POWER March 2013 - 46

WATER TREATMENT
has a big impact on the selection of the cycle
chemistry program. The design of any unit
must include a means to handle the ingress of
any impurities that may enter the system via
condenser tube leaks, air in-leakage, makeup
demineralizer operational issues, and the
like. If condensate polishers are included as
part of the plant design to remove any impurities
that find their way into the system, AVT
and OT chemistry programs can be utilized.
If no condensate polishers are included in the
plant design, phosphate or caustic treatment
programs should be employed to buffer or
neutralize any ingress of impurities and minimize
corrosion and deposition in the system.
Plant Operating Regimes Must Be
Considered
The proper water treatment process must
be selected for the operating conditions and
type of equipment, as just discussed. How
the equipment is operated is equally important,
as the water treatment process may be
remarkably different at a baseload plant, a
cycling plant, and one of the new generation
of fast-start plants.
Cycling Service Considerations. Although
phosphate treatment programs are
designed to provide good buffering capability
for drum units, cycling operation while
utilizing these treatment regimes has been
linked historically with phosphate hideout
problems, where the concentration of the
phosphate in the boiler/HRSG seems to " disappear
and reappear " as the unit makes significant
load changes. When the phosphate
" disappears, " operations staff typically try to
correct the situation by adding more chemical,
which usually results in an overfeed
situation when load changes again and the
phosphate " reappears. "
Any drum-level control problems that result
as part of the load swings can also result
in mechanical carryover of phosphate and
sodium from the drum to the superheater
sections and the steam turbine, leading to potential
deposition and corrosion. Oxygenated
treatment programs are also best suited for
steady load-type operations, whether the unit
is a drum or once-through type boiler.
Boilers operating in a cycling mode are
best served chemistry-wise by employing an
AVT program coupled with full-flow condensate
polishing to remove any contaminants
that may enter the cycle. Units with copperbearing
alloys in the steam system should
employ AVT(R) chemistry, where ammonia/
amine is added to the condensate/feedwater
to control pH and an oxygen scavenger
is also added to the condensate/feedwater
to minimize dissolved oxygen concentration.
Ferrous-only units are best served by
implementing an AVT(O) chemistry program
46
where ammonia/amine only is added to the
condensate/feedwater. It is recommended
that inorganic chemicals (such as ammonium
hydroxide and hydrazine) be used as the pH
adjuster and oxygen scavenger, but there are
numerous organic substitutions on the market
today that will also yield good results when
properly applied under the advice of a water
treatment expert.
Fast-Start or Rapid-Response Designs.
Several " fast-start " or " rapid-response "
HRSG designs have made their
way into the combined cycle market in
recent years. These units-developed usually
through collaborative efforts among
the steam turbine, gas turbine, and HRSG
OEMs-feature combined cycle power plant
designs that are intended for quick startups
and/or very quick and frequent load swings.
These designs, while providing the swift reaction
to the electric grid needs that today's
power market demands, also complicate
steam cycle chemistry issues.
Chemistry, like the HRSG and steam turbine
equipment, must now also be flexible
enough to respond to fast startups and/or
very quick and frequent load swings. These
plants cannot tolerate chemistry holds that
have been standard in traditional fossil units
and still meet their startup time or load swing
guarantees. Therefore, use of high-quality
makeup and maintenance of condensate/
feedwater purity are primary concerns for
projects that utilize a fast-start or rapid-response
design.
Such plants should ideally include permanent
condensate polishers as part of their
standard design to maintain condensate and
feedwater purity and minimize chemistry
holds. For Siemens' once-through Benson
boiler design, for instance, condensate polishers
are required because the HP portion
of the HRSG is designed to operate on OT
chemistry, and therefore condensate polishers
are a key part of this chemistry treatment
program. For other fast-start designs, AVT
chemistry programs coupled with a condensate
polisher are generally the best choice to
maintain a clean cycle and respond rapidly
to changes.
Other Water Chemistry Issues
Plants incorporating an air-cooled condenser
(ACC) for condensation of the steam turbine
exhaust have unique requirements for their
cycle chemistry treatment program.
The ACC design consists of a very large
surface area for condensation of the exhaust
steam. Though this large surface area works
well for heat transfer purposes, it can upset
the steam-water cycle chemistry. Newly
erected ACCs are difficult to completely
clean and tend to contribute a substantial
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amount of contaminants to the cycle during
initial startup and even during unit restart if
vacuum has been broken. The large surface
area also increases the likelihood of iron
transport in the system, particularly during
initial startup and during unit restarts, and
the potential for air in-leakage in the system.
Owners should seriously consider including
a condensate polisher when using an ACC.
FAC concerns are also common in the
ACC. In order to minimize FAC in an ACC,
the pH in the early condensate must be increased
above that required for an equivalent
water-cooled condenser. HP feedwater pH
should be maintained in the range of 9.6 to
9.8 to minimize FAC in the ACC. This may
require supplemental chemical injection for
the HP steam drum or HP feedwater.
Steam from auxiliary boilers is frequently
utilized in combined cycle power plants for
purposes such as pegging the deaerator during
startup, holding vacuum overnight, or
hotwell sparging. The purity of the steam
coming from the auxiliary boiler must be
the same as the steam produced in the main
cycle. Therefore, the chemical treatment program
utilized for the auxiliary boiler must be
compatible with the operating pressure and
temperatures of the main cycle, even though
the auxiliary boiler typically operates at lower
pressures and temperatures.
For instance, nonvolatile oxygen scavengers
are frequently used in industrial boilers
operating at low pressures (less than 800 psi).
However, if the industrial boiler is serving as
an auxiliary boiler that is supplying steam to
an HRSG with a HP pressure of over 800 psi,
volatile oxygen scavengers such as hydrazine
or carbohydrazide must be used in treating
the auxiliary boiler, just as in the main steam
cycle.
Consider Plant Staffing
The plant staffing plan and operating experience
level of the team may also affect the
choice of chemistry treatment for a unit. Managing
a high-performance chemistry program
(AVT or OT) requires tighter operating controls,
more supervision of plant makeup water
treatment systems and chemical additions,
and a higher-level knowledge of steam cycle
chemistry practices. An operations team supported
by a dedicated on-site plant chemist
or chemistry technician specifically trained
in these practices is preferable. If the facility
is unable to provide this level of support
and supervision of steam cycle chemistry, the
better option is to use a phosphate chemical
treatment program, which can be more forgiving
when system upsets occur. ■
-Colleen M. Layman (colleenlayman@
hdrinc.com) is the energy-water management
practice director for HDR Inc.
POWER | March 2013
http://www.powermag.com
POWER March 2013

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