POWER March 2013 - 43

WATER TREATMENT
the plant is designed to operate.
In some instances, however, the HRSG or
combustion turbine components may have
limits based on their design or operation that
are more stringent than the chemistry requirements
dictated by the steam turbine limits.
One specific example is a combustion turbine
design where IP steam is utilized for cooling.
In this instance, the purity of the IP steam to
meet the combustion turbine requirements
is more stringent than the purity that would
normally be required based on steam turbine
OEM limits. This is why it is important to
collect and review all chemistry guidelines
supplied by the OEMs for all major pieces
of equipment.
The addition of multiple separate pressure
circuits to the HRSG to supply IP or LP steam
for services such as deaeration and feedwater
heating has been an important improvement
in the steam cycle efficiency and has
been applied to most combined cycle power
plant designs. Steam from these IP and/or
LP circuits replaces the steam extraction
regenerative feedwater heating design used
in conventional steam power cycles. This
design combines separate HRSG circuits
with varying operating pressures within a
common HRSG casing but typically utilizes
a common condensate/feedwater system to
supply water to all of the circuits.
It's true that IP and LP circuits may have
less-stringent chemistry requirements than
the HP circuit strictly based on operating
pressure under standard water tube boiler
guidelines. But the intermingling of condensate,
feedwater, and steam systems within
the combined cycle power plant generally
requires all HRSG circuits to be restricted
to the same chemistry limits as the highest
pressure circuit.
Although originally intended to improve
simple cycle plant efficiency by absorbing
and utilizing the waste heat from a gas turbine
or turbines in a steam turbine, many
HRSGs are now designed to incorporate the
routine use of duct burners. The use of duct
burners can significantly increase the operating
pressures of all HRSG circuits and considerably
increase the unit's steaming rate.
When choosing operating chemistry limits
and the appropriate treatment for the HRSG,
operators and designers need to consider
these higher operating pressures as limiting
design conditions.
Operating experience has shown that the
most prevalent cause of phosphate hideout
in a HRSG is operation of the duct burners.
The degree of hideout experienced generally
varies with the firing capacity of the duct
burners: The greater the rate at which a unit
is duct fired, the more severe the hideout issues
experienced generally are. Frequency
March 2013 | POWER
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of duct burner use should be factored into
the chemistry selection process, and unit
testing during duct burner operation should
be performed to determine how a given
unit will respond if a phosphate treatment
program is selected for application at the
facility. If phosphate hideout is severe and
maintenance of proper drum chemistry becomes
a challenge, the operator may be better
served by implementing a caustic or AVT
program at the plant.
Several once-through HRSG designs are
available that do not include a steam drum
in one or more of the various HRSG pressure
circuits. Drum-type units utilize a steam
drum not only for water/steam separation but
also for concentration and removal of impurities
in the system. Once-through units, on
the other hand, are designed such that feedwater
is pumped into the HRSG in the liquid
phase and is converted to steam as it passes
through the heat exchangers in the HRSG.
No provisions are provided in this design for
concentration and mechanical removal of
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POWER March 2013

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