ASME B31.3 Process Piping — Inspection Requirements Made Simple

ASME B31.3 after start-up: where API 570 takes over

ASME B31.3 governs process piping until it enters service: design, materials, fabrication, examination and the leak test. API 570 governs the same piping afterwards: in-service inspection, rating, repair, alteration and re-rating. The two codes hand off rather than compete. API 570 says it is not a substitute for original construction requirements, and in-service repairs follow the principles of B31.3 (or the original construction code) as far as practical, with API 570 covering what B31.3 cannot.

The examination and leak-test sections above cover B31.3 during construction. This section covers the questions operating plants ask once the line is running. Which document do you open? Can B31.3 radiography criteria be applied to old welds? Does B31.3 set a minimum distance between welds? What does "B31.3 compliant" mean on a component datasheet? How do OSHA PSM and Canadian regulators fit in? ASME lists the 2024 edition of B31.3 as current, and API 570 is in its fifth edition (February 2024). Requirements below are paraphrased, not quoted. Confirm wording against licensed current copies.

The lifecycle handoff: who governs what

Design, fabrication, construction examination and the initial leak test are B31.3. Inspection planning, thickness monitoring, intervals, fitness decisions, repairs, alterations and re-rating of in-service piping are API 570, which points back to B31.3 principles for the technical details of new welds and replacement components.

A useful rule of thumb: if the question is about how piping should be built, B31.3 answers it. If the question is about how long it can keep running, how often to look at it, or what to do when you find damage, API 570 answers it, borrowing B31.3's engineering where needed.

Repairs, alterations and re-rating: "the principles of B31.3"

API 570 tells you to follow ASME B31.3, or the code the piping was built to, to the extent practical for in-service repairs and alterations. Where B31.3 cannot be followed because it was written for new construction, the inspector or piping engineer uses API 570.

This phrase does a lot of work. It means a replacement spool should meet B31.3 material, welding and examination expectations, and a new branch connection should be reinforced to B31.3 rules. It also accepts that some things are impossible in an operating plant: you may not be able to hydrotest a tie-in weld without taking a whole unit down, or you may be welding onto pipe that is thinner than its original nominal wall. API 570 provides the in-service alternatives, such as when a pressure test can be replaced by other NDE on a closure weld and what engineering approval is needed, and these decisions belong to the owner/user's piping engineer and authorized piping inspector.

Re-rating works the same way. A line can be re-rated to a new pressure or temperature when calculations are performed to the code the piping was built to or the latest edition of the applicable code. The maximum allowable working pressure formula in API 570's annex uses the B31.3 pressure design equation with corroded thickness. The thickness inputs come from in-service inspection data, which is why reliable UT thickness readings matter so much.

Repair organisation, authorization and documentation requirements sit in API 570. The construction code does not know who your authorized inspector is; API 570 does.

The radiography trap: B31.3 acceptance criteria are not in-service criteria

B31.3's radiography acceptance criteria apply to a random sample of new welds to judge the probable quality of all welds in a lot. API 570 cautions that applying those workmanship criteria to in-service welds may not be appropriate. Flaws found in service should be evaluated for their actual significance.

This is the most common technical misunderstanding between the two codes. During construction, B31.3 requires a stated percentage of welds to be examined, and the acceptance criteria are workmanship standards. They show whether the welder and procedure are producing acceptable quality, and if a sample fails, more welds are examined. The examination extents by fluid service are summarised in the table earlier on this page.

That sampling logic has consequences. A plant that has been operating for twenty years has thousands of welds that were never radiographed. If an in-service radiograph of one of them shows a slag line that B31.3 would reject, the weld has not suddenly become dangerous. It has been there since construction and has carried pressure for twenty years. API 570 recognises this and directs that such findings be assessed for their effect on integrity, considering the service, any active damage mechanism and whether the indication is growing. Assessments of that kind may draw on fitness-for-service methods, an engineering evaluation the owner commissions.

The opposite mistake also happens. Some programmes treat a weld that "passed RT at construction" as immune to cracking in service. Construction RT says nothing about later environmental cracking such as wet H2S damage, chloride stress corrosion cracking or caustic cracking. In-service damage needs in-service methods: shear-wave UT, phased array, TOFD or wet fluorescent MT, chosen for the mechanism.

