Considerations for High Pressure Boiler
Chemical Treatment – Equipment and
Chemistry
As the ammonia plant design envelope is continually pressed for larger and more efficient plants, the
boiler and steam systems become even more critical. In the middle 1960s, the then new single train
ammonia units came on-line with 1500 psig (104 bar) steam systems. At that time, experienced
ammonia producers said it was like “Operating a power plant with ammonia as the by-product.”
Today, such a comparison is still appropriate as heat fluxes, steam pressures, and design production
rates have increased. The plant designers and operators have choices when it comes to both the
boiler internal treatment and the feed equipment for proper dosing. This paper describes the various
systems available and the strengths and weaknesses of each.
Daniel M. Setaro
GE Water & Process Technologies
Thermal Power Systems has published
Consensus on Operating Practices for the Control
of Feedwater and Boiler Water Chemistry in
Modern Industrial Boilers.1 Many refer to these
practices as the ASME guidelines.
The
recommended feedwater quality for boiler
systems in the pressure range of concern is
shown in Table 1.
Chemical Treatment Considerations
For purposes of this paper, the targeted operating
boiler pressure range is 1500 to 2000 psig (104 to
140 bar) as this is the typical HP steam range
found in modern ammonia plants. In this
pressure range, the recommended feedwater
quality is quite pure so as to assure the highest
levels of steam purity. For further conciseness,
this paper will address phosphate based internal
treatment chemistries such as the coordinated
phosphate and congruent phosphate programs
commonly employed in high-pressure industrial
systems. In special cases all volatile treatment
programs are employed where there are serious
concerns for phosphate hideout problems. Much
more stringent demands are placed on boiler
feedwater quality when AVT chemistry is
utilized.
Table 1: ASME Feedwater Consensus Guidelines for
1501 to 2000 psig (104 to 140 bar) systems
BOILER FEEDWATER
ASME CONSENSUS
PARAMETER
Dissolved oxygen ppm
< 0.007
(mg/l), as O2, measured at
point prior to addition of
oxygen scavenger
Total iron ppm (mg/l), as
<0.010
Fe
Total copper ppm (mg/l),
<0.010
as Cu
ND [not detectable]
Total hardness ppm (mg/l)
, as CaCO3
pH @ 25oC
8.8 – 9.6
Boiler Feedwater
Chemical for preboiler
system protection
The feedwater quality task group of the industrial
subcommittee of the ASME Research and
Technology Committee on Water and Steam in
2006
279
Use only volatile alkaline
materials upstream of
attemperation water source*
AMMONIA TECHNICAL MANUAL
*As a general rule, the requirements for attemperation
spray water quality are the same as those for steam purity.
In some cases boiler feedwater is suitable. In all cases the
spray water should be obtained from a source that is free of
deposit forming and corrosive chemicals such as sodium
hydroxide, sodium sulfite, sodium phosphate, iron, and
copper. The suggested limits for spray water quality are:
<30 ppb (ųg/l) TDS, < 10 ppb (ųg/l) Na, <20 ppb (ųg/l)
SiO2, and it should be essentially oxygen free.
scavengers were developed to be a replacement
for hydrazine, as it has suspected carcinogenic
properties. Since a detailed treatise of volatile
oxygen scavenging is beyond the scope of this
paper, let’s leave it that small excess of
scavenger assures a reducing environment for the
formation of protective magnetite on the surfaces
of the boiler and steam systems. High purity
feedwater in all steel systems without dissolved
oxygen present is passive and protective at
elevated pH levels of 9.0 to 9.6. Minimizing the
amount of feedwater iron entering the steam
drum is a key component of successful internal
boiler water treatment. Even a very small
amount of transported iron oxide over time can
cause problems in boiler systems.
As shown in earlier work presented at the
American Power Conference, the release of iron
oxide from steel feedwater heaters is minimized
as the feedwater pH is increased to the 9.3 to 9.6
range.2 In a similar fashion, the feedwater in
route to the steam drum(s) in ammonia plants
should be at elevated pH levels to minimize
release of iron oxide from the various process
coolers and convection section economizers.
Data from an ammonia plant study indicated
similar reduction in iron oxide release from the
pre-boiler circuit as pH was increased. The data
shown in Table 2 comes from a paper presented
at this conference at an earlier date.3
Internal Treatment Chemistry for
Corrosion Control
Phosphate – a general review
Most high-pressure industrial boilers with high
purity feedwater use a phosphate-based chemical
treatment program for corrosion control. The
evolution of phosphate-based boiler chemistries
followed improvements in feedwater quality.
