Presentation on theme: "Managing Pressure Vessels with Known Flaws"— Presentation transcript:
1Managing Pressure Vessels with Known Flaws By Augusto Roveredo, Corrosion Project Manager, Sherritt Metals , and Ana Benz, Specialty Services, IRISNDT
2What Will Be Presented? How the Heads Were Replaced Information on the:Intricate Design and Construction of the Vessels.Inspections Performed and their Findings.Deciding factors for choosing whether to replace the heads.How a Temper Bead Welding Procedure Was Developed Based on the National Board Inspection Code RD-1000.Head and Nozzle Replacements.Project Milestones.What Was Found Inspecting the Removed Heads
3Leach 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.
4Leach Reactors- Background The lead lining is covered by special corrosion and erosion resistant brick layers.
5Results of NDE Inspections prior to Head Replacements 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.
6Results of NDE Inspections … Cont’d 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.
7Based on the NDE Inspections: 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 inch per year.Attempted to assess the possible growth rate and life expectancy associated with the possible head cracks.
8Should 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.
9Welding 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.
10Welding Procedure Optimization 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.
11Welding Procedure Optimization Weld Coupon C 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.
12Welding Procedure Optimization Weld Coupon C Macro of Weld Coupon CVery small HAZ, indicative of high cooling rates, high residual stresses and hardness values.
13Welding Procedure Optimization Weld Coupon D 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.
14Welding Procedure Optimization: Weld Coupon D Continued… 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.
15Welding Procedure Optimization Macro of Weld Coupon D Bigger HAZ, indicative of lower cooling rates.Lower hardness values.
17Welding Procedure Optimization Conclusion 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.
18Replacing the Heads and Nozzles – the Project Starts Preparation for an international project.Weld repairs.Weld final inspections and approval.
19Preparation: 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.
20Preparation: 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.
21Preparation: Purchase of New Nozzles Additional nozzles were purchased to replace thinned nozzles on the Leach Train 1 Vessels A and C.
23Preparation: 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.
24Preparation: Showing the Welders the Importance of Their Contribution 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.
25Showing 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)
26The Result of an Inadequate Post Weld Heat Treatment
27The Result of an Inadequate Post Weld Heat Treatment
28Execution: 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.
29Execution: Reactor Cut Line Being Prepared A cut line platform was designed, fabricated and installed to ensure safety for the workers.
30Execution: Radiograph Torch Cutting An oxygen and acetylene radiograph torch assembly was tack welded to the existing shell section.
49Weld Final Inspections and Approval: ASME Compliance 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.
50Weld Final Inspections and Approval: Radiography of the New WeldIn Situ Metallography and Hardness Tests
51Execution: Final Steps 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.
52Project 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.
53Project 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 losseslarge cracks that followed the deposited weld metal fusion linethe cracks appeared to have grown after fabricationsmall cracks on the manway nozzle
54Tasks 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.
55Extensive Nozzle Thickness Losses The nozzles of removed heads had extensive thickness losses, as expected.
56Head Manhole to Head Joint Had Large Cracks 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
57Morphology of Head Manhole to Head Joint Cracks The cracks appeared to have grown after fabrication since multiple fracture morphologies were apparent:Some fracture surfaceshad transgranular cleavage cracks indicative of a linear elastic fracture.Other fracture surfaces had welding related slag remnantsSome fracture surfaces had dimples indicative of a plastic overload.
58Fracture Surfaces Were Covered with Heavy Non-Metallic Layers Welding related slag remnants on fractures
59The 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.
60What 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-serviceDuring hydrostatic tests.During the most severe start-up and shutdownconditions.
61Thermal and Mechanical Stresses These analyses found that the most severe stressconditions for the weld crack occur during hydrostatictests at 1.3 times the service pressure.The most severe start-up and shutdown conditionsresulted in thermal stresses that were eithercompressive or significantly smaller than thehydrostatic test stresses.
62Thermal 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 postweld heat treatment were significant. Previously,they were considered insignificant since the weldhad been stress relieved.This section of the head is subjected to anunknown but significant source of stress.
63Thermal and Mechanical Stresses … Cont’d The residual welding stresses subsequent to postweld heat treatment were assessed.Considered the possibility that this section of thehead is subjected to an unknown but significantsource of stress.
65Stress Intensity Factor, Residual Stress + 700psi For a 700 psi hydrostaticstress and Method 1residual stresses, thestress intensity (thecrack driving force) isgreater than 30 ksiin for0.3WT to 0.7WT deepcracks.For cracks deeper than0.7WT, the stressintensity drops below30 ksiin.
66Stress 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.
67Stress Intensity Factor, Residual Stress + 900psi For 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.
68Why 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:It was measured for the head parent material. The head parent material is tougher than the fusion zone where the crack grew.The cleavage fractures noted are not consistent with a 198 ksiin toughness.
69Why a fracture toughness of 30 ksiin?… Cont’d Temperature versus Energy AbsorptionLocation212FFt-Lbs.122F70F-29FWeld Average-11564Head Average5337237
70The 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.
71What 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.