7.1 Radiographic Examination Techniques & Image Quality Indicators (Article 2)
Key Takeaways
- ASME Section V Article 2 mandates standard radiographic examination procedures, specifying radiation source selection, geometric unsharpness limits, Image Quality Indicator (IQI) sensitivity, and transmitted film density.
- Industrial gamma sources are selected based on material thickness: Selenium-75 (0.15 to 1.18 in. / 4 to 30 mm steel), Iridium-192 (0.25 to 2.5 in. / 6 to 64 mm steel; half-life 73.8 days), and Cobalt-60 (1.5 to 7.0 in. / 38 to 178 mm steel; half-life 5.27 years).
- Image Quality Indicators (IQIs) must be placed on the source side of the component; film-side placement is permitted only when the source side is physically inaccessible, requiring a lead letter 'F' marker adjacent to the IQI.
- Transmitted radiographic film density must be 1.8 minimum for X-ray and 2.0 minimum for gamma sources, up to a maximum of 4.0 for single-film viewing; composite viewing requires a minimum density of 2.6, and density across the area of interest cannot vary by more than -15% or +30% from the IQI density.
- Maximum allowable geometric unsharpness (Ug = F * d / D) per Table T-274.2 is 0.020 in. (0.51 mm) for material thickness under 2 in., 0.030 in. for 2 to 3 in., 0.040 in. for 3 to 4 in., and 0.070 in. for thicknesses exceeding 4 in.
Radiographic Examination Techniques & Image Quality Indicators (Article 2)
Radiographic Examination (RT) is one of the foundational volumetric nondestructive examination methods referenced by API 510 and ASME Section VIII, Division 1. By recording differential radiation absorption on photographic film or digital detector arrays, radiography allows inspectors to detect internal volumetric flaws (porosity, slag inclusions) and planar defects (incomplete penetration, lack of fusion, cracks) in pressure vessel shells, heads, and nozzle welds.
ASME Boiler and Pressure Vessel Code (BPVC) Section V, Article 2 establishes the mandatory requirements for conducting radiographic examination, verifying image quality, controlling optical density, and maintaining viewing environments. An API 510 Inspector must master Article 2 rules to audit radiographic reports, verify radiograph quality, and interpret code compliance.
1. Fundamentals of Radiographic Examination & ASME Section V Scope
Radiography relies on the attenuation of high-energy electromagnetic radiation (photons) as they pass through matter. Dense materials (or thicker sections) absorb more photons, resulting in fewer photons reaching the film (producing lighter regions). Voids, cracks, or thinner sections absorb fewer photons, allowing more radiation to expose the film emulsion (producing darker regions after chemical development).
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| BASIC RADIOGRAPHIC EXPOSURE GEOMETRY |
| |
| [ RADIATION SOURCE (Focal Spot F) ] |
| | |
| | |
| | Source-to-Object Distance (D) |
| | |
| v |
| ========================[ SOURCE SIDE IQI ]============================= |
| \\\\\\\\\\\\\\\\\\\\ WELD REINFORCEMENT ////////////////////////////// |
| ------------------------------------------------------------------------ |
| |
| BASE METAL THICKNESS |
| (t) |
| |
| ------------------------------------------------------------------------ |
| //////////////////////// WELD ROOT PASS \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\ |
| ======================================================================== |
| | Object-to-Film Distance (d) |
| v |
| -----------------------[ FILM-SIDE LEAD 'F' ]--------------------------- |
| [========================= RADIOGRAPHIC FILM ==========================] |
| [------------------------- LEAD SCREEN BACKING ------------------------] |
| [========================= LEAD LETTER 'B' ============================] |
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Code Scope & Responsibilities (T-210 / T-220)
- ASME Section V provides the methodology (how to perform the examination and verify image quality).
- ASME Section VIII, Division 1 (UW-51 & UW-52) and API 510 provide the acceptance criteria (what flaw sizes are rejectable).
- Written procedures are mandatory for all radiographic examinations governed by Article 2.
2. Industrial Radiation Sources: X-Ray Machines vs. Gamma Radioisotopes
Industrial radiography utilizes two primary radiation sources: electrically generated X-radiation and isotope-generated Gamma radiation.