Prior to the advent of demineralizers, sodium
phosphates were used to precipitate calcium to a
calcium hydroxyapatite that was a fluid sludge
removable by lower drum blowdown. When
demineralized water systems came about, the
need for hardness precipitation was replaced by a
need for phosphate buffering. The buffering
action of the phosphate treatment will minimize
the potential for acid and caustic based corrosion.
Keeping a clean heat transfer surface is key to
minimizing under-deposit corrosion cells.
Deposit minimization relies on both the
feedwater treatment and polymeric dispersants.
Besides causing boiler tube overheat failures,
deposits play a role in corrosion. Formation of
concentration cells under and within deposits can
lead to corrosive conditions at the tube metal
surface. As such, the maintenance of clean
boiler tube surfaces is the most important step
that one can take to minimize corrosive attack.
Table 2: Iron release from BFW circuit as function of
pH
Day
BFW pH
BFW Iron,
ppb (ųg/l), as
Fe*
15
8.4
40
16
8.5
35
17
9.1
11
18
9.1
9
19
9.0
<5
20
9.0
6
21
9.1
<5
22
9.0
<5
23
9.0
<5
24
9.0
6
*Data from ammonia plant study where deaerator outlet
iron was in 5 to 10 ppb range, as Fe
Feedwater Oxygen Control
To assure operation under a reducing
environment, volatile oxygen scavengers are
added to the deaerated boiler feedwater.
Commonly used chemical scavengers include
carbohydrazide, hydrazine, and hydroquinone.
Both carbohydrazide and hydroquinone based
AMMONIA TECHNICAL MANUAL
280
2006
developed around solubility data. Research data7
showed that pure sodium phosphate solutions at
572oF (300oC) form precipitates at a 2.85 to 1
sodium-to-phosphate ratio. This led researchers8
to propose an upper limit for maintaining
sodium-to-phosphate congruency.
Further
definition of the chemistry9 found two invariant
points, one at 2.85:1 and one at 2.15:1. These
boundaries became the upper and lower limits for
the congruent control utilized by most industrial
boiler systems today.
Coordinated and Congruent Phosphate
The first use of phosphate chemistry programs 4/5
for corrosion control dates back to the early
1940s when Whirl and Purcell developed
coordinated phosphate/pH for control of caustic
embrittlement. The basis of the chemistry is:
NaOH + Na2HPO4 Æ Na3PO4 + H2O
As shown above, feedwater caustic will react
with disodium phosphate in the boiler water
forming trisodium phosphate.
Even though congruent control can maintain pH
during minor contaminant ingress and to a minor
extent inhibit calcium deposition during hardness
intrusion, its primary purpose is to mitigate
under-deposit corrosion. It performs this task by
maintaining under-deposit sodium phosphate
solution chemistry between the congruency
limits. By doing this, the concentrated solution
under a boiler deposit does not result in either
caustic gouging or acidic phosphate corrosion.
Figure 1 shows the relative corrosion of carbon
steel boiler tubing operating at 590oF (310oC) as
a function of pH and concentration of corroding
species – HCl and NaOH.
In a similar fashion feedwater acidic species will
react with trisodium phosphate to form disodium
phosphate as follows:
HCl + Na3PO4 Æ Na2HPO4 + NaCl
By controlling the boiler chemistry to maintain
di-basic phosphate in the boiler (sodium to
phosphate molar ratio less than 3.0 to 1), caustic
(NaOH) cannot concentrate and cause corrosion.
This assures that all the sodium is associated
with phosphate and no free NaOH is in solution.
By maintaining the alkalinity as a captive
phosphate alkalinity, under-deposit concentration
of caustic is eliminated, thereby minimizing
caustic gouging potential.
