Quiz- Section 9 Acceptance Standards for NDT – CWI Part C- 10 Questions
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Section 9: Acceptance Standards for NDT
This section presents acceptance standards for imperfections found by radiographic, magnetic particle, liquid penetrant, ultrasonic, and visual testing. NDT shall not be used to select welds that are to be destructively tested for welder qualification.
9.2 Rights of Rejection
The acceptance criteria presented in this Section are based primarily on discontinuity length, given the fact that NDT methods, other than ultrasonic testing, can offer no information about a discontinuity’s depth into the weld. For this reason, the company retains the right to reject any weld that meets the limitations specified in this Section if, in their opinion, the depth of any acceptable discontinuity could be detrimental to the weld.
Section 9 defines each discontinuity and, for radiographic testing, clarifies the definition with a figure which shows a schematic. For each discontinuity, this section then provides acceptance criteria, which are typically expressed as both a limit on the size of any single indication as well as a limit on the total length of acceptable discontinuities per weld length. To be acceptable, welds must meet the criteria for each discontinuity as well as for the total accumulated flaws per weld as described in paragraph 9.3.1 2.
Rather than repeat the verbiage in Section 9, this part of the Study Guide will simply point the reader to the applicable paragraphs for the criteria for each discontinuity.
9.3 Radiographic Testing
Paragraph 9.3.1 provides the acceptance criteria for inadequate penetration without high-low, referred to as IP. The schematic is shown in Figure 1 3 on page 35.
Paragraph 9.3.2 provides the acceptance criteria for inadequate penetration due to high-low, referred to as IPD. This condition exists when inadequate penetration is associated with misalignment of the pipe. The schematic is shown in Figure 1 4 on page 36.
Paragraph 9.3.3 provides the acceptance criteria for inadequate cross penetration, referred to as ICP. This discontinuity can only exist in welds made from both the inside and outside of the pipe. A schematic of this is shown in Figure 1 5 on page 36.
Paragraph 9.3.4 provides the acceptance criteria for incomplete fusion, referred to as IF. API 1 1 04’s definition of this limits it to incomplete fusion that is open to the surface. A schematic of this is shown in Figure 1 6 on page 36.
Unlike many other welding standards, API 1 1 04 identifies two categories of incomplete fusion, based on whether it is open to the surface or not. While paragraph 9.3.4 addresses incomplete fusion open to the surface (IF), paragraph 9.3.5 provides criteria for incomplete fusion when it is not open to the surface. This type of incomplete fusion said to be the result of a “cold lap,” is referred to as IFD. A schematic of this is shown in Figure 1 7 on page 37.
Paragraph 9.3.6 provides the acceptance criteria for internal concavity, referred to as IC. A schematic of this is shown in Figure 1 8 on page 37. These criteria are stated in terms of the density of the radiographic image on the radiographic flam. Simply stated, this criterion permits any length of internal concavity as long as the weld thickness is no less than the thinner of the two pipe wall thicknesses being joined. When the minimum wall thickness has been violated, the criteria for burn-through in paragraph 9.3.7 apply instead.
Paragraph 9.3.7 addresses burn-through, referred to as BT. No schematic for this discontinuity is provided. Paragraph 18.104.22.168 defines it and the acceptance criteria are provided for pipe having a specified OD of 2.375 inches and greater in paragraph 22.214.171.124 and for pipe having a specified OD less than 2.375 inches in paragraph 126.96.36.199.
Paragraph 9.3.8 addresses slag inclusions and defines two categories – elongated slag inclusions, referred to as ESIs, and isolated slag inclusions, referred to as ISIs. No schematic is provided for these discontinuities. ESIs are typically linear, separated by approximately the width of the root bead, and usually located between the root bead and the hot pass. ISIs have an irregular shape and may appear anywhere in the weld. The acceptance criteria for slag inclusions are located in paragraph 188.8.131.52 for pipe having a specified OD of 2.375 inches and greater and in paragraph 184.108.40.206 for pipe having a specified OD less than 2.375 inches. The note on page 38 is an exception to the acceptance criteria
and specifies that ESIs separated by approximately the width of the root bead are to be considered a single ESI unless the width of either one exceeds 1 /32 inch, in which case the two indications are to be considered separately as ESIs.
Paragraph 9.3.9 addresses porosity and defines it in paragraph 220.127.116.11. Acceptance criteria are specified based on the types of porosity. Criteria for individual or scattered porosity are given in paragraph 18.104.22.168. Criteria for cluster porosity, referred to as CP, are given in paragraph 22.214.171.124, which only applies to the final or cover (cap) passes. For CP in other than the finish pass, the criteria in 126.96.36.199 apply. Criteria for hollow bead porosity, referred to as HB, are given in paragraph 188.8.131.52.