Fluid service categories vs API 570 piping classes

B31.3 classifies piping by fluid service (Normal, Category D, Category M, High Pressure, High Purity) to set construction rules. API 570 classifies in-service piping into Classes 1 to 4 by consequence of a leak to set inspection intervals. The two schemes overlap, but one does not convert directly into the other.

The construction category is fixed at design. API 570's scope list then decides whether the line is in the in-service programme at all. Process fluids, hydrocarbons, flammable and toxic services, hydrogen, fuel gas and flare systems, sour water, hazardous waste and cryogenic fluids are included. Several services are optional at the owner/user's discretion, including water, steam, steam condensate, boiler feed water and B31.3 Category D services. A Category D utility line built to B31.3 may or may not be in your API 570 programme, depending on what the owner decided and documented.

For piping in the programme, API 570 assigns a class. Class 1 covers the highest-consequence services, such as flammable services that can auto-refrigerate and cause brittle fracture. Class 2 covers most unit process piping. Class 3 covers flammable services that do not significantly vaporise and many off-site lines. Class 4 covers essentially nonflammable, nontoxic services. Each class has maximum intervals for thickness measurement and external visual inspection, and the thickness interval is also limited by remaining life. Owners may set intervals with RBI, which API 570 permits as an engineering programme they commission. Take the interval numbers from the current fifth edition, not from older copies circulating online. A Category M line is not automatically Class 1, and a Normal-service hydrocarbon line can be Class 1 or Class 3 depending on what happens if it leaks.

Pressure testing after repairs: B31.3 rules, API 570 judgement

B31.3 requires a leak test before new piping enters service. API 570 says pressure tests are not normally part of routine in-service inspection. When one is performed after a repair or alteration it follows B31.3, and the inspector decides whether one is needed at all.

This is one of the most useful practical differences between the codes. A tie-in weld on a live unit often cannot be hydrotested without taking down far more than the repaired spool. API 570 provides a documented route for accepting certain closure welds on NDE instead. That usually means full volumetric examination plus surface examination, with engineering review and the conditions set out in the code. B31.3 alone does not offer that route for in-service work, because it was written for systems that can be tested before start-up. When the owner's inspector does call for a pressure test after repair, the test pressure, medium and hold follow the B31.3 rules described earlier on this page.

Minimum distance between welds in ASME B31.3

ASME B31.3 does not set a general minimum distance between adjacent girth welds. ASME answered this in a published interpretation: apart from the branch-connection reinforcement rules, the code has no such requirement. Spacing limits found on projects come from owner or EPC specifications.

This question comes up constantly. Shop and site supervisors are told to keep welds a certain distance apart, assume the rule comes from the code, and cannot find it in B31.3. The rule is usually in the project piping specification. Common reasons owners impose one:

• Overlapping heat-affected zones. Two welds very close together put two weld thermal cycles and two residual-stress fields into the same short length of pipe, which some owners want to avoid in services where toughness or cracking resistance matters.
• Examination access. UT, PAUT and radiography all need room. A pup piece shorter than the probe footprint or film coverage can make a weld impossible to examine properly, both at construction and decades later under API 570.
• Reinforcement zones. Branch connections need reinforcement within a defined zone, and welds crowded into that zone complicate the calculation. This is the one area where the code itself constrains proximity.
• Fit-up and distortion. Very short spools are hard to align and may distort.

Rules of thumb such as a multiple of the pipe diameter, or a fixed minimum length for pup pieces, are company and project choices. If your specification has one, it governs your job, but it is not a B31.3 requirement. For in-service piping, weld proximity matters for a different reason: a repair sleeve or replacement spool welded close to an old weld creates exactly the access problem that makes later CML readings and crack examination unreliable. Plan repairs with the next inspection in mind, and ask us to review examination access before a spool is fabricated. (The interpretation is reported in secondary sources; check it in ASME's interpretation index for your edition.)

What "B31.3 compliant" means for filters, strainers and other components

A component is acceptable under B31.3 either because it is a listed component made to a standard the code accepts, or because it is an unlisted component whose pressure design has been qualified by a method the code permits. "B31.3 compliant" on a datasheet should tell you which route applies.

