Organic
Water Treatment Chemicals
Steam Boilers, Cooling Towers,
Hot And Chilled Closed Systems
Fuel Oil Treatment
(Home of D.M Concentrate)
Boiler
Feed Water And Return Lines Make
Up
Make-up
water is described as the final
treated water that is mixed with
returned condensate to form the
feed water, 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 uncommon 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.
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.
When Thermidaire Water
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.
Feed water System
The whole is only as strong
as the weakest of its parts and
this certainly applies to the
feed water system of the overall
boiler water cycle. The primary
functions of the equipment that
comprise the feed water 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:
Feed
water Flow Rate
Feed water Temperature
Heating Steam Temperature and
Pressure
Quantity
of vent
Significant deviation from
design conditions can result in
high oxygen levels in the feed
water. 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 feed
water 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 feed water.
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 was 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
Passivating 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 feed water 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 an established
schedule basis. Operators should
be trained as to how to respond
to those test results. The
sample points within the feed
water system are also very
important and should be reviewed
and changed as needed by you
water treatment consultant.
superheating Water
If
feed water is used for this
purpose, be certain that feed
water 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 feed water system. Careful
attention to it by a skilled
water treatment engineer is
recommended.
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 Water Consultants do
an up-to-date energy and
material balance around the
waterside 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
waterside 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.
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
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.
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 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, when fed to the boiler,
volatile 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
basically 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
Passivating Agent
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 Passivating 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 Passivating
agents. That is why corrosion
monitoring, always important,
becomes even more so when
treatment results depend on a
passivation chemical.
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 Water
Consultants include the
following:
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.
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.
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 feed water represents the
corrosion product 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.
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.
B&W
developed membrane filers, like
those available from Millipore,
originally 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.
Determining iron concentrations
analytically and assigning
average values determined to
actual membrane filters
collected during the same time
period does this.
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