For years congruent phosphate programs have
been applied with the assistance of a logarithmic
control chart as that shown in Figure 2. Several
upper molar ratio boundaries are presented
depending on the operating pressure of the
boiler. These boundaries were set to assure
minimal chance of phosphate precipitation at the
corresponding temperatures. Staying below the
upper molar ratio limit will minimize the chance
of caustic concentration and gouging of the tube
metal. The lower control boundary of 2.2: 1
molar ratio is set to minimize the chance of
acidic phosphate attack. When the boiler pH and
phosphate levels stay within the control
boundaries, the boiler water should be noncorrosive. The vector diagram indicates the
direction a point will go on the phosphate-pH
diagram if a certain chemical is added to the
system. Also shown is the affect of increased
continuous blowdown. In thinking of powdered
Coordinated phosphate programs were designed
to prevent caustic gouging, but such problems
persisted even when coordination was
maintained. The gouging was associated with
boiler deposits and it suggested that the gouging
was related to the extent of the deposition. 6 It
was also noted that phosphate precipitation
occurred at a lower ratio that the 3.0 originally
prescribed for coordinated control.
These observations made researchers re-think the
sodium phosphate buffer chemistry under the
boiler environments where problems were
observed. As a result of further studies of
solution chemistry and two-phase equilibrium, a
new definition of phosphate control was
2006
281
AMMONIA TECHNICAL MANUAL
Staying within the control boundaries of
phosphate and pH is key to successful operation
of high-pressure industrial boilers. But both of
the corrosion pathways are connected with
deposits. Figure 3 depicts caustic gouging
mechanism. Figure 4 shows a photograph of
acidic phosphate attack.
sodium phosphates, one can imagine the need for
three phosphates as well as caustic. When
dealing strictly with powders, this could be the
case, but normally only tri and di-sodium
phosphate will be required to counter normal
contaminant variation in the incoming boiler
feedwater. Most water treatment companies can
provide blends of sodium phosphate products at
various sodium to phosphate molar ratios.
Figure 1: Relative corrosion of carbon steel by HCI and
NaOH at 590°F (310°C). (Berl and vanTaack, 1930;
Partridge and Hall, 1939)
Figure 3: Porous deposits provide conditions that
promote high concentrations of boiler water solids,
such as sodium hydroxide (NaOH).
Figure 1: Relative corrosion of carbon steel by HCl and
NaOH at 590ºF (310ºC) (Berl and vanTaack, 1930;
Partridge and Hall, 1939)
Figure 4: Acidic phosphate caused corrosion of this boiler
tube.
Figure 2: Coordinated Phosphate/pH Control Chart
AMMONIA TECHNICAL MANUAL
282
2006
All Volatile Treatment [AVT]
In an AVT program only volatile chemicals are
added to the feedwater primarily for pre-boiler
circuit corrosion control. Normally, the AVT
chemicals include ammonia or amines for pH
control and an oxygen scavenger (hydrazine,
carbohydrazide, hydroquinone, etc.) for oxygen
control. Secondarily, these volatile chemicals
control boiler corrosion by maintaining a
protective magnetite film through pH and oxygen
control.
Since ammonia is highly volatile
typically, the boiler pH is 0.2 to 0.3 units lower
that the feedwater pH value when ammonia is
used. In addition the boiler water has essentially
no capacity to buffer the pH in event of
feedwater contamination. Consequently, where
possible, it is advisable to use an amine with a
low distribution ratio at boiler water temperature.
This will provide a small increase in the ability to
maintain boiler water pH in the event of acidic
ingress.
Phosphate
Control
Comments
Steam Purity
Concerns – Solids
Favored
Less
favored
Plants with
inadequate
steam
separations are
forced to AVT
Steam Purity
Concerns – Silica
Less
Favored
Favored
Higher (more
favorable) pH is
possible with
2006
Phosphate
Control
Ability to cope
with
contamination
Less
Favored
Favored
Phosphate Hideout
Concerns
Hydrogen Damage
(Embrittlement)
Concerns
Ability to cope
with dryout
conditions
Favored
Less
Favored
Favored
Favored
Less
Favored
Solubility of
Turbine Deposits
Less
Favored
Favored
Operator skill
level required
Same
Same
Less
Favored
Comments
PO4
PO4 allows
buffering of
both acidic &
caustic
contamination.
AVT has no
solids
PO4 buffers
against acidic
contamination.
Unfortunately,
these conditions
cannot be
predicted.
PO4 program at
higher pH
reduces silica
vaporization.
Constant vigilance over the proper operation and
maintenance of the makeup water pretreatment
system and the condensate polishing system is a
must. Earlier mention was made regarding
keeping the feedwater pH elevated in order to
assure minimal release of iron oxide from the
feedwater preheat train. For best results in an all
steel system, feedwater pH should be within the
range of 9.2 to 9.6.
