6.4 Porosity — Scattered (P), Distribution Charts & Hollow Bead Cross-Reference

6.4 Porosity — Scattered (P), Distribution Charts & Hollow Bead Cross-Reference

Gas porosity is the most common volumetric discontinuity in pipeline welding. API 1104:2021 Section 9.3.9 evaluates it with two criteria: individual pore size and distribution charts. When measuring a pore's radiographic indication, the maximum dimension applies.

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1. Formation and Prevention

Porosity is gas trapped by solidifying weld metal before it can escape the molten pool. Drivers: moisture, oil, rust, or paint on the bevel; damp electrodes; shielding gas blown away by wind; excessive arc length drawing in atmosphere. Prevention is consumable discipline (Section 4.2 storage rules), clean bevels, and weather protection (Section 7.5).

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2. Individual or Scattered Porosity (9.3.9.2)

Scattered porosity is a defect when ANY of the following holds:

• Individual pore size exceeds 1/8 in. (3 mm) in diameter OR 25% of the specified wall thickness, whichever is less;
  • t = 0.500 in. → 25% t = 0.125 in. → limit is 1/8 in.
  • t = 0.250 in. → 25% t = 0.0625 in. (1/16 in.) → limit is 1/16 in.
• The pore exceeds 25% of the thinner of the two specified wall thicknesses joined (unequal-wall joints);
• The distribution of scattered porosity exceeds the concentration permitted by Figure 20 (wall t ≤ 0.500 in. / 12.7 mm) or Figure 21 (wall t > 0.500 in.) — the standard's charts showing the maximum allowed gas-pocket distribution (assorted sizes, and "aligned — three or more" patterns with spacing rules based on multiples of t).

22nd-edition change candidates must know: the separate cluster porosity (CP) acceptance clause that existed in the 21st edition (1/2 in. cluster diameter) was eliminated in the 22nd edition. API confirmed the deletion was intentional. Evaluate a pore group by individual pore size (9.3.9.2) plus the Figure 20/21 distribution concentration. (CP still appears as a UT volumetric category in 9.6.)

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3. Hollow Bead (9.3.9.3) — Cross-Reference

Hollow bead porosity — elongated linear porosity in the root bead — is evaluated under 9.3.9.3, not the porosity charts: individual > 1 in. (25 mm), aggregate > 2 in. (50 mm) per 12 in. (300 mm), or aggregate > 8% of weld length are defects. See Section 6.3 of this guide for the full treatment.

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4. Radiographic Evaluation Protocol

1. Measure pore diameters with a calibrated comparator; the maximum dimension of the indication applies.
2. Compare the individual size against min(1/8 in., 25% t).
3. Lay the Figure 20/21 chart logic over the film: count and size distribution of pockets, including aligned groups of three or more.
4. Classify elongated root-centerline porosity as HB and evaluate it under 9.3.9.3 instead.
5. Remember 9.2: the company may reject any weld even when it appears to meet these standards if, in its opinion, the imperfection depth may be detrimental.

5. Porosity Shapes and the Charts' Logic

Section 9.3.9.1 notes porosity is generally spherical but may be elongated or irregular — including "wormhole" (piping) porosity. Whatever the shape, the maximum dimension of the radiographic indication is measured against the criteria.

The Figure 20 and Figure 21 distribution charts exist because pore COUNT and SPACING matter as much as pore size: a field of fine pores can be acceptable while a few coarse or tightly aligned pores reject the weld. The charts classify pockets by size (large / medium / fine), limit how many of each size may appear in the evaluated area, and treat aligned groups of three or more pores with spacing-based rules tied to multiples of wall thickness. Practical evaluation workflow:

1. Confirm wall thickness to select Figure 20 (t ≤ 0.500 in.) or Figure 21 (t > 0.500 in.).
2. Size every pore; apply the min(1/8 in., 25% t) individual limit first — one oversize pore rejects without chart work.
3. Compare the observed size distribution and alignment against the chart for the applicable thickness.
4. Route elongated root-centerline porosity to the hollow-bead criteria (9.3.9.3) instead.

MT and PT cross-reference (9.4 / 9.5.2): rounded surface indications are evaluated to the same 9.3.9.2 size limits, and PT treats any indication of 1/16 in. (2 mm) or less as nonrelevant.

6. Porosity vs. Hollow Bead — the Classification Decision

The classification decision comes before any measurement: spherical or irregular pockets scattered through the bead are porosity (9.3.9.2); an elongated tubular void on the root-bead centerline is hollow bead (9.3.9.3). Misclassifying HB as scattered porosity applies the wrong limit set in both directions — the 1/8-in. pore limit does not cap HB length, and the HB 1-in. individual allowance does not legitimize a 1-in. scatter of pores. When doubt exists about the imperfection type, the standard's standing note applies: verification may be obtained using other NDT methods (9.4/9.5/9.6 notes).