BOILER
WATER TREATMENT
The
One Area In Which You Can't Afford
Mistakes
Let
Thermidaire Consultants Protect Your
Boiler, It Is the Heart of Your System
At Thermidaire Consultants, when we
refer to boiler water treatment, we
mean the total system, not just
internal treatment. Thermidaire can
help you keep your systems running
efficiently and reliably. Use our
one-time survey consulting services or
continuing surveillance contract as
outlined on our Consulting Services page.
Protect your boiler water systems with
the insurance they need by adding to
your staff the experts from Thermidaire
Rice Water Consultants. These experts
will survey, evaluate, review,
monitor, establish testing and control
parameters, and recommend corrective
changes as required in the following
areas:
Remember
To Protect Your Equipment Not In
Service.
Let Thermidaire Consultants Provide
Recommendations On Stand-by Treatment.
Make-up
water is described as the final
treated water that is mixed with
returned condensate to form the
feedwater which enters the boilers.
The pre-treatment system is the
sub-system by which this make-up water
is produced from the plant's source
water (which may or may not be treated
before reaching the boiler
pretreatment system).
Indeed, it was not un-common in years
past but a rare case today that the
make-up water receives no pretreatment
or is simply processed in a cold
precipitation softening process.
See Liquid/Solids Separation Page for
more information on how we can help
with operation of hot or cold
precipitation softeners.
A step up but also much more commonly
seen in older plants are hot process
precipitation softeners. Although
these hot precipitation softeners are
labor intensive and a high maintenance
item, they are still occasionally
installed in new plants because of
their unique ability to economically
soften waters and reduce alkalinity
and total solids. These are effective
and reliable pre-treatment processes,
but they can also be very system
disruptive unless skillfully operated.
If your facility utilizes one of these
, you need the help of Thermidaire to
review their operation and to train
your staff in proper control
procedures.
Today, most modern pre-treatment
systems consist of either softening by
sodium cycle ion- exchange or some
other ion-exchange process to reduce
hardness and alkalinity or to remove
essentially all dissolved solids. In
addition, whether used alone or in
combination with one of the
ion-exchange processes, membrane
processes like reverse osmosis have
been found to add efficiency to the
pre-treatment process.
If not controlled and operated correctly, each
of these unit processes pose potential
threats that could result in boiler
operations failure. Ready access to
qualified, water treatment engineers
is advisable.
For information on how Thermidaire
Consultants can help you in this
pre-treatment area, see Water
Purification Page
When Thermidaire Consultants survey
your system, one of the things they
will determine is the recommended
quality make-up water for your
boilers. They can help you achieve the
minimum quality required if your
current system does not meet that
minimum. Potential consequences of
operation below minimum quality will
be presented along with modified
operations recommendations. More often
than not, however, we find that
pre-treatment systems have been
up-dated beyond that needed for boiler
operations. Believe it or not, this
too can be a problem if the internal
boiler water treatment program has
remained unchanged from what it had
been prior to the upgrade. But that's
another problem we at Thermidaire can
solve for you.
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Feedwater System
The whole is only as strong as the
weakest of its parts, and this
certainly applies to the feedwater
system of the overall boiler water
cycle. The primary functions of the
equipment that comprise the feedwater
system, aside from the pumps
themselves, are to pre-heat the water
and to make it less corrosive. Both
these functions are normally
accomplished with low pressure,
saturated steam. Corrosion is
controlled by the removal of carbon
dioxide and oxygen. To accomplish the
main function of oxygen removal,
design and operational considerations
must be such to result in the optimum
temperature, pressure, and surface
contact area for release and scrubbing
of the gases An initial survey and
continuing service review by Thermidaire
will include consideration of all
potential trouble spots. These include
the following::
Design
versus Operational Conditions:
Feedwater
Flow Rate
Feedwater Temperature
Heating Steam Temperature and Pressure
Quantity of vent
Significant deviation from design
conditions can result in high oxygen
levels in the feedwater. This can
happen just as easily if operating
conditions are below design as if they
are over design. Most deaerators are
designed for operation with saturated
steam; if operation has changed to the
use of superheated steam, this too can
result in high oxygen levels. If
insufficient saturated steam is
available, oxygen removal suffers.
