1February 2006

Managing Pressure Vessels with Known Flaws

By Augusto Roveredo, Corrosion Project Manager, Sherritt Metals , andAna Benz, Specialty Services, IRISNDT

2February 2006

A. How the Heads Were Replaced 1. Information on the:

Intricate Design and Construction of the Vessels. Inspections Performed and their Findings. Deciding factors for choosing whether to replace the

heads.

2. How a Temper Bead Welding Procedure Was Developed Based on the National Board Inspection Code RD-1000.

3. Head and Nozzle Replacements.

4. Project Milestones.

B. What Was Found Inspecting the Removed Heads

What Will Be Presented?

3February 2006

Leach Reactors- Background• Four trains with

four reactors each have been in a very corrosive service for almost 50 years.

• The 212 grade B carbon steel is clad with lead lining for corrosion protection.

4February 2006

Leach Reactors- Background The lead lining is covered by special corrosion

and erosion resistant brick layers.

Mechanical seal consisting oflead wool packing between the

Titanium sleeve and the Carbon Brick.

4-1/2" Carbon Brick on a 1/8" bed of AR-20 mortar

This is covered with a fillet type sealof AR-20 acid resistant mortar.

3" Acid brick on a 1/8" bed of AR-20 mortar

Original HBL

Typical Nozzle Detail of Protective Lining

A 4" wide band of panel leadaround the nozzle opening replacesthe original HBL due to repairsin this area over the years of service.

Titanium sleeve

Panel lead lining which nowreplaces the original HBL

An overlay of Alloy 20

A spiral wound metal gasket

Carbon Steel nozzle neck

Carbon Steel Flange

Carbon Steel Repad

5February 2006

• Internal inspections only allow one to observe the condition of the bricks.

• In 1998 and 1999, during routine clean out cycles, Trains 1 and 2 were subjected to a design code compliance and to a non-destructive evaluation.

• The reactor shells had insignificant wall losses and were in good condition.

• The reactor drain nozzle flange to cone connection had thinned areas and previous weld repairs that did not meet ASME Section VIII, Division 1 requirements.

Results of NDE Inspections prior to Head Replacements

6February 2006

• The heads had:

• Minute cracks on the external surface.

• Nozzles with extensive internal erosion/corrosion losses.

• Some localized eroded/corroded areas.

• Internal pits on the inside surface of the head, beneath manway repad covered surfaces.

• Large shell nozzles had cracks along the toes of welds joining the repad to the shell.

• The anchor bolts fixing the reactors to the ground concrete pads were corroded extensively.

• During these inspections, IRISNDT personnel were informed that the inside surfaces of some reactor heads had had cracks previously.

• Some openings did not meet the ASME Section VIII, Division 1, 1998 reinforcement requirements.

Results of NDE Inspections … Cont’d

7February 2006

• Fitness for service calculations were performed to determine the remaining life of the nozzles. These indicated that the nozzles would leak instead of rupturing, i.e. “leak before break”

• The corrosion rates of the head nozzles were estimated by 2003 to be as high as 0.040 inch per year.

• Attempted to assess the possible growth rate and life expectancy associated with the possible head cracks.

Based on the NDE Inspections:

8February 2006

Should the Nozzles or the Complete Heads Be Replaced?

The replacement of only nozzles would not answer all the concerns identified with the head condition or the remaining life expectancy.

Complete head replacement would allow for homogeneous lead lining. Nozzle replacement would be subject to installing panel leading in the overhead position.

9February 2006

Welding on the Vessels - Considerations

• The welding procedure had to be optimized to decrease lead loss during welding.

• Other challenges were:– Joining of A212 grade B carbon steel to SA 516

Grade 70 carbon steel. This entailed dealing with the A212’s coarse grain structure.

– Minimizing the residual stresses from welding. This encompassed trying to minimize the hardness values in the heat affected zone.

– Minimizing down time.

10February 2006

• As well as minimizing the lead melting, a second objective was to obtain welds similar or better in quality than the originally post weld heat treated welds.

• This implied obtaining welds with low residual stresses that would have low hardness values and reasonable impact toughness.

• Several procedures were developed as per ASME Section IX. The procedures were qualified.

