5.2 Radiographic Examination (UW-11, UW-51, UW-52)
Key Takeaways
- Radiographic examination extent under ASME Section VIII Division 1 is categorized into Full RT (UW-11(a)), Spot RT (UW-11(b)), and Partial RT (UW-11(c)), stamped on vessel nameplates as RT 1, RT 2, RT 3, or RT 4.
- UW-51 governs Full Radiography acceptance: any crack, lack of fusion, or incomplete penetration is strictly unacceptable, with elongated slag inclusions limited based on thickness ($1/4\text{ in.}$ for $t \le 0.75\text{ in.}$, $t/3$ for intermediate thicknesses, and $3/4\text{ in.}$ for $t > 2.25\text{ in.}$).
- UW-52 governs Spot Radiography: requires one 6-inch spot per 50-foot weld increment (or fraction) for each welder and process, exempts rounded indications (Appendix 4 does not apply), and allows slag up to $2/3 \times t$.
- When a UW-52 spot radiograph fails, two tracer spots must be examined in the same 50-ft increment; if either tracer fails, the entire 50-ft increment is rejected and must be either 100% radiographed or completely cut out and re-welded.
- Nameplate marking RT 1 requires 100% full RT on all Category A, B, C, and D butt welds, whereas RT 2 requires full RT on Category A/D and spot RT on Category B/C butt welds per UW-11(a)(5)(b).
Radiographic Examination (UW-11, UW-51, UW-52)
Radiographic Examination (RT) is the primary nondestructive volumetric testing method mandated by ASME Section VIII, Division 1 to verify the internal soundness of welded joints. Radiography detects internal planar flaws (cracks, lack of fusion, incomplete penetration) and volumetric discontinuities (slag inclusions, porosity, tungsten inclusions).
An API 510 Pressure Vessel Inspector must understand: (1) the required extent of examination codified in UW-11, (2) the vessel nameplate marking rules in UG-116, (3) the acceptance criteria for full radiography in UW-51, and (4) the spot radiography sampling and tracer protocols governed by UW-52.
1. Radiography Extent & Nameplate Markings (UW-11 & UG-116)
ASME Section VIII Division 1 paragraph UG-116 requires that every stamped vessel display one of four RT designations on its nameplate to indicate the extent of radiographic examination performed on its butt welds:
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| ASME NAMEPLATE RADIOGRAPHY MARKINGS (UG-116) |
| |
| [RT 1: FULL 100% RADIOGRAPHY] [RT 2: FULL CAT A / SPOT CAT B] |
| - All Cat A, B, C, D Butt Welds - Cat A & D: 100% Full RT |
| examined 100% Full Length - Cat B & C: Spot RT (UW-11(a)(5)) |
| - Cat A: E = 1.00 | Cat B: E = 1.00 - Cat A: E = 1.00 | Cat B: E = 0.85 |
| |
| [RT 3: SPOT RADIOGRAPHY] [RT 4: PARTIAL RADIOGRAPHY] |
| - Cat A & B Butt Welds Spot - Only specific portions or welds |
| examined per UW-11(b) / UW-52 radiographed |
| - 1 Spot (6") per 50 ft increment - Does not meet RT 1, RT 2, or RT 3 |
| - Cat A: E = 0.85 | Cat B: E = 0.80/0.85 - Joint efficiencies per Table UW-12|
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Detailed Definition of Nameplate Markings
| Nameplate Marking | Code Clause Reference | Radiographic Extent on Welded Joints | Resulting Joint Efficiency ($E$) |
|---|---|---|---|
| RT 1 | UW-11(a)<br>UW-51 | • 100% Full Radiography along the entire length of all Category A, B, C, and D butt welds.<br>• All welds satisfy UW-51 acceptance criteria.<br>• Mandatory for vessels containing lethal substances or vessels exceeding thickness thresholds. | $E = 1.00$ (Type 1)<br>$E = 0.90$ (Type 2) for all joints |
| RT 2 | UW-11(a)(2)<br>UW-11(a)(5)(b) | • 100% Full Radiography on all Category A and D butt welds.<br>• Spot Radiography on Category B and C butt welds per UW-11(a)(5)(b).<br>• Allows seamless components or full-RT shells to utilize $E = 1.00$ without requiring 100% RT on circumferential seams. | $E = 1.00$ for Cat A<br>$E = 0.85$ for Cat B (Type 1) |
| RT 3 | UW-11(b)<br>UW-52 | • Spot Radiography performed along all Category A and B butt welds in accordance with paragraph UW-52.<br>• 1 spot (min $6\text{ in.}$) per $50\text{ ft}$ increment. | $E = 0.85$ (Type 1)<br>$E = 0.80$ (Type 2) |
| RT 4 | UW-11(c) | • Partial Radiography: When only a portion of the vessel welds are examined, or where examination satisfies neither RT 1, RT 2, nor RT 3.<br>• Individual joint efficiencies are calculated per Table UW-12 for each specific seam. | Varies by joint ($0.45$ to $1.00$) |
2. UW-51 Acceptance Criteria for Full Radiography
When a vessel weld is designated for Full Radiography (UW-51), the entire length of the weld seam is radiographed. Radiographs are evaluated against strict mathematical flaw rejection thresholds.