Excess steam costs money. In
industrial systems, a good rule of
thumb is to keep the deaerator at
design pressure and temperature and to
keep the temperature in the storage
section 2 degrees F below that
temperature. If the temperature
difference is greater than 2 degrees,
insufficient steam is being used; if
less, then steam is being wasted.
Mechanical
Condition-
Routine
inspection of the deaerator to assure
sprays and trays are in place,
undamaged, and free of restricting
deposits is a must. Safe operation
also demands inspection for shell
integrity.
Testing
for Oxygen.
Industrial
deaerators should reduce the dissolved
oxygen to 7 ppb or less. Your
operators should routinely determine
the feedwater oxygen content to assure
the unit continues to operate
properly. Your water consultant should
establish the testing schedule, method
of test, sample point, and instruct
your operators in the fine points of
this delicate testing.
Chemical
Feed-
It
is normal , and recommended, practice
to feed an oxygen scavenger to the
feedwater. Direct to the storage
section of the heater is a common
injection point. With the emergence of
modern treatment chemicals, the choice
of feed is now much more difficult
than it had been when sulfite or
hydrazine were the only choices.
Hydrazine, particularly catalyzed
hydrazine, is still an effective
treatment for higher pressure boilers
but is now environmentally taboo in
most systems. Sulfite continues to be
widely used in low pressure systems
but has been displaced in higher
pressure systems with hydroquinone,
erythorbate, or DEHA and kindred
chemistries . These new chemistries
are superior in many cases to sulfite
because they act as pasivating agents
in addition to functioning as oxygen
scavengers.
Sodium sulfite can have the additional
disadvantage of interfering with the
pH/phosphate balance in coordinated
phosphate/pH control internal
treatment modes.
Other
Testing-
The
feedwater provides the final source
for determination of what contaminants
may actually be entering the boiler.
Tests for control and for contaminant
levels should be performed on a
established schedule basis. Operators
should be trained as to how to respond
to those test results. The sample
points within the feedwater system are
also very important and should be
reviewed and changed as needed by you
water treatment consultant.
Attemperation/desuperheating
Water
If
feedwater is used for this purpose, be
certain that feedwater source is
upstream from the chemical injection
point or that the chemicals being used
do not impart any dissolved solids to
the system.
As previously stated, there exists
much potential for problems in the
feedwater system. Careful attention to
it by a skilled water treatment
engineer is recommended.
Back to The Top
Condensate
In the section on "Heat
Recovery", we discuss the
economic advantages of increasing as
much condensate as possible to the
boiler cycle. In addition to the large
impact on system economics this heat
savings can have, additional savings
can be considerable based on the
reduced make-up water requirements.
Let Thermidaire Consultants do an
up-to-date energy and material balance
around the water side of your boiler
systems to show you what potential
dollars can be saved.
But there is more to returning
condensate than just the required
plumbing. If the source of the
condensate is from process heaters,
the effect on system operations caused
by that contaminant needs to be
accessed. A monitoring program needs
to be established so that the
condensate can be dumped if and when
necessary. A routine composite and
spot sampling program needs to be set
up to determine the presence of contaminants
and to track down their source should
they occur.
But even if no outside contamination
takes place, corrosion products from
the condensate system itself are often
the most serious threat to reliable
boiler operation. Condensate corrosion
control is therefore essential to
keeping your boiler water-side
surfaces clean. The discussion below
is therefore limited to condensate
corrosion control.
Condensate
Corrosion--Factors and Control
Let
Thermidaire work with your staff in
minimizing condensate corrosion.
-
Carbon
Dioxide
Breakdown of carbonate and
bicarbonate that enter the boiler
is the main source of carbon
dioxide. Left unchecked, this can
result in low pH condensate. This
has been observed as grooving in
sections of condensate lines that
are not completely filled with
water.
-
Oxygen
Oxygen can enter a condensate
system by many sources even if the
deaerating heater is functioning
properly. The oxygen, at its
worse, can result in deep pitting
of condensate lines. The
combination of oxygen and carbon
dioxide corrosion can be
particularly troublesome in
causing corrosion products to be
produced and transported to the
boiler.
-
Velocity
Although often not controllable,
high flow rates within the
condensate system can produce
extremely severe corrosion
conditions. This flow-assisted
corrosion is accelerated at low pH
and can be minimized by keeping
the pH above 9.0.
