Page 745 - ICMFF9

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Figure 2: Analyzed PSA vessels.
The PSA equipment
(Figure 2)
is a carbon steel
SA-516 gr. 70,
vessel whose
mechanical properties for the operational temperature
(40°C)
are: Young modulus
(207,000 MPa), Poisson ratio (0.3), Yield stress (262 MPa) and Ultimate tensile stress
(483 MPa), obtained from the ASME and data sheets.
Toughness is not obtained from tables. Instead, it is necessary to measure or
calculate. This work employs the Lower Bound Method, described by the API 579
[7]
.
56
036 .0 exp 084 .35.36 ;
110 min
 
ref
IC
TT
m MPa
K
(5)
In Eq. 5, T is the evaluation temperature,
40ºC,
and T
ref
is equal to
-6 °C,
obtained
from the Impact Test Exemption curve (ASME Section VIII Division 1 – UCS-66) for
the SA-516 gr. 70 and a thickness of
23.5 mm
(head thickness). Replacing all values in
Eq. 5, we obtain
m MPa m MPa
K
IC
158 ;
110 min
and then we select
m MPa
K
IC
110
, the maximum allowable toughness according to this methodology.
Material constants for using with Paris law was obtained from BS-7910 for steel in a
marine atmosphere. Point 1:
K=0, m=3.42, C=1.72x10
-13
; Point 2:
K=748
2
3
/
mmN
,
m=1.11, C=7.48x10
-7
.
Finite element model and fatigue results
Stresses were calculated using a linear elastic analysis on an axisymmetric Finite
Element model, implanted in the software Ansys Mechanical R. 11.0 SP1. Stresses were
used for both crack assessment and fatigue crack growth evaluation.
Figure 3
shows
both the geometric model and some mesh details from the interest regions.
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