API 510 vs ASME Section VIII: In-Service vs New Construction

Short answer: ASME BPVC Section VIII sets the rules for building a pressure vessel: design, materials, fabrication, examination, testing and stamping. API 510 sets the rules for keeping it in service: inspection, repair, alteration and rerating once the vessel is operating. API 510 reuses Section VIII's design formulas and examination rules where they fit. Where the two conflict for an in-service vessel, API 510 governs.

Engineers search "ASME Section VIII pressure vessels" and "API 510 vs ASME" for practical reasons. A purchase order or inspection contract cites both and someone needs to know which applies. A repair is being planned and the team is unsure whether it must meet the original construction code. Or a vessel is being rerated and nobody is sure which edition's stresses can be used. This guide compares the two documents across the vessel's life cycle and explains where they meet. It also covers the jurisdictional layer (National Board, state law, Canadian provinces) and what to put in a contract. Code references are to API 510, 11th edition (2022, with later errata), and the ASME BPVC 2025 edition. Confirm against the editions your jurisdiction and contracts invoke. Both codes are copyrighted, so this page summarises them and does not reproduce them.

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Two codes, two stages of a vessel's life

Section VIII is a construction code. API 510 is an in-service code. That one difference explains almost everything else.

ASME BPVC Section VIII, Rules for Construction of Pressure Vessels, has three divisions. Division 1 is the widely used design-by-rule code. Division 2 gives alternative rules with more rigorous design analysis and examination, and allows higher allowable stresses. Division 3 covers high-pressure vessels. Section VIII governs a vessel from design through fabrication, examination, the final pressure test, the manufacturer's data report and application of the ASME Certification Mark. ASME publishes the BPVC on a two-year cycle. The 2025 edition was released in July 2025. Construction contracts normally fix which edition applies.

API 510, Pressure Vessel Inspection Code: In-service Inspection, Rating, Repair, and Alteration, starts where construction ends. Its 11th edition applies to hydrocarbon and chemical process vessels that have been placed in service, unless a vessel is specifically excluded. Owners in other industries may apply it voluntarily. It covers vessels built to a construction code such as ASME. For Section VIII vessels it applies to Divisions 1 and 2, not Division 3. It also covers non-code vessels, jurisdiction-special vessels and nonstandard vessels. It also covers the pressure-relieving devices that protect those vessels. API 510 says plainly that it does not cover source inspection of newly fabricated vessels, and points readers to API RP 588 for that.

So the hand-off is clear. The fabricator, the Authorized Inspector and Section VIII deliver a stamped, documented vessel. The owner-operator, its API 510 inspector and API 510 then manage that vessel for the rest of its life.

The precedence rule: when the codes disagree

API 510 deals directly with how it relates to construction codes. It says the ASME BPVC and other construction codes are written for new construction. It also says most of their technical requirements for design, welding, NDE and materials can still be applied to inspection, rerating, repair and alteration of in-service vessels. If an in-service item cannot follow the construction code because that code is written for new construction, the requirements for design, material, fabrication and inspection must follow API 510 instead. And if API 510 and the construction code both cover an in-service vessel, or conflict, API 510 takes precedence.

API 510 also explains its own wording here. It uses the phrase "applicable requirements of the construction code" rather than "in accordance with the construction code." The difference matters in practice. A repair weld on a 30-year-old Division 1 vessel uses the construction code's welding, material and examination requirements where they apply. It does not have to recreate conditions that only existed in the fabrication shop, such as a full-vessel final hydrotest after every minor repair.

This is why a well-written in-service repair plan cites both documents. API 510 sets the framework: who authorizes the work, what counts as a repair or an alteration, when a pressure test is needed. Section VIII (and Sections V and IX, which it references) supplies the technical details: weld procedure qualification, NDE method and acceptance criteria, and design formulas.

Side-by-side comparison

How API 510 borrows Section VIII during service

Day to day, API 510 relies on construction-code engineering in at least four places.

Required thickness and MAWP. API 510 defines required thickness as the minimum thickness, without corrosion allowance, from the appropriate design-code calculations for pressure and structural loads. For a Division 1 vessel those are Section VIII's formulas for shells, heads and nozzle reinforcement. The MAWP for continued service is calculated with the applicable construction code. The calculation is allowed only if the vessel's essential details comply with that code: head, shell and nozzle reinforcement design, material specifications, allowable stresses, joint efficiencies, inspection acceptance criteria and cyclic-service requirements. The calculated MAWP cannot exceed the original MAWP unless the vessel is formally rerated. API 510 also requires the thickness used to be the measured thickness minus twice the corrosion expected before the next inspection. Section VIII has no equivalent, because it designs for a corrosion allowance that the owner then uses up in service.