Inline filters, strainers, sight glasses, flexible hoses and special fittings are frequent sources of confusion. For a listed component, the manufacturer should cite the product standard and pressure-temperature rating. For an unlisted component, ask for the design basis: calculation, proof test or experience-based qualification as the code allows, plus materials and rating. Some larger filter housings are designed as pressure vessels under ASME Section VIII instead. That changes the governing construction code, and later the in-service code too, because the vessel then falls under the owner's API 510 programme rather than API 570. ASME's change list for B31.3 2024 includes unlisted valves, so check the current edition when buying. In service, these components often become the weak point: thin bodies, dissimilar metals and dead spaces where corrosion concentrates. Include them in the CML plan rather than assuming they match the line.

NDE methods on each side of the handoff

Construction NDE under B31.3 is about weld quality: VT, RT or UT of a defined sample, and MT or PT where required. In-service NDE under API 570 is about damage: UT thickness at CMLs, profile radiography, corrosion mapping, guided wave screening, and crack detection methods chosen for the damage mechanism.

API 570 also identifies certain in-service ultrasonic shear-wave applications where examiners should have passed a performance demonstration. Check your edition and owner specification for when this applies. For in-service piping see the API 570 piping inspection guide and piping circuit CML inspection.

The regulatory overlay: PSM, jurisdictions and Canada

In the US, OSHA PSM requires inspection and testing that follow RAGAGEP, and for in-service process piping most employers cite API 570 while citing B31.3 for design and construction. In Canada, provinces adopt construction codes through CSA B51 and provincial regulations and reference API 570 for in-service inspection.

Under 29 CFR 1910.119(j), piping systems including valves are covered equipment. Inspection and testing must follow recognized and generally accepted good engineering practices, at frequencies consistent with manufacturers' recommendations and good engineering practice, and each inspection must be documented with the date, the person who performed it, an equipment identifier, a description of the test and the results. OSHA's RAGAGEP guidance treats "shall" provisions of a consensus code as mandatory and expects departures from "should" provisions to be documented. In practice, a facility's mechanical integrity programme cites B31.3 for new and modified piping and API 570 with API RP 574 for in-service inspection. For equipment designed to codes no longer in general use, the employer must document that it is designed, maintained, inspected, tested and operating safely.

US state boiler and pressure vessel laws generally focus on boilers and vessels rather than process piping, so the main legal driver for piping inspection is PSM (and EPA RMP where it applies). Confirm with your state if you are unsure.

In Canada, provincial regulators register and enforce pressure piping construction, typically referencing ASME B31.3 through CSA B51 and provincial pressure equipment legislation. For in-service work, Technical Safety BC lists API 570 among the codes referenced through its adoption of CSA B51, and Alberta applies API codes through ABSA's AB-506 owner-user requirements rather than adopting them directly. Ontario's TSSA framework has its own amendments. Requirements differ by province, so confirm with your provincial regulator.

How Atlantis supports both sides

Atlantis performs construction NDE to B31.3 (RT through licensed radiography crews where the work is, UT, phased array, TOFD, MT, PT and VT) and in-service NDE under the owner's API 570 programme: CML thickness surveys, profile RT, corrosion mapping and guided wave screening. Technicians are ASNT-certified under ASNT Level III oversight. Acceptance, interval and repair decisions stay with the owner's authorized piping inspector and engineer. We do not perform RBI or fitness-for-service assessments. Request a piping NDE quote; we reply within 24 hours.

More questions about B31.3 in operating plants

Is B31.3 used for in-service piping?

Not as an inspection code. In-service inspection, rating, repair and alteration fall under API 570 or the owner's equivalent RAGAGEP. B31.3 still matters in service: repairs follow its principles where practical, and its pressure-design equation supplies the required thickness used in remaining-life calculations.

What is the difference between API 570 and ASME B31.3?

B31.3 governs building new process piping; API 570 governs inspecting and maintaining that piping once in service. API 570 does not replace construction requirements, and B31.3 does not set inspection intervals. See also our B31.3 code overview.

Can I use B31.3 radiography acceptance criteria on in-service welds?

API 570 cautions that this may not be appropriate, because B31.3 criteria are workmanship standards applied to a sample of new welds. In-service indications should be evaluated for their effect on integrity by the owner's inspector and engineer. Ask about in-service weld examination.