Inspection intervals: half life, RBI, and the cap

An inspection interval is a promise about the future built from three numbers about the past. The measured thickness says where the wall is. The minimum required thickness says where it must stop. The corrosion rate says how fast it is getting there. Divide the difference by the rate and you have a remaining life. Halve it, compare it to the code's fixed maximum, take the lesser, and you have the interval. Every step is well defined by API 510, API 570 and API 653, and every step is a place where the record can lose the reason for the number. This paper covers the inspection interval calculation under the three standards, what RBI can change and cannot, and what the interval record must carry so that the next inspector understands why the date is what it is.

How is remaining life calculated from thickness data?

Remaining life is the time until the wall reaches its minimum required thickness at the current corrosion rate. The formula is the same in API 510, API 570 and API 653:

remaining life = (t actual minus t required) divided by corrosion rate

where t actual is the latest measured thickness at the CML, t required is the minimum required thickness for that component, and the corrosion rate is in thickness per year.

The corrosion rate is where judgment enters. API 510 and API 570 define two rates:

• Long-term rate: the initial thickness minus the current thickness, divided by the years between them. It smooths out measurement noise and describes the average life of the component.
• Short-term rate: the previous thickness minus the current thickness, divided by the years between the two most recent readings. It responds to a change in process conditions or a new mechanism.

The standards direct the inspector to use the rate that best represents current conditions, and many owner-user programs take the higher of the two as the governing rate by policy. A third figure, the maximum observed rate across the CMLs of a component or circuit, is often carried as well because it identifies the location that will govern the interval.

The consequences for the record are direct. Every remaining life must reference the CML, the two or three readings it was computed from, the rate selected and the reason for selecting it, and the tMin with the method that produced it. A remaining life with none of these is a date without a basis.

For new equipment or new service where no history exists, the standards allow the rate to be estimated from similar service, published data, or a short-interval initial survey. The record must flag the estimate as an estimate and record when actual data replaced it.

What are the API 510 interval rules for pressure vessels?

API 510 sets two inspection types with two limits each.

Internal or on-stream inspection. The interval shall not exceed the lesser of half the remaining life and ten years. Where remaining life is less than four years, the interval may be the full remaining life up to a maximum of two years. On-stream inspection may substitute for internal inspection when the conditions in the standard are met (general corrosion rate known and low, no environmental cracking concern, adequate external UT access, and others).

External inspection. The interval shall not exceed the lesser of five years and the required internal or on-stream interval. External inspections are visual and look for insulation damage, supports, external corrosion, and leaks.

Thickness measurement intervals follow the internal or on-stream interval. Pressure relief devices have their own interval rules based on service history.

RBI. API 510 permits an RBI assessment conducted under API 580 to increase or decrease the intervals above, including exceeding the ten-year internal limit, subject to the assessment being documented and reassessed at intervals not exceeding ten years or on process changes.

The record for a vessel interval therefore needs the governing remaining life, the half-life value, the ten-year cap, and which of the two produced the interval. If RBI is applied, it needs the RBI assessment reference and its own review date. An interval that reads "10 years" with no indication whether it hit the cap or fell short of it does not tell the next inspector how much margin remains.

What are the API 570 interval rules for piping?

API 570 assigns piping to classes by consequence of failure and sets maximum intervals by class.

• Class 1 (highest consequence, such as flammable services that auto-refrigerate, hydrogen sulfide services, and anhydrous hydrogen chloride): thickness measurements at five years, external visual at five years.
• Class 2 (most process piping): thickness at ten years, external visual at five years.
• Class 3 (flammable but not significantly vaporizing, and services not harmful to human tissue): thickness at ten years, external visual at ten years.
• Class 4 (non-flammable, non-toxic, such as steam and water): thickness optional, external visual optional, at the owner-user's discretion.

In every class the thickness interval also shall not exceed half the remaining life, and the lesser of the two governs. Injection point circuits designated as such have a thickness interval that should not exceed three years. Soil-to-air interfaces, buried piping, and mixing points have their own rules.

The class is itself a decision in the record. A line reclassified from Class 2 to Class 1 changes its interval from ten years to five whether or not the corrosion rate moved. The record must carry the class, the basis for the class, and the date it was set, and a reclassification must appear as a change with a reason.

API 570 also permits RBI under API 580 to set intervals different from the class maximums, with the same documentation and reassessment requirements as API 510.

How does API 653 set tank inspection intervals?

Storage tanks under API 653 follow a different logic because the governing component is usually the bottom, which cannot be measured from outside while the tank is in service.

