POWER November 2011 - 51

CONDENSER LIFE CYCLE
■ If the pH is lower than 7, the maximum
chloride level should be lowered.
■ This guide assumes a clean surface. If deposits
form, the pH can be significantly lower under
the deposits, and the chloride levels may be
much higher than the bulk water.
MIC. Microbiologically influenced corrosion
(MIC) is often confused with pitting corrosion
and typically occurs in water normally
considered benign. The term " influenced " is
used because the bacteria does not actively
cause the corrosion. The bacteria forms a
film or slime that creates a crevice. This isolates
the water chemistry on the metal surface
from the bulk water chemistry. The bacteria
may also metabolate a product that can be
very aggressive. Table 1 lists common bacteria
types known to influence corrosion.
The most common MIC attack in North
America is a result of the influence of manganese-fixing
bacteria (Figure 5). Although the
mechanism is complicated, it's also the most
likely type of attack. The bacteria assists in
oxidation of the soluble manganese to form
an insoluble manganese oxide (MnO2
) layer
on the metal surface. This creates a crevice.
When the operator detects an increase in condenser
backpressure, sliming is suspected,
and chlorination is initiated. The chlorination
intended to kill the bacteria and assist
in slime removal further oxidizes the MnO2
layer to a permanganate. Under that layer,
the combination of the generated hydrogen
and chloride ions reacts to form hydrochloric
acid. The acid attacks the passive layer of the
stainless steel, which initiates the attack.
Studies have found that manganese concentrations
as low as 20 ppb can initiate MIC.
This mechanism most commonly occurs with
TP 304 and TP 316, but higher molybdenumcontaining
grades and some duplexes have
also been attacked. In general, an alloy needs a
minimum CCT of 25C in the G48 crevice corrosion
test to be considered resistant to MIC.
The Effect of Material Properties
The mechanical and physical properties for
common copper base, titanium, and stainless
steel tubing are available from a number of
handbooks and online from tubing suppliers.
These properties-such as ultimate strength,
yield strength, and hardness-are important to
consider when selecting a new condenser tube.
One important concern is the tube's erosion
resistance. Erosion resistance is a function of
the ability of the protective layer to remain
attached to the substrate and the strength
(hardness) of the substrate directly below the
protective layer. Two types of erosion commonly
cause problems in the power industry:
flow-accelerated erosion-corrosion and water
droplet/steam impingement erosion.
Flow-Accelerated Erosion-Corrosion.
When the fluid velocity is so high that it will
actually " scrub " the protective film from the
metal surface, then flow-accelerated erosioncorrosion
is possible. Conversely, higher velocities
are desirable because they increase
the heat transfer rate and keep the tube surfaces
clean, reducing the surface resistance
to flow. In general, a velocity of 6 feet per
second (fps) is considered the minimum for
keeping the tube surface clean. Biofilms have
been known to develop at lower flow rates.
Table 2 summarizes the maximum safe fluid
flow velocities inside of condenser tubes of
different materials.
Older traditional condenser designs restricted
flow rates to the 6 to 7 fps range to
protect the copper alloy-based tubing. With
the elimination of copper in modern condensers,
modern flow rates are in the 9 to 10 fps
range. If your existing condenser has marginal
thermal performance and can handle
a higher cooling water flow rate (velocity),
then significant performance improvements
can be made, assuming pump modifications
to increase flow rate are possible. At the
higher flow rates, stainless steels and titanium
are immune to erosion and corrosion.
In many systems, the payback on investment
may be less than one year. Additionally, the
higher flow rates will result in cleaner tubes.
Water Droplet/Steam Impingement
Erosion. Under certain unique conditions, it
is possible to experience erosion of the tube
outside surface caused by the localized impact
of high-velocity water droplets (Figure
6). This can occur near diverter plates that
may focus the high-speed wet steam onto the
tube or during other upset conditions. It often
Table 1. Bacteria commonly associated with MIC. Source: Plymouth Tube Co.
Organism
Action
Thiobacillus
Desulfovibrio
Gallionella
Crenothrix
Spaerotilus
Nitrobacter
November 2011 | POWER
Sulfate reducer
Sulfate reducer
Mn/Fe fixer
Mn/Fe fixer
Mn/Fe fixer
Nitrate reducer
Problem
Produces H2SO4
Produces H2S
Precipitates MnO2, Fe2O3
Precipitates MnO2, Fe2O3
Precipitates MnO2, Fe2O3
Produces HNO3
www.powermag.com
51
occurs in steam dump areas when the outlets
are not properly designed.
The resistance of the tubes to this form of
erosion is a direct function of the hardness of
the metal substrate below the protective oxide.
In general, higher hardness provides higher
erosion resistance. Using a water droplet impingement
device developed by Avesta Sheffield,
alloys can be ranked by time to failure.
The process is to graphically plot material
hardness versus time to failure, so a relationship
can be determined. Other material grades
can then be added by comparing the hardness.
Using titanium grade 2 as the reference, with
a value of 1, the relative resistance of other
grades can be ranked, as presented in Table 3.
Hydrogen Embrittlement
Titanium and superferritic stainless steels,
such as S44660 and S44735, can embrittle
with exposure to monotonic hydrogen. This
commonly occurs in water systems that have
Table 2. Commonly accepted maximum
water flow rates for erosioncorrosion.
Source: Plymouth Tube Co.
Alloy
Admiralty
90/10 copper-nickel
70/30 copper-nickel
304/316 stainless steel
Titanium Grade 2
Super-ferritic stainless steel
Maximum velocity
6 fps
8 fps
10 fps
30+ fps
100 fps
100+ fps
5. Manganese MIC-related pitting
on TP 439 tubing after eight
months of baseload service. Courtesy:
Plymouth Tube Co.
6. Steam water droplet erosion of
stainless steel tubing in a nuclear
power plant. Courtesy: Plymouth Tube Co.
http://www.powermag.com
POWER November 2011

Table of Contents for the Digital Edition of POWER November 2011
