In API 570, how is the short-term corrosion rate calculated?

In API 570, how is the short-term corrosion rate calculated?

• A. Readings from the most recent inspection shall be used.
• B. Calculated using a fixed engineering value.
• C. Readings from the two most recent inspections shall be used.
• D. Average of the last three inspections.

Answer: (C)

The main idea is that short-term corrosion rate is determined from the latest interval between inspections. You take the change in wall thickness between the two most recent readings and divide by the time elapsed between those inspections. This gives the rate at which metal is currently corroding, reflecting the most recent conditions rather than older data. Using only the latest reading doesn’t provide a rate, a fixed engineering value isn’t site-specific, and averaging three inspections would smooth out recent changes, masking a developing issue. The two-point approach captures the immediate trend, which is exactly what short-term assessment is meant to reflect.

The main idea is that short-term corrosion rate is determined from the latest interval between inspections. You take the change in wall thickness between the two most recent readings and divide by the time elapsed between those inspections. This gives the rate at which metal is currently corroding, reflecting the most recent conditions rather than older data. Using only the latest reading doesn’t provide a rate, a fixed engineering value isn’t site-specific, and averaging three inspections would smooth out recent changes, masking a developing issue. The two-point approach captures the immediate trend, which is exactly what short-term assessment is meant to reflect.