POWER March 2013 - 44

WATER TREATMENT
impurities. Whatever contaminants are present
in the feedwater are transported to the
turbine in the steam. Therefore, condensate
and feedwater must be maintained very pure
and must match the steam turbine OEM's
steam purity requirements. Condensate polishing
is crucial to establishing and maintaining
this high level of purity in the cycle,
and chemistry choices are limited to OT and
AVT programs.
At drum pressures of 2,400 psi or higher,
contaminants such as sodium and chloride
present in the boiler water easily vaporize
with the steam and carry over to the steam
turbine. This is due to the " partition coefficient, "
or the ratio of the concentration of
these contaminants in the steam versus in the
water, which begins to increase rapidly at
pressures above 2,400 psi. Due to concerns
with carryover, phosphate and/or sodium
hydroxide addition is not recommended for
HRSG drums operating 2,400 psi or higher
drum pressures.
Condensate/Feedwater System. SelecIf
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tion of a cycle chemistry treatment program
is significantly simplified when the system
metallurgy is all ferrous and no copper alloys
are present in the cycle. This includes the
condenser tubes, feedwater heater tubes, condensate
and feedwater pumps, and any heat
exchangers belonging to a host that receives
steam supply from the plant. Copper alloys
possess excellent heat transfer characteristics
but are susceptible to ammonia attack and,
therefore, the presence of these alloys in the
system limits operating pH in the condensate
and feedwater to approximately 9.1. Operation
at this lower pH does also increase the
system's risk of FAC. Systems with copper
alloy components can not operate on AVT(O)
or OT. Chemistry treatment programs for
these units are limited to AVT(R), phosphate,
or caustic.
For all-ferrous units, all four types of
treatment programs are applicable. However,
it is strongly recommended that oxygen scavengers
not be utilized in all-ferrous systems
due to the potential link between a reducing
environment in the condensate/feedwater and
single-phase FAC. Two-phase FAC, on the
other hand, which is common in many HRSG
LP evaporator circuits, is not impacted by reducing
or oxidizing chemistry (the addition
or absence of an oxygen scavenger). To protect
against two-phase FAC, boosting the pH
of the LP evaporator by adding sufficient ammonia
(in AVT programs), TSP (in phosphate
treatment programs), or sodium hydroxide
(in phosphate and/or caustic programs) directly
to the LP drum to increase the pH of
the drum water above 9.6 is recommended.
Adding phosphate or caustic, however, is
not always possible in the LP evaporator. The
chemical treatment of the LP drum varies
depending on HRSG arrangement. In some
configurations, each of the HRSG drums
receives feedwater in parallel from the condensate/feedwater
system. In other configurations,
some of the drums are in series with
drum water from one drum (usually the LP)
providing feedwater to other steam drums
(usually IP and HP drums). In configurations
where the LP drum is the feedwater source
for higher-pressure (IP and HP) drums, phosphate
or sodium hydroxide cannot be added
to the LP drum to provide protection against
two-phase FAC. The same challenge exists
in designs where water from the LP drum
is used for steam attemperation. In designs
where water from the LP drum is used for
steam attemperation, the LP drum water
must meet the same purity requirements as
the steam itself and, therefore, the addition of
caustic or phosphate to the LP steam water is
not permitted.
Whether or not the condensate system
is designed to include condensate polishers
44
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POWER March 2013

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