Short answer
Remaining life is the wall thickness that you have above the minimum required thickness, divided by the corrosion rate. Example: 10.8 mm measured, 4.8 mm required and 0.40 mm/yr give 15.0 years. Use the higher of the short-term and long-term corrosion rates. Then limit the next inspection to the code maximum.
The formula
Remaining life (years) = (t_actual − t_required) / corrosion rate (mm/yr)
- t_actual: the thickness at the CML from the last reading, in mm.
- t_required: the minimum required thickness from the design code calculation, in mm. Reliatic shows it as t-min.
- Corrosion rate: the loss of wall thickness per year, in mm/yr.
API 510 and API 570 give this formula for vessels and piping. API 653 uses the same approach for tank shells.
Short-term and long-term corrosion rates
The long-term rate uses the first and the last reading. The short-term rate uses the last two readings. API 510 and API 570 ask the inspector to compare the two rates and to choose the rate that best reflects the current process.
Reliatic takes the higher of the two rates, which gives the shorter remaining life. If you enter an expected rate that is higher than both measured rates, Reliatic uses the expected rate.
Worked example
A CML on a pressure vessel has a nominal thickness of 12.7 mm and a t-min of 4.8 mm. The table shows three readings.
| Date | Thickness (mm) |
|---|---|
| 15 Mar 2015 | 12.7 |
| 15 Mar 2024 | 11.2 |
| 15 Mar 2025 | 10.8 |
- Long-term rate: (12.7 − 10.8) / 10 years = 0.19 mm/yr.
- Short-term rate: (11.2 − 10.8) / 1 year = 0.40 mm/yr.
- Rate used: 0.40 mm/yr, the higher of the two.
- Remaining life: (10.8 − 4.8) / 0.40 = 15.0 years from the last reading.
From remaining life to the next inspection
The inspection codes limit the next thickness inspection to half the remaining life and to a fixed maximum. Reliatic uses the lesser of the two values.
| Code | Applies to | Next inspection |
|---|---|---|
| API 510 §6.5.1.1 | Pressure vessels | Lesser of half the remaining life and 10 years |
| API 570 Table 2 | Piping, Class 1 | Lesser of half the remaining life and 5 years |
| API 570 Table 2 | Piping, Class 2 and 3 | Lesser of half the remaining life and 10 years |
| API 653 §6.3.3.2 | Tank shell, UT thickness | Lesser of half the remaining life and 15 years |
If the code of an asset is not known, Reliatic uses the shortest maximum, 5 years. If the piping class is not on record, Reliatic uses the Class 1 maximum.
API 510 lets a vessel with less than 4 years of remaining life use the full remaining life, up to 2 years. Reliatic keeps the stricter rule of half the remaining life.
In the example, half of 15.0 years is 7.5 years. On a pressure vessel, 7.5 years is less than 10 years, so the next inspection is due in September 2032. On Class 1 piping, the 5-year maximum governs: the next inspection is due in March 2030.
Data that the calculation needs
- Record the minimum required thickness for every CML. Without it, Reliatic blocks the calculation. It never reads an empty value as 0 mm.
- Record one reading per CML per day. If two readings have the same date, Reliatic keeps the later one.
- Check any reading that shows a thickness gain. A gain is often a measurement error or a reading at a different point.
- Reliatic treats a rate below 0.001 mm/yr as noise, not as wall loss. The code maximum then sets the next inspection.
Questions and answers
Which corrosion rate do you use for remaining life?
Compare the short-term and the long-term rate. The higher rate gives the shorter remaining life, which is the conservative answer. API 510 and API 570 ask the inspector to choose the rate that best reflects the current process.
What is t-min?
t-min is the minimum required thickness. You get it from the design code calculation, for example, ASME Section VIII for vessels or ASME B31.3 for piping. It does not include the corrosion allowance.
How often do you inspect a pressure vessel under API 510?
The interval for internal or on-stream inspection is the lesser of half the remaining life and 10 years. API 510 sets a separate limit of 5 years for external inspection. An RBI assessment can set a different interval, under the rules of API 510.
What if a CML shows no wall loss?
A rate of zero gives no finite remaining life. The code maximum then sets the next inspection. Reliatic treats a rate below 0.001 mm/yr as noise.