Remaining Life Calculator

Formula overview

Thicknesses in mm, ages and intervals in years, corrosion rate in mm/yr, remaining life in years.

API 579-1/ASME FFS-1, Part 4 (general) and Part 5 (local) metal loss:

  CR_long  = (t_initial − t_current) / age
  CR_short = (t_previous − t_current) / (t between readings)
  CR       = the governing (larger) of the two

  remaining life = (t_current − t_min) / CR

A MAWP-limited life is also reported, for the case where the pressure
the component can still hold — not t_min itself — sets the endpoint.

Worked example

Line installed at 8.0 mm, 10 years old, now measuring 6.8 mm.
Pressure design gives t_min = 4.5 mm.

CR_long        = (8.0 − 6.8) / 10 = 0.12 mm/yr
remaining life = (6.8 − 4.5) / 0.12 = 19.2 years

If the last two readings four years apart showed 7.6 → 6.8 mm, the
short-term rate is 0.20 mm/yr and governs instead:
remaining life = (6.8 − 4.5) / 0.20 = 11.5 years — corrosion has
accelerated, and the interval should follow the shorter answer.

Common mistakes

• Using the long-term rate when the short-term rate is higher. A rising short-term rate means conditions have changed, and averaging it away against a long benign history is exactly how a leak gets missed.
• Setting the next inspection at the full remaining life. Common practice is half the remaining life, so there is an opportunity to detect an accelerating rate before the wall reaches t_min.
• Applying general-metal-loss logic to a local pit. Part 4 averages thickness over a length; a discrete pit or groove is a Part 5 assessment, and treating it as general loss overstates the life significantly.

FAQ