POWER September 2015 - 44

WATER TREATMENT
piping materials where the bleach is stored. The level of nickel in the
bleach should be less than 0.1 mg/l and copper should be less than 1
mg/l.
Testing Bulk Chemicals Is Essential. It behooves the chemistry
personnel at the plant to check the concentration of hypochlorite in
each shipment of bleach before it is offloaded into their tank. One
plant questioned why it was suddenly not achieving a free available
chlorine residual in its cooling water, as it had in the past. A check of
the bleach bulk tank found that the concentration in the tank was only
2.5% bleach. Evidently, the previous bleach delivery was not the 10%
bleach the supplier claimed it was. Simple hypochlorite test kits are
available for this purpose.
Bromine Is Not a Biocide. Sometimes I hear people referring
to their cooling water chemistry as " bromine " chemistry. Bromine
is a reddish-brown gas and highly toxic (like its neighbor
in the periodic table-chlorine gas). Is not something that should
ever be generated at a power plant. The use of sodium bromide
to enhance the biocidal properties of bleach at higher cooling
water pH values, however, is very common. We discussed the
proper application and blending of these two chemicals to produce
the maximum amount of hypobromous acid in " Biofouling
Control Options for Cooling Systems " in the September 2007
issue (available in the Water Treatment Guidebook, available in
the Store at powermag.com.)
Is important to remember, however, that sodium bromide is not a
biocide; it is a simple salt, with no biocidal properties. It is only when
it is properly reacted with hypochlorous acid (not bleach-which is
predominantly hypochlorite ion) that it will form hypobromous acid,
which is the desired biocide.
Stabilized hypobromous acid is available a number of chemical
vendors. This eliminates the need to properly mix the bleach and sodium
bromide on site. However, it is more expensive to buy the stabilized
hypobromous acid than to generate it.
Scale Prevention for Power Plant Cooling Water
By far the most common cooling water scale is calcium carbonate,
though other scales can often be just as detrimental and far more difficult
to remove. These include calcium sulfate, calcium phosphate,
and various forms of silica deposits.
The Calcium and Conductivity Connection. The inverse solubility
of calcium compounds such as these cause scaling in condenser
tubes, which can severely affect heat transfer across the condenser,
increase the unit back pressure, and restrict the unit's generation. In
recirculating systems (cooling towers) the cooling tower blowdown is
often controlled by the conductivity of the cooling water. It is important
to remember that conductivity is only a proxy for the real control
parameter-calcium. Conductivity can be easily and continuously
monitored and therefore has a much better chance at controlling the
cooling water calcium than any other parameter.
However, there are cases where this proxy breaks down. If your
cooling tower makeup water comes from multiple sources, or if the
chemistry of these sources has changed over time, then the assumptions
for the cooling water chemistry control parameters (including
the conductivity control limits and chemical feed rates) will also have
changed.
Besides conductivity and pH, there should always be a specified
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limit on total hardness and calcium hardness in your cooling water
program. The lab should test the cooling water regularly for these
parameters, and these limits should always override the conductivity
limit. One zero-liquid discharge (ZLD) plant occasionally fed water
from the ZLD clarifier/softener effluent back to the cooling tower
when it could not be used in the downstream portion of the ZLD.
This water was high in conductivity but very low in calcium, which
dramatically skewed the conductivity control of the cooling tower and
the amount of anti-scalant being added.
Ensuring Consistent pH. The pH of cooling water is the other
critical factor for preventing scaling. If pH control with sulfuric acid
is part of your cooling water chemistry program, it should be understood
that it is a critical part.
A sulfuric acid pump malfunction or problem with the pH controller
for the pump can cause serious scaling or corrosion issues in the
cooling tower. Both the acid pump systems and pH control elements
on the cooling water should be redundant. There should either be two
separate cooling water pH probes at different locations, or the pH of
the cooling water should be checked frequently by the lab or operations
and compared to the pH of the online analyzer. Both the lab and
operations should be aware of the potential damage that can be caused
when they are not operating properly and know what actions to take
if problems arise.
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44
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POWER | September 2015
mendous benefit to maintain and clean a condenser. It goes without
saying that these chemical pumps are also critical.
Anti-scalants can be grouped into two broad categories: phosphonates
and polymers. There are polymers that have anti-scaling
properties and other polymers that act as a general dispersant
to keep silt from depositing on the condenser tubes. Often, a
cooling water treatment will contain both an anti-scalant and a
polymeric dispersant.
Is important to remember that the critical area for precipitation
is in the condenser tubes (see sidebar, next page). This is where
elevated temperatures will cause the precipitation. In some cases,
overtreatment of these chemicals is not only expensive but also
detrimental to other plant processes. In cooling lakes, precipitation
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POWER September 2015

Table of Contents for the Digital Edition of POWER September 2015
