POWER November 2011 - 40

CONDENSER LIFE CYCLE
Improving Condenser O&M Practices
Losses attributed to condenser tube leaks, fouling, and failures continue to
climb, costing the power generation industry an estimated half-billion
dollars annually in maintenance costs and loss of production. Investing in
an effective condenser maintenance program will reduce those expenses
in short order.
By Eric H. Fayard, Conco Systems Inc. and Michael J. Horn, Plastocor Inc.
M
aintaining clean condenser tubes is
of vital importance to reliable, efficient
power plant operation. Condenser
tubes may become lightly fouled with
soft organic deposits or severely scaled with
hardened minerals that are difficult to remove.
Success in cleaning the condenser requires
selecting the appropriate cleaning technology
for the specific fouling conditions.
Early identification of fouling characteristics
and a fundamental knowledge of cleaning
system capabilities are essential to determining
the most effective cleaning technology, as
well as the frequency of cleaning required.
A good engineered coating can also prevent
fouling of condenser tubes and renew life in
older systems.
Mechanisms of Condenser Tube
Fouling
Generally, two major problems result from
substances that adhere to interior tube surfaces:
loss of heat transfer and under-deposit
corrosion.
Internal tube fouling is nearly always detrimental
to heat transfer because it reduces
the efficiency of condensing steam and results
in less-efficient steam turbine operation,
as discussed in detail in the previous article,
" Optimizing Condenser Tube Selection. " Internal
tube fouling is also the most common
cause of under-deposit corrosion-related
tube failure.
This form of corrosion can be caused either
directly or indirectly. When the attack is direct,
the deposit itself contains corrosive substances
which, when concentrated at a localized site,
can cause loss of tube material. A common example
of this type of corrosion would be chloride
pitting. Indirect attacks can be caused by
several factors, including accumulation of deposits
that form a barrier between the cooling
water and tube material, allowing a corrosion
cell to form underneath, or by microbiologically
influenced corrosion (MIC). Pitting is
often a result, which can cause tube failure
well before the material's life expectancy.
In either situation, contaminants in cooling
water (which may be concentrated signif40
icantly
in a recirculating cooling tower) are
the precursors to what often results in major
damage to boiler tubing and steam turbine
materials, potentially resulting in huge economic
penalties in unit outages and equipment
repair and/or replacement. Investment
in the equipment that can remove and/or
prevent foulants from forming on condenser
tubes is a minor price to pay to produce the
best possible plant economics.
Options for Preventing and
Removing Foulants
Equipment options for preventing and/or removing
foulants can be classified according to
plant operating status: off-line or online. Offline
removal methods include chemical (acid
or chelate dissolution) and mechanical methods
(metal or plastic tube cleaners, brushes,
high-pressure water, and the like). Online preventive
measures include chemical treatments
(scale and corrosion inhibition, dispersants,
and biocides) and continuous recirculation of
sponge balls to remove soft deposits on the inner
wall of the condenser tubes.
Because condenser fouling can have such a
dramatic impact on cost-effective power plant
operation, deposition and corrosion should be
carefully monitored and the appropriate cleaning
technology used at optimal intervals. Here
is an overview of the main treatment options.
Chemical Treatment Methods. Several
chemicals, often in combination, are used to
control condenser tube fouling. Chemicals are
primarily utilized with recirculating cooling
towers, because the concentration of dissolved
constituents is significant, increasing the
threat of scaling and corrosion, and because
once-through cooling systems often discharge
directly into a river or lake, so chemicals are
restricted in the effluent. Typical chemical
treatment methods used today include adding
scale inhibitors, dispersants, biocides, corrosion
inhibitors, and chemicals for pH control.
Another common way of mitigating chemically
influenced problems is to minimize the
concentration of dissolved solids in the system,
particularly in cooling towers. A balance must
be achieved between minimizing water use and
www.powermag.com
avoiding excessive buildup of scaling and corrosive
constituents in recirculated water.
Some scales can be removed online by lowering
circulating water pH to the point of scale
dissolution by increasing the sulfuric acid feed
(pH of 5.8 or less for calcium carbonate and
4.0 for calcium phosphate). However, potential
risks to base metal and carbon steel piping
must be carefully considered when using
such harsh chemicals. In the case of online
chemical cleaning by pH reduction, one major
disadvantage is that you can't be sure of the
composition of the foulants.
Chemical scale removal can be very effective
and may be necessary if the deposits are silicabased.
However, chemical cleaning of condensers
is maintenance- and labor-intensive.
Mechanical Fouling Prevention. A
number of mechanical approaches have been
developed to prevent condenser tube fouling.
Screens can be installed to block large debris
from reaching the condenser while settling
ponds and/or clarifiers remove particulate
and some dissolved constituents. Filtration
systems, such as sand filters, can also be employed
to remove fine particulates, on either a
full-flow or a side stream partial-flow basis.
A number of online systems have been developed
that send cleaning objects-such as
sponge balls, brushes, or plastic scrapers-
through tubes with the cooling water flow. In
theory, these devices will mechanically wipe
the tube surface clean. In the more automated
systems, the cleaning devices are retrieved at
the condenser outlet and automatically returned
to the inlet. Other designs return the devices to
the inlet via backwashing. Because of the random
distribution of some of the cleaning devices,
not all tubes receive consistent cleaning.
Increasing cooling water flow through
the condenser can be effective in limiting
microbiological growth. This approach
will also keep cooling water temperatures
lower, possibly to a point at which scale is
less likely to form. However, higher flow
rates can cause increased rates of erosioncorrosion
in copper alloys.
Mechanical Removal Methods. Plant
operating economics favor removing any
POWER | November 2011
http://www.powermag.com
POWER November 2011

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