Time-Based vs Risk-Based Inspection Intervals: How the Codes Allow Both

Short answer: API 510, API 570 and API 653 each set default, time-based limits, usually a fraction of remaining life capped at a fixed number of years, and each also lets the owner/user set intervals through a risk-based inspection (RBI) assessment instead. RBI is defined by API RP 580 (program elements) and API RP 581 (a quantitative methodology). RBI can lengthen or shorten intervals, but it must be documented, reviewed and periodically updated, and some regulators restrict it.

This page explains the rules as the codes frame them, shows how the two approaches compare on the same equipment, covers the regulatory limits (including PHMSA's exclusion of RBI intervals for breakout tanks), and answers the questions owners ask about adopting API RP 580 and RP 581. Interval decisions belong to the owner/user, the inspector and the engineer under the code and the jurisdiction; nothing here is a recommendation for a specific asset. Atlantis does not perform RBI studies; this is context for planning the NDE that either approach needs.

What "time-based" means in each code

Time-based intervals are calculated from measured condition and capped by a maximum. They are simple to audit and do not depend on a consequence model.

API 510 (pressure vessels). API 510 caps the time between internal or on-stream inspections at half the vessel's calculated remaining life or 10 years, whichever comes first, unless an RBI assessment justifies something different. When remaining life is very short, the code allows a short interval tied to that remaining life rather than the half-life rule; the exact threshold is in the code. External visual inspections, again unless RBI supports otherwise, are due at five years or at the internal/on-stream interval if that is shorter. The inspector or engineer sets the interval within the owner/user's quality assurance system. These limits appear in the current eleventh edition (October 2022, with errata); confirm wording and clause numbers in your licensed copy.

API 570 (piping). Without RBI, API 570 bases intervals on the measured corrosion rate and remaining life, the piping service class, jurisdictional requirements, and the judgment of the inspector or piping engineer. Thickness measurement intervals are limited to half the remaining life or the code's recommended maximum for the circuit's class, whichever is shorter. Class 1 circuits (the highest consequence of a leak) carry the shortest recommended maximums, five years for both thickness and external visual inspection; lower classes get longer maximums, Class 4 is optional, and injection points have their own, shorter thickness interval. The current edition is the fifth edition (February 2024, with Addendum 1); read its interval table directly rather than relying on older editions.

API 653 (aboveground tanks). API 653 requires a visual external inspection by an authorized inspector at no more than five years, or sooner if a calculation based on the shell's remaining corrosion allowance and corrosion rate (a quarter of the remaining shell corrosion life) gives a shorter period. External ultrasonic thickness monitoring, where used, has its own maximum interval tied to whether the shell corrosion rate is known. Internal inspection intervals are set so that the tank bottom will still meet the standard's minimum bottom thickness at the next inspection; without RBI the standard sets an outer ceiling (20 years for a tank without a release prevention barrier, longer where one is installed), and a 10-year interval applies when corrosion rates are unknown and no similar-service data exist. The current text is the fifth edition (2014) with addenda; Addendum 4 (July 2025) revised the tank-bottom thickness measurement text, so check the current addendum.

What "risk-based" means and how each code allows it

RBI combines the probability of failure (driven by damage mechanisms, rates, and how effective past inspections were at finding them) with the consequence of failure (fluid, inventory, location, people, environment, business impact). The output is an inspection plan: methods, extent, and an interval or due date.

API 510 lets the owner/user use an RBI assessment to set internal, on-stream and external intervals, including intervals beyond the 10-year, half-remaining-life and five-year external limits. When an RBI-based internal or on-stream interval runs past the 10-year cap, the code requires the RBI assessment itself to be reviewed and approved by the engineer and the inspector on a fixed maximum cycle (10 years in the editions we reviewed), and sooner if process, equipment or consequence changes warrant it. Confirm the review requirement in the current edition.

API 570 allows an RBI analysis performed in line with API RP 580 to set inspection intervals or due dates and the type and extent of inspection. The analysis must assess both probability and consequence of failure, and API 570 points to API RP 581 as a methodology consistent with RP 580. In the 2016 fourth edition, an RBI assessment used to set intervals had to be updated after each inspection, on a maximum 10-year cycle, and after process or hardware changes, with approval by qualified personnel and the inspector. Check the 2024 fifth edition for current wording.

API 653 offers an RBI-based alternative for setting the internal inspection interval. The RBI result can lengthen or shorten the interval that the corrosion-rate method would give, including the outer ceiling. The assessment has to be approved by an authorized inspector together with an engineer experienced in tank design, foundations and corrosion, and re-reviewed periodically (the editions we reviewed set a 10-year maximum) or sooner if service changes. Confirm against the current edition and addenda.

