Power Engineering - February 2018

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.