API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

RISK-BASED INSPECTION METHODOLOGY, PART 2—PROBABILITY OF FAILURE METHODOLOGY 2-133
STEP 1: Determine the material of
construction, exposure temperature T
and the exposure hydrogen partial
pressure P H2 .
No, replaced with
upgraded material
STEP 2: Has HTHA damage
been observed in the
component?
Yes
Is the
component
Replaced in
Kind?
Yes, replaced
in kind
High
Susceptibility
No
No, not
replaced
Damage
Observed
STEP 3 1): Is the component
carbon or C-½ Mo steel?
Yes
STEP 3 1): Is the
component
operating > 177C (350F)
and H2pp > 0.345 MPa (50
psia)?
Yes
High
Susceptibility
No
No
STEP 3 2): Assign Susceptibility based on
operating condition and Figure 19.1
No
Susceptibility
STEP 4: Determine the damage factor for
HTHA, using Table 19.2.
20 Brittle Fracture DF
20.1 Scope
Figure 19.2—Determination of the HTHA DF
The DF calculation for ferritic components subject to brittle fracture due to low-temperature operation is
covered in this section. Low alloy steels subject to embrittlement at relatively high temperature are not part of
the scope in this section and are covered in Section 21.
20.2 Description of Damage
Brittle fracture due to low temperature operation or relatively low toughness is the sudden failure of a
structural component, usually initiated at a crack or defect. This is an unusual occurrence because design
stresses are normally low enough to prevent such an occurrence. However, some older equipment with thick
walls, equipment that might see low temperatures due to an upset, or equipment that has been modified
could be susceptible to varying degrees.