Tube-wall thinning caused
by single-phase FAC
3
Source: Photo courtesy of Chem Treat
Iron solubility as a function of pH and
temperature
4
NH3
(mg•kg- 1) pH
0.1 8.75
0.2 8.90
0.3 9.00
0.5 9. 20
1.0 9. 40
2.0 9.60
100
80
60
40
20
0
0 50 100 150
Temperature (°C)
Temperature (°F)
200 250 300 350
0 100 200 300 400 500 600
F
e
(
µg
•
k
g
- 1)
WWW.POWER-ENG.COM
units were typically designed with extensive
condensate/feedwater networks with perhaps a half-dozen feedwater heaters. A
common material for feedwater heater
tubes was some type of copper alloy. The
prevalent thinking was that any dissolved
oxygen (D.O) would cause feedwater system and boiler corrosion, and indeed dissolved oxygen can be very troublesome if
copper alloys are present or if oxygen accumulates in stagnant areas. Therefore,
virtually all feedwater systems for conventional high-pressure steam generators included a deaerator for oxygen removal. A
properly-functioning deaerator can lower
D.O. levels to 7 parts-per-billion (ppb).
Even this residual D.O. concentration
was still considered harmful, so supplemental chemical deaeration was a nearly
universal process at most plants. Hydrazine
(N2H4), typically supplied as a liquid in 35
percent concentration, was once the common reducing agent/oxygen scavenger.
Feedwater hydrazine residuals of perhaps
20 to 100 parts-per-billion (ppb) were suf-
ficient. Hydrazine treatment was coupled
with feed of ammonia or an amine to
maintain feedwater pH within a mildly
alkaline range, 8. 8 to 9. 1 for
mixed-metallurgy feedwater systems and
9. 1 to 9. 3 for all-ferrous systems.
NH3 + H2O ⇔ NH4+ + OH-
This program is known as all-volatile
treatment reducing [AV T(R)], and was de-
signed to maintain the protective magnetite
(Fe3O4) coating that forms on steel when
a unit is placed in operation.
Hydrazine has
long been a suspected carcinogen, so
alternative chemicals
such as carbohydra-zide, methyl ethyl
ketoxime, and others
emerged as alternatives. All still had the
same purpose, to
establish a reducing
environment in the
feedwater circuit,
thus inhibiting oxidation of metal.
AVT(R) 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.” [ 1]. Researchers learned from this accident and
others that the reducing environment produced by oxygen scavenger feed results in
single-phase flow-accelerated corrosion
(FAC).
The general effect of single-phase FAC
is outlined in the next illustration..
The attack occurs at flow disturbances,
e.g., elbows, valves, etc., in feedwater piping, HRSG low-pressure (and to some extent intermediate-pressure) evaporators
and economizers, attemperator piping, and
similar locations. As the following figure
illustrates, pH and temperature also have
a large influence on FAC; the latter of which
explains why LP economizers and evaporators, and attemperator lines, are particularly susceptible.
Corrosion reaches a maximum at 300o
F and is highly influenced by pH. This
author has attended several presentations
by the well-known steam generation chemistry expert, Dr. Barry Dooley (formerly of
the Electric Power Research Institute [EPRI]
and now with Structural Integrity Associates), who has commented on the numerous FAC-damaged HRSGs he has seen
around the world, many with only a few
years of operation. The reason; lack of
understanding of this chemistry.
So, what are solutions to this issue?
Nearly a half century ago, researchers in
Europe developed a program known as
oxygenated treatment (OT) to minimize
carbon steel corrosion in supercritical
steam generators. The key component of
the program was, and still is, deliberate
injection of oxygen into the condensate/
feedwater network to establish oxygen residuals of up to 150 ppb. What chemists
discovered is that in very pure feedwater
(cation conductivity ≤ 0.15 µS/cm), the
oxygen will intersperse and overlay magnetite to generate a tenacious and very
insoluble film of ferric oxide hydrate
(FeOOH). OT greatly minimizes single-phase FAC and can lower feedwater
iron concentrations to 1 ppb or less. OT
is now the preferred feedwater treatment
for most once-through utility steam generators around the world.
Author
Brad Buecker is Senior Technical Publicist
at Chem Treat.