• The first procedure tested resulted in hardness values as high as 382 HV. This value was unacceptable.

Welding Procedure Optimization

11February 2006

• This procedure is that described as Welding Method 3 in the National Board Inspection Code RD-1050.

• Preheat temperature min: 425 deg. F.

• Inter-pass temperature max: 450 deg. F.

• String or weave bead: stringer bead only.

• Initial and inter-pass cleaning: half bead technique first 4 passes 3/32” and 1/8” electrode,

grinding and wire brush between passes.• Travel speed (range):

4-8 inches per minute.

Welding Procedure OptimizationWeld Coupon C

12February 2006

Welding Procedure OptimizationWeld Coupon C

Macro of Weld Coupon C

Very small HAZ, indicative of high cooling rates, high residual stresses and hardness values.

13February 2006

• Preheat temperature min:

425 deg. F.• Inter-pass temperature max:

450 deg. F.• String or weave bead:

stringer bead only.• Initial and inter-pass cleaning:

half bead technique first 4 passes 3/32” and 1/8” electrode, grinding and wire brush between passes.

• Travel speed (range): 4-8 inches per minute.

No changes from Weld Coupon C.

Welding Procedure OptimizationWeld Coupon D

14February 2006

• Changed from Weld Coupon C.– Apply Butter Pass 1 along the joint

edges from the root to the face of the plate.

– Remove half of the Butter Pass 1 by grinding.

– Apply Butter Pass 2.– Grind cap flush and re-cap, then grind

flush again.

Welding Procedure Optimization:Weld Coupon D Continued…

15February 2006

Welding Procedure OptimizationMacro of Weld Coupon D

• Bigger HAZ, indicative of lower cooling rates.

• Lower hardness values.

16February 2006

226 196

254

230

226

226234

222

260

222

217196

238

266

248

217 15

xx

226187184

216 251 168219 226177

257

226226

171 210 193 212 212 171

158165

269

Welding Procedure OptimizationComparison of Hardness Values

Weld Coupon C Weld Coupon D

17February 2006

• The initial welding procedure coupons had hardness values in excess of 382 HV.

• With the additions of a buttering pass and higher preheat the HAZ hardness values dropped to a maximum of 269 HV.

• The HAZ impact properties were tested at –29°F. They met the ASME SA 516 Grade 70 requirements for operation at –29°F.

Welding Procedure OptimizationConclusion

18February 2006

• Preparation for an international project.• Weld repairs.• Weld final inspections and approval.

Replacing the Heads and Nozzles – the Project Starts

19February 2006

Preparation: Where Should the Heads Be Cut?

• The shell plate was inspected with ultrasound to ensure that the area to be cut was free of extensive laminations.

• The cutting line was chosen 18” below the head to shell seam; this would facilitate lead repairs.

• The vessels were strapped to size the new heads.

20February 2006

Preparation:Purchase of New Heads

• Two SA516 Gr. 70, 2:1 elliptical heads and shell assemblies with homogeneous lead lining were purchased for Leach Train 1 vessels‘ B and D.

• The nozzle design was changed from nozzles with repads to self reinforced nozzles.

• The diameter of the manway nozzle was increased from 24” to 30” in order to accommodate additional 2” acid brick lining.

• The heads were fabricated in Canada and carry a CRN registration.

21February 2006

Preparation:Purchase of New Nozzles

• Additional nozzles were purchased to replace thinned nozzles on the Leach Train 1 Vessels A and C.

22February 2006

Preparation:The Brick Lined New Heads

23February 2006

Preparation: Qualifying the Welders to the Procedure

• The Canadian company contracted to weld the heads and nozzles performed welding procedure qualification and welder performance qualification tests.

24February 2006

• For the reactor welds, improper welding techniques/workmanship could result in:– High residual stresses.– High hardness values.– Areas more prone to cracking than

those with lower residual stresses.

Preparation: Showing the Welders the Importance of Their

Contribution

25February 2006

Showing the Welders the Importance of Their Contribution

• An example was shown to the welders of a nozzle weld that failed due to high residual welding stresses (inadequate post weld heat treatment)

26February 2006

The Result of an Inadequate Post Weld Heat Treatment

27February 2006

The Result of an Inadequate Post Weld Heat Treatment

28February 2006

Execution: Project Milestones (What We Had to Achieve)

• Meet all requirements of ASME section VIII, Division 1.

