POWER October 2010 - 66

WATER TREATMENT
6. Frozen solid. This titanium condenser
tube failed when water left in the tube froze.
Courtesy: Andy Howell, Xcel Energy
and create a black magnesium oxide coating.
This coating can create an uneven
deposit layer that is more noble than the
stainless steel, making it the anode and
thus causing pitting corrosion.
Other Failure Modes
Mechanical vibration can result in catastrophic
condenser tube failures or leaks
that are very difficult to find because they
open and close depending on steam conditions.
The number of tube failures due to
vibration may increase suddenly after a
turbine upgrade that uses more steam or
creates a different set of harmonic vibrations
in the condenser.
When considering a turbine upgrade, it is
Stainless steel alloys are also susceptible
to enoblement corrosion. This form of
corrosion results from the combined action
of manganese in the water source, bacteria,
and the action of bleach or chlorine. Manganese
can be found in seawater, lakes, and
rivers. In freshwater sources, the levels of
manganese can spike seasonally with lake
turnover or spring runoff. Bacteria in biofilms
on the surface of the condenser tubes
can collect and precipitate the manganese
good to examine the condenser design to ensure
that the changed steam flow rate doesn't
create areas of mid-span collisions, fretting,
or fatigue in the condenser tubes. Regular visual
inspection of tube supports, NDE, and
modeling of the condenser with computational
fluid dynamics analysis are all tools
that can be used to determine the potential
for failures due to vibration.
Plugging of the tube sheet with debris
and macro-biological fouling (such as from
fish or clams) can be problematic for units
using cooling water from lakes, rivers, or
seawater. A small tear in a screen can result
in a large accumulation of debris on
the tube sheet that can plug a significant
number of the condenser tubes, forcing
the remaining water through the remaining
tubes at higher velocities. The plugged or
partially plugged tubes can develop calcium
carbonate deposits and silt accumulations
due to slower flows. These deposits
can then become havens for microbiologically
influenced corrosion.
Freeze protection is also important for
any condenser where temperatures drop
below freezing or any using cooling water
that can freeze. Some water boxes may not
drain completely, particularly if the drain
is partially blocked or rusted shut. Figure
6 shows a titanium tube that failed due to
freezing temperatures. ■
Special thanks to Ray Post of ChemTreat
and Andy Howell of Xcel Energy for their
contributions to this article.
-David G. Daniels (david_daniels@
mmengineering.com) is a principal of
M&M Engineering and a POWER
contributing editor.
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POWER October 2010

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