Prevention of the Material
Degradation of Boilers
4th March, 2014
Hiroki Kimoto
Idemitsu Kosan Co., Ltd.
Technical & Engineering Center
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Outline
□ Introduction
□ What is FAC ?
□ Experience of FAC damage
・ Damages in piping
・ Analysis of sample piping
・ Analysis of flow
□ Corrosion prevention
□ Conclusions
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Introduction
In refineries , boilers are the important component for supply
of power and steam. In the worst case, shutdown of steam and
power system can results in emergency shutdown of the
refining plants.
During emergency shutdown, flare gas is released and the
inner oil is drained out. This will increase CO2 emission
from flare and the bad odor from the drained oil.
It is very important to improve reliability of boilers for the
preservation of the environment as well as plant safety.
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Introduction
A circulation boiler consists of a drum Super heated Steam
and many water tubes .
The feed water pump increases the Steam Super heater
pressure of the boiler feedwater.
Steam Drum
Then, an economizer exchanges heat up
the feedwater temperature with exhaust
gas. The boiler water is injected into a Boiler Tubes
steam drum, guided to boiler tubes Economizer
through a downcomer, and steam is
generated. Furnace
The generated steam is separated into Feed Pump
steam and mist in a steam drum, and the
steam becomes superheated steam in a Water Drum
super heater.
schematic illustration of water
circulation in a boiler
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What is FAC ?
Significant piping failures caused by flow-accelerated corrosion (FAC)
at unit 3 secondary pipe of Mihama Power Station in 2004.
16 Mpa
325℃
6 Mpa
Steam generator 275℃
Orifice
Turbine Dynamo
Steam condenser
Reactor presser vessel
Feed water pump
Nuclear containment
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What is FAC ?
FAC is the chemical dissolution of surface oxide and metal. There are no physical or
microstructural changes in steel.
□ Static or low velocity : The oxide layer is stabile and Corrosion rate is low.
□ High velocity : The oxide layer is removed mechanically. Extend and location of
oxide layer removal can be changed as increasing velocity.
□FAC: As increasing velocity, the corrosion rate increases. Because the diffusion speed
of the ferric ion increases and the corrosion reaction is promoted.
FAC Erosion-Corrosion Erosion
Partial Disappearance Disappearance
Oxide Thin
Loss
layer
Base
Metal
Corrosion Corrosion Corrosion Mechanical loss
Low Velocity high
Relationship between Velocity and Damages
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What is FAC ?
FAC is affected by many factors..
pH: The corrosion rate decreases as the PH increases.
Dissolved oxygen (DO): The corrosion rate increases
when DO becomes less than a few ppb.
Temperature: The corrosion rate is maximized at150 ℃.
Material: The corrosion rate decreases as the Cr content increases.
Velocity: Corrosion rate increases as the flow rate increases.
Shape: Corrosion rate increases depending on piping geometry
Corrosion rate increases in areas of turbulent flows.
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Experience of FAC damage
In shutdown inspection, the localized thinning found BFW piping.
① Operating Conditions
Temperature: 165 ℃
Pressure: 16.14 MPa
Velocity : 4.1 m/ s
② Material & Others
Material : Carbon steel(ASTM A106Gr.B)
Damage Location : From feed pump to
outlet of economizer
Pipe Diameter : 8 inches
Thickness : 23 mm
Damage area
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Experience of FAC damage
(1) Visual observation of damaged piping
・Reddish scales are observed in some spots inside of the elbows.
・Thickness of the Reddish scale areas were thinner than the black area.
・Tiger skin stripe pattern was observed there.
Sample elbow
Reddish scale area
Tiger skin stripe
pattern
Color changed area of inside piping Enlarged view
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Experience of FAC damage
(2) Destructive test of piping
① Microscopic observation of cross section
The thin oxide layer was observed on the surface of base metal.
② X-ray diffraction pattern analysis
The scales are consist of magnetite(Fe3O4) , Hematite (Fe2O3)
and Wustite (FeO).
Cross section of damage pipe
Result of X-ray diffraction
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Experience of FAC damage
③Analysis of flow
Recently the capability of steam generation was increased up to about10%.
The average velocity of the boiler feedwater was 4.1 m/s, it reaches 5 m/s in
some spots. The velocity is high in elbows.
Result of the computation fluid dynamics
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Prevention of corrosion
There are several measures against FAC.
・Temperature changes
・Velocity
・Material
difficult to take all measures
Two factors in BFW quality are changed to prevent FAC.
pH Do
Before 8.5 < pH < 7 ppb
After 9.0 < pH a few ppb
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Summary
1. It is important to periodically inspect and keep track of conditions of
facilities.
2. The FAC can be caused by slight changes in operating conditions by
increasing the capability of steam generation.
3. Fortunately, the thickness reduction was found in an early phase,
and the prevention measures were implemented in our unit.
The system is properly operating today.
4. It is important to prevent these problems proactively before they occur.
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