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

2-36 API RECOMMENDED PRACTICE 581
4.8 Figures
STEP 1: Determine the corrosion rate,
C r,bm and C r,cm based on the material of
construction and process environment,
(see Annex B).
STEPS 2: Determine the time-in-service,
age tk, since the last inspection reading,
t rdi
STEP 3: For clad vessels calculate
the age rc using Equation (2.11).
STEP 4:
Is the component
a tank bottom?
Yes
Does the tank have a
release protection
barrier?
Yes
t min=0.254
cm (0.10 in.)
No
No
Determine S, E and t min
using the original
construction code or API
579-1/ASME FFS-1
t min= 0.127 cm
(0.05 in.)
STEP 5: Calculate the A rt
using Equation (2.12), (2.13), (2.14)
or (2.15).
STEP 6: Calculate the Flow Stress
using Equation (2.16).
STEPS 7: Calculate the Strength
Ratio parameter using Equation
(2.17) or (2.18).
STEP 8-10: Determine the number and
effectiveness category for inspections and
calculate Posterior Probabilities for inspection
using Equations (2.19) and (2.20).
STEPS 11-13: Calculate the DF
using Equations (2.21) – (2.22).
STEP 14:
Determine adjustment factors:
• Online Monitoring
• Injection/Mix Points
• Dead Legs
• Welded Construction
• Maintenance
• Settlement
Determine final damage
factor for thinning using
Equation (2.23).
Figure 4.1—Determination of the Thinning DF