{"slug":"/standards/asme-pt-mt-acceptance-criteria","title":"ASME PT and MT Acceptance Criteria (Section VIII Div 1)","description":"ASME Section V has no PT or MT acceptance criteria. Section VIII Div 1 Appendices 8 and 6 do: what is relevant, linear vs rounded, and what is rejected.","h1":"Penetrant and Magnetic Particle Acceptance Criteria in ASME","answer":"ASME Section V Articles 6 and 7 tell you how to perform liquid penetrant and magnetic particle examination but contain no acceptance criteria. For Section VIII Division 1 vessels the criteria are in Mandatory Appendix 8 for PT and Mandatory Appendix 6 for MT: indications larger than 1/16 in. are relevant, relevant linear indications are rejected, and rounded indications larger than 3/16 in. are rejected.","expansion":"The two appendices use the same rules. An indication is linear when its length is more than three times its width, and rounded when it is circular or elliptical with a length of three times its width or less. Four or more relevant rounded indications in a line, separated by 1/16 in. or less edge to edge, are rejected even when each one is individually acceptable. The size of the indication, not the size of the imperfection underneath it, is what gets measured. The criteria apply unless Division 1 specifies something more restrictive for a particular material or application, and they belong to Section VIII Division 1 only: ASME B31.1, B31.3, Section I and Division 2 each carry their own acceptance rules, so a vessel acceptance clause cannot simply be copied into a piping procedure.","source":"ASME BPVC Section VIII Division 1: Mandatory Appendix 6 (methods for magnetic particle examination), Mandatory Appendix 8 (methods for liquid penetrant examination) and UW-35 (finished longitudinal and circumferential joints); ASME BPVC Section V, Articles 6, 7 and 9; ASME B31.3 Table 341.3.2 and ASME B31.1 para. 136.4 for the piping codes' own criteria. Values are given in U.S. customary units as the appendices state them; confirm them against the edition the construction code invokes.","table":{"caption":"PT and MT acceptance rules in ASME Section VIII Division 1","columns":["Rule","Penetrant (Appendix 8)","Magnetic particle (Appendix 6)","What it means in practice"],"rows":[["How to examine","Section V Article 6","Section V Article 7","Technique only; no accept or reject values"],["Relevant indication","Major dimension over 1/16 in. (1.5 mm)","Major dimension over 1/16 in. (1.5 mm)","Smaller indications are not evaluated"],["Linear indication","Length more than 3 x width; rejected if relevant","Length more than 3 x width; rejected if relevant","No relevant linear indication is acceptable"],["Rounded indication","Rejected if over 3/16 in. (5 mm)","Rejected if over 3/16 in. (5 mm)","Measure the indication, not the pit beneath it"],["Rounded indications in a line","Four or more, 1/16 in. (1.5 mm) or less apart edge to edge: rejected","Same rule","Check alignment before counting"],["Questionable indications","Re-examined to decide relevance","Re-examined to decide relevance","Clean and re-test rather than guess"]],"note":"Section VIII Division 1 vessels only. B31.3 piping uses Table 341.3.2 and B31.1 power piping uses para. 136.4; Section I and Division 2 carry their own requirements."},"sections":[{"heading":"PT acceptance criteria in ASME","paragraphs":["For a pressure vessel built to Section VIII Division 1, penetrant acceptance criteria come from Mandatory Appendix 8, not from Section V. Section V Article 6 governs the procedure, materials, dwell, removal, development and the conditions for interpretation; Appendix 8 decides what is acceptable. A procedure, report or inspection and test plan that cites Article 6 as its acceptance standard has a gap that an Authorized Inspector will find.","Appendix 8 works in three steps. First, decide which indications are relevant: only those with a major dimension over 1/16 in. Second, classify each relevant indication as linear or rounded by its length-to-width ratio. Third, apply the rejection rules: any relevant linear indication, any rounded indication over 3/16 in., and any line of four or more rounded indications separated by 1/16 in. or less.","The appendix applies unless a more restrictive standard is specified for a particular material or application within Division 1, so check the material-specific parts of the code and the purchase specification before treating the appendix as the whole answer. Owners with sour or cyclic service often specify tighter limits in their own documents."]},{"heading":"MT acceptance criteria under Appendix 6","paragraphs":["Appendix 6 applies the same thresholds to magnetic particle indications, so a vessel's MT and PT acceptance clauses read alike. The differences between the methods sit in the technique, which Section V Article 7 controls: field direction, the requirement to examine each area twice with the field in roughly perpendicular directions, field adequacy checks and the particle type. An MT indication only means something if the field was strong enough and in the right direction to produce it, so an acceptance decision on an MT report starts with the technique record, not the indication.","Nonrelevant indications are a particular MT problem. Changes in section, the edges of a weld cap and magnetic writing can all hold particles with no discontinuity beneath them. They are resolved by re-examination after surface conditioning or by a second technique, and the outcome is recorded; they are not dismissed on sight. Fluorescent and visible techniques are judged by the same criteria; the technique changes the lighting conditions for interpretation, not the acceptance limits."]},{"heading":"Visual acceptance under Section VIII Division 1 (UW-35)","paragraphs":["People asking for ASME visual acceptance criteria are usually looking at the same vessel. Section V Article 9 sets how visual examination is done and contains no acceptance criteria. For finished longitudinal and circumferential butt joints, Division 1 states its requirements in UW-35: complete penetration and fusion, and a weld surface free enough of coarse ripples, grooves, overlaps and abrupt ridges and valleys that the required examinations can be interpreted properly.","The reduction in thickness at the edge of the weld, the undercut, may not exceed 1/32 in. or 10 percent of the nominal thickness of the adjoining surface, whichever is less, and may not take the material below the minimum required thickness. Weld reinforcement is limited by thickness through a table in UW-35; read the values from the edition that applies rather than from a summary."]