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

2-130 API RECOMMENDED PRACTICE 581
b) STEP 2—Has HTHA damage historically been observed in the component?
— If yes and component has not been replaced, assign susceptibility to Damage Observed and skip
to STEP 4.
— If yes and the component has been replaced in kind, assign susceptibility to High and skip to
STEP 4.
— If a component has been replaced with an upgrade in the materials of construction, the component
shall be re-evaluated in STEP 1 for the susceptibility based on the new material of construction.
c) STEP 3—Assign component susceptibility to HTHA as outlined below.
1) For Carbon and C-½ Mo Alloy Steels.
a) If the exposure temperature is >177 °C (350 °F) and the exposure hydrogen partial pressure is
>0.345 MPa (50 psia), assign a high susceptibility to HTHA.
b) If exposure temperature is ≤177 °C (350 °F) and the exposure hydrogen partial pressure is
≤0.345 MPa (50 psia), assign HTHA susceptibility to None.
2) For All Other Cr-Mo Low Alloy Steels.
a) If the exposure temperature is >177 °C (>350 °F) and exposure hydrogen partial pressure is
>0.345 MPa (>50 psia), calculate ∆T proximity to the API 941 curve using T and P H2 from
STEP 1. Assign HTHA susceptibility using Figure 19.1.
Note that the approach used in Figure 19.1 is an example guideline using 27.7 °C (50 °F)
increments. The 27.7 °C (50 °F) increments were used to represent relative changes in
susceptibility. It is the owner–user’s responsibility to customize the values to represent their
practice for determining HTHA susceptibility.
d) STEP 4—Determine the DF for HTHA,
HTHA
D f
, using Table 19.2 based on the susceptibility from
STEP 2 or STEP 3.
19.6.4 Consideration of Susceptibility
The time in service of component significantly affects susceptibility to HTHA and should be considered
during the HTHA review. Additionally, steels fabricated prior to 1970 may contain impurities and/or inclusions
that were introduced during fabrication. As these steels age, they may become more susceptible to HTHA for
similar process conditions compared to steels fabricated in 1980 or later. As a result, the owner–user may
choose more conservative guidelines by increasing the susceptibilities in Table 19.2.
19.7 Nomenclature
HTHA
D f
is the DF for HTHA
P H2
is the hydrogen partial pressure, MPa (psia)
T
is the operating temperature, °C (°F)