Managing Pressure Vessels with Known
Flaws
By Augusto Roveredo, Corrosion Project Manager, Sherritt Metals ,
and
Ana Benz, Specialty Services, IRISNDT
February 2006
1
What Will Be Presented?
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
February 2006
2
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.
February 2006
3
Leach Reactors- Background
The lead lining is covered by special corrosion
and erosion resistant brick layers.
A spiral wound metal gasket
Carbon Steel Flange
An overlay of Alloy 20
Carbon Steel nozzle neck
A 4" wide band of panel lead
around the nozzle opening replaces
the original HBL due to repairs
in this area over the years of service.
Panel lead lining which now
replaces the original HBL
Original HBL
Carbon Steel Repad
Titanium sleeve
Mechanical seal consisting of
lead wool packing between the
Titanium sleeve and the Carbon Brick.
This is covered with a fillet type seal
of AR-20 acid resistant mortar.
4-1/2" Carbon Brick on a 1/8" bed of AR-20 mortar
3" Acid brick on a 1/8" bed of AR-20 mortar
Typical Nozzle Detail of Protective Lining
February 2006
4
Results 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.
February 2006
5
Results 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.
February 2006
6
Based 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 0.040 inch
per year.
• Attempted to assess the possible growth rate
and life expectancy associated with the
possible head cracks.
February 2006
7
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.
February 2006
8
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.
February 2006
9
Welding 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.
February 2006
10
Welding 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.
February 2006
11
Welding Procedure Optimization
Weld Coupon C
Macro of Weld Coupon C
Very small HAZ,
indicative of
high cooling
rates, high
residual
stresses and
hardness values.
February 2006
12
Welding 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.
February 2006
13
Welding 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.
February 2006
14
Welding Procedure Optimization
Macro of Weld Coupon D
• Bigger HAZ,
indicative of
lower cooling
rates.
• Lower
hardness
values.
February 2006
15
Welding Procedure Optimization
Comparison of Hardness Values
Weld Coupon D
Weld Coupon C
226
196
196
222
254
177
217
216
219
226
251
168
238
257
230
260
266
226
226
226
x 184
x
269
165
248
226
234
187
158
217
226
15
171
210
193
212
212
171
222
February 2006
16
Welding 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.
February 2006
17
Replacing the Heads and Nozzles
– the Project Starts
• Preparation for an international project.
• Weld repairs.
• Weld final inspections and approval.
February 2006
18
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.
February 2006
19
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.
February 2006
20
Preparation:
Purchase of New Nozzles
• Additional nozzles
were purchased to
replace thinned
nozzles on the
Leach Train 1
Vessels A and C.
February 2006
21
Preparation:
The Brick Lined New Heads
February 2006
22
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.
February 2006
23
Preparation: 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.
February 2006
24
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)
February 2006
25
The Result of an Inadequate Post
Weld Heat Treatment
February 2006
26
The Result of an Inadequate Post
Weld Heat Treatment
February 2006
27
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.
February 2006
28
Execution:
Reactor Cut Line Being Prepared
• A cut line
platform was
designed,
fabricated
and installed
to ensure
safety for
the workers.
February 2006
29
Execution:
Radiograph Torch Cutting
• An oxygen and
acetylene
radiograph torch
assembly was tack
welded to the
existing shell
section.
February 2006
30
Execution:
Completed Torch Cut
February 2006
31
Execution:
Head Being Removed
February 2006
32
Execution:
Lead Being Removed by Torch
February 2006
33
Execution:
Bevel Being Cut by Torch
February 2006
34
Execution:
Bevel Prep Final Product
February 2006
35
Execution:
New Head Being Positioned
February 2006
36
Execution:
Installation of Heating Coils
February 2006
37
Execution:
Insulation Wrap Around Heating Coils
February 2006
38
Execution:
Stringer Beads Being Welded on Inside
Surface
February 2006
39
Execution: Dry Magnetic Particle
Examination of the Root Pass
February 2006
40
Execution:
Finished Internal Weld
February 2006
Finished External Weld
41
Execution:
External Weld Cap Ground Flush
February 2006
42
Execution:
Lead Panel of the New Joint
• Lead panel
joints were
inspected
with liquid
penetrant.
February 2006
43
Execution:
Nozzle Butter Pass
• Integral
nozzles were
manufactured.
• Welding trials
were
performed on
the removed
heads.
February 2006
44
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.
February 2006
45
Execution: Replacement Nozzle Fit-up
February 2006
46
Execution: Nozzle Completed Outside
Weld
February 2006
47
Execution:
Nozzle Completed Inside Weld
February 2006
48
Weld 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.
February 2006
49
Weld Final Inspections and Approval:
Radiography of the New Weld
February 2006
In Situ Metallography and Hardness Tests
50
Execution: 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.
February 2006
51
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.
February 2006
52
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
February 2006
53
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.
February 2006
54
Extensive Nozzle Thickness Losses
1. The nozzles of removed heads had extensive thickness losses, as
expected.
February 2006
55
Head Manhole to Head Joint Had Large
Cracks
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.
C
R
A
C
K
S
February 2006
56
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.
February 2006
Some fracture surfaces
had transgranular cleavage
cracks indicative of a linear
elastic fracture.
Other fracture
surfaces had welding
related slag remnants
57
Fracture Surfaces Were Covered with
Heavy Non-Metallic Layers
Welding related
slag remnants on
fractures
February 2006
58
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.
February 2006
59
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.
February 2006
60
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.
February 2006
61
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.
February 2006
62
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.
February 2006
63
Stress Intensity Factor, Residual Stress Only
February 2006
64
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 ksiin for
0.3WT to 0.7WT deep
cracks.
For cracks deeper than
0.7WT, the stress
intensity drops below
30 ksiin.
February 2006
65
Stress Intensity Factor, Residual Stress + 700psi
Assuming that the
fracture toughness of
the HAZ is 30 ksiin,
this predicts that brittle
cracks can reach a
0.7WT depth and then
arrest. This is
consistent with the
crack depths observed
to date.
February 2006
66
Stress Intensity Factor, Residual Stress + 900psi
For a 900 psi
hydrostatic stress
and Method 1
residual stresses,
the stress intensity
is greater than 30
ksiin for 0.2WT to
0.8WT deep cracks.
For cracks deeper
than 0.8WT, the
stress intensity
drops below 30
ksiin. This predicts
that brittle cracks
can reach a depth of
0.8WT and then
arrest.
February 2006
67
Why a Fracture Toughness of 30 Ksiin?
•
A 198 ksiin 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 ksiin toughness.
February 2006
68
Why a fracture toughness of 30 ksiin?… Cont’d
Temperature versus Energy Absorption
Location
Weld
Average
Head
Average
February 2006
212F
FtLbs.
-
122F
FtLbs.
-
70F
FtLbs.
115
-29F
FtLbs.
64
53
37
23
7
69
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.
February 2006
70
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.
February 2006
71