5.2 Radiographic Testing (RT) & Image Quality Indicators (IQI)

5.2 Radiographic Testing (RT) & Image Quality Indicators (IQI)

Radiographic Testing (RT) is the most widely utilized volumetric non-destructive examination method for cross-country pipeline girth welds. API Standard 1104 Section 11.1 outlines stringent procedure qualification standards, exposure geometry rules, optical density boundaries, image quality indicator (IQI) selection criteria, and darkroom processing parameters. Certified Welding Inspectors and NDT auditors must master these quantitative standards to evaluate radiograph validity before interpreting weld defects.

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1. Radiographic Procedure Qualification (Section 11.1.2)

Prior to performing production radiography, a detailed written RT procedure specification must be established and qualified by producing acceptable radiographs of production or test welds. The written procedure must explicitly specify:

1. Radiation Source: Type of source, manufacturer model, voltage rating for X-ray units (kVp), or isotope type and activity in Curies (Ci) or Gigabecquerels (GBq) for gamma sources.
2. Focal Spot / Source Dimension: Effective physical size of the X-ray focal spot (f) or gamma isotope capsule dimension.
3. Radiographic Film: Brand, film class (ASTM E1815 Class I or Class II fine-grain film), and screen type (front and rear lead intensifying screens, typically 0.005 in. / 0.125 mm thick).
4. Exposure Geometry: Single-wall exposure single-wall viewing (SWE/SWV, internal panoramic crawler), double-wall exposure single-wall viewing (DWE/SWV), or double-wall exposure double-wall viewing (DWE/DWV, elliptical or contact technique).
5. Source-to-Object Distance (D) & Object-to-Film Distance (d): Minimum SOD and maximum OFD geometry.
6. Processing Method: Manual tank chemical processing or automatic processor chemistry, developer temperature, immersion time, washing, and drying parameters.
7. IQI Details: Type (wire vs. hole-type), material group, placement rules, and essential wire diameter or hole size.

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2. Radiation Sources and Application Limits

Pipeline RT utilizes two primary categories of ionizing radiation sources under Section 11.1.2:

X-Ray Generators

• Internal Panoramic Crawlers: Battery-powered motorized units propelled inside the pipe to center an omnidirectional 360° X-ray tube directly aligned with the girth weld. Produces superior single-wall exposures with uniform exposure geometry and zero geometric magnification.
• Directional Portable X-Ray Tubes: External units positioned outside the pipe for double-wall exposures on small-diameter lines or tie-in welds.

Gamma-Ray Isotopic Sources

• Iridium-192 (Ir¹⁹²): Emits gamma radiation equivalent to approximately 460 to 600 kVp X-rays. Ideal for pipe wall thicknesses between 0.25 in. (6.4 mm) and 1.50 in. (38.1 mm).
• Cobalt-60 (Co⁶⁰): Emits high-energy gamma rays (1.17 and 1.33 MeV), equivalent to a 1.2 MV X-ray unit. Used primarily for thick-wall piping exceeding 1.50 in. (38.1 mm).

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3. Geometric Unsharpness Formula & Calculation

Geometric unsharpness (U_g) represents the penumbral shadow blur at the edges of a radiographic image, caused by the non-zero physical dimension of the radiation source. Excessive unsharpness degrades image sharpness and masks small cracks or lack of fusion.

The Mathematical Formula

$U_g = \frac{f \cdot d}{D}$

Where:

• U_g = Geometric unsharpness (in. or mm).
• f = Physical source size or focal spot dimension (in. or mm).
• d = Object-to-film distance (distance from source side of weld to film) (in. or mm).
• D = Source-to-object distance (distance from radiation focal spot to source side of weld) (in. or mm).

Code Limit and the Source-Distance Equation

API 1104:2021 (11.1.3.1) controls unsharpness by mandating a minimum source-to-object distance:

$D = \frac{S \times t}{k}$

where D = minimum distance from source/focal spot to the source side of the object, S = effective source size, t = weld thickness including reinforcement plus object-to-film distance, and k = 0.02 in. (0.5 mm) — the geometric unsharpness factor — for material up to 2.000 in. (50.8 mm) thick.

Example: an Ir-192 source with effective size S = 0.125 in. (3.18 mm) radiographing a 0.375 in. wall with film against the back wall (t = 0.375 in.) needs D ≥ 0.125 × 0.375 / 0.02 = 2.34 in. Any source placement at or beyond 2.34 in. satisfies the unsharpness requirement.

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4. Optical Density Limits & Darkroom Control

Film density refers to the quantitative measure of film blackening caused by exposure to radiation and chemical development, measured using a calibrated photoelectric densitometer.

Mandatory Density Ranges (API 1104 Section 11.1.12)

• Area of interest: transmitted H&D density must be not less than 1.8 nor greater than 4.0.
• Localized exception: small localized areas caused by irregular weld configurations may fall outside that band, but never below 1.5 or above 4.2.
• Viewing: the illuminator must be variable high-intensity, capable of the specified density range, and shielded against stray light; viewing rooms use subdued background lighting. Densitometers are calibrated annually with a calibrated step wedge.
• Archival quality: when the company requests it, film is processed and stored so images remain interpretable for at least three years.

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5. Image Quality Indicators (IQI) Selection & Placement Rules

An Image Quality Indicator (IQI), or penetrameter, validates that a radiograph possesses adequate sensitivity and contrast to reveal fine volumetric and planar discontinuities.

Wire-Type vs. Hole-Type IQIs

API 1104 accepts both wire-type (ASTM E747 / ISO 19232-1) and hole-type (ASTM E1025) IQIs:

• Wire-Type IQIs: Consist of a series of six parallel metallic wires of calibrated diameters encased in clear plastic. Wire-type IQIs are widely preferred in pipeline RT due to direct alignment across weld reinforcement.
• Hole-Type IQIs: Rectangular metallic plaques with calibrated thickness containing three drilled holes (1T, 2T, 4T hole diameters, where T is plaque thickness).

Selection Rules (API 1104:2021 Tables 17-19)

The essential wire (or hole) is selected on the weld thickness of the thinner joining member plus reinforcement:

IQI Placement Rules (11.1.6)

1. Panoramic single exposure (internal source, SWE/SWV): a minimum of four IQIs spaced approximately equally around the circumference — wire type placed across the weld, hole type adjacent to the weld on the film/PIP/detector side.
2. DWE/DWV: one IQI on the source side, across the weld for wire type or adjacent to it for hole type.
3. Multiple-exposure techniques: where the interpreted image length exceeds 5 in. (130 mm), two IQIs are used — one within 1 in. (25 mm) of the end of the interpreted length and one at its center; at 5 in. (130 mm) or less, one centered IQI.
4. Separate blocks: when placement on the weld is impractical, an IQI on a block of radiographically similar material of the same thickness may be used, placed no closer than the source side of the weld.
5. Shims: hole-type IQIs use shims radiographically similar to the pipe and equivalent in thickness to the average weld reinforcement, sized so the plaque outline shows on at least three sides.