Catastrophic failure
due to flow-accelerated
corrosion
2
Gray-black magnetite
coating in steam
generators under
reducing conditions
1
Source: www.girardchemical.com
During the heyday of coal-fired power plant
construction and operation in the last century, many lessons were learned regarding
correct water/steam chemistry control in
high-pressure, fossil-fuel steam generators.
Even seemingly minor issues had the potential to cause serious problems, and some
corrosion-induced failures led to injuries
and death of plant personnel.
Progress in transferring these lessons to
the combined cycle power industry has
often been slow, and a number of outdated
chemistry concepts continue to appear in
the specifications for new combined cycle
plants. Some problems are magnified by
the unique geometrical features of heat
recovery steam generators (HRSGs) as compared to their coal unit counterparts.
This article will examine three of the
most important issues in this regard, as
outlined below:
• Unless the condensate/feedwater system
of the HRSG contains copper alloys
(very rare), an oxygen scavenger should
not be part of the chemical treatment
program. Use of oxygen scavengers, a
more accurate term is reducing agent,
induces flow-accelerated corrosion
(FAC) of plain carbon steel. FAC has
caused catastrophic piping and tube
failures at a number of facilities over the
Maintaining
High Combined
Cycle HRSG
Efficiency and
Reliability
BY BRAD BUECKER
last three decades, and it continues to
occur at many plants.
• Tri-sodium phosphate has served as the
primary chemical for boiler water treatment in many base-loaded coal units,
and the chemistry is often employed in
HRSGs. Even at steady load, phosphate
treatment is problematic due to the
phenomenon known as phosphate
hideout. In cycling units, hideout may
make phosphate chemistry extremely
difficult to control.
• While a strong focus should always be
placed on operating chemistry, off-line
chemistry control is frequently neglect-
ed. Air in-leakage into water-filled steam
generator networks during down times
can significantly damage tubes, piping,
turbine blades and rotors, and other
equipment. Given the regular cycling
nature of most power plants in today’s
environment, the potential for air in-
gress and subsequent corrosion may be
extensive.
FORGET THE
OXYGEN SCAVENGE
When this author began his power career
in 1981, common wisdom said that any
dissolved oxygen which entered the condensate/feedwater system of utility boilers
was harmful. At that time, large base-loaded steam generators were the norm. Such
