POWER September 2010 - 58

WATER TREATMENT
boiler water chloride or sulfate level would
be due to contamination.
Analysis of the boiler water is often far
more sensitive to contamination than a steam
or feedwater analysis for cation conductivity,
as the boiler blowdown valve is generally
closed or very close to closed. Contaminants
in the feedwater are also concentrated hundreds
of times in the boiler water, making
them far easier to detect. A boiler that sees an
increase of 100 ppb of chloride in the boiler
water is in effect measuring an increase in
the feedwater chloride level of 0.5 ppb in a
boiler with 200 cycles of concentration. This
increase is unlikely to be detected by cation
conductivity, particularly in the presence of
organic acids and carbon dioxide.
Online sodium analyzers on feedwater
may also be an excellent method for catching
a condenser tube leak early, before it has
a chance to contaminate the steam or cause
boiler chemistry problems. These online analyzers,
which can be purchased from a number
of excellent vendors (including Swan
Analytical Instruments and Hach), now claim
detection limits into the low parts per trillion
range and can be very reliable. For some
plants, direct analysis of chloride, sulfate,
and sodium at low levels can be performed
by ion chromatography, using a benchtop
unit or even an online monitor.
Hide and Go Seek
If the decision is made that the plant must
take an immediate forced outage, then there
is also a concern about whether the leak can
be quickly found and plugged. The ability
to detect a leak is the product of its size, the
conductivity or sodium in the cooling water,
and the method used to detect the leaks.
When half of the condenser can be safely
isolated while the unit is running (or at least
while there is still vacuum on the condenser)
any number of methods have been tried to find
condenser tube leaks. Some operators have
used food wrap, candles, rubber stoppers, and
shaving cream. However, helium detectors are
more sensitive and reliable and can quickly
locate even very small condenser tube leaks.
The latest helium leak detectors have become
much more portable and easier to operate.
Though the design of each plant is somewhat
different, an approximation of the size of
a condenser tube leak can be made by looking
at the cooling water chemistry, the feedwater
chemistry, and some basic fluid dynamics.
As an example, consider the typical cooling
water analysis summarized here, where
the water comes from a local surface source
and the pH of the cooling water is adjusted
with sulfuric acid:
■ Specific conductivity (µS/cm): 4,000
■ Calcium (ppm as CaCO3
■ Magnesium (ppm as CaCO3
): 770
): 350
■ Sodium (ppm): 340
■ Chloride (ppm): 370
■ Sulfate (ppm): 1,200
Let's also assume that the cooling water
pressure in the water box is 25 psig and that
the unit produces about 1 million pounds of
steam an hour, meaning that the flow through
the hotwell is also about 1 million pounds per
hour, or 2,000 gpm.
Suppose that there were a sudden increase
of sodium at the condensate discharge of 5
ppb caused by a condenser tube leak. The increase
of sodium in the condensate would be
unmistakable, as it is far above any normal
fluctuation from other potential sources of
contamination. Such a leak would also produce
feedwater chloride of approximately 5
ppb and sulfate of 16 ppb. Although it is difficult
to predict precisely at these low levels,
you may also see an increase of about 0.2 µS/
cm in cation conductivity from this leak. In
58
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POWER September 2010

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