5.2 Radiographic and Ultrasonic Testing (RT, UT) per ASME Sec V

Radiographic Testing (RT) per ASME Section V, Article 2

Radiographic testing uses penetrating radiation (X-rays or Gamma-rays) to pass through a weld or pipe wall, exposing a film or digital detector on the opposite side. Thick or dense areas absorb more radiation, appearing lighter on the film, while thin areas or voids (like cracks or gas pockets) allow more radiation through, appearing darker.

Source Selection and Film Density

• Radiation Sources:
  • X-rays: Generated electrically, allowing control over energy level (voltage) and exposure time. Best for thin-walled piping.
  • Gamma-rays: Emitted by radioactive isotopes. The most common isotopes used in piping inspections are:
    • Iridium-192 (Ir-192): Used for steel thicknesses ranging from 0.25 to 3.0 inches (6 to 75 mm).
    • Cobalt-60 (Co-60): Used for heavy wall thicknesses ranging from 1.5 to 5.0 inches (38 to 125 mm).
    • Selenium-75 (Se-75): Ideal for thin-walled or small-bore piping, offering high contrast on steel under 1.0 inch.
• Radiographic Density Limits: Film density is a measure of the blackening of the film. Too light (low density) or too dark (high density) makes flaws invisible.
  • X-ray source: The density through the weld and area of interest must be between 1.8 and 4.0 for single-film viewing.
  • Gamma-ray source: The density must be between 2.0 and 4.0 for single-film viewing.
  • Density Variation: The density at any point in the area of interest must not vary by more than -15% or +30% from the density measured through the body of the Image Quality Indicator (IQI).

Image Quality Indicators (IQIs) and Geometric Unsharpness

• IQI Selection and Placement: An IQI (penetrameter) is a device placed on the component to verify the sensitivity of the radiograph. It can be a hole-type (metal plate with holes of specific diameters) or a wire-type (a set of parallel wires of increasing thickness). Wire-type IQIs are the industry standard for piping welds.
  • Thickness Basis: The IQI thickness or wire diameter is selected based on the nominal single-wall thickness plus allowed weld reinforcement.
  • Placement: The IQI must be placed on the source side of the component (closest to the radiation source). If this is physically impossible (e.g., double-wall piping where access is restricted), the IQI may be placed on the film side (closest to the film). If a film-side IQI is used, a lead letter "F" must be placed on the film holder next to the IQI to identify its placement.
• Geometric Unsharpness ($U_g$): Geometric unsharpness is the fuzziness at the edges of a radiographic image caused by the physical size of the radiation source (focal spot or source capsule).
  • Formula: $U_g = \frac{F \cdot d}{D}$ Where:
    • $F$ = Source size (focal spot size or radioactive capsule diameter, in inches or mm).
    • $d$ = Distance from the source-side of the object to the film/detector (typically the component thickness plus any stand-off).
    • $D$ = Distance from the radiation source to the source-side of the object (source-to-object distance).
  • ASME Limits: ASME Section V, Article 2, Table T-276 sets maximum allowable geometric unsharpness. For material thicknesses under 2.0 inches (50 mm), the maximum allowable $U_g$ is 0.020 inches (0.51 mm).

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Ultrasonic Testing (UT) per ASME Section V, Article 4

Ultrasonic testing uses high-frequency sound waves (typically 1 to 10 MHz) to detect internal flaws or measure wall thickness.

Wave Types and Transducers

UT utilizes different wave modes depending on the application:

• Longitudinal (Straight) Waves: Longitudinal waves travel perpendicular to the entry surface. They are highly effective for locating laminar discontinuities (parallel to the surface) and for measuring wall thickness. Straight-beam transducers are the standard tool for routine corrosion profiling.
• Shear (Angle) Waves: Shear waves travel at an angle (usually 45, 60, or 70 degrees) through the material. As they bounce off the internal surfaces, they are highly sensitive to planar flaws that are perpendicular or angled to the pipe surface, such as crack-like flaws, lack of fusion, and lack of penetration in welds.

Calibration and Scanning

• Calibration Blocks: UT systems must be calibrated for distance (sweep range) and sensitivity (gain) before any inspection. Calibration blocks must be made of the same material group and have a thickness within a specified range of the component being inspected. Standard blocks include the IIW block and specific step blocks.
• Calibration Frequency: Calibration must be verified at the start and end of each examination, with any change of transducer, cable, or battery, and at least every 4 hours of continuous use.
• Scanning Patterns: Scanning must be performed with a raster pattern, overlapping the transducer path by at least 10% of the transducer width to ensure full coverage.

High-Temperature Thickness Measurements

When performing UT thickness measurements on piping that is operating at elevated temperatures (above 150°F / 65°C), the inspector must apply a correction factor.

