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Tube Failure
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The Total Condenser Performance Workshop
Tubing Failure Mechanisms, Materials Selection 2. Feedwater heater tube
The Total Condenser PerformanceTM Workshop
Failure Case Study 1ECT Tubesheet Map
The Total Condenser PerformanceTM Workshop
Failure Case Study 1Vibration Failure
The Total Condenser PerformanceTM Workshop
Failure Case Study 2ECT Tubesheet Map
The Total Condenser PerformanceTM Workshop
Failure Case Study 2Desuperheater Zone Damage• Two likely causes:
– Chloride induced stress corrosion cracking(SCC)• Chlorides concentrated near steam inlet aswet/dry conditions prevent rinsing• Chloride source likely condenser tube leaks
– Thermal fatigue• Occurs when cage distorts preventing tubingfrom expanding and contracting• Tube alternates bending up and straightening
Stress Corrosion Cracking
Desuperheating zone damage
SCC
Thermal Faigue
SCC -What is the driver?
TP 304, 316 Need Very Low Residual Stress
The Total Condenser PerformanceTM Workshop
Failure Case Study 3ECT Tubesheet Map(Failures primarilyin condensing zone)
The Total Condenser PerformanceTM Workshop
Failure Case Study 3Condensing Zone Damage
The Total Condenser PerformanceTM Workshop
Failure Case Study 3Condensing Zone DamageFailure found within two weeks ofstart-up!– Below extraction pipe in condensingzone
– Cause – Cl Contamination fromimproper cleaning before start-up!
– Tube was found to have very highresidual stress – 130 MPa
The Total Condenser PerformanceTM Workshop
Stainless Steel Changes• 25 years ago:– Feedwater heater tube makers (Plymouth, Trent, Allegheny,ITP) understood needs for reliable FWH tubing.– EPCs and fabricators included specials in specifications
• Today:– Competition has force pricing to dominate purchaserequirements– Most current tube makers focus on low cost manufacturingwith no specials.– Most current tube makers and many EPCs have nounderstanding of feedwater heater tubing needs.– Feedwater heater failures have become very common!
The Total Condenser PerformanceTM Workshop
Base ASME Specification Choices
• SA 213, SA 249, and SA 268 – Allowedby Code but lacking any details forfeedwater heaters
• SA 688 and SA 803- Developed forminimum feedwater heater service
• Need to include additional specialrequirements for reliable heater tubing!
What Need to be Specified?
Alloy selection Seamless vs. welded ASME specification Welding processes Cold working Heat treatment NDE
Eddy Current Ultrasonic Pressure testing
Residual stress In-process destructive
tests Corrosion tests Supplier qualification Quality plan review
Welding Processes
TIG Plasma
Laser
In-Line Cold Working Processes
Bead Forge Reciprocating Roll Down
Cold Drawing
Danger of No Cold Work
Do Not Allow Weld Polishing!
In-Line Heat Treatment
Very rapid temperature increase and quench Great for ferritic alloys! Time too short for
austentics!
Furnace Annealing
Much longer time for homogenization Needed for TP 304, TP
316, and AL6XN FWH which require long time for diffusion
Elimination of ferrite provides much lower background eddy current noise
Slower cooling rates Detrimental to ferritics
Non-Destructive Testing Choices
Eddy Current A 1016 Base: .031” Drilled Hole A 688/803 S1: Longitudinal & Transverse
Notches A 688 S2: Most stringent “Select Grade”
Ultrasonic testing: Longitudinal OD/ID Notches
Pressure Testing
ECT –Strengths & Weaknesses
Differential technique sensitive to abrupt imperfections (transverse)
Signal is volume related Can find non-through wall imperfections Attenuation reduces sensitivity on ID surface
– results can be very different from ID testing Won’t detect gradually growing imperfections
(longitudinal) CW process that “irons” walls provides low
background noise for better signal to noise ratio
Ultrasonic – Strengths & Weaknesses
Sensitive to longitudinal defects
Signal related to reflected area of imperfection
Defects do not need to have volume.
Sensitive to both OD and ID imperfections
Imperfections do not need to be through-wall
UT Notch Position
Case B: Extending from best fit radius relative to tube ID
Case A: Extending from weld surface
Total notch depth much more!
Make sure you don’t get this!
Residual Stress Testing
Recommended Maximum Residual Stress for 300 Series FWH
Straight Sections Hoop stress – 5 ksi (34MPa) maximum*
▪ Now- 3k psi (20.5MPa) is available! Longitudinal Stress – 3 ksi (34MPa)
maximum Bend region
8 ksi (55MPa) maximum*“Feedwater Heaters: Replacement Specification Guidelines”, Part 1.4- Tubing Selection
and Preparation, EPRI Final Report GS-6913, Project 2504-5, August 1990.
Why Not ASTM G36?
G36 indicates residual stresses are high enough to cause failure!This shows levels of 10 KSI to 12 KSI (68 MPa -82 MPa)Tubes in service experience stress from multiple sources including residualG36 does not provide actual stress value nor provide cushion for other sources
What Stress Level Should We Use?
Grade 500°F 600°F304 17.5 16.6 W12, G5304 14.8 14.1 G5, G24304 11 10.4 G24
ASME B&PV Code, Section II, Part D, Table 1AMaximum Allowable Stress Values (s in ksi) for Ferrous Materials
Values may be interpolated for intermediate temperatures.Notes W12 (100%) and G24 (85%) are weld efficiency and apply only to weldedtubing.Note G5 (35% increase) allows distortion and applies to both welded andseamless.
SA 688 weldedtubing
Notes concerning allowed stress level
Global Alloy ConcernsCarbon and Alloy Steels• Addition of Ti to Carbon and Alloy steels– Added in boiler tubes for formability
• Tramp alloys from poor quality scrap in Carbon andAlloy steels.– Ni, Cr, Mo, and Cu
• T11, T22 – No universally accepted process for heattreating. Without this, anything may be OK.
• These grades very sensitive to carbon pickup• Requires nitrogen blanketing during shut-downs –Major damage has occurred to heaters not cared for
Global Alloy ConcernsStainless Steels• Chinese and Indian stainless steels can have very high carbon unless otherwise specified.
• Tube inside surface may be heavily contaminated.
How Do We Order Reliable Tubing?Needs
1. Ensure minimum quality needs
2. Eliminate rolling/expansion problems
3. Ensure future low noise ID ECT testing
4. Eliminate small weepers5. Reduce chloride SCC potential
6. Eliminate longitudinal defects7. Ensure full wall thickness
SolutionsSpecify A688/ A803 req’tsRequire quality plan approvalVerify user listSpecify cold working using both
OD and ID toolingSpecify A688/A803 S1 or S2Specify A249 S7, 1.0 Ratio or
lowerSpecify ID cold work toolingSpecify air-under-water testSpecify 20.5 MPa max. long. &
axial stressSpecify Ultrasonic testingSpecify “No weld polishing”
Most Importantly……
Know your supplier!
There are no ASTM/
ASME Police!