Process piping code for petroleum, chemical, and pharmaceutical plants — design, materials, fabrication, examination, testing, and inspection requirements.
Scope
ASME B31.3 — Process Piping — is one of the four primary piping codes in the ASME B31 family (B31.1 Power Piping, B31.3 Process Piping, B31.4 Pipeline Transportation of Liquids, B31.8 Gas Transmission). B31.3 is the dominant code for piping in refineries, petrochemical plants, chemical plants, pharmaceutical plants, and similar process industries — encompassing the vast majority of API 570 in-service inspection scope. It covers piping design (allowable stress, wall thickness, support spacing), material selection, fabrication (welding, bending, branch connections), examination (RT, UT, MT, PT, VT), pressure testing, and quality assurance. B31.3 is published roughly every two years (latest editions 2022, 2024) and applies to construction-phase piping. API 570 covers in-service inspection of existing B31.3 piping. The code is structured: Chapter I (Scope, definitions), Chapter II (Design), Chapter III (Materials), Chapter IV (Standards for piping components), Chapter V (Fabrication, assembly, erection), Chapter VI (Examination, inspection, testing), Chapter VII (Nonmetallic piping), Chapter VIII (Severe cyclic conditions), Chapter IX (High-pressure piping), Chapter X (High-purity piping).
Code compliance is only demonstrable if the evidence behind it is: the procedure revision in force, the inspector's certification state and the instrument's calibration status at the time of test. Atlantis NDT provides ASNT Level III consulting for procedure and written-practice work against this code, training toward the certifications that reference it, and inspection management software that keeps that evidence recoverable years later. Request a consultation.
How a standard like this is applied in an inspection programme
A standard is only half of the requirement. It defines how an examination is performed and, in some cases, how results are classified — but the acceptance criteria that decide whether a component stays in service normally come from the construction or in-service code governing the item, not from the examination standard itself. Confusing the two is one of the more common findings in a procedure review: a procedure that correctly cites the examination standard but applies acceptance criteria from the wrong code or the wrong edition.
What has to be in place for compliance to be demonstrable
A written procedure qualified against this standard for the specific materials, thickness ranges and geometries in scope — not a generic procedure covering everything
Personnel certified for the method and level under ASNT SNT-TC-1A, ANSI/ASNT CP-189, NAS 410 or ISO 9712, current on the date the examination was performed
Equipment, probes and reference standards in calibration on that date, with traceability to a national standard under ISO 17025
The applicable edition of the standard recorded against the examination, so historical work stays assessed under the edition then in force
Technique sheets under the same revision control as the procedure above them — the most frequently uncontrolled document in an otherwise compliant quality system
Edition changes
When a new edition is issued, new work moves to it from a defined effective date that you set and record; work already performed stays assessed under the edition in force at the time. Retrospectively applying a new edition to historical dispositions invalidates the original acceptance decision and creates a substantially larger problem than the one being solved.
Where this usually goes wrong
Not in the technical content, but in reconstruction. An auditor picks an issued report and asks which procedure revision applied, who performed the work and whether they were qualified on that date, and whether the instrument and reference blocks were in calibration. Programmes that hold only current state can answer none of those. Binding the document revision, the qualification state and the calibration state to each inspection record as it is created turns that from an investigation into a lookup.
ASME B31.3 is the construction code for process piping in chemical plants, refineries and related facilities. Its defining feature is that examination extent is set by fluid service category rather than applied uniformly — the more damaging a release would be, the larger the proportion of welds that must be volumetrically examined.
Category D fluid service covers non-flammable, non-toxic streams at modest pressure and temperature, and is satisfied by visual examination. Normal fluid service, which covers most process piping, adds random radiography or ultrasonic examination of a small percentage of circumferential butt welds on top of full visual examination. Severe cyclic conditions raise that to complete volumetric examination of girth and miter groove welds, because fatigue crack initiation at weld discontinuities is the governing failure mode rather than gross fabrication error. Category M applies where a single exposure could cause irreversible harm and carries its own reinforced requirements. High pressure piping under the code's separate chapter is examined in full. The practical implication is that a fabricator cannot quote examination scope until the fluid service category is fixed, and a category assigned loosely at design stage propagates into either unnecessary radiography or an under-examined system.
Source: ASME B31.3 Process Piping, including the examination requirements of Chapter VI and the acceptance criteria table for welds; ASME Boiler and Pressure Vessel Code Section V for examination technique; ASME Section IX for welding procedure and performance qualification; API 570 for the in-service regime that follows commissioning.