-
Other
gases
Other gases that can be corrosive
and present in the condensate
system include ammonia, hydrogen
sulfide, and sulfur dioxide. The
most common of these is ammonia.
Copper corrosion can be as serious
as iron corrosion and is made even
more serious in the presence of
copper complexing agents such as
ammonia. Again, oxygen in
combination with these gases
increases copper corrosion.
-
Neutralizing
Amines
Neutralizing amines, when fed to
the boiler, volatilize with the
steam and enter the condensate
system. These amines are weak
bases and will therefore
neutralize any carbon dioxide present
and will raise the pH of the
condensate. If oxygen levels are
very low, these neutralizing
amines can, by themselves,
effectively control condensate
corrosion. However, knowing which
ones to feed, how much, and how to
control can be a difficult and
confusing decision for the typical
user. There are more than a dozen
amines in common use. Each amine
is unique in certain characteristics,
each of which affect how the amine
functions at a given point in a
given condensate system. Those
amine characteristics include
basicity value, molecular weight,
distribution ratio, and hydrolytic
thermal stability. Since it is
necessary to prevent corrosion
from the point of initial steam
condensation to the far ends and
back of condensate systems, a
blend of neutralizing amines is
normally fed.
-
Volatile
Pasivating Agents
If oxygen is present, the
neutralizing amines alone will not
control corrosion. Fortunately,
chemical treatments have been
developed which will transport
with the steam and will, in
addition to possibly reacting with
oxygen, function as pasivating
agents to prevent corrosion. But
as is often the case, such new
alternatives bring with them a
whole new set of potential
problems and set of rules. Which
product to use where, when, and
how are questions that are best
answered by an expert in water
treatment chemistry who is also
highly knowledgeable about your
system. Control and testing are
the main problems with these pasivating
agents. That is why corrosion
monitoring, always important,
becomes even more so when
treatment results depend on a
passivation chemical.
MONITORING
Corrosion
monitoring is a major task for any
water system. Relative pure
waters, like condensate, make the
task even more difficult. Some of
the methods used and recommended
by Thermidaire Consultants include
the following:
-
Test
Coupons Steel and copper
corrosion coupons have been
used in condensate systems for
many years. While there are
concerns as to how accurately
they reflect the actual
corrosion rate within a given
system, the do provide a good
relative measure of trends in
long term corrosion. A
consistent, continuing program
using coupons at the same
locations and for the same
duration of time is therefore
most meaningful. The locations
and plumbing for the coupons
must be carefully engineered
to avoid meaningless results.
-
pH
Monitoring While not a direct
measurement of corrosion,
continuous measurement of
condensate pH can be very
helpful in systems that depend
on neutralizing amines for pH
control. Other monitors, such
as for conductivity, are also
helpful to guard against
condensate contamination.
Location of sample points and
method of sampling are
critical and should be
established by a qualified
water treatment engineer.
-
Iron
and Copper Testing Another old
but proven effective standby
is iron and copper testing.
The sample points, method of
collection, and analytical
procedures are more critical
in trace metal testing than in
any other analysis. Incorrect
results are much worse than no
results at all. The sample
program should be established
to collect samples at a
pre-determined interval. The
final feedwater represents the
corrosion products load
actually entering the boiler
and can be a good indicator of
the expected cleanliness of
boiler surfaces over a period
of time. Composite as well as
spot samples should be taken.
However, since the slightest
change in flows can make
sample results worthless, any
composite samples collected
should be collected from a
continuously flowing sample
and with a proven condensate
composite sampler.
-
Corrosion
Test Monitors Electronic
corrosion test monitors have
now been developed that can be
effectively used in condensate
systems. Older corrosion test
meters were not capable of
this because of the need for
the water being tested to
exhibit a minimum
conductivity. As with any of
the monitoring methods,
results from these should be
evaluated over a long period
of time and compared with
prior results versus actual
inspections.
-
Membrane
Filters Membrane filers, like
those available from
Millipore, were developed
originally by B&W for
monitoring of the corrosion
test load in returned
condensate. Many plants have
used this method quite
successfully for years. But it
requires time, skill, and
careful dedication to record
keeping for this type of
monitoring to be meaningful.
While "standard"
membrane stains are available,
in practice, the iron
concentrations from a given
system need to be established
over a long period of time.
