Control of Flow-Accelerated
Corrosion in Steam Generators
By
Brad Buecker, Process Specialist
Kiewit Power Engineers
Preface
In 2012,
the author gave a paper on flow-accelerated corrosion at the 73rd
Annual International Water Conference. [1]
The following discussion incorporates many details from
that paper, and also includes new information.
Introduction
Thanks to
extensive efforts by personnel at organizations such as the
Electric Power Research Institute (EPRI), the International
Association of the Properties of Water and Steam (IAPWS) [2],
and by power plant chemists around the world, it has been shown
that the reducing environment produced by oxygen scavengers
initiates and propagates flow-accelerated corrosion (FAC) in the
feedwater system and other components of high-pressure steam
generators, including heat recovery steam generators (HRSGs).
Since 1986, FAC-induced attack has caused numerous
failures, some with fatalities, at a number of power plants in
the U.S. Yet,
specifications for many new power plants both domestically and
globally, and in which the condensate/feedwater system contains
no copper alloys, continue to call for oxygen scavenger feed to
the condensate.
This document is a response to this misguided trend.
Old Chemistry
When I began my utility
career in 1981, conventional wisdom said that any dissolved oxygen which entered
the condensate/feedwater system of utility boilers was harmful.
At that time, over 50 percent of the power produced in the U.S. came from
coal. Coal-fired units typically
have complex condensate/feedwater networks with numerous feedwater heaters.
Common feedwater heater tube materials during the heyday of large coal
plant construction included Admiralty metal (a copper-zinc alloy) for
low-pressure heaters and often copper-nickel alloys for high-pressure heaters.
The prevalent thinking during this era was that any trace of dissolved
oxygen (D.O) would cause corrosion, and indeed dissolved oxygen can cause severe
corrosion of copper alloys. Therefore, virtually all feedwater systems for
high-pressure steam generators were equipped with a deaerator for
non-condensable gas removal. A
properly operating deaerator can lower D.O. concentrations to 7
parts-per-billion (ppb).
However, even this residual
D.O. concentration was still considered harmful, so chemical deaeration was also
adopted at virtually all plants.
The workhorse for many years was hydrazine (N2H4), a
reducing agent which reacts with oxygen as follows:
N2H4
+ O2
®
2H2O + N2
Hydrazine proved advantageous because it does not add any dissolved solids to
the feedwater, it reacts with oxygen in a one-to-one weight ratio, and it is
supplied in liquid form at 35% concentration.
More
importantly, hydrazine will passivate oxidized areas of piping and tube
materials as follows:
N2H4
+ 6Fe2O3
®
4Fe3O4 + N2
+ 2H2O
N2H4
+ 4CuO
®
2Cu2O + N2
+ 2H2O
Hydrazine residuals were
typically maintained at relatively low levels of perhaps 20 to 100
part-per-billion (ppb). Oxygen
scavenger treatment was coupled with feed of ammonia or an amine to maintain
feedwater pH within a mildly alkaline range, which evolved to 9.0 to 9.3 for
mixed-metallurgy feedwater systems and 9.2 to 9.6 for all-ferrous systems.
NH3 + H2O
Ű
NH4+ + OH-
The program of
ammonia/reducing agent feed became known as all-volatile treatment reducing
[AVT(R)].
Due to the suspected
carcinogenic nature of hydrazine, alternative chemicals such as carbohydrazide,
methyl ethyl ketoxime, and others gained popularity.
Regardless, all still had the same purpose, to establish a reducing
environment in the feedwater circuit, thus inhibiting oxidation of metal.
The technique became a standard in the industry.
“This changed in 1986.
On December 9 of that year, an elbow in the condensate system ruptured at
the Surry Nuclear Power Station [near Rushmere, Virginia.]
The failure caused four fatalities and tens of millions of dollars in
repair costs and lost revenues.” [3].
Researchers learned from this accident and others that the reducing
environment produced by oxygen scavenger feed results in single-phase
flow-accelerated corrosion (FAC).
.
Fig. 1.
Single-phase FAC. Note the
orange peel texture. Photo courtesy
of Dave Johnson, ChemTreat.
The attack occurs at flow
disturbances such as elbows in feedwater piping and economizers, feedwater
heater drains, locations downstream of valves and reducing fittings,
attemperator piping, and, most notably for the combined-cycle industry, in
low-pressure evaporators.
The effect of single-phase
FAC is outlined in the next illustration.
Fig. 2.
Photo of tube-wall thinning caused by single-phase FAC.
Photo courtesy of Dave Johnson, ChemTreat.
Wall thinning occurs
gradually until the remaining material at the affected location can no longer
withstand the process pressure, whereupon catastrophic failure occurs.