• Complete the installation of the two heads as outlined in an 11 day schedule.

• Complete the project within budget.

• Complete the project with zero loss time accidents.

29February 2006

Execution:Reactor Cut Line Being Prepared

• A cut line platform was designed, fabricated and installed to ensure safety for the workers.

30February 2006

Execution: Radiograph Torch Cutting

• An oxygen and acetylene radiograph torch assembly was tack welded to the existing shell section.

31February 2006

Execution:Completed Torch Cut

32February 2006

Execution:Head Being Removed

33February 2006

Execution:Lead Being Removed by Torch

34February 2006

Execution:Bevel Being Cut by Torch

35February 2006

Execution:Bevel Prep Final Product

36February 2006

Execution:New Head Being Positioned

37February 2006

Execution:Installation of Heating Coils

38February 2006

Execution:Insulation Wrap Around Heating Coils

39February 2006

Execution:Stringer Beads Being Welded on Inside

Surface

40February 2006

Execution: Dry Magnetic Particle Examination of the Root Pass

41February 2006

Execution:

Finished Internal Weld Finished External Weld

42February 2006

Execution:External Weld Cap Ground Flush

43February 2006

Execution:Lead Panel of the New Joint

• Lead panel joints were inspected with liquid penetrant.

44February 2006

Execution:Nozzle Butter Pass

• Integral nozzles were manufactured.

• Welding trials were performed on the removed heads.

45February 2006

Execution: Preparing the Head Surface

where the New Nozzle Was to Be Inserted

• The prepared base metal bevelled surfaces were subjected to black on white magnetic particle inspections.

46February 2006

Execution: Replacement Nozzle Fit-up

47February 2006

Execution: Nozzle Completed Outside Weld

48February 2006

Execution:Nozzle Completed Inside Weld

49February 2006

• A North American Authorized Pressure Vessel Fabrication Inspector performed all the checks that would have been required in North America for an equivalent repair and modification.

Weld Final Inspections and Approval: ASME Compliance

50February 2006

Weld Final Inspections and Approval:Radiography of the New Weld In Situ Metallography and Hardness Tests

51February 2006

• Bricks were replaced in the closing seam area.• Hydrostatic testing was not required since the

repair weld was radiographed. Nevertheless, the vessels were hydrostatically tested.

• A new deck was installed.• The temporary work platform was removed.

Execution: Final Steps

52February 2006

Project Milestones: Conclusion

• Meet all requirements of ASME section VIII, Division 1…..Accomplished.

• Complete the installation of the two heads as outlined in the 11 day schedule….Completed one day under schedule.

• Complete the project within budget…. Came in under budget.

• Complete the project with zero loss time accidents…. Completed without a loss time accident.

53February 2006

Project Completed? – What Was Found Inspecting the

Removed Heads• The Head D manhole and several of its

nozzles were cut. Their cross-sections were examined and several deficiencies were found:– extensive thickness losses

– large cracks that followed the deposited weld metal fusion line

– the cracks appeared to have grown after fabrication

– small cracks on the manway nozzle

54February 2006

Tasks Once the Cracks Were Identified

• Fully characterize and identify main cracks and other deficiencies.

• Determine fracture mode.• Perform finite element and fitness for

service evaluations to determine the major stresses that contributed to the failure.

• Determine remaining life.• Develop repair and replacement plans.

55February 2006

Extensive Nozzle Thickness Losses

1. The nozzles of removed heads had extensive thickness losses, as expected.

56February 2006

2. The head manhole to head joint had large cracks that followed the deposited weld metal fusion line between the deposited weld metal and the head.

CRACKS

Head Manhole to Head Joint Had Large Cracks

57February 2006

Morphology of Head Manhole to Head Joint Cracks

3. The cracks appeared to have grown after fabrication since multiple fracture morphologies were apparent:

Some fracture surfaces had dimples indicative of a plastic overload.

Some fracture surfaces had transgranular cleavage cracks indicative of a linear elastic fracture.