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| INDUSTRIAL RADIATION SOURCE MATRIX |
| |
| [X-RAY GENERATORS] [GAMMA RADIOISOTOPES] |
| - Continuous energy spectrum - Discrete monoenergetic lines |
| - Adjustable kV and mA - Fixed photon energies per isotope|
| - Switched on/off electrically - Emits continuously (decay) |
| - Higher contrast / sensitivity - Highly portable field cameras |
| - Heavy, requires field power - Requires no power source |
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Gamma Radioisotope Characteristics (Article 2 & API 577)
| Radioisotope | Chemical Symbol | Average Photon Energy | Half-Life ($t_{1/2}$) | Practical Steel Thickness Range | Key Inspection Applications |
|---|---|---|---|---|---|
| Selenium-75 | $^{75}\text{Se}$ | $\approx 0.22\text{ MeV}$ (range $0.066\text{--}0.40\text{ MeV}$) | $119.8\text{ days}$ | $0.15\text{ in.} \text{ to } 1.18\text{ in.} (4\text{ to } 30\text{ mm})$ | Thin-wall piping, small-diameter vessel nozzles; high radiographic contrast close to X-ray. |
| Iridium-192 | $^{192}\text{Ir}$ | $\approx 0.38\text{ MeV}$ (range $0.20\text{--}0.61\text{ MeV}$) | $73.8\text{ days}$ | $0.25\text{ in.} \text{ to } 2.50\text{ in.} (6\text{ to } 64\text{ mm})$ | Standard refinery workhorse; used for vessel girth seams, shell welds, and piping. |
| Cobalt-60 | $^{60}\text{Co}$ | $1.25\text{ MeV}$ ($1.17\text{ and } 1.33\text{ MeV}$) | $5.27\text{ years}$ | $1.50\text{ in.} \text{ to } 7.00\text{ in.} (38\text{ to } 178\text{ mm})$ | Heavy-wall reactor vessels, thick forged flanges, heavy castings; highly penetrating. |
[!NOTE] Isotope Decay & Exposure Calculations: Radioisotopes decay according to $A(t) = A_0 \cdot e^{-\lambda t} = A_0 \cdot (0.5)^{t / t_{1/2}}$. Because radioactive sources decay continuously, exposure times must be recalculated daily using decay charts or source calculators to achieve target radiographic film densities.
3. Image Quality Indicators (IQIs) / Penetrameters
An Image Quality Indicator (IQI), historically termed a penetrameter, is a standardized gauge placed on the part during radiography to verify radiographic technique sensitivity, contrast, and resolution. An IQI does not measure flaw sizes—it proves that the radiograph has sufficient definition to reveal flaws of a specified magnitude.
ASME Section V Article 2 recognizes two standard IQI designs:
- Hole-Type IQIs (Plaques per Table T-233.1 / ASTM E1025)
- Wire-Type IQIs (Per Table T-233.2 / ASTM E747)
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| HOLE-TYPE VS. WIRE-TYPE IQIs |
| |
| [HOLE-TYPE IQI (PLAQUE)] [WIRE-TYPE IQI (ASTM E747)] |
| +-------------------------------+ +---------------------------+ |
| | 25 4T 1T 2T | | [1] [2] [3] [4] [5] [6] | |
| | --- (O) (o) (O) | | | | | | | | | |
| | AISI | | | | | | | | | |
| +-------------------------------+ | SET A ASTM DIN FE | |
| - Plaque thickness T = 2% of nominal +---------------------------+ |
| - 1T Hole dia = 1 x T - Set of 6 encapsulated | |
| - 2T Hole dia = 2 x T (Essential Hole) wires of increasing dia | |
| - 4T Hole dia = 4 x T - Essential Wire must be | |
| - Material alloy matches base metal clearly visible on film | |
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Hole-Type IQI Mechanics
- A rectangular metallic plaque whose thickness $T_{\text{plaque}}$ is nominally 2% of the material thickness being radiographed.
- Contains three precision drilled holes:
- $1T$ Hole: Diameter equals $1 \times T_{\text{plaque}}$
- $2T$ Hole: Diameter equals $2 \times T_{\text{plaque}}$ (Standard Essential Hole)
- $4T$ Hole: Diameter equals $4 \times T_{\text{plaque}}$
- Standard radiographic quality level is $2\text{-}2T$ (2% plaque thickness, $2T$ essential hole must be visible on the processed radiograph).
