2.2 Thickness Inspection Intervals (Half-Life & Visual Rules)

2.2 Thickness Inspection Intervals (Half-Life & Visual Rules)

Calculating when to inspect in-service piping is one of the most critical responsibilities under API 570. The code establishes two primary types of scheduled inspections for each class of piping: thickness measurements (which assess internal metal loss using ultrasonic testing [UT] or radiography [RT]) and external visual inspections (which assess external corrosion, paint failure, support alignment, and signs of leakage). The timing of these inspections is determined by combining the calculated remaining life of the piping system with the maximum allowable intervals defined by the piping class.

The Half-Remaining-Life Rule

The fundamental calculation for thickness inspection intervals is the half-remaining-life rule. This rule states that the interval between thickness measurements must not exceed half of the remaining life of the piping circuit.

To determine the thickness inspection interval, you must follow these steps:

1. Determine the Corrosion Rate (CR): Calculate the corrosion rate of the system. This can be the Long-Term Corrosion Rate (LTCR) or the Short-Term Corrosion Rate (STCR), typically using the higher of the two values to remain conservative.
2. Calculate the Remaining Life (RL): Use the remaining life formula: RL = (t_actual - t_minimum) / CR Where:
   • t_actual is the actual thickness measured during the most recent inspection (in inches or millimeters).
   • t_minimum is the minimum required thickness (retirement thickness) for the pipe section, determined by pressure design calculations (per ASME B31.3) or structural minimums, whichever is greater.
   • CR is the corrosion rate (in inches/year or millimeters/year).
3. Apply the Half-Life Division: Divide the calculated remaining life by 2: RL / 2.
4. Compare against the Maximum Class Interval: Compare the half-life value to the maximum thickness measurement interval permitted for that specific class of piping. The scheduled interval must be the lesser of these two values.

Maximum Permitted Inspection Intervals

API 570 establishes maximum time limits between inspections to prevent systems from operating too long without evaluation, even if they have a very low corrosion rate and a long remaining life.

The table below details the maximum intervals for each piping class:

Important Distinctions

• Class 1: Because of the high hazard level, both thickness measurements and external visual inspections must occur at least every 5 years (or half-life, whichever is shorter).
• Class 2: Thickness measurements can go up to 10 years, but external visual inspections must still be conducted every 5 years. This is because external damage mechanisms like Corrosion Under Insulation (CUI) or atmospheric corrosion can develop rapidly.
• Class 3: With lower hazard levels, both thickness measurements and external visual inspections have a maximum interval of 10 years.

Mathematical Worked Examples

Let's go through three distinct cases to see how these rules are applied on the API 570 exam.

Case A: Short Remaining Life (Class 1)

• Piping System: Class 1 hydrocarbon vapor line.
• Actual Thickness ($t_{\text{actual}}$): 0.280 inches.
• Minimum Required Thickness ($t_{\text{minimum}}$): 0.220 inches.
• Calculated Corrosion Rate ($CR$): 0.015 inches per year.
• Calculated Remaining Life (RL): RL = (0.280 - 0.220) / 0.015 = 0.060 / 0.015 = 4.0 years
• Applying Half-Life: RL / 2 = 4.0 / 2 = 2.0 years
• Comparison with Class Limit: The maximum thickness interval for Class 1 is 5 years. Since 2.0 years is less than 5 years, the thickness measurements must be scheduled in 2.0 years.
• External Visual: Scheduled in 4.0 years (or the standard Class 1 maximum of 5 years, but often aligned with thickness or kept at 5 years maximum).

Case B: Long Remaining Life (Class 2)

• Piping System: Class 2 crude oil line.
• Actual Thickness ($t_{\text{actual}}$): 0.380 inches.
• Minimum Required Thickness ($t_{\text{minimum}}$): 0.260 inches.
• Calculated Corrosion Rate ($CR$): 0.005 inches per year.
• Calculated Remaining Life (RL): RL = (0.380 - 0.260) / 0.005 = 0.120 / 0.005 = 24.0 years
• Applying Half-Life: RL / 2 = 24.0 / 2 = 12.0 years
• Comparison with Class Limit: The maximum thickness interval for Class 2 is 10 years. Since 12.0 years exceeds the 10-year limit, the maximum interval governs. The thickness measurements must be scheduled in 10.0 years.
• External Visual: Must be scheduled in 5.0 years (Class 2 maximum visual interval).

Case C: Threshold Case (Class 3)

• Piping System: Class 3 lubricating oil line.
• Actual Thickness ($t_{\text{actual}}$): 0.310 inches.
• Minimum Required Thickness ($t_{\text{minimum}}$): 0.250 inches.
• Calculated Corrosion Rate ($CR$): 0.006 inches per year.
• Calculated Remaining Life (RL): RL = (0.310 - 0.250) / 0.006 = 0.060 / 0.006 = 10.0 years
• Applying Half-Life: RL / 2 = 10.0 / 2 = 5.0 years
• Comparison with Class Limit: The maximum thickness interval for Class 3 is 10 years. Since 5.0 years is less than 10 years, the half-life governs. The thickness measurements must be scheduled in 5.0 years.
• External Visual: Scheduled at the Class 3 maximum of 10.0 years (though good practice may align it with the 5.0-year thickness measurement).

Exam Traps & Tips

• The "Lesser of" Rule: Always select the lesser of the calculated half-life ($RL/2$) and the maximum class interval. The exam will frequently offer the full remaining life or the half-life without checking the class limit as distractor options.
• Remaining Life vs. Inspection Interval: Read the question carefully. Is it asking for the remaining life of the pipe (which is $RL$), or is it asking for the next thickness measurement inspection interval (which is the smaller of $RL/2$ and the class limit)?
• External vs. Thickness Differences: For Class 2 piping, the thickness measurement interval can be up to 10 years, but the external visual inspection is capped at 5 years. Do not mix these up.
• Corrosion Rate Zero: If the corrosion rate is calculated as 0 (no metal loss detected), the remaining life is mathematically infinite. Under these conditions, the maximum class interval (5 years for Class 1, 10 years for Class 2 and 3) automatically governs the thickness measurement schedule.