 
content of the system to be reduced by half if no additional phosphate was 
added. 
Monitor of phosphate content and pH and control of chemical feed was 
achieved by using an FIA apparatus equipped with a pH monitor and 
computer, as described above. To determine the pH and phosphate content of 
the system, a filtered sample stream ran to the FIA apparatus and passed 
through the pH meter. A signal representing the pH was sent to the 
computer on a continuous basis. The sample stream then ran to a selector 
valve. At designated intervals, the selector valve directed the sample 
stream through a 20 .mu.l sample loop in an injection valve. The injection 
valve switched the sample in-line and the ascorbic acid reagent, which 
functioned as a carrier, pushed the sample ahead. The sample mixed with 
the molybdate reagent at a 90.degree. angle T-connector. The ascorbic acid 
and molybdate reagents were prepared in the preferred manner as described 
above. From the T-connector, the sample/reagent mixture entered a 25 foot, 
0.02 inch internal diameter heating coil at about 40.degree. C. Travel 
time from injection of the sample through the heating coil was 
approximately 10 minutes. From the heating coil, the sample/reagent 
mixture passed through an air filter to remove any extraneous bubbles 
which may have formed. Next the sample/reagent mixture flowed through a 
colorimeter with a 1 cm path length and 650 mn filter. The signal was sent 
to the computer which calculated the phosphate content from standardized 
data obtained periodically by injecting orthophosphate standards. The 
computer then calculated the sodium to phosphate ratio from the pH valve 
and the phosphate content. Adjustments to the feed pumps were 
automatically made by the computer based on the sodium phosphate ratio. 
The test was run for 48 hours. The results are presented in FIGS. 3, 4, 
and 5. 
FIG. 3 shows the relationship between pH and phosphate over 44 hours with 
the, asterisks representing the average values for these parameters taken 
over a 4 hour period. 
The lines labeled "2.8:1" and "2.2:1" represent pH and phosphate values 
which correspond with these two sodium to phosphate ratios and define the 
molar ratio box. FIG. 4 separately plots pH and phosphate against time. As 
can be seen from these figures, the initial pH and phosphate content of 
the system were higher than the target level for these parameters. FIG. 5 
plots chemical feed of the mono and tri sodium phosphate species over 
time. As can be seen from that figure, the computer fed a higher amount of 
monosodium phosphate initially to bring the pH within range, and a higher 
amount of trisodium phosphate when the pH dropped slightly below target 
(at about hour 12). Following that time, the computer maintained the 
system at or near the target values by adding trisodium phosphate to 
monosodium phosphate in a ratio of approximately 3:1. From the initial 
reading, it took the system approximately 4 hours to get within the target 
box, and approximately 12 hours after that to get at or near the target. 
Example II 
Example I was repeated. Prior to beginning the test, sulphuric acid was 
intentionally added to the system to bring pH below target. Through the 
FIA analysis it was confirmed that pH was below target, and also that the 
phosphate content was within target. The results are presented in FIGS. 6, 
7 and 8. As can be seen in FIG. 8, which plots chemical feed of the mono 
and tri sodium phosphate species over time, the computer initially fed 
trisodium phosphate to bring the pH up within range. Once target values 
were reached for both pH and phosphate, trisodium phosphate and monosodium 
phosphate were fed to the system in a ratio of approximately 3:1. As can 
be seen from FIGS. 6 and 7, it took the system approximately 4 hours to 
get within the target box and approximately 8 hours to get at or near the 
target. 
Example III 
A boiler system was established with the following conditions and control 
parameters, as if being operated at a pressure of 1500 psig: 
System volume: 35 gallons 
Boiler water phosphate target: 8.3 ppm total phosphate 
Boiler water pH target: 9.61 
Two chemicals being fed: monosodium phosphate and trisodium phosphate 
Half-life: between about 7-8 hours 
High purity water 
N.sub.2 purged system 
The methods of Example I were repeated in a system operating under the 
above parameters. The results are presented in FIGS. 9, 10 and 11. As was 
determined by the FIA apparatus, the system had initial conditions of low 
phosphate content and low pH. To compensate for these conditions, the 
computer significantly increased the feed of both phosphate species to 
bring pH and phosphate within the target box, as is illustrated in FIG. 
11. Once target values were reached for both of these parameters, the 
computer fed trisodium phosphate and monosodium phosphate to the system in 
a ratio of approximately 3:1. As can be seen from FIGS. 9 and 10, it took 
the system approximately 16 hours to get at or near the target.