What ASME B31.1 Actually Requires You to Examine, and When

Where B31.1 governs and where B31.3 takes over

B31.1 prescribes requirements for the design, materials, fabrication, erection, test, inspection, operation and maintenance of power piping systems, including steam, water, oil, gas and air services. Its home is electric power generating stations, industrial and institutional plants, geothermal heating systems and central heating and cooling systems. B31.3 covers piping found in petroleum refineries and in chemical, pharmaceutical, hydrogen, textile, paper, semiconductor and cryogenic plants. The dividing line is the plant and the jurisdiction over it, not the pressure or the temperature on the line list.

Boiler external piping is where the two worlds meet. Para. 100.1.2(A) brings within B31.1 the piping for power boilers generating steam or vapour above 15 psig, and high temperature water boilers above 160 psig or 250F. That piping begins where the boiler proper terminates, at the first circumferential welding joint, the face of the first bolted flange, or the first threaded joint, and extends to and including the valves required by para. 122.1. Terminal points themselves count as boiler external piping.

The consequence is administrative, and it is expensive when missed. Piping between the terminal points and those valves requires Data Reports, inspection and stamping under Section I of the ASME Boiler and Pressure Vessel Code, and the welding must be done by an organisation authorised to use the appropriate Code symbol. A single isometric can therefore carry two jurisdictions. Compare the process piping route on the B31.3 explainer before you build one examination plan for both.

Inspection and examination are different jobs with different owners

Para. 136.1 puts inspection on the Owner. The Owner's Inspector is designated by the Owner and is an employee of the Owner, of an engineering or scientific organisation, or of a recognised insurance or inspection company acting as the Owner's agent. That inspector may not represent or be employed by the piping manufacturer, fabricator or erector, unless the Owner is also the manufacturer, fabricator or erector. Independence here is a Code requirement, not a procurement preference.

The experience bar is explicit: not less than ten years in the design, manufacture, erection, fabrication or inspection of power piping. Each satisfactorily completed year toward a recognised engineering degree counts as one year of experience, capped at five years total. Where the Owner delegates an inspection function, the Owner remains responsible for determining that the delegate is qualified to perform it. For boiler external piping, para. 136.2 escalates further and requires an Inspector employed by an ASME accredited Authorized Inspection Agency, working to PG-104 through PG-113.

Examination is the other function entirely. Para. 136.3.1 defines it as what the manufacturer, fabricator, erector or a party authorised by the Owner performs: visual, radiographic, ultrasonic, eddy current, liquid penetrant and magnetic particle methods. Any degree of examination or acceptance standard beyond what the Code requires is a matter of prior agreement between the fabricator and the Owner. Getting that agreement into the ITP in writing is what makes a defensible NDT report package later.

How to read the mandatory examination table without getting it wrong

The table has two axes and three service columns. Down the side sit the weld types: butt welds both girth and longitudinal, welded branch connections, and fillet, socket, attachment and seal welds. Across the top sit the service conditions: temperatures over 750F at all pressures; temperatures between 350F and 750F inclusive with all pressures over 1,025 psig; and all others. Find the cell, and the cell names the method. Welds not covered by the table require visual examination only.

For butt welds, the first column turns on size alone. Over NPS 2 takes RT or UT; NPS 2 and smaller takes MT or PT. The second column adds a thickness gate that catches people out: RT or UT applies to welds over NPS 2 only when thickness exceeds 3/4 in. (19.0 mm), and everything at 3/4 in. or less drops to visual. Note 1 defines the thickness of a butt weld as the thicker of the two abutting ends after end preparation, so a transition weld is judged on the heavier side.

Branch connections are keyed to the branch size, not the header, and the size threshold moves to NPS 4. Two notes control the sequence and the substitution. RT or UT of branch welds is performed before any nonintegral reinforcing material is applied, so pad welds go on after the volumetric shot. In place of that volumetric examination, surface examination is acceptable when performed at the lesser of half the weld thickness or each 1/2 in. (12.5 mm) of weld thickness, plus all accessible final weld surfaces. Fillet welds up to 1/4 in. (6 mm) throat attaching nonpressure parts are exempt from the PT or MT requirement.