Absolute Rejections (Zero Tolerance Flaws)
Under ASME UW-51(b), the following planar discontinuities are strictly unacceptable, regardless of their length, depth, or location:
- Any type of Crack (longitudinal, transverse, crater, or star cracks).
- Lack of Fusion (LOF) (failure of weld metal to fuse with base plate or adjacent beads).
- Incomplete Penetration (LOP) (failure of weld root to penetrate through the full joint thickness).
Elongated Inclusions (Slag Inclusions & Piping)
Elongated volumetric indications (such as continuous or intermittent slag lines) are evaluated based on the nominal thickness ($t$) of the weld:
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| ASME UW-51 SLAG INCLUSION MAXIMUM LENGTH LIMITS |
| |
| [WELD THICKNESS RANGE] [MAX ALLOWABLE SINGLE LENGTH] |
| |
| t <= 0.75 in. (19 mm) -----------------------------> 1/4 in. (6.0 mm) |
| |
| 0.75 in. < t <= 2.25 in. (19 mm < t <= 57 mm) -------> 1/3 * t |
| |
| t > 2.25 in. (57 mm) -----------------------------> 3/4 in. (19.0 mm) |
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Aligned Slag Inclusions Grouping Rule
A group of aligned slag inclusions is unacceptable if the aggregate length of all indications exceeds $t$ within a weld length of $12t$, unless the separation distance between consecutive imperfections exceeds $6L$ (where $L$ is the length of the longest indication in the group).
Rounded Indications (Porosity & Tungsten Inclusions)
Rounded indications (indications whose length is $\le 3$ times their width) are evaluated per ASME Section VIII, Division 1, Mandatory Appendix 4:
- Appendix 4 provides sets of visual comparison charts representing maximum allowable concentrations of small, medium, and large pores for specific plate thickness brackets (e.g., $t < 1/8\text{ in.}$, $1/8\text{ in.}$ to $1/4\text{ in.}$, up to $t > 4\text{ in.}$). Porosity exceeding the chart density is rejectable.
3. UW-52 Spot Radiography Rules & Sampling Frequency
Spot Radiography (UW-52) is a statistical quality-control sampling protocol. It does not inspect the entire weld but provides assurance that the welding process and welder performance remain within acceptable quality bounds.
Sampling Increments & Dimensions
- Frequency: At least one spot radiograph must be taken for each $50\text{ ft}$ ($15\text{ m}$) of weld increment or fraction thereof.
- Welder Separation: Sampling must be performed separately for each welder or welding operator and for each welding process used on the vessel.
- Minimum Spot Length: Each spot radiograph must have a minimum length of $6\text{ inches}$ ($150\text{ mm}$) along the weld seam.
- Spot Selection: The Authorized Inspector reserves the right to select the exact physical locations of all spot radiographs.
UW-52 Acceptance Criteria vs. UW-51 Comparison
| Discontinuity Type | UW-51 Full RT Criteria | UW-52 Spot RT Criteria | Key Practical Difference |
|---|---|---|---|
| Cracks, LOF, LOP | UNACCEPTABLE (Zero tolerance) | UNACCEPTABLE (Zero tolerance) | Identical strict zero tolerance for planar defects. |
| Elongated Slag Length | Max $1/4\text{ in.}$ for $t \le 3/4\text{ in.}$;<br>Max $t/3$ for $3/4 < t \le 2.25\text{ in.}$;<br>Max $3/4\text{ in.}$ for $t > 2.25\text{ in.}$ | Max $2/3 \times t$ regardless of thickness (up to max allowable limits) | UW-52 allows twice the slag length permitted under UW-51 ($2/3t$ vs $1/3t$). |
| Rounded Indications (Porosity) | Strictly evaluated against Appendix 4 Charts | EXEMPT / NOT EVALUATED | Rounded porosity is acceptable under UW-52; Appendix 4 charts do NOT apply! |
4. UW-52 Tracer Radiography Protocol (Handling Defective Spots)
When a spot radiograph fails to meet the acceptance criteria of UW-52, the manufacturer cannot simply grind out the spot, reweld it, and ignore the surrounding weld seam. ASME Section VIII mandates a strict Tracer Radiography Protocol:
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| ASME UW-52 TRACER PROTOCOL STEP-BY-STEP |
| |
| [INITIAL 6" SPOT RADIOGRAPH] ---> REJECTED (Slag > 2/3t or Crack/LOF/LOP found) |
| | |
| v |
| [MANDATORY ACTION] -------------> Take TWO ADDITIONAL TRACER SPOTS (6" each) |
| in the same 50-ft weld increment |
| | |
| +---------------------+---------------------+ |
| | | |
| v v |
| [CASE 1: BOTH TRACERS PASS] [CASE 2: EITHER TRACER FAILS] |
| - 50-ft increment is accepted. - ENTIRE 50-FT INCREMENT IS REJECTED! |
| - Grind out, repair, and re-RT - Must either: |
| the original defective spot only. (A) 100% Radiograph entire length, |
| repair all defects, OR |
| (B) Cut out entire weld, reweld, |
| and re-examine completely. |
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Step-by-Step Breakdown of the Tracer Protocol:
-
Step 1: Defect Identified in Initial Spot:
- A $6\text{-inch}$ spot radiograph reveals a defect violating UW-52 (e.g., a crack, lack of fusion, or a slag inclusion measuring $> 2/3 \times t$).