Even with the best programs, safeguards, and
instrumentation
in
place,
feedwater
contamination cannot be completely eliminated.
As such, there is potential for troublesome
deposits to develop. Consider a system that
averages only 10 ppb (ųg/l) of feedwater iron and
produces an average of 650,000 pounds per hour
(295 tonnes per hour) of steam.
It will
accumulate 57 pounds (26 kg) of iron deposit (as
Fe) in one year. This amount of deposit is not
that significant if it is spread uniformly over the
boiler system’s entire heat transfer surface. But
deposits do not distribute uniformly, and in
systems with varying heat fluxes and circulation
patterns, the majority of the deposition can
accumulate in small areas.
Table 3: Comparison of Congruent Phosphate to All
Volatile Treatment
AVT
AVT
Minimizing Deposition
It is prudent to have 100% polishing of the
makeup and condensate return streams as the
feedwater should be maintained below 0.2
microSiemens/cm cation conductivity.
Any
identified in-leakage of contaminants to the
feedwater such that the cation conductivity
exceeds
0.2
microSiemens/cm
requires
immediate implementation of a congruent
phosphate program to protect against small levels
of either acidic or alkaline contamination.
Depending on the magnitude of the
contamination, more serious measures may be
required.
Parameter
Parameter
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AMMONIA TECHNICAL MANUAL
dispersants under high pressure research boiler
conditions.
The use of a polymeric agent to control the
deposition of feedwater contaminants, even in
systems with good feedwater quality, is usually
of benefit.
In systems with demineralized
makeup and condensate that is polished, iron will
be the major contaminant to address. So in the
case of the modern day ammonia plant, an
effective iron dispersant that will function
effectively at higher temperature and pressure
would be necessary. A polymer with phosphate
functional groups has proven to be exceptionally
useful in this regard.
Listed below are the highlights of our experience
with two different dispersants in 1500 psig
ammonia plant boiler systems. At GE Water &
Process Technologies, laboratory and field trial
work has indicated the two dispersants for
consideration in high pressure industrial plants
are polymethacrylate (PMA) and poly
(isopropenyl phosphonic acid). Of these two, the
latter (branded as HTP) is the stronger performer.
Monitoring the boiler system cleanliness is of
key importance. Even with the aid of effective
polymeric dispersant, no system will exhibit
100% iron transport. With that in mind, plant
operators should assess the various ways to
measure deposit tendency and accumulation
within the boiler circuits. In fired boilers, a
planned testing program for cutting tube sections
for deposit weight density and deposit
composition determinations could be initiated.
Tube selection, removal techniques, handling,
and test method can significantly affect the
results obtained. Guidance in these matters can
be found in references 10, 11, and 12. On-line
analysis of deposit accumulations through use of
strategically placed chordal thermocouples can
be useful.13 Chordal thermocouples are special
devices that measure the temperature gradient
through the tube wall of a fired boiler. Data
collected is an indirect measure of the
accumulation of boiler tube internal deposits.
Research Boiler Deposit Inhibition
Comparison (1450 psig)
% DEPOSIT INHIBITION
100
0
HTP
PMA
PAAM
PAA
NONE
EQUIVALENT POLYMER DOSAGE
Figure 5: Relative Effectiveness of Various Boiler Water
Dispersants
Iron transport data from 1500 psig ammonia
plant boiler system locations indicate improved
performance by the HTP polymer.
Plant A: Internal treatment program was
congruent-phosphate/pH with PMA polymeric
dispersant. Boiler feedwater iron levels were in
the range of 5 to 10 ppb throughout the duration
of the trial period. Below Tables 4 and 5
summarize the iron transport data in the
ammonia plant field trial with PMA dispersant
followed by HTP dispersant feed.
Lastly, in the case of high purity systems, it is
good to regularly monitor the iron levels in the
feedwater and boiler water for an approximation
of iron transport. While not without flaws, such
data will give good qualitative trends particularly
when going from no dispersant to an added
dispersant or from on type of polymeric
dispersant to another.
Figure 5 shows a bar chart summarizing the
relative effectiveness of various boiler water
AMMONIA TECHNICAL MANUAL
284
2006
program was just a phosphate based congruent
program.
Table 4: Comparison of Boiler Water PMA dispersant
levels with Iron levels.