Following is a brief examination of the chemistry behind single-phase
FAC. When a steam generator is
placed in service, carbon steel feedwater piping and waterwall tubes form a
layer of protective iron oxide known as magnetite (Fe3O4).
Magnetite is actually a composition of FeO (with iron in a +2 oxidation
state) and Fe2O3 (with iron in a +3 oxidation state).
The combination of a
reducing environment and localized fluid flow disturbances causes dissolution of
ferrous ions (Fe+2) from the metal and metal oxide matrix.
Fig. 3.
Diagram of single-phase FAC mechanism.
Source: Reference 3.
Results from EPRI show that
iron dissolution is greatly influenced by not only reducing conditions but also
by solution pH and temperature.
Fig. 4.
Carbon steel matrix dissolution as a function of pH and temperature.
Source: Reference 3.
As Figure 4 illustrates,
corrosion reaches a maximum at 300o F.
Thus, feedwater systems and HRSG low-pressure evaporators are
particularly susceptible locations.
Also note the influence of pH on the corrosion characteristics.
The quest to maintain a
non-detectable oxygen residual in feedwater systems led to FAC issues at many
coal-fired power plants. I observed
this directly at one of my two former utilities.
At the plant in which I worked, a feedwater heater drain line failed due
to FAC, shutting down an 800 MW supercritical unit.
Infinitely more serious was FAC-induced failure of an attemperator line
in 2007 at another of the utility’s stations, which killed two workers and
seriously injured a third.
This brings us to the heart
of this document. In large measure,
coal plant personnel have recognized the problem of single-phase FAC, and have
adopted alternative feedwater treatment methods to mitigate the issue.
However, I regularly review combined-cycle proposals in which the
developer specifies an oxygen scavenger feed system.
Solutions to Single-Phase FAC and Changing the
Oxygen Scavenger Mindset
The following discussion
primarily focuses upon HRSGs, but the general chemistry is also applicable to
coal-fired units, and indeed most of the original developments came at
coal-fired plants. The figure below
illustrates the generic layout of a very common HRSG style, a triple-pressure
unit.
Fig. 5.
A common HRSG style, three-pressure drum design.
HRSGs by their very nature
typically have many waterwall tubes with short-radius elbows.
Thus, the HRSG contains many spots for single-phase FAC, particularly in
the low-pressure (LP) economizer and evaporator, where the temperatures are at
or near 300oF. A primary
method to mitigate this attack is selection of proper feedwater treatment, which
we will now examine.
Approximately 40 years ago,
researchers and chemists in Germany and Russia began using a program known as
oxygenated treatment (OT) to minimize carbon steel corrosion and iron
dissolution in supercritical steam generators.
The key component of the program was, and still is, deliberate injection
of pure oxygen into the condensate/feedwater network to establish oxygen
residuals of up to 300 ppb. What
chemists discovered is that in very pure feedwater (having a cation conductivity
≤ 0.15 µS), the oxygen will intersperse and overlay magnetite to generate a
tenacious and very insoluble film of ferric oxide hydrate (FeOOH).
OT typically lowered feedwater iron
concentrations to 1 ppb or less, and, as researchers have subsequently
confirmed, greatly minimized single-phase FAC.
Now, OT is the preferred feedwater treatment for most once-through
utility steam generators around the world.
Common in the United States is an oxygen residual range of 30 to 150 ppb,
with a recommended pH range of 8.0 to 8.5.
Although OT has been
successfully applied to drum boilers, another program has evolved that is very
popular for condensate/feedwater in drum units.
It is known as all-volatile treatment oxygenated [AVT(O)].
With AVT(O), oxygen that enters from condenser air in-leakage is allowed
to remain without any oxygen scavenger/metal passivator treatment.
It should be noted at this point that OT and AVT(O) are not permissible
for feedwater systems containing copper alloys, as the oxygen would simply be
too corrosive to the metal.
At this point, an issue
regarding HRSG configuration must be introduced to proceed with the discussion.
Figure 5 illustrates a three-pressure system with an LP circuit that EPRI
refers to as a “stand-alone LP (SALP) configuration. [4]
As can be seen, separate branches from the main feedwater line feed each
circuit. In many multi-pressure
HRSGs, the entire feedwater stream flows through the LP circuit for pre-heating
with distribution then to the IP and HP circuits.
When researchers first
developed AVT(O), the following guidelines appeared.
·
Feedwater pH range, 9.2-9.6
·
Feedwater D.O. concentration, 1-10 ppb
·
Condensate and feedwater cation conductivity, ≤ 0.2 µS/cm
It is now known that an
even higher pH of up to 9.8 or thereabouts in the LP circuit will further
inhibit single-phase FAC.
Complexity arises in this regard from the HRSG configuration.
