H2Tech - Q3 2022 - 32

MAINTENANCE AND RELIABILITY
Material properties. The tensile properties
and fracture toughness for the vessel
material (SA516 Gr. 70) used for the FFS
assessment are summarized in TABLE 6.
FAD assessment results. The cracks
found in each vessel have been assessed
using an FAD. Selected FADs are shown
in FIGS. 6 and 7. The assessment revealed:
* Most of the cracks assessed for
vessels D01-D12 fell within the
FAD envelope; therefore, they are
acceptable for continued operation.
* The crack, D10-N02-2 (the crack
in the top nozzle weld in the D10
vessel) was outside the FAD
TABLE 5. Summary of the remaining lives of all vessels
Critical flaw size
Vessel
D01
D02
D03
D04
D05
D06
D07
D08
D09
D10
D11
D12
Crack ID
D01-H02-2
D02-N01-1
D03-N02-2
D04-H02-1
D05-N01-2
D06-N01-3
D07-N02-1
D08-N01-3
D09-H02-1
D10-N02-2
D11-N02-1
D12-N02-2
Depth, mm Length, mm Flaw type
36.12
32.6
29.7
31.5
28.9
29.9
28
31
39.8
34.2
24.2
29
36.17
51.4
74.4
63.4
72.3
52.4
49.8
72.3
71.7
34.6
55.4
49.2
Cycles
Embedded 37,753
Embedded 195,918
Surface
Surface
Surface
Surface
Remaining life
Days
377
171,266
134,316
191,203
138,942
Embedded 203,140
Surface
Surface
124,678
127,234
Embedded 30,232
Surface
Embedded 87,478
TABLE 6. Material properties assumed in the FFS assessment
Yield stress1
Location, weld ID
N01, N02
N01, N02
N01, N02
H01, H02, H03, V01, V02
H01, H02, H03, V01, V02
H01, H02, H03, V01, V02
1
Temperature,
°C
20
95
121
20
95
121
,
Mpa
260
260
260
260
260
260
124,029
1,959
1,712
1,343
1,912
1,389
2,030
1,246
1,272
302
1,240
874
envelope, indicating the current
crack size was unacceptable (FIG. 7).
* The crack D01-H02-2 (the crack
in the middle body weld in the
D1 vessel) was inside the FAD
envelope, but with a very small
margin, as shown in FIG. 6. This
implies a potential risk of this crack
being critical in the near future, even
if its current crack size is acceptable.
Yr
1
5.3
4.6
3.7
5.3
3.8
5.5
3.4
3.4
< 1
3.4
2.3
Refined FAD assessment. The crack
lengths used for the initial FAD assessments
were determined based on a conservative
assumption of equaling them to
the weld size. The FAD assessments for
the most critical cracks, D01-H02-2 (Vessel
D01) and D10-N02-2 (Vessel D10)
were refined by using the measured crack
lengths, which were provided after the
initial assessment.
The measured length for D01-H02-2
and D10-N02-2 were 16 mm and 12 mm,
respectively; however, the a/c (heightto-length)
ratio exceeded the valid range,
therefore the closest valid ratio of 0.97
was applied. The refined assessment revealed
that both cracks fell inside the FAD
envelope (as seen in FIG. 8), indicating
that both cracks were acceptable.
UTS1
Mpa
485
485
485
485
485
485
Lower bound values have been assumed as per ASME Sec. II Part D
TABLE 7. The most critical flaws identified for each PSA vessel
Vessel
D01
D02
D03
D04
D05
D06
D07
D08
D09
D10
D11
D12
Defect ID Crack type
D01-H02-2 Embedded
D02-N01-1 Embedded
Surface
Height,
mm
31.7
11.1
D03-N02-2
D04-H02-1 Embedded
D05-N01-2 Embedded
D06-N01-3 Embedded
D07-N02-1 Embedded
D08-N01-3 Embedded
D09-H02-1 Embedded
D10-N02-2 Embedded
D11-N02-1 Embedded
D12-N02-2 Embedded
32 Q3 2022 | H2-Tech.com
10.4
18
7
11.4
8
15.3
22.3
30.2
8.2
12.4
Length,
mm
40
45
45
40
45
45
45
45
40
45
45
45
Ligament,
mm
3.7
18.7
-
5.2
8.8
9.6
17.6
2.8
6.3
7.4
8.3
11.7
Crack location
close to
External surface
External surface
External surface
External surface
External surface
External surface
External surface
External surface
External surface
External surface
External surface
External surface
,
Fracture toughness,
Kmat
(N/mm3/2
3,225
3,276
3,276
3,225
3,276
3,276
)
REMAINING LIFE ASSESSMENT
Damage mechanism and crack
growth. The PSA vessels have been subjected
to a cyclic loading frequency of
100 loading cycles per day. This cyclic
loading operation was believed to be the
major driving force of the crack growth,
so a fatigue cracking mechanism was applied
in the remaining life assessment.
The remaining life is determined when
the crack continues to grow until it reaches
critical flaw size.
Multiple scenarios exist for how the
crack could grow to reach critical flaw
size:
* The initial flaw could grow from
being an embedded flaw, to
breaking the surface, then through
the wall before reaching its final
critical flaw size.
* The initial flaw could grow from
being an embedded flaw, to
through the wall to reach its final
critical flaw size.
* The initial flaw could grow from
being an embedded flaw, to breaking
the surface, which could then
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