Understanding Film Placement
What Is Film Placement?
Correct film placement is essential for producing radiographs that provide complete coverage of the inspection area while meeting the requirements of the applicable radiographic technique and inspection standard.
This calculator helps determine the optimum position, spacing and overlap of radiographic films around a pipe or cylindrical component. By entering the pipe dimensions and the number of films, the tool calculates the recommended layout to achieve consistent circumferential coverage.
Why Film Placement Matters
Proper film positioning reduces the likelihood of missed areas, unnecessary repeat exposures and excessive film usage. Correct positioning helps to:
- Achieve complete weld coverage.
- Maintain the required image quality.
- Reduce repeat radiographs.
- Optimise film usage.
- Improve inspection efficiency.
- Meet radiographic procedure requirements.
Incorrect placement can leave unexamined areas or create excessive overlap that increases cost without improving inspection quality.
How the Calculator Works
The calculator uses the pipe circumference together with the selected number of films to determine:
- Centre position of each film.
- Angular spacing around the pipe.
- Film start and end positions.
- Circumferential coverage.
- Film overlap.
- Overall inspection layout.
If film length and minimum overlap are entered, the calculator also verifies whether complete coverage can be achieved.
Exposure Techniques
SWSI (Single Wall Single Image)
The radiation beam passes through one wall of the pipe and produces a single image on the film. Best suited for:
- Small to medium diameter pipe.
- High quality weld inspection.
- Critical fabrication work.
- Most code compliant butt weld examinations.
DWDI (Double Wall Double Image)
Both pipe walls are exposed simultaneously, producing two wall images on a single radiograph. Typically used for:
- Small diameter piping.
- Limited access locations.
- Situations where SWSI is impractical.
DWSI (Double Wall Single Image)
The beam passes through both walls, but only one wall is interpreted. Often used where access is restricted while maintaining acceptable image quality.
Film Overlap
A small overlap between adjacent films helps ensure that no part of the weld or inspection area is missed. Too little overlap may leave gaps in coverage. Too much overlap increases film consumption, exposure time and processing costs without adding inspection value. The required overlap should always follow the applicable inspection procedure or client specification (typically 20mm to 25mm, or 1 inch).
Typical Film Sizes
| Film Size |
Typical Applications |
| 60 × 240 mm | Small bore pipe |
| 80 × 300 mm | Pipe welds |
| 100 × 400 mm | Medium diameter pipe |
| 100 × 480 mm | General fabrication |
| 120 × 480 mm | Large diameter pipe |
| 350 × 430 mm | Pressure vessels and plate welds |
Film selection depends on pipe diameter, weld width and the chosen exposure technique.
Best Practices & Guidelines
Best Practice Workflow
- Measure the pipe outside diameter or circumference.
- Select the appropriate radiographic technique.
- Choose the correct film size.
- Determine the minimum overlap required.
- Calculate the number and position of the films.
- Mark each film location around the pipe.
- Position IQIs, identification markers and lead letters.
- Perform the exposures and verify complete circumferential coverage.
Engineer's Tips
- Measure the actual pipe outside diameter before planning the layout.
- Confirm the required overlap in the applicable inspection procedure.
- Mark film positions on the pipe before starting exposures.
- Verify that weld identification and IQIs can be placed without interfering with the area of interest.
- Consider source position and beam coverage when selecting film locations.
- Keep film spacing consistent to simplify exposure planning and reporting.
Factors That Affect Film Placement
- Pipe outside diameter & Circumference.
- Film size & Required overlap.
- Exposure technique & Weld width.
- Inspection standard.
- Available access & Source location.
All should be considered before commencing radiography.
Common Mistakes
- Forgetting to include the required film overlap.
- Uneven spacing between films.
- Using incorrect pipe dimensions.
- Selecting a film length that is too short for the required coverage.
- Ignoring the requirements of the radiographic technique sheet.
- Assuming equal spacing always provides complete coverage.
- Not marking film positions before exposure.
Common Technique Selection Guide
| Technique |
Typical Application |
Image Quality |
Access Required |
| SWSI | Medium to large diameter pipe | Excellent | Access to both sides |
| DWSI | Restricted access piping | Very Good | Single side access |
| DWDI | Small diameter pipe | Good | Single side access |
Frequently Asked Questions & Quick Facts
Why is film overlap necessary?
Overlap ensures there are no unexamined gaps between adjacent radiographs and provides complete inspection coverage around the component.
How many films should I use?
The required number depends on the pipe circumference, film size, required overlap and the applicable radiographic technique.
Should the films always be equally spaced?
For most circumferential inspections, equal spacing provides consistent coverage. However, special geometries or restricted access may require alternative layouts.
Does film length affect coverage?
Yes. Longer films cover a greater portion of the circumference, reducing the number of exposures required.
Can this calculator replace a radiographic procedure?
No. The calculator is intended to assist with planning. Final film placement should always comply with the approved radiographic procedure and the applicable inspection standard.
Did You Know?
- Proper film planning can significantly reduce repeat radiographs and improve inspection productivity.
- Larger diameter pipes generally require more films to achieve complete circumferential coverage.
- Excessive film overlap increases material costs without necessarily improving image quality.
- SWSI is generally the preferred technique whenever access and geometry allow, as it produces superior image quality compared with double wall techniques.
- Accurate film positioning helps ensure consistent image interpretation and complete weld examination.
- IQI Placement: Ensure Image Quality Indicators (IQIs) are correctly placed for each film segment. Codes usually require source-side placement. If the source side is inaccessible, film-side placement is permitted provided the appropriate lead letter ("F") indicator is utilized.
- Geometric Unsharpness (Ug): Always verify your Source-to-Object Distance (SOD) against the Object-to-Film Distance (OFD). Maintaining the correct ratios is critical to achieving acceptable geometric unsharpness as dictated by your governing code.
Important Notice: This calculator is intended as an engineering planning and educational tool. The calculated film positions and overlap values are theoretical and should be verified against the approved radiographic procedure, applicable inspection standards, customer specifications and site requirements before production radiography is carried out. Proper film placement, identification markers, IQI positioning and exposure techniques remain the responsibility of qualified radiographic personnel.
Film positions are equally distributed around the entered circumference starting at Top Dead Center (0°). Errors and omissions excepted.