Internal inspection. The interval is set so the bottom will not corrode below its minimum remaining thickness before the next internal inspection, using the measured bottom thickness, the minimum thickness required for the bottom (which depends on whether there is a release prevention barrier), and the bottom corrosion rate. The standard caps the interval at twenty years where no RBI assessment is performed and thirty years where a documented RBI assessment supports it. Where the corrosion rate is unknown, the initial internal interval is limited to ten years.

External inspection. A visual external inspection at intervals not exceeding five years or a fraction of the remaining corrosion allowance, whichever is less. Ultrasonic thickness measurement of the shell at intervals set by the shell corrosion rate, with a fixed maximum of fifteen years where the rate is known and five years where it is not.

The shell courses follow the hydrostatic minimum thickness formula in API 653, so the interval record for a tank carries a different set of inputs from a vessel: tank diameter, liquid height, specific gravity, and joint efficiency for the shell; bottom thickness measurements, the release prevention barrier status, and the bottom rate for the internal interval.

Tanks are where the longest intervals live, and therefore where the record must be most careful. A thirty-year interval justified by RBI in 2026 will be examined by people who have not yet joined the company.

What can risk-based inspection change about the interval?

RBI under API 580 and API 581 replaces a rule-based interval with one derived from the probability and consequence of failure. API 581 provides the quantitative method; many programs use a semi-quantitative approach where probability and consequence are each scored on a scale and combined into a risk rank.

What RBI changes:

• It can extend intervals beyond the fixed maximums in API 510, API 570 and API 653, where the assessed risk supports it.
• It can shorten intervals where risk is high even though the half-life rule would allow longer.
• It can redirect inspection effort toward the damage mechanisms that dominate risk and away from ones that do not.
• It can choose the technique, not just the date, since effectiveness of inspection is part of the probability model.

What RBI does not change:

• It does not remove the requirement for a documented remaining life at the CML level. The half-life value should still be computed and recorded even when RBI overrides it, so the override is visible.
• It does not remove the reassessment obligation. The RBI assessment must be reviewed at set intervals and after process changes, equipment changes, or new damage mechanisms.
• It does not remove the owner-user's approval. The interval set by RBI is an owner-user decision with the assessment as its basis.

The interval record under RBI therefore carries more, not less: the probability and consequence scores and their basis, the risk rank, the resulting interval, the rule-based interval it replaced, the approver, and the reassessment date.

What must the interval record carry?

Across all three standards, an interval record that will survive an audit or a handover carries:

1. The CML or component whose remaining life governs, with its readings history.
2. tMin and the method that produced it (code, formula, inputs).
3. The corrosion rates computed (long-term, short-term, maximum), the governing rate selected, and the reason.
4. Remaining life from those inputs.
5. The half-life value and the code maximum for the equipment type and class, and which one produced the interval.
6. The class (piping) or the RBI rank and assessment reference, where applied.
7. The interval and the next due date it produces.
8. Any adjustment and its reason: shortened for a finding, extended under RBI, reset after a repair or a change of service.
9. Who set the interval and who approved it, with dates.
10. Change history so that a shortened or lengthened interval is visible as a change rather than a replacement.

Most programs have items 7 and 8. Fewer have 5 and 10. An interval with no visible cap and no visible history is a date that appears to have been chosen.

How intelliSPEC handles it

intelliINTEGRITY computes the interval from the record and keeps the reasons with it.

• Three rates, one governing. The corrosion summary computes the short-term, long-term, and maximum rates from the CML history and takes the maximum as governing, with remaining life as (t minus tMin) divided by that rate. All three rates remain visible on the CML, which carries 64 fields including the latest thickness and the tMin with its method.
• tMin by equipment type. Minimum thickness routes by classification to ASME B31.3 for piping, ASME VIII Division 1 for vessels, or API 653 for tanks, returning the design code and formula with the number, so the remaining life stands on a sourced tMin.
• The half-life cap applied. The API 510 and API 570 interval calculator caps the interval at half the remaining life, so an extension cannot happen silently.
• Semi-quantitative RBI. API 580 and API 581 semi-quantitative scoring combines probability and consequence into a rank from 1 to 5, stored against the equipment so the RBI basis for an interval is on the same record as the rule-based value.
• A change history that cannot be edited away. Every update to an interval or a CML carries the actor, tenant, request id, and a SHA-256 hash chain that a verify routine can recompute for auditors. Audit events are never auto-deleted.
• Records as executed. Completed records pin the configuration they ran with, so a 2026 interval record renders in 2046 exactly as it was set.

Definitions are in the glossary. For how interval programs sit inside a refinery, a chemical plant, or a terminal, see oil and gas, chemical, and pipelines and terminals.