Visual examination is the floor, and it carries real acceptance criteria

Every weld covered by the table receives a visual examination in addition to whatever specific method the cell names. Welds not requiring RT, UT, MT or PT by the Code or by the engineering design are judged acceptable when they meet the visual acceptance standards of para. 136.4.2 and the pressure test requirements of para. 137. Visual examination is performed to the methods of Section V, Article 9, and may be conducted during fabrication and erection to verify that design and WPS requirements are being met.

The acceptance list in 136.4.2(A) is short and absolute. Unacceptable are: cracks on the external surface; undercut on the surface greater than 1/32 in. (1.0 mm) deep; weld reinforcement greater than Table 127.4.2 permits; lack of fusion on the surface; incomplete penetration, applicable only when the inside surface is readily accessible; any other linear indication greater than 3/16 in. (5.0 mm) long; and surface porosity with rounded indications over 3/16 in. (5.0 mm), or four or more rounded indications separated by 1/16 in. (2.0 mm) or less edge to edge in any direction.

Two of those hinge on definitions that belong in the procedure. A rounded indication is circular or elliptical with a length less than three times its width, which makes everything longer a linear indication and therefore subject to the 3/16 in. limit. And reinforcement is not a single number: Table 127.4.2 grades the maximum cap height by base metal thickness and by design temperature, tightening as the service gets hotter. Both trace back to the method requirements collected in ASME BPVC Section V.

MT and PT acceptance are the same four rules, twice

Magnetic particle examination under 136.4.3 is performed to Article 7 of Section V; liquid penetrant under 136.4.4 to Article 6. Both paragraphs then run the same evaluation logic. Indications are not necessarily defects, and anything believed to be nonrelevant must be re-examined to verify whether actual defects are present, with surface conditioning permitted first. Nonrelevant indications that would mask indications of defects are themselves unacceptable. The mechanics of the penetrant method are worked through in detail on the Article 6 requirements page.

The relevance threshold is a major dimension greater than 1/16 in. (2.0 mm). Linear indications are those whose length is more than three times the width; rounded indications are circular or elliptical with a length less than three times the width. Then the acceptance rule, identical in both paragraphs: any cracks or linear indications are unacceptable; rounded indications with dimensions greater than 3/16 in. (5.0 mm) are unacceptable; four or more rounded indications in a line separated by 1/16 in. (2.0 mm) or less edge to edge are unacceptable.

The fourth rule is the density limit that gets forgotten on large surfaces: ten or more rounded indications in any 6 square inches (3 870 square millimetres) of surface are unacceptable, with the major dimension of that area not to exceed 6 in. (150 mm) and the area taken in the most unfavourable location relative to the indications being evaluated. Both paragraphs also state the trap plainly: an indication may be larger than the discontinuity causing it, and the size of the indication, not the discontinuity, is the basis of acceptance or rejection.

Radiographic acceptance: the length ladder in para. 136.4.5

Radiography is performed to Article 2 of Section V with one deliberate deviation. The requirements of T-285 are to be used as a guide but not for the rejection of radiographs, unless geometric unsharpness exceeds 0.07 in. (2.0 mm). That single sentence resolves most film-quality arguments on a power piping job, and it is worth quoting into the radiographic procedure so the argument never starts.

The acceptance standards open with an absolute. Any type of crack, or any zone of incomplete fusion or penetration, is unacceptable, with no length allowance at all. After that comes the elongated indication ladder, keyed to t, the thickness of the thinner of the two members at the weld. An elongated indication is unacceptable when its length exceeds 1/4 in. (6.0 mm) for t up to 3/4 in. (19.0 mm) inclusive; one third of t for t from 3/4 in. to 2-1/4 in. (57.0 mm) inclusive; and 3/4 in. (19.0 mm) for t over 2-1/4 in.

Three more rules close it out. Any group of indications in line is unacceptable when the aggregate length exceeds t in a length of 12t, except where the distance between successive indications exceeds 6L, L being the longest indication in the group. Porosity in excess of the charts in Appendix A-250 of Section I is unacceptable. And root concavity is unacceptable when there is an abrupt change in density, as indicated on the radiograph, which turns a density reading into an accept or reject decision. The method trade-off against ultrasonics is set out on RT versus UT for weld inspection.

