POWER March 2016 - 49

FUNDAMENTALS
2. Critical cooling. Given the importance of the generator, its cooling water system deserves
special attention. Courtesy: M&M Engineering
surfaces evenly. This produces a general, but
overall low level of corrosion.
One of the advantages of this treatment is
that it is thought to be very environmentally
benign, although as long as the closed-loop
system remains closed, there should be no
impact on the environment.
Monitoring Closed-Loop Cooling
Water
Key to keeping your closed-loop system
functioning properly is regular monitoring.
Whatever the active agent is in your treatment
(nitrite, molybdate, or polymer) the
concentration must be regularly monitored.
Generally, weekly testing is sufficient unless
the levels of the treatment are dropping. (You
won't know that if you are not monitoring
regularly.) Because the carbon steel and copper
corrosion treatment are typically blended
into one product, low levels of treatment may
affect more than just the carbon steel piping.
The pH of the water should also be tested
3 Narrow passage. Stator cooling water flows through the narrow channels of the stator
bar. Courtesy: M&M Engineering
regularly. Considering the amount of pH
buffering in chemical treatment, the pH of
the water should be rock solid. Drops in pH
may indicate bacterial contamination, particularly
with the nitrite-based treatments.
Another thing that can drop the pH is leaks in
the system, which bring in fresh demineralized
water makeup.
Be on the lookout for other signs of bacterial
contamination, such as slimy growth in
any sightglass or flow indicators, or septic
smells when the sample is collected. Plate
and frame heat exchangers have a very large
surface and small spacing for heat exchange
between the plates. Bacterial contamination
can not only seriously affect heat transfer, but
it also can cause pinhole leaks in the stainless
steel plates. Depending on the pressure of the
closed-loop versus open-loop system at this
point, the bearing cooling water may leak
out, or the open cooling water may leak in.
Remember that it is much easier to prevent
bacterial contamination than it is to try to recover
from a system that is severely contaminated.
be concentrated in areas of historical political
unrest, and over the years, molybdate prices
have varied dramatically. That price variability
can make molybdate treatment competitive
with nitrite-or far more expensive.
Ironically, in closed-loop systems that are
very tight, dissolved oxygen levels can drop,
and thus minimize the effectiveness of a molybdate
treatment (which requires dissolved
oxygen to form a passive layer). Experts recommend
a minimum of 1 ppm of dissolved
March 2016 | POWER
oxygen in molybdate-treated systems.
Polymer Treatments. Polymer treatments
have been used for many years to
prevent scale and corrosion product accumulations
in open cooling towers. Similar polymers
are also now sold for use in closed-loop
systems. It appears that the polymer acts as a
dispersant for any corrosion products or scale
that might form, so it prevents corrosion by
keeping the surface clean and ensuring that
any dissolved oxygen in the water attacks all
www.powermag.com
Stator Cooling Water Systems
The stator cooling water system is a very special
closed loop for a couple of reasons. First,
it protects one of the most critical pieces of
equipment-the generator. There is only one
metal of concern in this system: copper. And
this system must remain very clean, even
pristine. Small temperature increases in stator
cooling bars can restrict load on or even
shut down the generator. Therefore, this system
requires special understanding, attention,
and monitoring (Figure 2).
Stator cooling water is contained in a closedloop
system that cools the copper stator bars
in water-cooled generators. The copper coils
49

http://www.powermag.com

POWER March 2016

Table of Contents for the Digital Edition of POWER March 2016