Chemical Engineering March 2023 - 32

representative of the steam generator
type shown in Figure 1. We will
refer to these data several times in
the following discussion.
Even in low-pressure steam generators,
impurity ingress control is
important. Observe the low limits for
feedwater hardness. For decades,
a popular method of makeup-water
hardness removal has been sodium
softening by ion exchange. However,
the authors and colleagues
have seen numerous instances of
hardness excursions
caused by
inadequate softener operation and
maintenance. Even more alarming
are cases where operators were
directed to bypass the system and
feed raw water directly to the boilers
when the softener malfunctioned.
The typical result of hardness excursions
is shown in Figure 2.
Calcium carbonate (CaCO3) is the
most common scale, but other deposits
are possible, including calcium
sulfate and magnesium and calcium
silicates. All deposits are strong thermal
insulators and can cause tube
overheating and failure (Figure 3).
Calcium carbonate forms from the
inverse solubility with temperature of
dissolved calcium and bicarbonate
alkalinity (HCO3
-).
Ca2+ + 2HCO3
scaling
- + heat 
CaCO3 + CO2 + H2O
some
(1)
As added protection against this
mechanism,
plant
makeup systems include an ionexchange
dealkalizer or split-stream
dealkalizer or downstream forceddraft
decarbonator. With acid feed
ahead of the decarbonator, most
alkalinity converts to carbon dioxide,
which then vents from the unit.
H+ + HCO3
-  CO2 + H2O
(2)
It is not unheard of to simultaneously
see poor performance from
both the softener and the decarbonator,
which greatly increases
the potential for CaCO3 scale formation
in the boilers.
Another benefit of makeup dealkalization
is that alkalinity, upon reaching
the boiler, is in large measure
converted to CO2 via the following
reactions:
2HCO3
CO3
32
- + heat 
2- + CO2 + H2O
(3)
CO3
2- + heat  CO2 + OH-
(4)
Carbon dioxide flashes off with
steam and can increase the acidity
of the heat exchanger condensate
and condensate-return piping when
the CO2 redissolves.
CO2 + H2O  H2CO3 
H+ + HCO3
-
(5)
Although the pH generated by
Reaction (5) has a relatively mild
lower limit, the acidity is more than
enough to cause significant carbonsteel
corrosion
in condensate-return
systems (Figure 4).
Even with a properly operating
softener and decarbonator, many
other dissolved ions pass directly to
the steam generators. These include
chloride, sulfate and silica. While
lower-pressure boilers can tolerate
some dissolved solids, adherence
to the conductivity guidelines shown
in Table 1 often requires frequent or
steady blowdown, which wastes
water
and
energy.
Furthermore,
conditions may arise where the ions
induce direct corrosion. Some of
these mechanisms are explored in
the next section.
A makeup-treatment method being
adopted at some facilities is reverse
osmosis (RO) installation or retrofit.
RO lowers the concentration of virtually
all dissolved ions. Reliable RO
design and operation requires detailed
feedwater analyses. A typical
design includes upstream particulate
filters, or perhaps even micro- or ultrafiltration,
to minimize mechanical
fouling of RO membranes [2].
Condensate impurities
For many boilers like the type shown
in Figure 1, as well as other units,
ASME feedwater guidelines are
very important. Apart from makeup,
feedwater conditions are often significantly
influenced by condensate
return chemistry.
enter
trial boilers, including co-generation
units. This power plant's generating
fleet included a relatively small,
low-pressure (1,250 psig) unit with a
condensing steam turbine. The condenser
was on once-through cooling
from an artificial lake. After a preplanned
maintenance outage, plant
chemists discovered the feedwater
total dissolved solids (TDS) concentration
had increased from normally
low parts-per-billion levels to values
approaching 1 part per million (ppm).
The laboratory staff traced the leak
back to the condenser and requested
an immediate unit shutdown to find
and repair the leak. Plant management
refused because the utility was
selling power (another example of
the " water is water " mindset). This
condition continued for three weeks.
Plant chemists worked diligently
to maintain boiler water chemistry
as close to normal as possible, but
within two to three months, boiler
tubes began failing with regularity.
Eventually, the entire boiler had to be
re-tubed at a cost many times higher
than the company made from the
power sales. Metallurgical analyses
revealed underdeposit corrosion that
led to hydrogen damage throughout
the boiler (Figure 5).
The equation below represents a
well-known reaction that can occur
under boiler-tube deposits.
MgCl2 + 2H2O + heat 
Mg(OH)2 + 2HCl
(6)
One product of this reaction is hydrochloric
acid (HCl). HCl can definitely
cause corrosion on its own,
Because of the
enormous variety of materials produced
at industrial plants, many
compounds may potentially
the condensate return from heat
exchanger leaks or other sources.
Unfortunately, as with makeup-water-system
operation, condensatereturn
chemistry is frequently neglected
until a major upset occurs.
A classic example comes from
the power industry, but can easily
apply to many heavy-duty indusChemTreat
FIGURE
2. This layered boiler-tube scale is caused
by multiple hardness incursions
ChemTreat
FIGURE 3. A boiler tube that has bulged and blistered
by overheating
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