Ultrasonic acceptance, and the recording requirement most bidders miss

Para. 136.4.6 sends ultrasonic examination to Article 4 of Section V and then adds requirements that Article 4 does not contain. The equipment used shall be capable of recording the UT data, so that a third party can analyse it and so that a subsequent examination is repeatable. Manual UT is permitted only where physical obstructions prevent the use of a recording system, and then only with the approval of the Owner. On a B31.1 job, encoded ultrasonics is the default and manual scanning is the exception you have to ask for.

The personnel clause is equally specific. NDE personnel performing and evaluating UT shall be qualified and certified in accordance with their employer's written practice and the requirements of para. 136.3.2. Beyond that, the personnel, the procedures and the equipment used to collect and analyse the data must have demonstrated their ability to perform an acceptable examination using test blocks approved by the Owner. Demonstration on Owner-approved blocks is a separate hurdle from certification, and it needs scheduling. The underlying method requirements are covered on the Article 4 UT requirements page.

Acceptance runs in two stages. Any discontinuity producing an indication greater than 20% of the reference level must be investigated far enough for the examiner to determine its shape, identity and location. Then: discontinuities evaluated as cracks, lack of fusion or incomplete penetration are unacceptable regardless of length. All other discontinuities are unacceptable when the indication exceeds the reference level and the length exceeds 1/4 in. (6.0 mm) for t up to 3/4 in.; one third of t for t from 3/4 in. to 2-1/4 in.; and 3/4 in. (19.0 mm) for t over 2-1/4 in. The length ladder is identical to the radiographic one.

NDE personnel qualification under B31.1 is not automatically SNT-TC-1A

Para. 136.3.2 requires that personnel performing nondestructive examination of welds be qualified and certified for each method under a program established by their employer, built on five minimum elements. Those are: instruction in the fundamentals of the method; on-the-job training long enough to assure adequate assimilation of the appearance and interpretation of indications; an eye examination performed at least once each year; an oral or written examination plus a performance examination administered by the employer; and recertification of anyone whose work has not included that method for a year or more, along with recertification after substantial changes in procedures or equipment.

Two alternatives sit alongside it. As an alternative to the whole program, the requirements of ASME Section V, Article 1 may be used to qualify NDE personnel. And personnel qualified to AWS QC1 may be used for the visual examination of welds. Neither route makes a national certification scheme mandatory by default, which surprises auditors who arrive expecting one.

This is where paperwork audits fail on power piping jobs. Owners routinely invoke ASNT SNT-TC-1A or ANSI/ASNT CP-189 through the purchase specification rather than through the Code, and the written practice then has to satisfy both documents at once. Where a program has to be reconciled against a contract that names a different scheme, an outsourced Level III can write and sign the practice, the examinations and the certification records against a single defensible baseline.

Five places B31.1 and B31.3 diverge on the job

The trigger differs. B31.1 keys extent to weld type and service condition and then demands examination of every weld that lands in the table. B31.3 keys extent to fluid service category in its examination clause, running from visual alone for Category D up to full volumetric examination for severe cyclic conditions, with a sampled percentage of girth welds in between for normal fluid service. Anyone moving from a refinery to a power station discovers that the percentage they used to negotiate no longer exists.

The acceptance vocabulary differs. B31.1 keeps its criteria inside Chapter VI, one paragraph per method, with the same length ladder shared by radiography and ultrasonics. B31.3 collects criteria in a single acceptance table keyed by weld type and fluid service, which makes it look more configurable and makes it easier to apply the wrong row. B31.1 is the harsher document on one specific discontinuity: any zone of incomplete fusion or penetration on a radiograph is rejected outright, with no length allowance to argue over.

Choosing between them is not an engineering preference. The plant, the jurisdiction and the Owner's specification decide, and boiler external piping is B31.1 administered under Section I regardless of what the rest of the drawing says. Where a scope crosses that line, the examination plan needs to change at the terminal point, not at the plot boundary. Getting the split written correctly into a technical procedure before fabrication starts is cheaper than re-shooting a header, and our team will review a draft ITP against both codes on request.

Request a consultation