API RP 580 and RP 581. API RP 580, "Elements of a Risk-Based Inspection Program", sets the minimum and recommended elements of an RBI program; its fourth edition was published in August 2023. API RP 581, "Risk-Based Inspection Methodology", provides calculation procedures for probability and consequence of failure for fixed equipment, piping, tanks, pressure relief devices and heat exchanger bundles; its fourth edition was published in January 2025. RP 580 tells you what a credible program must contain; RP 581 is one detailed way to do the calculations.

Side-by-side: time-based vs risk-based

A worked example, described qualitatively

Take two vessels in the same unit. Vessel A is a carbon steel knockout drum in a clean, dry hydrocarbon service with a low, stable corrosion rate and a long remaining life. Under time-based rules its internal interval is capped at 10 years even though half its remaining life is far longer. An RBI assessment that shows low probability of failure (no credible cracking mechanism, a good thickness history from effective on-stream UT, process conditions held inside integrity operating windows) and moderate consequence could justify going past 10 years, with the assessment reviewed and approved by the engineer and inspector at no more than 10-year intervals.

Vessel B sits in wet H2S service. Its general corrosion rate is low too, so the time-based calculation might give the full 10 years. An RBI assessment, however, would weigh the cracking susceptibility, the low effectiveness of thickness readings at finding HIC or SOHIC, and the toxic consequence of a release. The likely outcome is a shorter interval, or the same interval with a different method set, such as wet fluorescent magnetic particle or shear-wave and phased array examination of welds and nozzles, rather than more thickness readings.

The lesson is that RBI is not a tool to stretch intervals. It moves inspection effort toward where risk is, which can lengthen some intervals and shorten others. Both vessels still need NDE whose effectiveness matches the damage mechanism.

Regulatory and jurisdictional overlay

PHMSA breakout tanks. 49 CFR 195.432(b) requires operators to inspect atmospheric and low-pressure steel aboveground breakout tanks according to API Std 653, but excludes section 6.4.3 (the alternative internal inspection interval). The rule states that the risk-based procedures in that section may not be used to determine intervals, and operators who had set intervals that way had to re-establish them using the corrosion-rate method of section 6.4.2. So a terminal with both breakout and non-jurisdictional tanks may run two interval regimes on similar equipment.

OSHA PSM. 29 CFR 1910.119(j)(4) requires inspection and testing that follow recognized and generally accepted good engineering practices, with frequencies consistent with manufacturers' recommendations and good engineering practice, and more frequent if prior operating experience shows it is needed. An RBI program built to API 580 can be part of RAGAGEP, but the owner must be able to show the assessment is documented, current and followed.

State boiler and pressure vessel laws. Some jurisdictions set their own inspection frequencies or require approval of owner-user programs before code intervals or RBI intervals apply. Confirm with your jurisdiction.

Canada. In Alberta, ABSA publishes AB-505, risk-based inspection requirements for pressure equipment, alongside AB-506 for owner-user inspection and servicing; RBI-based intervals in an Alberta owner-user program must meet those ABSA documents. Other provinces regulate through their own authorities, such as TSSA in Ontario. Confirm requirements with the provincial regulator.

Is adopting RP 580 and RP 581 worth it?

An RBI program is an engineering assessment the owner/user commissions and owns, carried out by personnel qualified under API RP 580, with the inspector and engineer approving its outputs. The points below are context for that owner decision, not a service offer.

Owners who already hold API 510 and API 570 often ask whether adding RP 580 and RP 581 to support a formal RBI program is worthwhile if they are not running quantitative RBI software. The codes give a way to think about it.

RP 580 is the governing reference if you set intervals by RBI under API 510 or 570: API 570 explicitly ties RBI to API 580 and requires the analysis to include its planning elements. If you intend to exceed any time-based cap, an API 580-conformant program is the route the codes describe, and a qualitative or semi-quantitative program can meet RP 580 without RP 581 software. RP 581 matters if you want a quantitative, reproducible methodology across many assets; it is a detailed calculation framework, and implementing it by hand at scale is impractical, which is why most users apply it through software. If your equipment is mostly well inside the time-based caps and your goal is better targeting rather than longer intervals, a damage mechanism review and stronger inspection plans under API 510/570 may give most of the benefit.

On the cost of the documents themselves (for example whether a subscription bundle is cheaper than individual purchases), API and its distributors publish current pricing; compare their quotes directly, because bundles and single-copy prices change. This page does not track publication prices.