-
Step 2: Take Two Tracer Spots:
- The manufacturer must immediately take two additional spot radiographs (tracer spots) at locations designated by the inspector in the same $50\text{-foot}$ increment of weld.
- These tracer spots must be taken away from the original failed spot to test the statistical quality of the remaining weld.
-
Step 3: Evaluate Tracers (The Dual-Pass Requirement):
- Subcase A (Both Tracers Acceptable): If both tracer spots satisfy UW-52 criteria, the $50\text{-foot}$ increment is considered acceptable. The manufacturer must then gouge/grind out the defective area identified in the initial spot, execute a weld repair, and re-radiograph that specific repair area.
- Subcase B (Either Tracer Fails): If both tracer spots do not meet UW-52 criteria, or if either of the two tracer spots fails to meet UW-52 criteria, the entire 50-foot weld increment is officially rejected.
-
Step 4: Remedying a Rejected Increment:
- When an increment is rejected under Subcase B, the manufacturer has two code-approved choices:
- Option 1 (100% Radiography): Radiograph the entire remaining length of weld in that $50\text{-foot}$ increment. All defective areas revealed by this 100% examination must be repaired, and all repairs must be re-radiographed.
- Option 2 (Complete Cut-Out): Completely remove and cut out the entire $50\text{-foot}$ weld increment, re-prepare the joint edges, re-weld the seam from scratch, and re-inspect with a new spot radiograph.
- When an increment is rejected under Subcase B, the manufacturer has two code-approved choices:
5. Comprehensive Summary & Synthesis Table
| Inspection Feature | ASME UW-51 (Full Radiography) | ASME UW-52 (Spot Radiography) |
|---|---|---|
| Examination Extent | 100% of the entire length of designated weld seams | At least 1 spot ($6\text{ in.}$ minimum) per $50\text{ ft}$ of weld or fraction |
| Sampling Unit | Continuous weld run | $50\text{ ft}$ increment per welder and per process |
| Cracks & Planar Flaws | Zero tolerance (Unacceptable) | Zero tolerance (Unacceptable) |
| Slag Inclusion Limits | $1/4\text{ in.}$ ($t \le 3/4\text{ in.}$); $t/3$ ($3/4 < t \le 2.25\text{ in.}$); $3/4\text{ in.}$ ($t > 2.25\text{ in.}$) | $2/3 \times t$ regardless of plate thickness |
| Rounded Indications (Porosity) | Must satisfy Mandatory Appendix 4 Charts | Acceptable / Appendix 4 does not apply |
| Action on Flaw Discovery | Repair defective area and re-radiograph repaired zone | Take 2 tracer spots; if either fails, reject entire $50\text{ ft}$ increment |
| Nameplate Marking Impact | Marks vessel as RT 1 or RT 2 ($E = 1.00$) | Marks vessel as RT 3 ($E = 0.85$) |
A vessel manufacturer is performing spot radiography per ASME Section VIII Division 1 paragraph UW-52. A 6-inch spot radiograph on a Category A longitudinal butt weld reveals a cluster of rounded porosity indications. How should this radiograph be evaluated under UW-52?
During fabrication of an ASME Section VIII pressure vessel, a required UW-52 spot radiograph on a 50-foot shell seam increment reveals an elongated slag inclusion measuring 0.75 inches on a 0.75-inch thick plate (which exceeds the 2/3*t limit). What is the mandatory next step required by the ASME Code?
An ASME Section VIII Division 1 pressure vessel nameplate is stamped with 'RT 2'. What does this marking indicate regarding the radiographic examination performed on the vessel?
Under ASME Section VIII Division 1 paragraph UW-51 for Full Radiography, what is the maximum permitted single length of an elongated slag inclusion in a weld with a nominal plate thickness of t = 1.50 inches?