This is done by determining
iron concentrations
analytically and assigning
average values determined to
actual membrane filters
collected during the same time
period.
Monitoring of your condensate
is essential toward protection
of, not just the condensate
system itself, but more
importantly, to the continued
reliability of the system
boilers.
Back To The Top
Boiler
Water Internal Treatment
-
Corrosion
Control
Coordinated Phosphate
-
Scale
and Deposit Control
Conventional
Phosphate
Chelating
Agents
Polymer
Treatment
CORROSION
CONTROL
Since iron rapidly reacts with
water at high temperature, it
would not be possible to
operate a boiler at high
pressure for very long except
for the protection afforded by
formation of a uniform
magnetite layer. The job for
internal treatment therefore
becomes one of promoting and marinating
that uniform layer. This job
is made difficult because
either high or low pH can
destroy the protective
magnetite. To further
complicate matters, the
process of generating steam
can result in concentration
mechanisms that provide
chemistry conditions different
from those found in the bulk
boiler water. Caustic
corrosion occurs in high
pressure boilers due to this
concentration phenomenon.
Coordinated phosphate control
was developed as a treatment
scheme to prevent this type
corrosion.
Coordinated Phosphate Control
There
are several forms of this
treatment approach commonly
practiced. In this form of
treatment, phosphate is used
as a buffering agent to assure
the boiler water pH will stay
within the desired range. At
lower pressures, the general
term captive alkalinity may be
used and relatively broad
control ranges are acceptable.
As operating pressures
increase, the allowable phosphate
residual is reduced and the
control range is narrowed.
Congruent control is the term
usually given to this type
treatment in high pressure
boilers. Phosphate can be
added in one or a combination
of several forms ranging from
mono-sodium phosphate to
tri-sodium phosphate. The mono
form is the most acidic form
and the tri- is the most
alkaline. By adjusting the
type of phosphate fed, the pH
of the boiler water can be
controlled within a
pre-defined range. This
control range will have been
defined such that, in theory,
no free caustic will be
present even under conditions
of a concentrating mechanism.
The above is an
over-simplification of a
complicated treatment scheme.
Be certain to retain a
qualified water treatment
consultant to review your boiler
water needs and to establish
required control ranges.
Conventional
Phosphate
In
boilers operating under 900
psig, particularly in very low
pressure boilers, scale and
deposits may be more of a
problem than corrosion. In
these cases, your water
treatment consultant needs to
advise you in regard to the
preferred method of internal
treatment. The choices will
usually be conventional
phosphate, chelating agents,
or "all polymer". A
combined treatment of two or
all three of these chemistries
is also possible. But the
decision is not based only on
operating pressures. Depending
on the make-up water
characteristics, some form of
coordinated phosphate
treatment may still be needed.
Conventional phosphate is more
often the choice when the
make-up water is of sodium
zeolite softening quality or
worse . Under these
conditions, scale and deposits
are the major threat and costs
and other considerations make
chelants or all polymer not
possible.
Phosphate is used as a
precipitating agent to react
with and cause calcium to
precipitate in a non-adherent
form. To assure that,
sufficient alkalinity must be
maintained in the boiler water
and the phosphate residual
kept within prescribed range.
It may also be necessary to adjust
the silica concentration in
reference to the phosphate in
order to assure precipitation
of the magnesium in a
non-adherent form.
Supplemental use of polymers
is now common practice.
Chelating
Agents
The
chelating agents NTA and EDTA
came into use in the 1960's
and have been the salvation of
many trouble plagued boiler
water systems. Today, EDTA is
the preferred chelants because
of cases of chelants corrosion
which occurred with NTA when
the chemical was not properly
controlled or was misapplied.
EDTA is a safer chelants to
use because it tends to
decompose as its concentration
increases, thus providing a
built-in safe-guard against
concentrated chelants attack.
Any concentration of free chelants
may increase the risk of
velocity related corrosion .
Chelants are usually used in
boilers operating under 800
psig and with make-up water of
sodium zeolite quality. The
chelating agents form soluble
complexes with calcium and
magnesium, and to a lesser
extent, iron oxide. These
soluble complexes are removed
in the blowdown water. Chelants
treatment is almost always
supplemented with polymers .
