bellows attachment weld
TRANSCRIPT
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1© 2010 CITGO Petroleum Corporation.
Failures of High Alloy Expansion Bellows
in Refinery Catalytic Cracking Units
Gerald W. Wilks
Senior Corrosion and Materials Engineer
CITGO Petroleum Corporation
Lemont Refinery
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Presentation Summary
� What a Catalytic Cracking Unit Does
� Where Expansion Bellows Are Used
� Bellows Failure Mechanisms
� The Reactor Overhead Line Failure
• Failure Description
• False Indication of Sensitization
• Weld Geometry Problems
• Field Repairs vs. Shop Repairs
� The Regenerator Off Gas Line Failure
• Failure Description
• Cracking during the turnaround.
• Cracking after start-up
• Long term plans
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To Fractionator
To Boiler
Expansion Joints in the Reactor and RegeneratorThe unit uses heat and catalyst to break large molecules into smaller molecules.
510-565°°°°C
690-735°°°°C
Expansion Joints
Cyclones
Cyclones
Bellows - 321 Stainless
Bellows – Incoloy 800
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What an Expansion Joint Looks LikeThe dimension shown are for the off gas system joint.
OD 213.3 cm
34.3 cm 39.4 cm
1.56 m
2.23 m 2.23 m2.24 m
1.83 m
39.4 cm 45.7 cm
Fixed Supports
Weather Shields
The expansion joint accommodates movement of the upstream and downstream piping by compression and rotation of the bellows. On some expansion joints the center pipe between the expansion bellows is fixed and on others it can tilt to help deal with the pipe elevation changes. At the operating temperature the expansion joint is straight and it is distorted when the joint is cooled for shut down.
Not to Scale
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FCC Regenerator Overhead System
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Failure Mechanisms for Expansion Bellows
� Creep: High temperatures due to purging failure orinsulation failure. Operations with higherthan design pressure – higher stress. Long-term deformation and rupture.
� Fatigue: High stress and cyclic loading.
� Dew Point Corrosion: H2SO4 condensing in the bellows.
� Sensitization: High temperatures lead to chromium carbides forming in the alloy grain boundaries makingthe surrounding alloy non-stainless.
� Polythionic Stress Metal sulfides form during operations andCorrosion Cracking: react with air and moisture when the unit cools
to form polythionic acids that can attacksensitized stainless steels.
� Pressure Overloads: High internal pressure leading to rupture.
� Temperature Upsets: High temperature leading to short termbulging and rupture.
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Failure Mechanisms for Expansion Bellows� Chloride Cracking: Alloys with more than 32% nickel content are
more or less immune. Some high Mo
stainless steels are immune.
� Alloy Embrittlement: Nickel alloys and stainless steels embrittle at
elevated temperatures due to the formation of
brittle phases: sigma, alpha prime, etc.
� Wet H2S Cracking: More than 50 ppm H2S and liquid water
must be present - unlikely for higher alloys.
� Caustic Cracking: Nickel alloys are immune and stainless steels
have some resistance. Liquid water must be
present.
� Erosion: Gas and catalyst flow impacting the bellows
surface causing wall loss. This is usually the
result of failure of internal shrouds.
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� Expansion Joint Design:
• The temperature of the bellows needs to be maintained below the range where long term degradation occurs – below 455ºC.
• The in Regenerator off gas systems the temperature needs to be above the temperature at which H2SO4 dew point corrosion occurs – above 150ºC.
• Weld designs should allow thorough inspections of the welds.
Flow Direction
These Rings Were Welded to the Bellows by the Bellows
Manufacturer
Braded Stainless Steel
Rope Seals
Internal Shroud to Protect the Bellows
Internal and External Ceramic Fiber Insulation Sized so that the Bellows Temperature is
325-650°FWeather Shielding Bolted on One Side
The Rings Are Welded to the Expansion
Bellows Housing with Full Penetration Welds
Flow Direction
These Rings Were Welded to the Bellows by the Bellows
Manufacturer
Braded Stainless Steel
Rope Seals
Internal Shroud to Protect the Bellows
Internal and External Ceramic Fiber Insulation Sized so that the Bellows Temperature is
325-650°FWeather Shielding Bolted on One Side
The Rings Are Welded to the Expansion
Bellows Housing with Full Penetration Welds
Failure Mechanisms for Expansion Bellows
150-455ºC
Consider Installing a Temperature Indicator
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The Reactor Overhead LinePrevious Bellows Attachment Weld Design
1¼Cr-½Mo Pipe
Failure Occurred Along
This Fusion LineTwo Ply Bellows321 Stainless Steel
Bellows Fab Shop
Resistance Weld
Bellows Attachment Weld Joint Geometry
~⅛”
~9/16”
Bellows Attachment WeldE309 Stainless Steel
1¼Cr-½Mo Pipe
Failure Occurred Along
This Fusion LineTwo Ply Bellows321 Stainless Steel
Bellows Fab Shop
Resistance Weld
Bellows Attachment Weld Joint Geometry
~⅛”
~9/16”
Bellows Attachment WeldE309 Stainless Steel
14.3 mm
3.2 mm
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Improved Attachment Weld Design
Pipe Wall
Two Ply Bellows
Bellows Fab Shop
Resistance Weld
Bellows Attachment Weld Joint Used in the Nuclear Industry
~⅛”
⅛” Deep Machined Groove and Bevel on the Pipe Wall OD
Full Penetration Bellows Attachment Weld
E309 or E347
Pipe Wall
Two Ply Bellows
Bellows Fab Shop
Resistance Weld
Bellows Attachment Weld Joint Used in the Nuclear Industry
~⅛”
⅛” Deep Machined Groove and Bevel on the Pipe Wall OD
Full Penetration Bellows Attachment Weld
E309 or E347
3.2 mm
3.2 mm Deep Machined Groove
and Bevel on the Pipe Wall OD
E309, E347 or ENiCrMo-3
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Another Improved Attachment Weld Design
The bellows attachment weld is made in the shop improving weld quality. The ring attachment welds do not involve mixed metallurgy and are larger welds.
