POWER October 2010 - 62

WATER TREATMENT
Taming Condenser Tube Leaks,
Part II
In Part I of this two-part report we examined the various chemical forces at
work in condenser tube leaks, the steam plant components placed at risk,
and the suite of instrumentation most capable of providing early warning
of a leak. Assuming you were able to repair the leak and quickly resume
operation, the next step is to identify the damage mechanisms that caused
the problem so you can minimize future leaks.
By David G. Daniels, Mechanical & Materials Engineering
C
ondenser tube leaks are caused by corrosion
or damage mechanisms that
affect the entire condenser. Those
mechanisms will continue to cause additional
leaks until the root cause(s) of the failures are
identified and changes are made to materials,
water treatment, or plant operations to eliminate
them. This article describes some of the
more common condenser tube failure mechanisms
and provides a brief discussion of how
they might be prevented.
The first step is to obtain a sample of the
tube that failed for metallurgical examination
to determine the failure mode. Nondestructive
evaluation (NDE) techniques, such
as eddy current testing, may be needed to
provide an indication of the scope of the
damage, but they cannot pinpoint a failure
mechanism. Together, metallurgical testing
plus some type of NDE can lead you to the
root cause of the failure.
Condenser Tube Materials
Common condenser tube materials include
various grades of copper brasses such as admiralty
brass, 90:10 copper-nickel (Cu-Ni),
and 70:30 Cu-Ni. In copper alloy condensers,
it is not uncommon to find higher-grade
materials, such as stainless steel tubes, used
in the air removal section.
Series 300 stainless steel alloys, such as
304 and 316L, are now fairly common in new
construction in both freshwater and seawater
applications. Higher-grade duplex stainless
steels, such as AL6XN and 6 Mo stainless
steel alloys, may also be used. Superferritic
stainless steel, such as Sea-Cure, has also
been used for cooling water where high levels
of chloride are present. These improved
materials are required when using brackish
and other alternative water sources.
The duplex and superferritic stainless
steels are far more resistant to stress corrosion
cracking than the Series 300 alloys. The table
62
Corrosion and pitting resistance of various stainless steel alloys. Source:
J.C. Tverberg, " Performance of Superferritic Stainless Steels in High Chloride Waters, " presented
at Condenser Technology: Seminar and Conference, EPRI, Palo Alto, Calif., 2002
Alloy
Type 439
Type 304L
Type 316L
Type 317L
Type 317LM
Type 317LMN
Type 2205
AL-6XN
SEA-CURE
UNS number
S43035
S30403
S31603
S31703
S31725
S31726
S32205
N08367
S44660
PREN
18
19
24
28
32
33
34
45
46
lists stainless steel materials with their Pitting
Resistance Equivalent Number (PREN)
and Critical Crevice Corrosion Temperature
(CCCT). The maximum chloride value is the
value below which chloride crevice corrosion
will not occur.
The standardized testing conditions under
which these values were developed are much
worse than conditions normally experienced
using traditional sources of cooling water.
The best approach is to use this information
as a comparison of the relative corrosion
resistance of one alloy against another,
not to set limits for cooling water chemistry.
Not shown in the table are titanium alloys
that are also used in seawater applications
or when the cooling water supply is high in
chlorides.
Each of these materials has its advantages
and disadvantages. As noted above, certain
stainless steel alloys are more resistant to
chloride pitting and corrosion than other alloys.
Copper alloys, such as 90:10 and 70:30,
though often discounted due to their lower
resistance to erosion, have a natural resiswww.powermag.com
CCCT
<-2C
(28F)
<-2C (28F)
<-2C (28F)
2C (36F)
16C (61F)
20C (68F)
24C (75F)
45C (113F)
52C (125F)
Max. chloride (ppm)
100
100
500
1,000
2,500
5,000
7,500
18,000
20,000+
tance to microbiologically influenced corrosion
(MIC) caused by the toxicity of copper
ions to many living species. However, this
same toxicity may create a problem if the
plant's cooling water discharge is returned to
a public waterway. In such cases, a plant may
be required to maintain the copper concentration
of the discharged cooling water below
very low levels.
Often, when retubing an existing condenser,
the material selection may be limited
due to the lower heat transfer coefficients of
stainless steel and titanium alloys compared
with copper alloys. Titanium alloys, though
typically considered immune to corrosion
from the cooling water side, have failed due
to impingement from saturated steam dump
lines in the condenser. They can also suffer
from failures if steel tools scratch and penetrate
the protective titanium oxide layer.
Common Condenser Tube
Failure Mechanisms
In our experience, the most common causes
of condenser tube failure are related to eroPOWER
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