POWER September 2015 - 45

WATER TREATMENT
Anti-Scalant: Not Too
Much; Not Too Little;
Just Right
There are a variety of scaling indices
(Ryzner, Langelier, and Practical Scale
Index) and an excellent software package
(www.frenchcreeksoftware.com) that
can be used to predict the potential for
scaling to form in the condenser tubes.
These can help a plant to adjust the antiscalant
chemical feed to its " just right "
level. Use of these indices requires accurate
and regular testing of a number of
parameters that your laboratory should
be doing already.
of calcium carbonate in the shallow areas
of the lake is something to be encouraged,
as it reduces the soluble calcium levels in
the water. Overtreatment has prevented this
" self-softening " mechanism, resulting in a
sudden massive scaling when the anti-scalant
could hold no more.
In ZLD plants where the cooling tower
blowdown water is treated and recycled, the
amount of anti-scalant used in the cooling
tower can have severe consequences in the
precipitation processes generally used as part
of the ZLD treatment. Therefore, the amount
of phosphonate or polymer treatment that is
fed to the cooling water also needs to be adjusted
frequently, based on a variety of chemistry
and physical factors. Those chemistry
factors include pH, total hardness, calcium,
total alkalinity, phosphate, phosphonate (in
cooling towers), and silica. Physical factors
include the temperature of at the condenser
outlet and the velocity of the water through
the tubes.
In some power plants, velocity of the cooling
water through the condenser tubes can be
changed by the number of circulating water
pumps in use. The use of one pump versus
two, or two pumps versus three, can make
a significant difference in the ability of the
anti-scalant to keep the tubes free of scale for
the few seconds that the water is in the hot
condenser tubes.
Biofilms create a natural resistance to flow
through the tube, thus slowing the velocity.
Debris can also plug or partially plug tubes,
reducing the velocity through the tube. Keeping
the tubes clear and water boxes clean
gives anti-scalants the best possible opportunity
to prevent scale accumulation.
The condition of the cooling water intake
screens can have a significant effect on
scaling of the condenser tubes. Some cooling
water systems get additional help from
September 2015 | POWER
side-stream filtration of the cooling water.
Removing particulate from the cooling water
enhances the effectiveness of the chemical
treatment.
Because the critical area for the anti-scalant
is the condenser tubes, is important that
the anti-scalant chemical is well mixed in the
cooling water flow just prior to the tubesheet.
Adding the chemical too close to the water
box may not provide sufficient mixing; adding
it far away from the water box can reduce
its effectiveness by having it consumed on
the walls of the cooling water pipe before it
reaches the condenser.
Approaches to Controlling
Corrosion
Gone are the days when we could prevent
corrosion in cooling water systems with an
active corrosion inhibitor (remember the
days of zinc chromate?) More recently, the
two primary chemical methods for controlling
carbon steel corrosion in the cooling
system have been to raise the pH of the
cooling water close to 9.0 or to add orthophosphate
to the cooling water treatment
to form an iron phosphate protective layer
on the carbon steel.
There are issues with both of these approaches.
Raising the cooling water pH to
around 9.0 drastically reduces the ability
of conventional bleach to control microbiological
growth. Even hypobromous acid effectiveness
drops off significantly at high
pHs. Therefore, microbiological control of
these systems can be a daunting task requiring
higher treatment levels or the addition of
nonoxidizing biocides. And biofilm growth
leads to corrosion.
When the pH is controlled with sulfuric
acid, the additional orthophosphate used in
the treatment can have unwanted side effects.
In cases where the calcium level in the tower
becomes excessive, calcium phosphate or calcium
sulfate can precipitate out on a variety
of surfaces. These deposits can be tenacious
and difficult to remove. In cases where orthophosphate
is used as part of the treatment
program, it is very important to determine the
amount of orthophosphate and the amount of
calcium hardness in the cooling water on a
regular basis and calculate the potential for
calcium phosphate saturation.
In cases where cooling tower makeup is a
treated wastewater, the incoming water can
have varying levels of orthophosphate, making
control of the orthophosphate level in the
cooling water very difficult.
Orthophosphate also acts as a " fertilizer "
for a variety of microbiological species,
and this may affect the amount and types of
biocides used to maintain a relatively clean
condenser and cooling tower. Cooling water
www.powermag.com
POWER POINTS
Best Practices for Control
of All Water Treatment
Challenges
Ensure consistent control of
biofilms in the cooling system.
Don't get it clean-keep it clean.
Make the best use of your
bleach: Get it fresh, keep it cool
and out of the sun, and test
every delivery.
Test your cooling water treatment
program assumptions regularly
(with lab testing) and adjust
accordingly. Add just what you
need.
Monitor, monitor, monitor-for
corrosion, scaling, and biological
growth.
systems operating at pH values just above 7
need to carefully monitor a variety of parameters
to ensure that they are neither corroding,
nor scaling, nor accumulating biofilms.
For those condensers that have copper alloy
tubing, or where the cooling water also
comes in contact with copper alloy tubed
heat exchangers, copper corrosion is typically
controlled with a type of azole. Tolytriazole
(TTA) is in common use, though it is
been shown to be very sensitive to bleach.
One manufacturer (Mexel USA) claims to
have developed a type of filming amine capable
of laying down the protective chemical
layer on the surface of the circulating water
piping to prevent corrosion. The company
claims that this slick surface has other benefits,
such as minimizing the ability of biofilms
to stick to the surface. This would be a
very interesting development, if it proves to
be durable and cost-effective for large cooling
water systems.
At the very least, it is critical that corrosion
rates in the cooling water are monitored
on a regular basis. Corrosion coupon racks
are a minimum. But they only tell you what
has happened generally over the past 30, 60,
or 90 days. To determine the instantaneous
corrosion rates on the circulating water system
linear polarization resistance probes (for
example CORRATER probes) have been
shown to provide reliable information on a
real-time basis. ■
-David Daniels (david_daniels@
mmengineering.com) is a frequent contributor
to POWER and senior principal scientist
at M&M Engineering Associates Inc.
45
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POWER September 2015

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