12. TCPW 2015 - Tube Failure Mechanisms (Part 2)

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

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Failure Case Study 1Vibration Failure

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Failure Case Study 2ECT Tubesheet Map

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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

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Failure Case Study 3ECT Tubesheet Map(Failures primarilyin condensing zone)

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Failure Case Study 3Condensing Zone Damage

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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

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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!

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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!