Corrosion rate and remaining life calculator for piping
Long-term and short-term corrosion rates, remaining life and the next thickness inspection for one condition monitoring location (CML), worked out the way API 570 practice does. Four thickness readings and two spans of years from your inspection records.
The first reading on record at this CML, or the nominal thickness if there is none.
From that reading to today's.
The reading before today's. Leave blank if there is only one.
From the previous reading to today's.
Today's reading at the same marked spot.
The greater of the pressure-design and structural minimum thickness.
Piping class
Class 2: maximum 10 years between thickness inspections and 5 between external visual inspections.
How the calculation works
Corrosion rates and remaining life
- Long-term rate = (t initial - t actual) / years between them
- Short-term rate = (t previous - t actual) / years between them
- Remaining life = (t actual - t required) / governing rate
The calculator governs on the higher rate. Where the lower one reflects current operation better, an inspector can use it and record why; that is a judgement the readings alone cannot make.
The next thickness inspection
The lesser of half the remaining life and the maximum interval for the piping class: 5 years for Class 1 and 10 years for Classes 2 and 3, with external visual inspections at 5, 5 and 10 years. The owner sets Class 4. These maximums are as published summaries of the 5th edition of API 570 (2024) give them; check them against your copy of the code and your inspection plan.
In South Africa, regulation 11(1)(e) of the Pressure Equipment Regulations, 2009 requires piping to be inspected to the relevant in-service standard, which for process piping is usually API 570.
Corrosion rate and remaining life, explained
How do you calculate the corrosion rate of a pipe?
Take two thickness readings at the same CML and divide the loss of wall by the years between them. The long-term rate uses the first reading on record, the short-term rate the one before this; with 6.02 mm fifteen years ago and 5.10 mm today, the long-term rate is 0.061 mm a year.
How is remaining life calculated under API 570?
Remaining life is the wall left above the required thickness divided by the governing corrosion rate: (t actual - t required) / rate. At 5.10 mm, with 4.50 mm required and 0.07 mm a year, the remaining life is 8.6 years, and the next thickness inspection is due in 4.3 years, half of it.
Which corrosion rate governs, long-term or short-term?
Normally the higher one, because it is the conservative choice. An inspector can choose the other and record why, for example when a process change has slowed the corrosion. A short-term rate much higher than the long-term one usually means something in the process has changed and is worth finding.
The calculation is only as good as the readings behind it
Inspecable does not calculate corrosion rates; it keeps the readings you calculate them from. CML wall-thickness readings sit on the isometric for each line, a reading below the minimum you have set raises a corrective action, and the external visual inspection runs on an Android phone with a photo per item, offline.