Day
PMA Level,
Boiler Water Iron, ppb
ppm
(ųg/l) as Fe
1
15
44
2
15
38
3
15
40
4
15
37
5
30
73
6
30
51
7
30
65
8
30
56
9
60
96
10
60
114
11
60
119
12
60
100
13
90
109
14
90
115
15
90
123
16
90
94
Figure 6: HTP Trial Test – Demonstrated Iron
Transport Improvement
Table 5: Comparison of Boiler Water HTP Dispersant
Levels with Iron Levels
Day
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
HTP Level,
ppm
12
12
12
12
12
15
15
15
15
15
20
20
20
20
20
40
40
40
40
40
Polymeric dispersants can generally be supplied
as separate liquid products or they can be
blended with the phosphate based liquid products
at the proper concentrations. Working with a
qualified water treatment specialist to obtain the
chemical treatment program to meet your
system’s needs is just the beginning point. One
must select the chemical feed and control system
that meets their needs and budget. In the next
section there will be descriptions of the various
feed systems that are available. It has been the
author’s opinion that one should only consider
the high-end feed and control equipment if they
are strongly committed to the proper calibration
and maintenance required.
Boiler Water Iron , ppb
(ųg/l) as Fe
382
311
346
361
333
487
511
530
492
481
671
634
660
628
647
1031
1006
1025
1013
1046
Chemical Feed and Control
Considerations for Congruent
Phosphate Internal Treatment
A less rigorous HTP trial test was performed at
another ammonia plant location. The results
shown in Figure 6 indicate similar iron transport
improvement as demonstrated in the previous
ammonia plant site. In this case neither plant
used a dispersant as the internal treatment
2006
Standard Day Tank Systems
Powdered Chemicals – For those without
concerns of the added labor costs and time
required for handling powdered sodium
285
AMMONIA TECHNICAL MANUAL
powders, the requirement and limitations of such
a liquid day tank system is similar to that of
powders.
phosphate chemicals, use of such chemicals is
certainly the most economical from a chemical
cost standpoint. In such a case, a day tank is
equipped with a dissolving basket, mixer, and
properly sized pump to deliver the made down
solution to the steam drum’s chemical feed
header.
Operators should always log the
amounts of each phosphate chemical, caustic and
polymeric agent [if in the program] that is
charged to the day tank along with time of day
and the volume of the tank when charging and
mixing began. The plant needs to decide how
changes will be made based on the boiler test
results – change the mixture recipe in the day
tank while leaving the pump speed and stroke
constant, or changing the pump settings. It is
important that all shifts of operators follow the
same procedure. The operators should log the
day tank’s sight glass reading on a once every
two-hour basis at a minimum. Manual changes
and responses are made based on operator testing
of boiler water for phosphate and pH. Normally
this testing is done once per shift – every eight to
twelve hours. While this type of system bears
the lowest capital cost, it does not do provide the
capability to react to system variations in a
timely fashion.
To minimize the handling and possible errors
involved in using one liquid product and caustic,
proprietary blends of low and high sodium to
phosphate ratio products are formulated. In such
a system there are two internal treatment
products, or a matched-pair. Each product will
have the same amount of phosphate and
polymeric agent [if included]. But each will be
formulated to have a specific sodium to
phosphate ratio -- one high and one low. That
way the operator will not have to handle and
measure out caustic at each makedown of
chemical. The operator will add the required
amounts of the two products to the day tank
based on results of boiler water phosphate and
pH measurements.
Depending on product
requirements, systems can be set up to deliver the
liquid products from bulk containers into a
measuring pot prior to the day tank. This
minimizes operator’s exposure to chemicals.
Such a liquid system should allow for less
potential operator error as compared to powders
and caustic or the one liquid product and caustic
to the day tank. The capital cost of the day tank
system would be approximately the same as that
for powders.
The matched pair product
application should allow for less day tank batchto- batch variation.
Liquid Chemicals – Water treatment companies
have the capability to blend sodium phosphate
and caustic to provide liquid products at a given
sodium to phosphate molar ratio. Within limits
of solubility and compatibility, product
formulations can be made to include polymeric
dispersants. Due to the possible variation of
sodium in the boiler feedwater, the use of one
specific blend is a challenge. If there is a desire
to use just one blend, then it is best to have a low
sodium to phosphate blend and have the
operators add the necessary amount of caustic to
the day tank to yield the proper blend in the day
tank for the current situation. That way, the
caustic will be able to react to feedwater
variation.