Fluid service category and the examination extent it triggers under B31.3
Fluid service
Typical scope
Volumetric examination extent
Category D
Non-flammable, non-toxic, modest pressure and temperature
Visual examination; no volumetric requirement
Normal
Most process piping in a refinery or chemical plant
Full visual plus random volumetric examination of a percentage of girth butt welds
Severe cyclic conditions
Piping subject to significant cyclic loading
Complete volumetric examination of girth and miter groove welds
Category M
Service where a single exposure could cause irreversible harm
Reinforced requirements beyond Normal fluid service
High pressure (separate chapter)
Pressure above the scope of the main code body
Complete volumetric examination
Acceptance criteria come from the code's weld acceptance table for the applicable fluid service; the examination technique itself comes from ASME Section V.
The Owner's Inspector is a defined role, not a job title
B31.3 places responsibility for verifying that the installation meets the code on an Owner's Inspector, and it sets explicit qualification requirements — substantial documented experience in the design, fabrication or inspection of industrial pressure piping, with formal engineering education permitted to offset part of that experience requirement.
This is distinct from the examiners who perform the NDT. Examiners are qualified by training and experience to perform the examinations and are certified under the employer's programme; the Owner's Inspector verifies that the examinations required by the code were performed and that the results meet the acceptance criteria. Conflating the two is a common finding on smaller projects where one person is assumed to cover both.
The delegation route matters as well. The Owner's Inspector may delegate work to qualified personnel but retains the responsibility, which is why the delegation and the qualifications behind it belong in the project quality plan rather than in an informal arrangement on site.
Where B31.3 stops and API 570 starts
B31.3 is a construction code. It governs design, materials, fabrication, examination and testing up to the point the system is accepted, and it does not set in-service inspection intervals. Once the plant is operating, API 570 governs, with its own circuit classification, condition monitoring locations and interval logic.
The handover is where records are most often lost. The as-built thickness, the material certifications, the weld map and the radiographs are the baseline every subsequent corrosion rate is computed against, and a programme that starts from the first in-service survey rather than from construction data has thrown away its long-term corrosion rate.
The practical measure is to treat construction records as an inspection deliverable rather than a project closeout formality — indexed, retrievable, and mapped to the circuits the in-service programme will actually use.
Why the random radiography percentage causes so much argument
Normal fluid service requires volumetric examination of a proportion of welds rather than all of them, and the code is explicit that the sample must be representative — spread across welders, procedures and positions rather than concentrated where access is easiest.
A sample taken from whichever welds the crew could reach with a source on a Friday afternoon is not representative, and it defeats the statistical logic that justifies examining a fraction rather than the whole. Where a welder's work is disproportionately absent from the sample, the sample says nothing about that welder.
The code also requires that when a weld selected for examination is rejected, additional welds by the same welder are examined. That progressive requirement is the mechanism that turns a sample into a control, and skipping it removes the reason the reduced scope was acceptable in the first place.
What does ASME B31.3 cover?
Design, materials, fabrication, examination, inspection and testing of process piping in refineries, chemical plants, pharmaceutical and related facilities. It is a construction code covering the system up to acceptance; in-service inspection of the same piping is governed afterwards by API 570.
How much radiography does B31.3 require?
It depends entirely on fluid service category. Category D requires visual examination only, Normal fluid service requires random volumetric examination of a percentage of girth butt welds, severe cyclic conditions require complete volumetric examination of girth and miter groove welds, and high pressure piping under the separate chapter is examined in full.
What is Category M fluid service?
A service in which a single exposure to a small quantity, caused by leakage, could produce irreversible harm to persons on breathing or bodily contact. It carries reinforced examination and testing requirements beyond Normal fluid service, reflecting that the consequence of even a small release is severe.
Who qualifies as an Owner's Inspector under B31.3?
Someone with substantial documented experience in the design, fabrication or inspection of industrial pressure piping, where formal engineering education may offset a defined portion of the experience requirement. The role verifies that required examinations were performed and met acceptance; it is distinct from the examiners who perform the NDT.
Does B31.3 set the NDT technique or the acceptance criteria?
It sets the acceptance criteria and the extent of examination; the technique comes from ASME Section V. This is the same division as elsewhere in the ASME system — Section V says how an examination is performed, and the construction code says how much of it is required and what passes.
What happens when a radiographed weld is rejected?
The code requires additional welds by the same welder to be examined, progressively. That escalation is what turns a percentage sample into a genuine control on workmanship, and omitting it removes the basis on which examining a fraction rather than the whole was acceptable.
Frequently asked
Can ultrasonic examination substitute for radiography under B31.3?
Where the code permits it for the applicable fluid service and the procedure is qualified and demonstrated accordingly. The substitution is a documented decision with technique justification behind it, not an operational convenience.
Does B31.3 apply to pipelines?
No. B31.4 covers liquid transportation pipelines and B31.8 covers gas transmission and distribution, and in the United States both sit under PHMSA regulation through 49 CFR Parts 195 and 192 respectively.