POWER October 2010 - 64

WATER TREATMENT
sion, corrosion, or some form of mechanical
damage. The following discussion is
meant to guide your initial examination of
the failed tubes and to help you close in on
the root cause.
Erosion. Cooling water erosion of condenser
tubes occurs in areas of high turbulence
and velocity such as in the first few
inches of the inlet to the condenser tube, or
in areas where there is an anomaly in the
tube, such as a deposit or foreign object that
the water must flow around. Figure 1 shows
the latter case.
Erosion is worse during periods of high
flow rates or when sand, fly ash, or other
abrasive materials are entrained in the water.
High localized flow rates can occur
when the tube sheet is partially blocked by
debris. Copper-nickel alloys have twice the
resistance to erosion of admiralty brass. But
there is at least an order of magnitude difference
between copper-nickel alloys and
stainless steel or titanium alloys. Erosion
can also affect the tube sheet and create
leaks in the crevice between the tube and
tube sheet.
Erosion can also occur from the steam
side. Emergency dump lines or drain headers
can impinge on the condenser tubing
below in a very localized area. In one case,
the proximity of a reheater dump header
caused a high-velocity water and steam
mixture to impinge on the top row of a condenser
with titanium tubes (Figure 2). The
plant is seawater-cooled, and the resulting
tube leak caused massive contamination of
the turbine requiring an extended outage to
clean and inspect the turbine and associated
valves and piping. Erosion of support
plates may also increase the propensity for
vibration damage.
Erosion on the waterside can be minimized
by keeping the tube sheet and tubes clean and
free of debris and operating the cooling water
flow rates within the design range. Regular
inspection can detect waterside erosion. Tube
inserts, both metal and plastic, have been
used to protect the first few inches of the condenser
tube from further erosion and leaks.
Care should be taken when using very high
pressure cleaning equipment on condenser
tubes. Excessive pressure or inexperienced
operators can damage tubes and create more
problems than they solve.
Regular and careful visual inspection of
the steam side of the condenser should locate
areas where steam erosion is occurring.
Steam/water shields can then be installed to
deflect impingement, or the affected tubes
can be plugged or replaced with a more erosion-resistant
alloy.
Waterside Corrosion. One of the most
common cooling water-side corrosion
64
mechanisms in condenser tubing is microbiologically
influenced corrosion (Figure
3). It is far too common in stainless steel
tubing and less common, but still possible,
in copper alloy tubing. A very few cases of
corrosion in titanium tubes have even been
ascribed to MIC.
Stainless steel condenser tubing is protected
from corrosion by a thin layer of metal oxide.
When a biofilm forms over the tube, the
chemistry between the biofilm and the condenser
tube often becomes corrosive to the
oxide layer and underlying metal. The more
well-established, thick, and microbiologically
diverse the biofilm is, the more likely it
is that MIC can form underneath it.
In nearly any water-containing system-
and certainly in cooling towers and oncethrough
cooling applications-biofilms
cannot be eliminated; they can only be controlled.
(See " Biofouling Control Options for
Cooling Systems, " September 2007 in our
online archives at www.powermag.com.)
The most common control strategy for
biofilm in cooling water is the use of bleach
to produce hypochlorous acid, an effective
biocide. In some cases the bleach is combined
with sodium bromide to produce hypobromous
acid, which is a better biocide in
alkaline pH cooling water. In cooling towers,
the effectiveness of bleach or bleach/
bromide can be enhanced with biodispersants.
Bleach or bleach/bromide, applied
properly and consistently, will typically
provide adequate control of biofilms and
minimize MIC.
Mechanical cleaning, whether with brushes,
scrapers, foam balls, or high-pressure
water lancing, is the only cleaning technique
that can effectively remove all biofilm in a
condenser tube. Units that regularly use one
or more of these mechanical cleaning methods
are far less likely to see condenser tube
failures due to MIC or other issues with biofilm,
regardless of the biocide program that
is in use.
Titanium tubes have a very tenacious and
chemically inert passivation layer that protects
them from nearly all corrosion mechanisms.
It is very difficult to accumulate
enough of the right kind of bacteria in a conventional
power plant cooling system to create
an environment that would be corrosive to
titanium oxide. However, titanium tubes are
still subject to biofilm accumulations that can
slow the flow through a condenser and cause
problems in the cooling tower when biofilm
accumulates on the fill.
Steamside Corrosion. Corrosion is also
often found on the steamside of a condenser
tube. Admiralty brass tubing is susceptible
to both ammonia grooving and ammoniainduced
stress corrosion cracking.
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POWER | October 2010
2. Leaky titanium tubes. Wet steam
caused the erosion of these titanium condenser
tubes. Courtesy: M&M Engineering
1. Cratered brass. Admiralty brass tubing
that carries the cooling water in a condenser
can erode because of excessive water velocities
or because the water is contaminated.
Courtesy: M&M Engineering
3. MIC damage. This stainless steel
condenser tubing was attacked by microbiologically
influenced corrosion (MIC). Courtesy:
M&M Engineering
4. Groovy tubes. Too much ammonia in
the cooling water caused grooving on both
sides of a condenser tube support. Courtesy:
Andy Howell, Xcel Energy
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POWER October 2010

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