In such a case, the equipment
described above [other than the mixing basket]
will be used for mixing and delivery of the
chemical solution. Other than not handling
AMMONIA TECHNICAL MANUAL
Automated
Control
of
Congruent
Phosphate/pH Treatment – Maintaining
effective control of boiler water chemistry can be
difficult. Some of the system variables that are
changing are the steam rate, boiler cycles,
blowdown flow, boiler feedwater sodium level,
and dosing chemistry and rate. So with manual
adjustment of blowdown, chemical makedown,
and dosing rate, the operator will have to base
changes on experience factor and best prediction.
Inaccurate predictions can have a negative
impact on the boiler chemistry.
286
2006
With this in mind, GE Water & Process
Technologies developed an automated system for
control of congruent phosphate / pH systems.
Most automated chemical feed systems utilize a
controller with a typical proportional-integralderivative (PID) algorithm to maintain a federate
passed on a primary control variable and a
secondary trim variable. But congruent control
requires a more complex algorithm that
accommodates the interdependent nature of the
pH and phosphate as well as the dynamics of the
boiler and the chemical feed network.
The continuous pH measurement of the
blowdown uses feedback control that compares
the on-line pH to the controller set point. Based
on the deviation from the set point, the controller
adjusts the feed ratio of two custom formulated
products with different sodium to phosphate
molar ratios.
To assure the needed accuracy of chemical feed,
the feed system was re-engineered to allow for
precise neat feed and verification by a patented
Verifeed system. Reliable low-pressure metering
pumps supply neat chemical to the automatic
makedown module (AMM) that is kept flooded
with high purity water. Note the controller
output changes the dose rate of the chemical
[high or low ratio product] to maintain the pH set
point and the phosphate concentration via mass
balance. From the AMM, the dilute chemical is
fed via normal high-pressure pump to the steam
drum. The HP pressure pump is normally set at a
high delivery rate and left unchanged.
In the automated system described in Figure 7,
two variables are used to control the chemical
dosage: blowdown flow rate and boiler water pH.
Knowing precisely the amount of phosphate fed
and the blowdown flow rate, the phosphate
concentration in the boiler is calculated by mass
balance. A blowdown flowmeter is required, and
the phosphate is fed based on blowdown.
Typically, the blowdown valve will be fixed to
optimize cycles, with changes occurring during
startup, shutdown, or when in upset situations.
Since pH measurement is very critical to the
control scheme, a sample conditioning station is
mandatory as it incorporates precise control and
verification of sample flow and temperature to
maximize the reliability of the on-line pH
measurement.
This automated system maintains the boiler
water chemistry in control as it responds to
changes in feedwater chemistry and steaming
rates as they occur. The frequency of pH and
phosphate testing can be reduced. The periodic
testing of pH will serve to assist in the calibration
of the on-line pH probe. Alarm systems are built
in to indicate any system failures such as loss of
sample flow; chemical feed pump failure, and
loss of level in the make-down module.
Communication with the plant’s DCS is possible.
The computerized data acquisition allows for a
review of historical operating data, resulting in
easier system diagnostics and simplified
troubleshooting.
Figure 7: Schematic of Boiler Precision System
2006
287
AMMONIA TECHNICAL MANUAL
providing phosphate data more frequently, this
trial tool allowed for a measure of the
controller’s robustness in regard to responding to
system upsets. Feedwater sodium concentration
changes can have a marked affect on the boiler
water chemistry. An on-line sodium analyzer
was installed on the BFW sample to
continuously measure this component. Data
from the analyzer would define periods of upset
such that the ability of the controller system’s
response could be measured. These on-line
analyzers are not used in standard installations of
the controller system.
Phosphate/pH Controller Field Trial Results
The following lists the pre-trial conditions: neat
chemical feed to steam drum with a single blend
of phosphate and polymer; boiler water pH and
phosphate were manually measured once per
eight hour shift; operators made adjustments to
chemical feed and blowdown based on sample
results and operating conditions. Due to the neat
chemical feed, no continuous changes could be
made in the sodium-to-phosphate ratio. Despite
the complexity of periodic manual chemical feed
and blowdown adjustments, the control was “in
the box” most of the time prior to the installation
of the controller. See Figure 8.