1¼Cr-½Mo Ring
Two Ply Bellows
Inconel 625 LCF
Bellows Fab Shop
Resistance Weld
Reinforcing Ring Weld Joint
~⅛”
¼” or ⅜”
Bellows Attachment Weld to
the Ring - a Shop Weld with
E309 Stainless Steel or
ENiCrMo-3
1¼Cr-½Mo Pipe~9/16”
Ring Attachment Weld
with E7018-B2
1¼Cr-½Mo Ring
Two Ply Bellows
Inconel 625 LCF
Bellows Fab Shop
Resistance Weld
Reinforcing Ring Weld Joint
~⅛”
¼” or ⅜”
Bellows Attachment Weld to
the Ring - a Shop Weld with
E309 Stainless Steel or
ENiCrMo-3
1¼Cr-½Mo Pipe~9/16”
Ring Attachment Weld
with E7018-B2
14.3 mm
6.35 mm
3.2 mm
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The Reactor Overhead Line Bellows FailureFire Damaged Area of the Bellows
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Insitu Metallography of the Reactor BellowsThis indicated the steel was sensitized.
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Insitu Metallography of the Reactor Bellows
Electrolytically Etched with Oxalic Acid Original Magnification - 500XThis indicated the 321 was sensitized.
West Bellows Outside of Fire Area
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Sensitization of Stainless Steels and Inconel 625Sensitization of Stainless Steels and Inconel 625Could the bellows be sensitized?
Average Reactor Temperature ~982 °°°°F
Time at Temperature - Hours
500
525
550
475
625
600
Average Reactor Temperature ~527°°°°C
Time at Temperature - Hours
Te
mp
era
ture
--°° °°
C
575
650
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Laboratory Testing Results Metallography of the Cross Section of the Bellows
Electrolytically Etched with Oxalic Acid Original Magnification 500X No indication of sensitization was detected in the laboratory.
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Sensitization Did Not OccurThere was some chromium carbide precipitation, but it wasn’t sensitized.
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Why the 321 Stainless Appeared to be Sensitized
Bellows OD SurfacePits Due to Oxidation
Polishing led to a surface with pits
outlining the grain boundaries so it
looked sensitized after electrolytic
etching with oxalic.
Etched with Glyceregia
Original Magnification
50X
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Unusual Weld Features Tack Welds that Weren’t Melted by the Attachment Weld
Remnant of a Tack Weld
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Weld Fracture Surface Features Two Views of the Fracture
Split Between Plies
Tack Weld Cavity
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Resistance Weld DefectsLack of fusion defects across the resistance welds.
Etched with Oxalic Acid Original Magnification 60X
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Weld Defects – Large Inclusions in WeldsSome cracks were found near inclusions – red arrows.
Original Magnification - Near Actual Size
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Reactor Overhead Line Bellows Failure
� Primary cause was attachment weld defects.
• Unmelted tack welds
• Lack of fusion at tack welds and along weld joint.
• Attachment welds couldn’t be thoroughly inspected with ultrasonic testing due to fillet weld geometry
• The attachment welds were dirty – field installation.
� Secondary cause was the defects in the
resistance welds – relatively large defects at the
attachment welds.
� Sensitization of the 321 stainless had not occurred.
� Bellows replaced with Inconel 625 bellows.
� Weld geometry improved.
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Inconel 625 Embrittlement Above 610°°°°CThis is a possible problem for Inconel 625
2000 Hours of Service
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The Regenerator Off Gas Bellows FailureMultiple Jagged Cracks Throughout the Incoloy 800 Bellows
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Cross Section of the Regenerator System BellowsCracks running in all directions – not stress driven.
Original Magnification – 20X
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Sensitization Curves for Incoloy 800This alloy sensitizes over a wide range of temperatures.
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Metallographic Test for SensitizationThe corroded boundaries indicate it was sensitized.
Electrolytically Etched with Oxalic Acid Original Magnification – 200X
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Carbides Fill Grain BoundariesThe grain boundaries are corroded indicating sensitization.
Etched with Glyceregia Original Magnification 1500X
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The Regenerator Off Gas Bellows Failure
� Sensitization was identified as the cause of the failure.
� Systems upgraded to keep the bellows cooler –more purge steam and more internal insulation.
� Bellows replaced with Inconel 625 LCF bellows:
• Reduced chance of sensitization.
• Increased resistance to H2SO4 corrosion.
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Questions?