Paragraph 9.3.1 0 provides the acceptance criteria for cracks. Cracks (previously referred to as ‘C’ ) are prohibited, except for shallow crater cracks or star cracks 5/32 inch in length or less. These cracks are acceptable and it is worth noting that API 1 1 04 is probably the only construction code that explicitly permits cracks of any size. The note at the bottom of this paragraph defines these shallow crater cracks or star cracks as the cracks at the terminations of welds due to shrinkage. These shrinkage cracks are basically solidification cracks that occur when the welding arc is terminated suddenly while the weld puddle surface is still fat. Solidification and further shrinkage cause the weld surface to become concave and pull away from the center, leaving cracks. These cracks, however, can be easily prevented with proper welding techniques.
Paragraph 9.3.1 1 addresses undercutting, which is divided into two categories depending on whether it is on the outside surface of the pipe, referred to as EU, or whether it is on the inside of the pipe, referred to as IU. The acceptance criteria here provide limits for the length of undercut when it is discovered using radiography. However, the depth of undercut is just as important, if not more so, than the length. As a result, the note sends the reader to subsection 9.7 for undercut criteria when the undercut is accessible for visual and mechanical measurement, i.e., accessible for depth measurement. Subsection 9.7 on page 46 sends the reader to Table 4 also on page 46 for the permissible lengths of undercut as a function of depth.
Paragraph 9.3.1 2 provides acceptance criteria for the sum of all acceptable indications in a single weld, referred to as the accumulation of imperfections (previously referred to as ‘AI ’ ). In addition to the acceptance criteria for each individual discontinuity as provided in paragraphs 9.3.1 through 9.3.1 1, the sum of the lengths of all of the different acceptable flaws, excluding IPD, EU, and IU, in any given weld may not exceed the limits given in this paragraph.
Paragraph 9.3.1 3 requires that base metal (pipe or fitting) discontinuities discovered during radiography of the welds be reported to the company for disposition.
9.4 Magnetic Particle Testing
Paragraph 9.4.1 states that not all indications produced by magnetic particle testing (MT) are the result of weld imperfections. The examiner must be able to distinguish between indications produced by magnetic and metallurgical variations and those produced by imperfections. Indications produced by magnetic and metallurgical variations are to be considered nonrelevant. In addition, indications having a maximum dimension of no more than 1 /1 6 inches are also considered to be nonrelevant unless proven otherwise.
After that, all relevant indications are then considered to be the result of weld imperfections and they are divided into two categories, depending on their aspect ratio. Those whose length is more than three times their width are referred to as linear indications. Those having a length three times their width or less are rounded indications.
Paragraph 9.4.2 provides the acceptance criteria for linear indications and refers the reader to paragraphs 184.108.40.206 (individual or scattered porosity) and 220.127.116.11 (cluster porosity) for the acceptance criteria for rounded indications.
Paragraph 9.4.3 addresses flaws found in base material during MT of a weld and requires that these be reported to the company for disposition.
9.5 Liquid Penetrant Testing
The requirements for liquid penetrant testing (PT) are exactly the same as those stated above for MT.
9.6 Ultrasonic Testing
Paragraph 9.6.1 .1 emphasizes that the indications produced by ultrasonic testing (UT) are not necessarily defects. The difference between relevant indications, which are those that result from weld imperfections, and nonrelevant indications, which are the result of changes in the weld geometry, reinforcement profiles, internal chamfering, or other geometric issues must be understood.
Paragraph 9.6.1 .2 classifies relevant indications as linear when their longest dimension is parallel to the direction of welding.
Paragraph 9.6.1 .3 classifies relevant indications as transverse when their longest dimension is across
the direction of welding.
Paragraph 9.6.1 .4 classifies three-dimensional indications as volumetric.
Paragraph 9.6.2 provides acceptance criteria for indications found by UT based on the definitions in the following paragraphs:
(a) 18.104.22.168 for linear surface (LS) indications.
(b) 22.214.171.124 for linear buried (LB) indications.
(c) 126.96.36.199 for transverse (T) indications.
(d) 188.8.131.52 for volumetric cluster (VC) indications.
(e) 184.108.40.206 for volumetric individual (VI ) indications.
(f) 220.127.116.11 for volumetric root (VR) indications and.
(g) 18.104.22.168 for the accumulation of relevant indications.
Paragraph 9.6.3 addresses imperfections in the base metal found during UT of welds and, similar to RT, MT, and PT, requires disposition of these flaws by the company.
9.7 Visual Acceptance Standards for Undercutting
Paragraph 9.7.1 reminds the reader that these criteria supplement, but do not replace, VT acceptance criteria found elsewhere in the Standard.
Paragraph 9.7.2 refers the reader to Table 4 on page 46 for acceptance criteria for undercut when visual and mechanical means can be used to determine depth. The table states, for instance, that there is no limit on the length of undercut in a weld if it is no deeper than the lesser of 1 /64 inch or 6 % of the wall thickness. As undercut becomes deeper, the length permitted decreases.