Sometimes, a small amount of
phosphate is also used as a
tracer material. Your water
treatment consultant needs to
weigh this option closely
since it is possible for
phosphate to successfully
compete with the chelating
agent for calcium, thus
negating some of the benefits
of the chelating agent. It
should be recognized, however,
that even with phosphate as a
tracer, a chelants treated
boiler will be cleaner than
the same boiler treated with
conventional phosphate.
If applied and controlled
correctly, chelating agents
will result in corrosion free
boiler surfaces that are much
cleaner than that possible
with phosphate treatment. .
All-Polymer
Treatment
All-polymer
treatment is the most modern
internal treatment chemistry.
The polymers that are used are
multi-functional and act to
keep precipitating cautions
like calcium in a soluble form
as well as to disperse any precipitants
that do form. The high polymer
concentration that defines
this program also is effective
in keeping incoming suspended
matter, primarily iron oxide,
from adhering on the heat
transfer surfaces.
All polymer treatment is not
normally applied to feedwater
containing more than 1 ppm
hardness or to systems in
which occasional high hardness
contamination is known to be a
problem. If calcium hardness
exceeds the capacity of the
all-polymer, there exists the
potential for calcium sulfate
formation. This calcium
sulfate that forms under the
high temperature conditions of
boiler waters is very hard and
adherent and could result in
tube failure from uver-heating
.
All-polymer treatment may be
the most appropriate for your
system. However, as with all
treatment alternatives,
improperly applied or applied
in the wrong situation could
result in un-planned boiler
outages.
Back To The Top
Steam
Purity
Need
For Concern
Steam is generated in boilers
and is used for many purposes.
If it is used to turn a
turbine, then we are
particularly concerned that
the steam contain no
contaminants that could
deposit on the turbine
surfaces. Even the relatively
pure water that results from
deionization pretreatment
contains some amount of dissolved
salts. As the feedwater
concentrates in the boiler,
these salts increase in
concentration. Steam that is
generated will contain a small
amount of water, the amount
being defined by the term
steam quality. Whatever
dissolved solids are contained
in the boiler water will be
contained, in the same
proportions, in the steam. The
amount of entrained water will
depend on boiler design,
design and effectiveness of
the steam separation
equipment, boiler operating
pressure, foam-inducing
contaminants in feedwater, and
the presence or absence of
chemical anti-foaming agents.
Volatile Contaminants
Certain substances, like
silica, that are solids at
ordinary temperatures can
become volatile at boiler
operating temperatures. Above
600 psig, you need to be
concerned about the
relationship between total
silica in the boiler water,
operating pressure, and boiler
water alkalinity. Allowing
silica to exceed
pre-determined limits could
result in silica deposition in
the turbines. Other salts also
exhibit this volatility;
however they are not of
concern at the relatively low
pressures encountered in
industrial boilers.
Monitoring
Steam purity testing should be
performed on a routine basis
to assure system reliability.
Recommended procedure is for
continuous monitoring with a
sodium analyzer. At the very
least, if there are power
generating turbines or turbine
pumps in the system, routine
grab samples should be
analyzed for sodium. Sample
points, method of sampling,
and analytical procedures must
be conducted in accordance
with strict guidelines. Sodium
is a pervasive substance and
sample contamination can be a
major problem when looking for
a few ppb. Procedures have
been developed to minimize
this contamination potential.
Back To The Top
Boiler
Blowdown Heat Recovery System
This section is limited to the
potential dollars that can be
saved by reducing boiler
blowdown rate and by
optimizing heat recovery of
heat contained in the boiler
blowdown. Thermidaire
Consultants can perform these
calculations for you. Although
not discussed here, an added
consideration that Cyrus Rice
can evaluate for you is the
impact that varying the amount
of returned condensate has on
your system energy balance.
All systems have some means of
dissipating the pressure
contained in boiler blowdown
waters. In most cases, this is
a blowdown flash tank and the
low pressure flashed steam
from this is usually
recovered. Often, this low
pressure steam is used to
provide heat and steam to the
deaerating heater. If this is
the case, 100% of that energy
is recovered.
Hot water is discharged from
the flash tank at the
saturation temperature
corresponding to the pressure
in the flash tank. If your
plant does not already recover
this heat in a heat recovery
heat exchanger, you will be
amazed at the Btu's that is
being sent to waste.
Contact Thermidaire for your boiler water
treatment needs and we will show you the
potential for saving energy
dollars.
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