What either approach needs from NDE

Both approaches depend on inspection data, and RBI depends on it more, because inspection effectiveness feeds the probability calculation. A thickness survey with the wrong coverage gives a low-effectiveness credit regardless of how many readings it has. Practical points:

• Thinning. UT thickness at well-placed CMLs is the base; automated or phased array corrosion mapping gives area coverage where corrosion is localized.
• Cracking. Thickness readings do not find cracks. Wet H2S, amine, caustic and chloride cracking need surface methods (WFMT, PT) and volumetric methods (shear-wave UT, PAUT, TOFD) on welds and high-stress areas.
• CUI. Screening methods such as profile radiography, pulsed eddy current or guided wave narrow down where to strip insulation for direct measurement.
• Tank bottoms. MFL floor scanning with UT prove-up supports the bottom thickness projections both the corrosion-rate and RBI methods in API 653 rely on.
• Records. Calibration, procedure, coverage maps and examiner certification must be traceable so the inspector can credit the inspection in either regime.

Common mistakes

• Treating RBI as an interval extension tool and ignoring its outputs that shorten intervals.
• Letting the RBI assessment age past its required review, which leaves intervals unsupported.
• Crediting thickness surveys as effective for cracking mechanisms.
• Applying API 653 RBI intervals to PHMSA-jurisdictional breakout tanks.
• Mixing editions: interval tables and clause numbers differ between editions; work from your licensed current copy.
• Assuming the software output is the decision; the codes require review and approval by qualified people, including the inspector.

How Atlantis supports this

Atlantis NDT does not perform RBI or fitness-for-service assessments and does not set intervals. We perform the NDE that time-based and risk-based plans call for: UT thickness and CML surveys, corrosion mapping, PAUT, TOFD, WFMT/MT, PT, tank-floor MFL and guided-wave screening, by ASNT-certified technicians under ASNT Level III oversight. Results, coverage and examiner records go to the owner's API-certified inspector and engineers, who stay responsible for the decisions. See inspection services, pressure vessel inspection, piping circuit and CML inspection and corrosion mapping, or send us your inspection plan for a quote within 24 hours.

Related: inspection effectiveness categories, remaining life and next inspection date, integrity operating windows (API 584), RBI explained: API 580 vs API 581, how corrosion rate drives inspection intervals, API 653 tank inspection intervals explained.

FAQ

What is API 581?

API RP 581, "Risk-Based Inspection Methodology", provides quantitative procedures for calculating probability and consequence of failure and building inspection plans for fixed equipment, piping, tanks, pressure relief devices and exchanger bundles. The fourth edition was published in January 2025.

What is API 580?

API RP 580, "Elements of a Risk-Based Inspection Program", sets the minimum and recommended elements for developing, implementing and maintaining an RBI program. The fourth edition was published in August 2023.

We hold API 510 and API 570 already. Is it worth adding RP 580 and RP 581 if we're not running quantitative RBI software?

RP 580 is the reference API 570 ties RBI to, so it matters if you plan to set intervals by RBI, including qualitative programs. RP 581 is mainly valuable when you run a quantitative methodology, usually through software. If you stay within time-based limits, a stronger damage-mechanism-based plan may deliver most of the benefit.

Can RBI extend an API 510 internal inspection beyond 10 years?

API 510 allows an RBI assessment to justify going beyond the 10-year and half-remaining-life limits, provided the RBI assessment is itself reviewed and approved by the engineer and inspector on the code's required cycle, and sooner if conditions change. Confirm the current edition's wording.

Can RBI shorten intervals?

Yes. API 653 says an RBI assessment may lengthen or shorten the interval, and the same logic applies under API 510 and 570. High-consequence or cracking-prone equipment often gets shorter intervals or more effective methods.

Can I use RBI intervals for breakout tanks?

Not for PHMSA-regulated breakout tanks. 49 CFR 195.432(b) excludes API 653 section 6.4.3 and requires intervals from the corrosion-rate method.

What is API RBI software?

Software that implements RBI calculations, commonly following API RP 581. It is a calculation aid; the codes still require documented assessments reviewed and approved by qualified personnel and the inspector.

How often must an RBI assessment be updated?

The editions we reviewed required updates after each inspection and at least every 10 years (API 570, 2016 edition), re-review at no more than 10 years (API 653), and engineer and inspector review on a 10-year maximum cycle when an API 510 interval exceeds 10 years. Check the current edition of each code.

Is time-based inspection still acceptable under OSHA PSM?

Yes. PSM requires RAGAGEP-based inspection at appropriate frequencies; the time-based rules in API 510, 570 and 653 are widely used as RAGAGEP.

Does Alberta allow RBI?

ABSA publishes AB-505 with risk-based inspection requirements for pressure equipment. Programs must meet ABSA's documents; confirm details with ABSA.

Need inspection data that supports your chosen interval method? Request a quote for NDE or ask about method selection by damage mechanism.