POWER March 2016 - 51

FUNDAMENTALS
5. Copper oxides. Feather-like copper
oxide structures are typical of low-dissolved
oxygen, neutral pH conditions. Courtesy:
M&M Engineering
6. Random deposits. Deposits like
these inhibit flow through the cooling strand in
the stator bar. Courtesy: M&M Engineering
tail ( " Forgotten Water: Stator Cooling Water
Chemistry " in the December 2007 issue of
POWER). Of those power plants that have
water-cooled stators, there is almost an even
split of high- and low-oxygen regimes, with
the slight numerical advantage to those operating
in the high-dissolved oxygen regime.
High dissolved oxygen requires maintainconditions,
the pits created by any localized
corrosion are very small. The problem is the
release of the copper oxides from one area
that collect in another. The copper " corrosion "
rate can be very low, but conditions that
accelerate the rate at which the copper oxide
is released into the water can be very detrimental
to the condition of the system. The
release rate of the copper oxides may also be
affected by pH and by temperature.
There is somewhat of a vicious cycle effect
here. An increase in the stator cooling
water temperature increases the release rate
of copper oxides into the water. Deposits
and partially plugged strands can reduce the
water flow rate. The slower the water travels
through the stator bar, the warmer it becomes.
This cycle can cause a stator cooling problem
to come out of nowhere.
The best way to avoid problems of copper
corrosion and keep the stator bars flowing
and cool is to know what treatment option
you are going to use and to monitor the stator
water-cooling system appropriately.
Monitoring Stator Water
Unimpeded flow through all stator bar openings
is critical to operation of the generator
and minimizing the transport of particles
into places where they can cause blockage.
That's why stator cooling water should be
continuously monitored for conductivity and
dissolved oxygen. The conductivity is critical
and generally is kept below 0.5 µS/cm. (Your
original equipment manufacturer may have
different limits.)
Monitoring the health of stator water systems
includes not only the chemical parameters
like dissolved oxygen and conductivity.
It also involves looking at a variety of related
temperatures and pressures for trends that
can predict an approaching problem.
Online (in-situ) monitoring for conductivity
and dissolved oxygen is recommended in
stator systems. Grab sampling is not usually
done due to the amount of water required to
March 2016 | POWER
flush out sample lines before one can be sure
of getting an accurate sample, which then
needs to be replaced. This can be a problem.
The pressure differential across the cartridge
filters and the mixed-bed deionizer are
also important. The frequency with which the
filter needs to be changed, due to particulate
plugging, is an indication of corrosive conditions
in the system. Not changing the cartridge
filters or replacing the demineralizer
resins when the differential pressure calls for
it will accelerate the plugging of the stator
bars. The recommendation is to replace the
demineralizer resins if the conductivity exceeds
0.5 µS/cm or if the pressure differential
across the deionizer exceeds 15 psid. As a
precautionary measure, the resins should be
changed every 18 months to two years.
Cartridge filters that are normally supplied
are 5 µm, and some plants have found that
moving to a 1 µm cartridge filter is helpful.
Any particles that are trapped by the filters
or by the deionization resins are particles that
cannot reattach to the surface of the stator
cooling channel.
Monitoring the makeup water usage in a
stator cooling system is also important. If the
system is operating under a low-dissolved
oxygen regime, makeup water brings in the
only dissolved oxygen but also carbon dioxide
that will lower the pH of the stator cooling
water and increase copper oxide release rate,
which will accelerate temperature problems.
Similarly, if there is a significant pressure
drop across the deionizer or across the cartridge
filters, or there are issues with the stator
cooling water pumps, these can all slow
the flow of the stator cooling water through
the system, accelerating the release of the
copper oxide and increasing the potential
for plugging.
Choose Your Treatment Option and
Stick with It
In a previous article we discussed treatment
options for stator cooling water in more dewww.powermag.com
ing
greater than 2 ppm of dissolved oxygen in
the stator cooling water at all times. This forces
the copper to the CuO form of the oxide and
maintains the passivation layer with very little
release of oxide into the cooling water. Any
level below 1 ppm should generate immediate
action to correct the problem. Often just
leaving the head tank vented is sufficient to
keep the stator cooling water oxygenated, but
if hydrogen leaks into the cooling water, it can
displace the oxygen and cause corrosion. The
open head tank also allows in carbon dioxide,
which can lower the pH of the stator water,
increasing corrosion. Some plants put carbon
dioxide absorbers on the head tank vent to remove
it, before the air enters the tank.
Operating a low-dissolved oxygen regime
requires that the stator cooling water always
maintain less than 20 ppb of dissolved oxygen,
and ideally as low as possible. This
generates the Cu2
O passive layer but over
a longer period of time. The formation of
the cuprous oxide is limited by the amount
of dissolved oxygen in the water. Low dissolved
oxygen can be maintained by keeping
the system leak free to limit the amount
of makeup water. Oxygen may also leak in
through flanges, pumps, and seals. In some
cases, oxygen leaks in through a loose connection
or flange that creates a venturi effect,
sucking air in. Some plants blanket the
head tank with nitrogen, or even hydrogen, to
preclude any air. The low-oxygen condition
needs to be maintained during major outages,
and that may take some planning.
Increasing the pH of the stator cooling water
also has been shown to be very effective at
reducing copper corrosion and release rates.
However, fewer utilities have opted to go for
the extra steps required to create and maintain
an alkaline pH.
Alkaline pH treatment can improve either
the low- or high-dissolved oxygen regime.
The target pH for the water is generally considered
between 8.5 and 9.0 and may be obtained
by adding small amounts of caustic to
the water or by substituting sodium exchange
resin for the hydrogen form cation resin in
one of the deionizer vessels and metering the
water through this exchanger until the pH
reaches the desired level. ■
-David Daniels (david_daniels@mmengineering.com)
is a frequent contributor to
POWER and senior principal scientist at
M&M Engineering Associates Inc.
51
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POWER March 2016

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