Figure 9 shows the chemistry results after the
controller system was installed. For the trial
period studied, the control was “inside the box”
for the entire duration. The pH target of 9.9 was
controlled with a precision of 0.04 pH units and
the phosphate concentration target of 18.0 ppm
was maintained within a precision of 0.5 ppm
during the same time period.
Figure 8: Boiler Chemistry Control Prior to System
Installation
The controller system was installed as
previously described such that two products that
differed only in their ratio of sodium-tophosphate could be fed to the automatic
makedown module via low pressure metering
pumps.
The dilute chemical solution was
pumped to the steam drum via high pressure
pump.
Boiler water pH was continuously
measured and the blowdown flow was measured
using a orifice plate flow meter. The metered
blowdown results were verified by tracer studies.
Figure 9: Boiler Chemistry Control after System
Installation
During the field trial, the ability of the controller
to respond to changes in blowdown flow was
assessed as follows:
Blowdown flow was
decreased via a 35% step change. The results,
shown in Figures 10 and 11, show chemical feed
was correctly ratioed during the flow change as
reflected by the phosphate and pH results.
To assure our trial system accuracy, a phosphate
analyzer was installed to continuously measure
boiler water phosphate residual.
Besides
AMMONIA TECHNICAL MANUAL
288
2006
Figure 12 shows the ability of the controller to
respond to an excursion in feedwater sodium
content. During the trial there was pretreatment
system upset that caused the sodium level in the
BFW to rise from 50 to 170 ppb followed by a
plateau of 100 ppb. During this period, the pH
control of the boiler water was maintained in
control (9.85 to 9.95).
Figure 12: pH Control during Feedwater Sodium
Excursions
Summary
The boiler precision control system provides
very high levels of boiler chemistry control and
reliability. Plant operating personnel uses their
once per shift manual testing for confirmation of
the on-line pH measurement and the output of
the controller’s algorithm. The boiler chemistry
control is “in the box” resulting in improved
system reliability. The data acquisition system
allows for improved diagnostics and system
troubleshooting.
Review of data regarding
chemical dosage, boiler blowdown dynamics,
and operating data allows for increased reliability
and system improvement.
Figure 10: Phosphate Control during Blowdown Flow
Change
References
1. American Society of Mechanical Engineers.
“Consensus on Operating Practices for the
Control of Feedwater and Boiler Water
Quality in Modern Industrial Boilers,” 1994
2. “Control of Iron Pick-up in Cycles Utilizing
Steel Feedwater Heaters,” from Proceedings
of the American Power Conference, Chicago,
1966.
3. Improved Water Treatment for High-Pressure
Boilers,
AIChE
Ammonia
Safety
Symposium, November, 1991.
Figure 11: pH Control during Blowdown Flow Change
2006
289
AMMONIA TECHNICAL MANUAL
4. T. Purcell, S. Whirl, Trans. ASME 64, (1942)
p. 397
5. Whirl, S. F.; Purcell, T.E.; “Protection
Against
Caustic
Embrittlement
by
coordinated
Phosphate
pH
Control,”
International Water conference, p. 45
(Engineers’
Society
of
Western
Pennsylvania, Pittsburgh, PA, 1945
6. H. Klein, Combustion, (1962), p. 1.
7. M. Ravich, L. Scherbakova, Izvest Sektora
Fiz, Khim, Anal., Inst Obschei Neorg. Khem,
Akad Nauk SSSR 26, (1955), p. 248
8. V. Marcy, S. Halstead, Combustion, 1964, p.
45. Panson, et. Al., J. Electrochem. Soc. 122,
7 (1975), p. 915
9. Mayer, P., “Information Required for Boiler
Tube
Failure
Investigation”,
NACE
Publication 7H290, 1990
10. Atwood, K.L., Hale, G.L., “A Method for
Determining the Need for Chemical Cleaning
of High Pressure Boilers”, American Power
Conference, Chicago, IL, 19717
11. Selby, K.A., ET. AL., “Evaluation of Boiler
Tube Deposit Weight Density Methodology”,
NACE Corrosion 97, New Orleans, LA 1997
12. Strub, J.W., “The Use of Chordal
Thermocouples for Monitoring the Thermal
Resistance of Boiler Waterside Deposits”,
American Power Conference, Chicago, IL,
1961
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2006