POWER November 2011 - 20

Steam Turbine Cleaning Using
Chemical Foams
In the May issue, we discussed the importance of either preventing
copper plating of the high-pressure (HP) steam turbine rotor
or finding a good foam or mechanical removal means of restoring
lost efficiency. In that article we noted that " copper deposits
typically form on the stationary nozzle block or first-stage stationary
blades " and that those deposits usually result in a steam
turbine loss of capacity at a rate of about 2 to 3 MW per month.
We also stated that the " general rule of thumb is that there is a
reduction of about 1 MW of generating capacity for each 1 to 2
pounds of deposit that accumulates on the HP turbine.
Danny Foster, product line manager for Aquilex HydroChem, suggests
that this rule of thumb has some limitations, based on his
experience of chemical cleaning more than 450 steam turbines
over the past 25 years. Foster points out that although copper may
plate throughout the HP turbine stages, the deposits on the early
stages of the HP turbine have a much larger effect on performance
than on the last stages. Foster is undoubtedly correct.
That earlier article also discussed the water chemistry issues
related to copper loss from the condenser and other feedwater
heaters that finds its way to the HP turbine blades. In this article,
we look at the process of cleaning the blades of the copper.
Sources of Copper
Dissolved copper oxide from the corrosion of copper tubing in condensers
and feedwater heaters often carries over into the boiler water
cycle and-with the right combination of temperatures, pressures,
and water chemistry-can plate out on the blades in the HP section
of a steam turbine. These deposits change the aerodynamics of the
steam path components and result in reduced unit generating capability
and increased heat rate (Figure 1).
Foam cleaning of steam turbines has been found to be an acceptable
and cost-effective method of removing deposits from
turbines without disassembly. The entire cleaning process can
be completed in 24 hours or less but will require about a fiveday
outage to cool the turbine to the desired temperature and
perform all the required mechanical tasks. Once the process is
complete, the turbine can be placed back into service, and full
capacity is normally restored.
The necessary connection points into the steam turbine must
be carefully selected. Low-pressure steam combined with the
cleaning foam is injected into the steam turbine and removed
through the cold reheat line. Other drains and vents are also used
during the cleaning process, such as a spent solvent drain, anti1.
The cost of copper. Elemental copper or copper oxide plating
out on the high-pressure turbine blades (left) and on the inside of
superheater tubes (right) can cause a significant loss of power output
from the steam turbine over time. The only way to restore the lost capacity
is to remove the copper. Courtesy: Aquilex HydroChem
foam injection points, and a system vent. The location of these
connections are steam-turbine-specific. Careful consideration is
also given to sealing of the turbine valve stems, shaft seals, and
condenser and feedwater heater protection.
The process of cleaning begins with equipment setup and premixing
chemicals the day before starting the scheduled cleaning,
while waiting for the steam turbine to cool to approximately
170F. Typically, 3,000 gallons of solution are premixed. Making
all the connections to the turbine requires about 10 to 12 hours,
spread over two days, including during turbine cooldown.
The cleaning process uses foaming CuSol I, Aquilex HydroChem's
copper oxide removal chemistry, with injections of an oxidizer
to remove elemental copper. Following completion of the
cleaning process, the turbine is rinsed with foamed condensate
followed by a saturated steam rinse until the conductivity of the
rinse water reaches an acceptable level. CuSol I is a proprietary
chemical that rapidly dissolves and removes copper oxide. Gaseous
oxygen or hydrogen peroxide is injected into the foam as an
oxidizer to convert elemental copper to copper oxide. At no time
during the cleaning is the turbine exposed to a corrosive agent.
The cleaning residual is nonexplosive.
How to Clean a Turbine
The cleaning process begins with the steam turbine on turning
gear and the first-stage inner metal temperature at 170F. First,
clean condensate water is injected with only the foaming agent
to pressure test all the turbine connections and to adjust the
foam quality.
If no problems are found during this test stage, the chemical
foam process begins. The chemical liquid solution is pumped from
the premixing tank at 10 gallons per minute and then through a
heat exchanger used to preheat the solution to 170F, to match
the turbine metal temperature. The heated solution enters the
foam generator, where air is introduced and mixed with the solution
in a static mixer. The result is a chemical foam discharge
with the consistency of shaving cream.
The foam enters the turbine steam chest through a proprietarydesign
chemical injection valve installed in place of one of the four
turbine control valves. With only one control valve in the open position,
foam is forced through the HP turbine inlet nozzle block by the
continuous flow of foam into the steam chest. As the foam passes
through the rotating and stationary blades, the copper deposits are
dissolved into the solution. The copper-laden solvent exits the last
stage of the HP turbine into the cold reheat piping, where an antifoam
solution is injected to collapse the foam back to a liquid.
The spent solvent is then drained from the piping through the
cold reheat low point drain valve, and a waste pump transports
the solution to the waste tank located outside in the wasteholding
area. Upon collapsing the waste foam back to a liquid,
the air and gas generated are vented through a cold reheat
safety valve that is disassembled for that purpose. This process
continues until all 3,000 gallons of the premixed solution have
passed through the HP turbine. During the chemical cleaning,
each control valve is cycled open for 1 hour to provide equal
cleaning through all quadrants of the inlet nozzle. This cycling of
the valves continues without interruption for the duration of the
cleaning process (approximately 5 hours).
A mobile laboratory is brought on site to perform continuous
sample analysis of the solution drained from the turbine for copper
20
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POWER | November 2011
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POWER November 2011

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