Rerating. API 510 lets a vessel's design temperature, minimum design metal temperature or MAWP be changed only when certain conditions are met. Calculations by the manufacturer or an experienced owner-operator engineer must justify the change. The rerating must follow the vessel's construction code, or the latest edition if all essential details comply. Recent inspection records must support the new conditions. A pressure test or an NDE alternative is required. The engineer must accept the rerating. API 510 sets special rules for vessels designed before the 1999 addenda to Section VIII, which reduced the design margin and raised allowable stresses. Vessels built to editions before 1968 must keep their original design stress basis. This is where in-service work depends most on construction-code history.

Repairs and alterations. Weld procedures and welders are qualified to ASME Section IX. Materials and NDE follow the applicable construction-code requirements, with API 510 setting out where alternatives are allowed. These include repair welding methods that replace post-weld heat treatment under defined conditions. API 510 lists acceptable repair organizations. They include ASME certificate holders, holders of a National Board R stamp, owner-operators repairing their own vessels under the code, and others accepted by the jurisdiction.

Pressure testing. When a code hydrostatic test is needed after a repair or alteration, API 510 says the minimum test pressure should follow the construction code's rules. For Division 1 vessels it uses the construction-code basis tied to the allowable-stress edition. It is 130 percent of MAWP, corrected for temperature, for vessels using the 1999-addenda (or later) allowable stresses, and 150 percent of MAWP for vessels using the earlier stresses. That mirrors Section VIII's own test rule for the matching edition. API 510 adds in-service safeguards that do not come up in a fabrication shop. These include reviewing supports and foundations before a hydrotest, controlling metal temperature against brittle fracture, and protecting relief devices set below the test pressure.

A worked example, described qualitatively

Take a 1985-vintage carbon steel Division 1 separator in a gas plant. The nameplate MAWP is set by the shell. UT thickness surveys at the corrosion monitoring locations show general thinning on the lower shell course, still above required thickness, plus a nozzle that needs a weld repair.

Under API 510, the inspector uses the measured thicknesses and the governing corrosion rate to calculate remaining life. The inspector then checks that the vessel can keep its MAWP until the next inspection, using Section VIII's shell formula with the measured thickness less twice the expected corrosion. The nozzle repair is authorized by the inspector under API 510. It is welded to a Section IX-qualified procedure and examined by the NDE method and acceptance criteria the repair plan specifies, drawn from the construction code. The inspector then decides whether a pressure test is needed or whether NDE can replace it.

Now say the owner wants to raise the MAWP to debottleneck the plant. That is a rerating. Because the vessel was designed before the 1999 addenda, API 510's rules on using current allowable stresses apply. The engineer must justify the change, recent thickness data from an internal or on-stream inspection must support it, and a pressure test (or an accepted alternative) is required. If the rerating is accepted, an additional nameplate is attached. At no point does the vessel go back under Section VIII as a new vessel. Section VIII supplies the calculations, and API 510 manages the process.

NDE under each code

Both codes use ASME Section V for NDE methods, with different emphasis.

• At construction, Section VIII decides which welds are examined, by which method and to what extent (full or spot radiography, or UT in place of RT where permitted), and sets the acceptance criteria. Personnel are qualified under the manufacturer's written practice to the documents Section VIII references, such as ASNT SNT-TC-1A. The examinations are a one-time verification of fabrication quality. For a plain-English summary see ASME Section VIII Division 1 NDE requirements.
• In service, API 510 points NDE at damage mechanisms rather than fabrication flaws. The tools are UT thickness at CMLs, scanning UT and corrosion mapping for localized loss, and wet fluorescent MT or phased array UT for environmental cracking. TOFD and PAUT are used for crack sizing, and external visual and CUI screening also play a part. On-stream inspection can replace internal entry only under the conditions API 510 lists. The inspector evaluates and accepts NDE results under API 510.
• After an in-service repair, the two codes meet again. The repair plan usually calls for construction-code examination methods and acceptance criteria, applied under API 510's authority.

A practical consequence is that a "Section VIII NDE procedure" written for a fabrication shop may not suit in-service corrosion mapping or crack detection. In-service procedures need to cover surface condition, temperature, coatings and the specific damage mechanism. They should be reviewed by a Level III against both Section V and the owner's API 510 inspection plan. For Section V requirements see ASME BPVC Section V.

Where the National Board and jurisdictions fit

Neither API 510 nor Section VIII is law on its own. US states and cities, and Canadian provinces, decide which apply.