Other fracture surfaces had welding related slag remnants

58February 2006

Fracture Surfaces Were Covered with Heavy Non-Metallic Layers

Welding related slag remnants on fractures

59February 2006

The Welds Had Relatively Low Hardness Values

Often, welding related fractures with cleavage cracks can be the result of hydrogen embrittlement. However, these cracks typically develop in the heat affected zone in metal of hardness values significantly greater than those measured here.

60February 2006

What Caused the Cracks? Finite Element and Fitness for Service Analyses

The cracks are subsurface and consequently are subjected to relatively low stress intensity values. The thermal and mechanical stresses were evaluated for the following conditions:

•While the vessels are in-service•During hydrostatic tests.•During the most severe start-up and shutdown conditions.

61February 2006

Thermal and Mechanical Stresses

•These analyses found that the most severe stress conditions for the weld crack occur during hydrostatic tests at 1.3 times the service pressure. •The most severe start-up and shutdown conditions resulted in thermal stresses that were either compressive or significantly smaller than the hydrostatic test stresses.

62February 2006

Thermal and Mechanical Stresses … Cont’d

Even the hydrostatic stresses in the nozzle to head weld were relatively low; this suggested the following possibilities: 

•The residual welding stresses subsequent to post weld heat treatment were significant. Previously, they were considered insignificant since the weld had been stress relieved.•This section of the head is subjected to an unknown but significant source of stress.

63February 2006

Thermal and Mechanical Stresses … Cont’d

•The residual welding stresses subsequent to post weld heat treatment were assessed.•Considered the possibility that this section of the head is subjected to an unknown but significant source of stress.

64February 2006

Stress Intensity Factor, Residual Stress Only

65February 2006

Stress Intensity Factor, Residual Stress + 700psi

For a 700 psi hydrostatic

stress and Method 1

residual stresses, the

stress intensity (the

crack driving force) is

greater than 30 ksiin for

0.3WT to 0.7WT deep

cracks.

For cracks deeper than

0.7WT, the stress

intensity drops below

30 ksiin.

66February 2006

Stress Intensity Factor, Residual Stress + 700psi

Assuming that the fracture toughness of the HAZ is 30 ksiin, this predicts that brittle cracks can reach a 0.7WT depth and then arrest. This is consistent with the crack depths observed to date.

67February 2006

Stress Intensity Factor, Residual Stress + 900psiFor a 900 psi hydrostatic stress and Method 1 residual stresses, the stress intensity is greater than 30 ksiin for 0.2WT to 0.8WT deep cracks. For cracks deeper than 0.8WT, the stress intensity drops below 30 ksiin. This predicts that brittle cracks can reach a depth of 0.8WT and then arrest.

68February 2006

Why a Fracture Toughness of 30 Ksiin?

• A 198 ksiin was assessed after measuring the J Fracture Toughness for the head parent material. However, this value was not used for the critical crack length assessments since:

1. It was measured for the head parent material. The head parent material is tougher than the fusion zone where the crack grew.

2. The cleavage fractures noted are not consistent with a 198 ksiin toughness.

69February 2006

Why a fracture toughness of 30 ksiin?… Cont’d

Temperature versus Energy Absorption 

Location 212F Ft-

Lbs.

122FFt-

Lbs.

70FFt-

Lbs.

-29FFt-

Lbs.

Weld Average

- - 115 64

Head Average

53 37 23 7

70February 2006

The Remaining Life Cannot Be Assessed

• The material ahead of the crack could consist mainly of slag and/or non-fusion (as noted in some sections examined). This material would have negligible toughness in which case the weld would fracture through thickness. The fillet weld joining the repad to the manway could maintain the parts from separating from the vessel. However, this weld also has cracks, non-fusion and slag.

• The reactors may have been subjected to stresses greater than those considered here.

• The cracks will likely continue growing and linking around the circumference. The stress intensity values for the cracks to link around the manhole circumference are greater than those for the crack to propagate through the thickness.

71February 2006

What Next?

• Continue the manhole to head joint crack sizing inspections

• Continue the hydrostatic tests prior to start-up. If the manway to head weld fails, it would likely fail during the hydrostatic test. 

• Prioritize which heads should be replaced assessing which are the deeper cracks.