- Higher sensitivity levels include $2\text{-}1T$ (2% plaque, $1T$ hole visible) and $1\text{-}1T$ (1% plaque, $1T$ hole visible).
Wire-Type IQI Sets (ASTM E747 / ASME Table T-233.2)
Wire IQIs consist of six parallel wires of increasing diameter encased in transparent plastic. ASTM E747 groups wires into four standard sets:
| Wire Set | Encapsulated Wire Numbers | Wire Diameters (inches) | Wire Diameters (mm) | Typical Thickness Application |
|---|---|---|---|---|
| Set A | 1, 2, 3, 4, 5, 6 | 0.0032, 0.004, 0.005, 0.0063, 0.008, 0.010 | 0.08, 0.10, 0.13, 0.16, 0.20, 0.25 | Up to $0.25\text{ in.} (6.4\text{ mm})$ |
| Set B | 6, 7, 8, 9, 10, 11 | 0.010, 0.0125, 0.016, 0.020, 0.025, 0.032 | 0.25, 0.32, 0.40, 0.51, 0.64, 0.81 | $> 0.25\text{ in.} \text{ to } 0.75\text{ in.} (6.4\text{ to } 19\text{ mm})$ |
| Set C | 11, 12, 13, 14, 15, 16 | 0.032, 0.040, 0.050, 0.063, 0.080, 0.100 | 0.81, 1.02, 1.27, 1.60, 2.03, 2.54 | $> 0.75\text{ in.} \text{ to } 2.00\text{ in.} (19\text{ to } 51\text{ mm})$ |
| Set D | 16, 17, 18, 19, 20, 21 | 0.100, 0.126, 0.160, 0.200, 0.250, 0.320 | 2.54, 3.20, 4.06, 5.08, 6.35, 8.13 | $> 2.00\text{ in.} \text{ to } 8.00\text{ in.} (51\text{ to } 203\text{ mm})$ |
4. IQI Selection, Placement Rules, Shims & Lead Markers
Nominal Thickness Determination for IQI Selection (T-276.1)
- For welds with reinforcement, the governing thickness used to select the IQI is the nominal base metal thickness plus allowable weld reinforcement (both sides, crown plus root).
- Backing rings or strips are included in the thickness calculation if they remain in place during radiography.
Governing Thickness = Nominal Plate Thickness + Internal Reinforcement + External Reinforcement
IQI Placement Rules (T-277.1)
- Source-Side Placement (Mandatory Default): The IQI must be placed on the source side of the component being examined.
- Film-Side Placement Exception: When physical accessibility prevents source-side placement (such as an enclosed vessel with no internal access and a panoramic central source is impossible):
- The IQI may be placed on the film side.
- A lead letter "F" (at least $1/4\text{ in.} / 6\text{ mm}$ high) must be placed adjacent to or on the IQI.
- The essential hole or wire requirement is typically one size smaller (more stringent) to compensate for the reduced geometric unsharpness on the film side.
- Orientation: Wire IQIs must be laid perpendicular to the weld axis so that the wires cross the weld reinforcement and HAZ.
Use of Shims Under IQIs (T-277.3)
When an IQI is placed on the base metal adjacent to a weld that has reinforcement, the base metal is thinner than the weld joint. To prevent the IQI from experiencing an artificially high exposure:
- A shim of radiographically similar material must be placed underneath the IQI.
- The thickness of the shim must equal the total weld reinforcement thickness, ensuring the optical density through the IQI matches the optical density through the weld area of interest.
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| SHIM PLACEMENT UNDER HOLE IQI |
| |
| [ HOLE IQI ] |
| +--------------+ |
| | 20 (o)(O) | |
| +--------------+ /\\\\\\\\ WELD CROWN REINFORCEMENT |
| | SHIM (t=t_r) | / \\\\\\\ |
| +-------+--------------+----+ \\\\\\+--------------------------+ |
| | | \\\\\| | |
| | BASE METAL | \\\\| BASE METAL | |
| | THICKNESS | \\\| THICKNESS | |
| | | \\| | |
+-----+---------------------------+---------+--------------------------+------+
| Shim Thickness (t_shim) = Total Weld Reinforcement Thickness (t_reinforce) |
+-----------------------------------------------------------------------------+
Backscatter Radiation & Lead Letter "B" Marker (T-223 / T-284)
Radiation scattering off walls, floors, or equipment behind the film holder creates non-image-forming fog that degrades radiograph contrast.