If the low-pressure circuit is SALP, tri-sodium phosphate (Na3PO4)
or caustic (NaOH) can be utilized to establish the desired pH range.
However, if the LP circuit serves as the pre-heater for the IP and HP
circuits, these solid alkalis cannot be employed, as they could be directly
transported to the steam via the attemperators.
Only ammonia or an amine is suitable in this case.
Space does not permit a detailed discussion of ammonia and amine
chemistry and their likenesses and differences, but perhaps this issue can be
addressed in another paper.
Dissolved oxygen guidelines
have changed also. The original
AVT(O) program relied solely on oxygen that entered the condensate through
condenser air in-leakage. But, as
most experienced plant chemists and engineers know, this can be quite variable.
The old rule of thumb for proper condenser conditions is a limit of 1
scfm (standard cubic feet per minute) of air in-leakage per 100 MW of capacity.
However, I have worked with units in which the air in-leakage ratio was
significantly higher, but where the condenser vacuum pumps had sufficient
capacity to remove the gases. If
the feedwater dissolved oxygen concentration is too low, not enough oxygen may
be present to form the FeOOH protective layer.
Modern guidelines now call for a 5 to 10 ppb dissolved oxygen
concentration in the feedwater. In
oxygen deficient systems, one method that has been employed to improve chemistry
is to close the deaerator vents. In
others, a supplemental feed of pure oxygen may be required.
This scenario closely relates to OT, as mentioned above.
Single-phase FAC control
can also be addressed in large measure by materials selection.
For HRSGs in the design phase, preparing the specifications to include
the addition of a small amount of chromium to FAC-susceptible spots virtually
eliminates the corrosion. A primary
example is LP waterwall elbows. Fabrication
of the elbows from 1Ľ or 2Ľ chrome alloy (T-11 and T-22, respectively) can
provide great benefit. While this
alloy addition adds some cost to the project, the materials are quite resistant
to FAC.
Two-Phase FAC
Many steam generators,
regardless of type, are susceptible to two-phase FAC.
As the name implies, this corrosion mechanism occurs where water flashes
to steam, resulting in a mixed-phase fluid.
For conventional units, feedwater heater shells and heater drains are
common locations for two-phase FAC, but this equipment is not common for HRSGs.
However, deaerators also experience two-phase fluid flow.
As fluid flashes upon entering a deaerator, oxygen departs with the
steam. Thus, the water that
impinges upon metal surfaces does not maintain an oxidizing environment.
Also, the pH of entrained water droplets within the steam is usually
lower than the bulk water pH. The
combination of these factors often initiates FAC.
Fig. 5.
Two-phase FAC in a deaerator.
Photo courtesy of Tom Gilchrist (ret.), Tri-State G&T.
As has been noted, elevated
pH will help to mitigate FAC, but the HRSG configuration dictates how the
maximum treatment allowed. If the
LP system is utilized for heating of feedwater to the IP and HP circuits, solid
alkali treatment (tri-sodium phosphate or caustic) of the LP circuit is not
permissible. Control of pH can only be
accomplished by ammonia or an amine.
As with single-phase FAC, a
method to combat two-phase FAC is fabrication of susceptible locations with
chromium-containing steel. Again,
this adds cost to the project.
References
1.
B.Buecker, “The Continuing Crusade Against Oxygen Scavenger Use in All-Ferrous
Steam Generators”; from the Proceedings of the 73rd Annual Meeting,
International Water Conference, November 12-16, 2012.
(The paper received the 2013 IWC Paul Cohen Award.)
2.
The IAPWS offers many documents for free viewing.
Their web site is www.IAPWS.org.
3.
Cycle Chemistry Guidelines for Combined-Cycle/Heat Recovery Steam Generators
(HRSGs),
EPRI, Palo Alto, CA: 2006. 1010438.
4.
Comprehensive Cycle Chemistry Guidelines for Combined-Cycle/Heat Recovery Steam
Generators (HRSGs),
EPRI, Palo Alto, CA: 2013.
3002001381.
Biography
Brad Buecker is Process
Specialist with Kiewit Power Engineers, Lenexa, KS.
He has over 33 years of experience in the power industry much of it with
City Water, Light & Power in Springfield, IL and at Kansas City Power & Light
Company’s La Cygne, KS generating station.
Buecker has written many articles and three books on steam generation
topics, and he is a member of the American Chemical Society, the American
Institute of Chemical Engineers, the American Society of Mechanical Engineers,
the Cooling Technology Institute, the National Association of Corrosion
Engineers, the ASME Research Committee on Power Plant & Environmental Chemistry,
and the Electric Utility Chemistry Workshop planning committee.
He has a B.S. in Chemistry from Iowa State University with additional
course work in fluid mechanics, energy and materials balances, and advanced
inorganic chemistry.