POWER November 2011 - 48

CONDENSER LIFE CYCLE
Condenser Tube Failure Mechanisms
The operating environment within a condenser is extremely harsh, and in spite
of the designer's best intentions, sometimes tubes made of the best materials
fail. The most important tube failure mechanisms typically result
from different forms of corrosion and erosion. When it's time to select new
condenser tube material, you'll need to consider the projected operating
environment and failure mechanisms that material will be subjected to.
By Daniel S. Janikowski, Plymouth Tube Co.
T
he steam surface condenser is the largest
heat exchanger in a power plant, and
the quality of the tubing within the condenser
is critical for reliable plant operations.
As discussed in " Optimizing Condenser
Tube Selection, " earlier in this issue, a plant
engineer has several materials from which to
choose when specifying a replacement set of
condenser tubes.
Those specifications must go well beyond
mere material selection, however. The environment
in which the tubes will operate, now
and in the future, also must be described in the
specifications. Upset and unusual operating
conditions are often the cause of premature
failure for tubing and piping materials in the
power plant. Other failure modes are caused
by changes in water chemistry due to leaks in
other parts of the system, corrosion from unexpected
sources, the impact of improper lay-up
practices, and the effect of corrosion product
transport to other parts of the system. This article
discusses those failure mechanisms, what
causes them, and how these types of failure
can be avoided in the future.
Steam-Side Failure Mechanisms
Because the total concentration of all contaminants
in the steam cycle water chemistry is usually
measured in parts per billion, steam cycle
water chemistry is normally considered nonaggressive
to most alloys. However, by actively
condensing the steam, the condenser will concentrate
gases that don't condense at the same
temperature as the steam. The most common
contaminates include gases like oxygen, nitrogen,
and ammonia (from decomposing oxygen
scavengers). Stainless steels and titanium are
resistant to these gases, but the copper-based
alloys can be attacked by ammonia; admiralty
brass and aluminum brass are the most susceptible.
The ammonia can cause two types of failures:
ammonia grooving and stress corrosion
cracking (Figures 1 and 2).
Dissolved copper, originating in the
copper-nickel alloy condenser tubes, is also
transported in the condensate throughout the
system. The solubility of the copper decreases
48
dramatically when the pressure significantly
increases. Plating of surfaces often occurs in
the boiler where it plates on the interior surfaces
of tubes and on high-pressure turbine
blades. Deposits on the boiler can depress
the melting point of the boiler tube material,
causing premature failure. This is called liquid
metal embrittlement. When copper deposits
in the turbine (Figure 3), it can cause as
much as 5% decrease in power generation-
worth millions of dollars each year.
Cooling Water Attacks
In addition to general corrosion, many condenser
tubes are susceptible to galvanic-driven
mechanisms such as pitting and crevice corrosion.
These failure mechanisms usually have
two stages: an incubation or initiation period
and a propagation mode. The time of initiation
is rarely determinable. It could be as short
as in a few weeks or may take years. Once
initiated, the propagation mode can proceed
rapidly, driven by the electropotential between
the depassivated location and the surrounding
area. Other tubes can be biologically attacked.
The following sections describe each of these
different forms of corrosive attack.
Pitting. Pitting corrosion is a highly localized
attack that can result in through-wall
penetration in very short order. Failures may
occur in less than four weeks. Once a pit is
initiated, the environment in the pit is more
aggressive than the bulk solution because the
material in the pit itself is stagnant. The pH
in a pit can drop to below 2. When this occurs,
the surface inside the pit activates. The
potential difference between the pit and the
more noble surrounding area is responsible
for a galvanic attack. As the surface area of
the anode (pit) is small and the cathode (the
passive surface surrounding the pit) is large,
current density can be quite high. For TP 316
in seawater, the voltage difference between
the active site (a pit) and the passive region
surrounding it can be 0.4 volts. These two factors
will result in very high localized corrosion
rates. Through-wall pitting in condenser
tubes has occurred in less than three weeks.
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The most common initiator of stainless
steel pitting is the presence of chlorides in the
condensate. Several alloying elements-such
as chromium, molybdenum, and nitrogen-
promote chloride resistance in this group of
alloys. Not all of the alloying elements have
the same effect. By investigating the impact
1. Ammonia grooving of aluminum
brass. Courtesy: Plymouth Tube Co.
2. Intergranular stress corrosion
cracking of admiralty brass. Courtesy:
Plymouth Tube Co.
3. Copper deposits on the control
stage of a high-pressure steam
turbine. Courtesy: Pacificorp
POWER | November 2011
http://www.powermag.com
POWER November 2011

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