- A lead letter "B" (minimum $1/2\text{ in.} / 13\text{ mm}$ high and $1/16\text{ in.} / 1.5\text{ mm}$ thick) must be affixed to the back of each film cassette/holder.
- Acceptance Rule: If a light (low-density) image of the letter "B" appears on a darker background of the processed radiograph, protection from backscatter is insufficient, and the radiograph must be rejected and re-shot.
- If a dark image of "B" appears, or no image appears, backscatter shielding is acceptable.
5. Radiographic Film Density & Calibration Requirements (T-282)
Transmitted Film Optical Density ($D$) is a quantitative measurement of film blackening, defined as:
Where $I_0$ is incident light intensity and $I_t$ is transmitted light intensity. A density of $2.0$ transmits $1/100$ (1%) of incident light; a density of $4.0$ transmits $1/10,000$ (0.01%) of incident light.
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| ASME SECTION V ARTICLE 2 FILM DENSITY LIMITS |
| |
| [X-RAY RADIOGRAPHS] [GAMMA RADIOGRAPHS] |
| - Single film viewing: - Single film viewing: |
| 1.8 Minimum 2.0 Minimum |
| 4.0 Maximum 4.0 Maximum |
| |
| [COMPOSITE VIEWING (DOUBLE FILM)] |
| - Composite Density: 2.6 Minimum to 4.0 Maximum |
| - Individual Film Density in Composite: 1.3 Minimum Each |
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Comprehensive Optical Density Thresholds (ASME T-282.1)
| Radiation Source | Single Film Minimum Density | Single Film Maximum Density | Composite Film Minimum Density | Composite Film Maximum Density |
|---|---|---|---|---|
| X-Ray Sources | 1.8 | 4.0 | 2.6 (each film $\ge 1.3$) | 4.0 |
| Gamma Sources (Ir-192, Co-60, Se-75) | 2.0 | 4.0 | 2.6 (each film $\ge 1.3$) | 4.0 |
Density Variation Limits (T-282.2)
The optical density within the area of interest (the weld deposit and adjacent heat-affected zone) must remain consistent with the density measured through the IQI:
- Allowable Density Range: The density in the area of interest shall not vary by more than -15% to +30% from the density measured through the essential hole of the hole-type IQI or adjacent to the essential wire of the wire-type IQI.
Densitometer Calibration Protocols (T-262)
- National Standard Step Tablet: Densitometers must be calibrated against a certified national standard step tablet (NIST-traceable) at least every 90 days.
- Periodic Verification: Calibration verification must be performed at the beginning of each shift, after 8 hours of continuous operation, or whenever the instrument is energized/relocated.
- Verification Tolerance: The densitometer readings must match the step tablet values within $\pm 0.05$ density units.
6. Geometric Unsharpness ($U_g$) Physics & Code Limits (T-274.2)
Geometric Unsharpness ($U_g$) is the penumbral blurring of edges on a radiograph caused by the physical dimensions of the radiation source (focal spot or isotope pellet diameter) and the geometry of the exposure setup.
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| GEOMETRIC UNSHARPNESS FORMULA |
| |
| F * d |
| Ug = --------- |
| D |
| |
| Where: |
| Ug = Geometric Unsharpness (inches or mm) |
| F = Source focal spot size or physical isotope dimensions (in. or mm) |
| d = Object-to-Film Distance (OFD) (source-side of object to film) |
| D = Source-to-Object Distance (SOD) (source to source-side of object) |
| |
| Note: Source-to-Film Distance (SFD) = D + d |
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Maximum Allowable Geometric Unsharpness (ASME Table T-274.2)
| Material Thickness Range ($t$) | Maximum Allowable $U_g$ (inches) | Maximum Allowable $U_g$ (mm) |
|---|---|---|
| Under $2.0\text{ in.} (50\text{ mm})$ | $0.020\text{ in.}$ | $0.51\text{ mm}$ |
| $2.0\text{ in.} \text{ to } 3.0\text{ in.} (50\text{ to } 75\text{ mm})$ | $0.030\text{ in.}$ | $0.76\text{ mm}$ |
| $> 3.0\text{ in.} \text{ to } 4.0\text{ in.} (75\text{ to } 100\text{ mm})$ | $0.040\text{ in.}$ | $1.02\text{ mm}$ |
| Greater than $4.0\text{ in.} (100\text{ mm})$ | $0.070\text{ in.}$ | $1.78\text{ mm}$ |
Calculation Example:
An inspector audits a shot on a $1.50\text{ in.}$ thick shell plate using an Iridium-192 source with a focal size $F = 0.150\text{ in.}$. The source-to-object distance $D = 20\text{ in.}$ and object-to-film distance $d = 1.65\text{ in.}$ (plate thickness plus standoff).