},{"heading":"How a Level III applies the criteria on a job","paragraphs":["The procedure's acceptance clause should name the construction code, the appendix and the edition, and the report should record indications against it rather than a bare accept. Indications are measured after the development time the procedure sets, with a scale that can resolve 1/16 in., and the indication is recorded as found, even though a bleeding penetrant indication is larger than the imperfection beneath it, because the appendix evaluates the indication. Records should identify each relevant indication by location and size even when it is acceptable, so a later examination can tell growth from a new indication.","Where an indication is rejectable, the imperfection is removed by grinding, the area is re-examined by the same method, and the remaining thickness is checked against the design minimum before any repair welding. On a multi-code job the Level III also decides which criteria apply where: a nozzle weld built to Division 1, the piping attached to it built to B31.3 and a support welded to AWS D1.1 can sit within a metre of each other and carry three different acceptance standards. The inspection and test plan should assign the standard weld by weld."]},{"heading":"Common mistakes","paragraphs":["Citing Section V Article 6 or 7 as the acceptance standard. Treating 1/16 in. as the acceptance limit rather than the relevance threshold. Measuring the imperfection after light grinding instead of the indication as developed. Counting rounded indications without checking whether four or more fall in a line. Accepting an MT result without evidence that the field was checked and applied in two directions. Copying Appendix 8 criteria into a B31.3 piping procedure, or B31.3 criteria into a vessel procedure. Recording no relevant indications on a surface that was never properly cleaned, where background could mask a real indication. Calling for magnetic particle examination on austenitic stainless steel or another non-ferromagnetic material, where the method cannot work and penetrant is the surface method to use."]},{"heading":"Other ASME codes and where their criteria live","paragraphs":["ASME B31.3 states its weld acceptance limits by type of imperfection in Table 341.3.2, and the engineering design can make them stricter. ASME B31.1 carries its own magnetic particle and penetrant acceptance standards in para. 136.4. Section VIII Division 2 and Section I contain their own examination and acceptance requirements. Section V supports all of them with method rules and none of them with acceptance values, so when a client asks for ASME PT acceptance criteria, the first question back is which ASME code the component is built to.","For the method rules themselves, see [ASME Section V Article 6](/standards/asme-section-v-article-6) and [Article 7](/standards/asme-section-v-article-7); for the volumetric criteria on the same vessels, see [Section VIII Division 1 joint efficiency and required NDE](/standards/asme-section-viii-div-1)."]}],"facets":[{"q":"Does ASME Section V contain PT acceptance criteria?","a":"No. Article 6 sets how a liquid penetrant examination is performed: materials, surface preparation, dwell, removal, development and the conditions for interpretation. Acceptance criteria come from the construction code, which for Section VIII Division 1 vessels is Mandatory Appendix 8."},{"q":"What size of penetrant indication is relevant under Section VIII Division 1?","a":"Only indications with a major dimension greater than 1/16 in. (1.5 mm) are relevant. Smaller indications are not evaluated, although questionable indications are re-examined to decide whether they are relevant."},{"q":"Are any linear indications acceptable under Appendix 8?","a":"No relevant linear indication is acceptable. An indication is linear when its length is more than three times its width, so a relevant crack-like indication is rejected whatever its length, once it is above the relevance threshold."},{"q":"Are the MT acceptance criteria the same as PT in Section VIII Division 1?","a":"Yes. Appendix 6 for magnetic particle uses the same relevance threshold, the same linear and rounded definitions and the same rejection rules as Appendix 8. What differs is the technique, which Section V Article 7 controls."},{"q":"Can Appendix 8 criteria be used for ASME B31.3 piping?","a":"Only if the engineering design or the contract invokes them. B31.3 states its own weld acceptance limits in Table 341.3.2, and those, together with any stricter owner requirements, govern process piping."},{"q":"What undercut does Section VIII Division 1 allow on butt welds?","a":"UW-35 limits the reduction in thickness at the edge of the weld to 1/32 in. or 10 percent of the nominal thickness of the adjoining surface, whichever is less, and the material may not be reduced below the minimum required thickness at any point."}],"faq":[{"q":"Does ASME Section V contain PT acceptance criteria?","a":"No. Article 6 sets how a liquid penetrant examination is performed: materials, surface preparation, dwell, removal, development and the conditions for interpretation. Acceptance criteria come from the construction code, which for Section VIII Division 1 vessels is Mandatory Appendix 8."},{"q":"What size of penetrant indication is relevant under Section VIII Division 1?","a":"Only indications with a major dimension greater than 1/16 in. (1.5 mm) are relevant. Smaller indications are not evaluated, although questionable indications are re-examined to decide whether they are relevant."},{"q":"Are any linear indications acceptable under Appendix 8?","a":"No relevant linear indication is acceptable. An indication is linear when its length is more than three times its width, so a relevant crack-like indication is rejected whatever its length, once it is above the relevance threshold."},{"q":"Are the MT acceptance criteria the same as PT in Section VIII Division 1?","a":"Yes. Appendix 6 for magnetic particle uses the same relevance threshold, the same linear and rounded definitions and the same rejection rules as Appendix 8. What differs is the technique, which Section V Article 7 controls."},{"q":"Can Appendix 8 criteria be used for ASME B31.3 piping?","a":"Only if the engineering design or the contract invokes them. B31.3 states its own weld acceptance limits in Table 341.3.2, and those, together with any stricter owner requirements, govern process piping."},{"q":"What undercut does Section VIII Division 1 allow on butt welds?","a":"UW-35 limits the reduction in thickness at the edge of the weld to 1/32 in. or 10 percent of the nominal thickness of the adjoining surface, whichever is less, and the material may not be reduced below the minimum required thickness at any point."}]}