• The Physics of Hot Steel: As steel heats up, its density decreases slightly, and its elastic modulus drops. This causes the acoustic velocity to slow down. Because the UT gauge is calibrated for the faster velocity of cold steel, it takes longer for the sound to return, which the gauge interprets as a thicker wall.
• Correction Rule: To compensate for this velocity drop, the inspector must subtract 1% of the measured thickness for every 100°F (55°C) that the pipe temperature exceeds the calibration block temperature.
• Formula: $$$$

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Comparison of Volumetric Methods: RT vs. UT

Choosing between RT and UT depends on the specific piping geometry, suspected damage mechanisms, and accessibility.

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Worked Scenario: High-Temperature UT Correction and Geometric Unsharpness

Scenario 1: UT Thickness Correction

An inspector is measuring the remaining wall thickness of an operating Class 1 hydrocarbon piping line. The line is operating at 650°F (343°C). The UT thickness gauge was calibrated on a carbon steel step block at an ambient temperature of 70°F (21°C). The gauge displays a thickness reading of 0.500 inches (12.70 mm).

To calculate the actual wall thickness, the inspector applies the high-temperature correction factor:

1. Calculate the temperature difference: $\Delta T = 650^\circ\text{F} - 70^\circ\text{F} = 580^\circ\text{F}$
2. Calculate the percentage correction: $\text{Correction} = \frac{580^\circ\text{F}}{100^\circ\text{F}} \times 1\% = 5.8\%$
3. Apply the correction factor (subtract 5.8% from the reading): $t_{\text{actual}} = 0.500 \times (1 - 0.058) = 0.500 \times 0.942 = 0.471\text{ inches}$

The actual thickness of the pipe is 0.471 inches (11.96 mm). If the inspector had recorded the uncorrected value of 0.500 inches, they would have significantly overestimated the remaining life and safety margin of the pipe.

Scenario 2: Geometric Unsharpness ($U_g$) Calculation

A radiographic exposure is set up on a pipe weld with a nominal wall thickness of 1.0 inch. The weld reinforcement is 0.125 inches, and the film is placed in direct contact with the weld. The radiation source size (capsule diameter) is 4 mm (0.157 inches). The source-to-object distance ($D$) is 18 inches.

1. The distance from the source-side of the weld to the film ($d$) is the wall thickness plus reinforcement: $d = 1.0 + 0.125 = 1.125\text{ inches}$
2. Calculate the geometric unsharpness: $U_g = \frac{F \cdot d}{D} = \frac{0.157 \times 1.125}{18} = 0.0098\text{ inches}$
3. The maximum allowable geometric unsharpness for a material thickness under 2.0 inches per ASME Section V, Article 2, is 0.020 inches.

Since $0.0098 < 0.020$, the exposure setup meets the ASME Section V requirements for geometric sharpness.

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Exam-Focused Traps and Tips

• Film-Side IQI Identification: If an exam question asks how to identify a film-side IQI on a radiograph, the answer is a lead letter "F" appearing on the film.
• RT Density Range: Remember the numbers 1.8 (X-ray min) and 2.0 (Gamma-ray min) up to 4.0 (max). The exam loves to ask what happens if a film-viewing density is 1.9 for a gamma-ray exposure (it is unacceptable because it is below the minimum limit of 2.0).
• High-Temperature UT Trap: The UT gauge overestimates thickness on hot steel. You must subtract thickness to find the actual value. Don't add it!

ASME Section V Article 23 / SE-797 — Manual Ultrasonic Thickness Measurement

API 570 exams test ASME Section V Article 23, SE-797 only — the standard practice for measuring thickness by manual ultrasonic pulse-echo contact method. This is how inspectors and examiners generate the CML thickness data that feed corrosion rates, remaining life, and MAWP.

Scope Inspectors Must Know

• Pulse-echo contact technique with a dual-element or single-element transducer coupled to the OD
• Calibration on a reference block of similar material and velocity before and after a series of readings
• Couplant control — air gaps and heavy scale give false lows or no signal
• Temperature effects — hot surfaces change velocity; use temperature compensation or wait for cool-down per procedure
• Multiple readings at a CML — report the minimum credible reading after rejecting invalid echoes

Procedure Discipline (SE-797 Paragraph 7 Concepts)

1. Verify instrument and probe identity match the written procedure
2. Calibrate on a known thickness block spanning the expected measurement range
3. Clean the contact area; remove loose scale that blocks coupling
4. Obtain a stable back-wall echo; do not accept a mid-wall echo as thickness unless scanning for laminations under a different procedure
5. Record location, reading, instrument, probe, and calibration status for the thickness database

Exam trap: confusing flaw detection UT (ASME V Article 4/5 style volumetric exams) with SE-797 thickness gauging. API 570 thickness CMLs are SE-797-style measurements unless the question explicitly calls for weld flaw UT.