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

2-70 API RECOMMENDED PRACTICE 581
10.10 Figures
pH of Water
STEP 1: Determine the susceptibility for
cracking using Table 10.2
CO 3
PWHT?
Cracks
present?
Yes
High
Susceptibility
Cracks
Removed?
No
STEP 2: Determine the severity index from
Table 10.3.
Yes
No
FFS
STEP 3: Determine the time in-service,
age, since the last inspection.
STEP 4: Determine the number of inspections
and the corresponding inspection
effectiveness category for all past inspections
using Table 2.C.9.2.
STEP 5: Determine the base damage
factor for carbonate cracking using
Table 6.3.
STEP 6: Calculate the escalation in the
damage factor using Equation (2.29).
Figure 10.1—Determination of the ACSCC DF
11 SCC DF—Polythionic Acid Stress Corrosion Cracking (PASCC)
11.1 Scope
The DF calculation for components subject to PASCC is covered in this section.
11.2 Description of Damage
PA and sulfurous acid are major considerations in the petroleum-refining industry, particularly in catalytic
cracking, desulfurizer, hydrocracker, and catalytic reforming processes. These complex acids typically form
in sulfide containing deposits during shutdown (or ambient) conditions when the component is exposed to air
and moisture. The acid environment, combined with susceptible materials of construction in the sensitized or
as-welded condition, results in rapid intergranular corrosion and cracking. Preventive measures to reduce or
eliminate PASCC include flushing the component with alkaline or soda ash solution to neutralize sulfides
immediately after shutdown and exposure to air or purging with dry nitrogen during the shutdown to prevent
air exposure, according to recommended practices established by NACE (RP0170).