API 653 inspection intervals are not fixed schedules. They are calculated values based on measured wall thickness, established corrosion rates, and the retirement thickness required for structural adequacy. Getting this calculation right determines whether a tank is inspected too frequently (unnecessary cost and downtime) or not frequently enough (undiscovered deterioration and integrity risk).
This article explains the calculation method for both external and internal inspection intervals, covers the data inputs required, and explains how risk-based inspection (RBI) as an alternative to the prescriptive formula changes the interval picture.
The Two Interval Types Under API 653
API 653 defines inspection intervals for two distinct inspection types. They are calculated independently and managed separately:
- -External inspection interval: The interval for inspections conducted while the tank is in service, without entry. Covers external shell condition, external coating, settlement, roof condition, and external attachments. API 653 sets a maximum of 5 years for external inspection regardless of corrosion rate or tank condition.
- -Internal inspection interval: The interval for inspections requiring tank decommissioning and entry. This is the comprehensive inspection covering floor plate, internal shell surfaces, internal coating, and the floor-to-shell junction. The internal interval is calculated from corrosion rate data and is subject to a prescriptive maximum.
Establishing the Corrosion Rate
The corrosion rate is the fundamental input to the interval calculation. It is expressed in millimetres per year (mm/yr) or mils per year (mpy) and represents the average wall loss rate at the location being assessed.
Corrosion rates are established from sequential thickness measurements at the same location. The rate is calculated as:
Corrosion rate = (Previous thickness - Current thickness) / Time between measurements (in years). A minimum of two measurements at the same location are required to establish a rate.
For new tanks or tanks with no prior inspection data, a conservative estimated rate must be applied. API 653 Section 7 provides guidance on using operating experience from similar service conditions when measured rates are not available.
The corrosion rate used in interval calculations should represent actual measured conditions at the inspection location. Shell plates in different courses or at different orientations can have different rates. The calculation should be performed using the local rate for each area being assessed, not a single average rate for the entire tank.
Calculating the Internal Inspection Interval
The internal inspection interval is calculated for each shell course individually using the following approach:
- -Step 1: Determine actual measured thickness: Use the UT thickness measurements from the most recent inspection. For the interval calculation, use the minimum measured thickness found in the course being assessed.
- -Step 2: Calculate minimum required thickness: Per API 653 Section 4, the minimum allowable thickness for each shell course is calculated from the tank's original design (or the actual product specific gravity if greater than design). This is the retirement thickness. Shell material below this thickness is structurally inadequate for the specified service.
- -Step 3: Calculate remaining corrosion allowance: Remaining CA = Minimum measured thickness - Minimum required thickness (retirement thickness). This is the total thickness the shell can lose before reaching the retirement limit.
- -Step 4: Apply the corrosion rate: Internal inspection interval = Remaining CA / Corrosion rate. This gives the time until the shell reaches retirement thickness at the measured corrosion rate.
- -Step 5: Apply prescriptive maximum: API 653 Table 6-1 sets the maximum internal inspection interval at 20 years (for tanks in active service with established corrosion rate data) or shorter for specific situations. The calculated interval cannot exceed this maximum.
The shortest interval from any assessed shell course governs the overall internal inspection interval for the tank. A tank with 15 corrosion-based years of remaining life on all shell courses but with one nozzle area showing 5 years of remaining life must be internally inspected within 5 years.
Floor Plate Interval Considerations
Floor plate interval calculations follow the same corrosion rate approach, but floor corrosion adds complexity because corrosion can occur on both sides of the floor: from the product side (internal corrosion) and from the foundation side (external corrosion under the floor plate).
Internal corrosion rates on floor plates are established from UT thickness surveys or floor plate scanning during internal inspections. External (under-floor) corrosion is more difficult to characterize because it depends on the condition of the foundation, the presence of water ingress, and the effectiveness of cathodic protection (if installed).
For floor plates, the minimum retirement thickness is 2.5mm (0.1 inch) per API 653. Localized pitting in floor plates is evaluated against additional pitting criteria in the standard.
Prescriptive Intervals vs RBI Intervals
| Approach | Basis | Typical Result | When Appropriate |
|---|---|---|---|
| Prescriptive (API 653 formula) | Corrosion rate + retirement thickness | 2 to 20 years for internal; 5 years for external | All tanks; required when RBI is not performed |
| Risk-Based Inspection (API 581) | Probability of failure x consequence of failure | May extend or compress prescriptive interval | When operator wants to optimize inspection schedule across a fleet of tanks |
RBI does not automatically extend intervals. On high-consequence tanks (large capacity, product that is difficult to contain, proximity to population), RBI may result in shorter intervals than the prescriptive formula calculates because the consequence factor elevates the overall risk. RBI is a tool for optimization, not a mechanism for postponing inspection.
Practical Example: Shell Course Interval Calculation
Consider the bottom shell course of a tank in crude oil service. The previous inspection measured a minimum thickness of 10.8mm. This inspection measures 10.1mm. The time between inspections was 7 years.
Corrosion rate = (10.8 - 10.1) / 7 = 0.10 mm/yr.
The minimum required thickness for the bottom course (calculated from the design basis) is 7.2mm.
Remaining CA = 10.1 - 7.2 = 2.9mm.
Calculated internal inspection interval = 2.9 / 0.10 = 29 years.
Applying the API 653 maximum of 20 years, the internal inspection interval is set at 20 years.
However, the external inspection interval remains at 5 years maximum. The next external inspection must occur within 5 years regardless of the internal interval.
The external and internal intervals are independent. The tank above would need an external inspection within 5 years even though the internal interval is 20 years.
Who Performs the Interval Calculation?
Inspection interval calculations under API 653 must be performed by or reviewed by an API 653 Authorized Inspector. The inspector is responsible for the accuracy of the thickness data, the validity of the corrosion rate established, and the correctness of the retirement thickness calculation.
For complex tanks with multiple courses, extensive pitting, or active damage mechanisms, the interval calculations can be extensive. Norman QC performs API 653 interval calculations as part of complete inspection reports or as standalone desk assessments when field data from client inspection teams is provided.
For API 653 inspection and interval assessment services in Alberta, or for remote engineering assessments using client-supplied UT data, see the contact page for scope and fee information.
FAQs
Can I use an RBI assessment to avoid internal inspection entirely?
No. API 653 does not allow RBI to eliminate internal inspection. It allows the interval to be adjusted (extended or compressed) based on risk analysis. Internal inspection must still be performed; RBI changes when it is required, not whether it is required.
What happens if inspection was overdue when it was finally performed?
If a tank's internal inspection was performed after the calculated interval expired, the inspection is overdue. The API 653 inspector documents this in the inspection report. The next internal inspection interval is calculated from the most recent inspection date. Operating a tank beyond its calculated inspection interval is a risk management and potentially a regulatory compliance issue depending on jurisdiction.
Does corrosion inhibitor use affect the interval calculation?
Corrosion inhibitor programs can reduce the actual corrosion rate in service. However, for the purpose of interval calculations, API 653 requires using measured corrosion rates from actual inspections. If the inhibitor program is recent and has not been validated by sequential thickness measurements showing a reduced rate, the previously measured rate must be used until new measurement data supports a revised rate.