• Construction: most US jurisdictions require pressure vessels to be built to ASME Section VIII and often registered with the National Board. In Canada, CSA B51 adopts the ASME code for construction, and the vessel design is registered with the province (the Canadian Registration Number, or CRN).
• In service: many jurisdictions adopt the National Board Inspection Code (NBIC) for in-service inspection, repair and alteration. Some accept API 510 for refinery and petrochemical vessels, especially under owner-user inspection programmes. API 510 itself notes that other in-service codes exist (it names NB-23, the NBIC's earlier designation) and that it was written for refining and petrochemical needs. For the NBIC side see API 510 vs NBIC commissioned inspector.
• Federal overlay: at PSM-covered US facilities, OSHA 1910.119(j) requires inspection and testing to follow recognized and generally accepted good engineering practice. API 510 and Section VIII are the documents most facilities use to meet that requirement for vessels.
• Canada: in Alberta, ABSA's AB-506 sets in-service inspection and servicing requirements and references API 510 alongside provincial rules. Repairs and alterations follow provincial requirements. Other provinces (TSSA, TSBC, TSASK) have their own rules.

Always confirm with the authority having jurisdiction which in-service code applies, who may repair, and whether repairs need a jurisdictional inspector's acceptance.

Planning an in-service repair? Ask us to scope the NDE and procedure review.

Common mistakes when mixing the two codes

• Specifying "inspect to ASME VIII" for an operating vessel. Section VIII has no in-service inspection intervals or remaining-life rules. An in-service scope should name API 510 (or NBIC) and the owner's inspection plan.
• Asking for an ASME Authorized Inspector on in-service work. The AI role belongs to construction and, in some jurisdictions, to repairs. In-service inspection under API 510 needs an API-certified inspector. See ASME Authorized Inspector vs API inspector.
• Counting corrosion allowance as available metal. API 510's required thickness excludes corrosion allowance. Remaining life is calculated against required thickness, not nominal thickness minus allowance.
• Rerating with current allowable stresses without checking edition history. Vessels designed before the 1999 addenda, and especially before 1968, have specific limits.
• Hydrotesting by habit. API 510 does not require a pressure test after every repair. It may be the wrong choice where brittle fracture, support loads or stainless steel chloride exposure are concerns. NDE alternatives exist under defined conditions.
• Applying API 510 to a Division 3 vessel. API 510 explicitly excludes Division 3.

How Atlantis supports this

Atlantis NDT performs the NDE both codes depend on. That means construction-phase weld examination to ASME Section V procedures for fabricators and EPCs, and in-service NDE for owner-operators running API 510 programmes. In-service work includes UT thickness and CML surveys, corrosion mapping, PAUT and TOFD, MT, PT, RT through licensed radiography crews, and CUI screening. ASNT-certified technicians do the work under ASNT Level III oversight, including procedure review against Section V. For API 510 work, Atlantis delivers results to the owner's API-certified inspector, who stays the inspector of record. Atlantis is not an Authorized Inspection Agency and does not stamp, rerate or approve repairs. See pressure vessel inspection services and API 510 inspection support. Request a quote within 24 hours.

Frequently asked questions

What is ASME Section VIII for pressure vessels?

ASME BPVC Section VIII is the construction code for pressure vessels. It covers design, materials, fabrication, examination, testing and certification. Division 1 is design-by-rule, Division 2 gives alternative rules, and Division 3 covers high pressure.

What is the difference between API 510 and ASME Section VIII?

Section VIII governs building a vessel. API 510 governs inspecting, repairing, altering and rerating it after it enters service. API 510 uses Section VIII formulas and examination rules where they apply.

Which code takes precedence for an in-service vessel?

API 510. It says that where API 510 and the construction code both cover an in-service vessel or conflict, API 510 takes precedence.

Does API 510 apply to ASME Section VIII Division 2 and 3 vessels?

It applies to Divisions 1 and 2 but not to Division 3.

Do I need an ASME Authorized Inspector for an API 510 inspection?

Not for routine in-service inspection, which needs an API-certified inspector. Jurisdictions may require an Authorized or commissioned inspector for some repairs and alterations, so confirm locally.

Is a hydrotest required after every pressure vessel repair?

No. API 510 normally requires one after alterations and major repairs. For other repairs it is at the inspector's discretion, and NDE alternatives are allowed under defined conditions.

Can I rerate a vessel using the current ASME Section VIII allowable stresses?

Sometimes. API 510 allows it if the vessel's essential details comply. Special rules apply to vessels designed before the 1999 addenda, and pre-1968 vessels keep their original stress basis.

Is API 510 or NBIC used for in-service vessels?

That depends on the jurisdiction and the industry. Many jurisdictions adopt NBIC, and many accept API 510 for refinery and petrochemical vessels. Confirm with your chief boiler inspector or provincial regulator.

Does corrosion allowance count toward remaining life?

Remaining life is measured down to the required thickness, which excludes corrosion allowance. Any metal above required thickness counts, including what was originally corrosion allowance.

What NDE does Atlantis perform on pressure vessels?

UT thickness and CML surveys, corrosion mapping, PAUT, TOFD, MT, PT, RT and CUI screening, under Level III oversight. Results go to the owner's inspector.

Related: ASME Section VIII glossary entry, API 510 code guide and PRD inspection intervals under API 510.