Hydrocarbon Processing - October 2021 - 78

Water Management
plants discovered that in high-purity feedwater
(cation conductivity ≤ 0.15 μS), the
deliberate injection of a small amount of
oxygen (to establish a DO concentration
of 50 ppb-300 ppb) and elimination of oxygen
scavenger feed would cause the grayish-black
magnetite (Fe3
O4) layer on carbon
steel surfaces to become interspersed
and overlaid with a different oxide layer,
known variously as α-hematite and ferric
oxide hydrate. With rigorously maintained
chemistry, this oxide forms a much tighter
bond than magnetite and greatly minimizes
FAC. The program gained the name
of oxygenated treatment (OT) and was
adapted as the replacement for AVT(R) at
many supercritical units around the world,
although it is unacceptable for units with
copper-alloy tubed feedwater heaters, as
the combination of oxygen and ammonia
will cause severe copper alloy corrosion.
Subsequently, the Electric Power
Research Institute (EPRI) developed a
program to replace AVT(R) for drum
units with AVT(O), which stands for allvolatile
treatment oxidizing. If the condensate/feedwater
system contains no
copper alloys (which is true for virtually
all modern HRSGs), then AVT(R) is not
recommended. In brief, with AVT(O)
chemistry, as with OT, the oxygen scavenger
feed is eliminated. A small residual
concentration (5 ppb-10 ppb) of dissolved
oxygen is required at the economizer
inlet. Ammonia or an ammonia/
neutralizing amine blend is still utilized for
pH control. High-purity condensate (cation
conductivity ≤ 0.2 µS/cm) is required
for AVT(O), but when proper conditions
are established, the magnetite becomes
overlaid and interspersed with the tighterbonding
α-hematite. The layer is noticeable
for its distinct red color (FIG. 10).
OT and AVT(O) have proven so successful
that the major power chemistry research
organizations strongly recommend
the elimination of oxygen scavengers in all
power steam generators unless the feedwater
system contains copper alloys.
However, AVT(O) and OT often
cannot be employed at industrial plants
because the feedwater does not meet
the purity limits previously listed. Dissolved
oxygen would cause serious carbon
steel corrosion. Also, many industrial
heat exchangers have copper alloy tubes,
which, as has been noted, can suffer serious
attack from the combination of ammonia
(including ammonia generated by
neutralizing amine decomposition) and
oxygen. Amine/oxygen scavenger treatment
is required in these cases to minimize
corrosion.
This issue and others must be factored
into the selection and monitoring of condensate/feedwater
treatment programs,
including those at industrial plants that
have switched to high-purity makeup systems
but still see impurity ingress from the
condensate return.
Formerly, sodium softening to reFIG.
8. Surface view of single-phase FAC.
Note the orange-peel texture.
move hardness was the primary treatment
method at many industrial plants. This
fundamental technique had both advantages
and disadvantages, with low cost
being a primary benefit. However, softening
system upsets or inadequate effluent
monitoring may cause severe boiler tube
scaling, as the authors and their colleagues
have observed at many facilities. A dramatic
example is shown in FIG. 11.
In many cases, notably at refineries
and similar industrial facilities with highpressure
boilers, the modern makeup
arrangement is reverse osmosis (RO),
perhaps followed by ion exchange (IX)
demineralization. However, at any plant
where a change is made from softened to
higher-purity water, careful evaluation of
feedwater treatment chemistry is recommended.
Simple sodium-softened water
still contains the alkalinity from the raw
supply. This alkalinity can help protect
the feedwater piping. Without feedwater
treatment modifications, a change to highpurity
water may lead to the corrosion issues
noted previously, including FAC.
Continuing evolution of film-forming
products. Research and development
continues to improve film-forming products
(both amine-based and those based
on other compounds with alternative active
groups) for steam system metal protection.
Decades ago, filming amine chemistry
was applied to steam generators, with
a common compound being octadecylamine
(ODA, C18
H39N). The amine group
attaches to the metal surface, and the hydrophobic
organic " tail " extends into the
fluid to shield the metal (FIG. 12).
However, poor control and lack of
knowledge often lead to problems with
ODA applications, including formation
of gelatinous spheroids, or " gunk balls "
in the common vernacular, which fouled
steam generators. More advanced amine
and non-amine film-forming products
(FFP) have been developed. Most of these
operate best in mildly basic conditions, so
neutralizing amine treatment is still necessary.
Product feed is based on the residual
concentration that best protects metal
surfaces. Regular iron monitoring can be
very important in evaluating and fine-tuning
programs. Some products will actually
cause dissolution of porous iron oxide
deposits during initial application, which
can sometimes be falsely assumed as base
metal corrosion.
Much of the effort to date has been concentrated
in the power industry, and literature2
provides
details of a recent successful
FIG. 9. Photo of tube wall thinning caused by
single-phase FAC.
78 OCTOBER 2021 | HydrocarbonProcessing.com
FIG. 10. Properly passivated surfaces in a
unit on AVT(O). Photo courtesy of Dan Dixon,
Lincoln Electric System.
case study of a film-forming amine (FFA)
application. This chemistry is also expanding
into the refining and petrochemical industries.
Successful products provide true
corrosion inhibition by forming a chemi
http://www.HydrocarbonProcessing.com
Hydrocarbon Processing - October 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - October 2021