- Evaluation: For thickness $< 2.0\text{ in.}$, maximum allowable $U_g = 0.020\text{ in.}$. Since $0.0124\text{ in.} \le 0.020\text{ in.}$, the geometric setup is acceptable.
7. Radiographic Viewing Facilities, Densitometers & Interpretation Protocols
Radiographic film interpretation requires controlled optical environments to ensure subtle flaw indications are visible:
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| RADIOGRAPH VIEWING ENVIRONMENT RULES |
| |
| [VIEWING ILLUMINATOR] [AMBIENT DARKROOM ENVIRONMENT] |
| - Variable intensity transmitted light - Subdued lighting |
| - Minimum light intensity: - Eliminate reflections from |
| 1000 cd/m2 or 100 foot-candles film surface |
| - Masking frames to block glare - Minimum 5-10 min eye |
| around film edges adaptation time |
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Key Viewing & Interpretation Standards (T-290)
- Illuminator Capacity: Illuminators must supply variable-intensity transmitted light capable of clearly reading optical densities up to 4.0.
- Masking: High-intensity light escaping around the outer perimeter of the film creates peripheral glare that severely impairs visual acuity. Masking plates or diaphragms must be used to block all light outside the film border.
- Eye Dark Adaptation: Interpreters entering the darkroom or viewing area from bright daylight must allow sufficient time (typically $5\text{ to } 10\text{ minutes}$) for rhodopsin regeneration and dark adaptation before evaluating radiographs.
8. Common Exam Traps & Real-World Pitfalls in Radiographic Examination
[!WARNING] Trap 1: The Backscatter 'B' Image - Light vs. Dark: A favorite API 510 question asks: "During interpretation, a dark image of the letter 'B' appears on the film. Is the radiograph acceptable?"
- Rule: Only a LIGHT image of the letter 'B' on a darker background indicates excessive backscatter and causes REJECTION.
- A dark image of 'B' is simply an overexposure artifact or secondary reflection and is ACCEPTABLE unless it obscures the weld area.
[!IMPORTANT] Trap 2: Minimum Density Confusion (X-Ray vs. Gamma): Do not mix up the minimum density limits: 1.8 for X-ray versus 2.0 for Gamma sources. Both share the same single-film maximum limit of 4.0.
[!TIP] Trap 3: IQI Selection Governing Thickness: When selecting an IQI for a welded joint, always include the weld reinforcement thickness added to the nominal plate thickness. If base metal is $0.500\text{ in.}$ with $0.125\text{ in.}$ reinforcement on both crown and root, the governing thickness is $0.500 + 0.125 + 0.125 = 0.750\text{ in.}$, which changes the required essential wire or hole designation!
An API 510 inspector reviews an industrial gamma radiograph taken with an Iridium-192 source on a 1.0-inch thick pressure vessel shell weld. When examining the film on an illuminator, a distinct LIGHT image of the lead letter 'B' appears against the darker background of the radiograph. What action does ASME Section V Article 2 require?
What are the minimum and maximum transmitted film optical density limits permitted by ASME Section V Article 2 for a single radiograph exposed using an Iridium-192 gamma ray source?
When radiographing a vessel circumferential seam where the inside of the vessel is physically inaccessible, an inspector must place the wire-type IQI on the film side. Which of the following is MANDATORY under ASME Section V Article 2?
An exposure setup uses an X-ray tube with a focal spot size (F) of 0.120 in. (3.0 mm). The distance from the radiation source to the source-side of a 1.25-inch thick vessel shell (D) is 24.0 in., and the distance from the source-side of the shell to the film (d) is 1.50 in. What is the calculated geometric unsharpness (Ug), and is it acceptable per ASME Section V Table T-274.2?