What is the formula for minimum sod for pipe?

SOD_min = F x OFD / Ug_max; Panoramic: OFD = wall, SOD available = ID/2; SWSI: OFD = wall, SOD available = ID; DWSI/DWDI: OFD = OD, SOD chosen by the technician; SFD_min = SOD_min + OFD. Minimum source-to-object distance for panoramic, single-wall and double-wall pipe techniques, and whether the geometry available inside the pipe can meet the unsharpness limit.

What should I check before relying on the result?

The Ug limit is taken on nominal single-wall thickness even where the beam penetrates two walls. Double-wall techniques put the source-side wall a full outside diameter from the film, which is what drives the long standoff. Panoramic geometry is fixed by the pipe - if it fails, no exposure adjustment will fix it. Verify against the governing edition: ASME BPVC Section V, Article 2, T-274.2 and T-271.2 (2023).

Minimum SOD for pipe — RT Calculator

The unsharpness rule Ug = F x OFD / SOD is the same on pipe as on plate, but the object-to-film distance depends entirely on which technique is used, and it varies by more than an order of magnitude between them.

For a panoramic shot with the source centred inside, the film is wrapped on the outside surface and only the wall thickness separates the weld from the film, so OFD = wall. But the source-to-object distance is fixed by the pipe itself at ID/2 – you cannot back off. On small-bore pipe that is a hard geometric constraint: if F x wall / (ID/2) exceeds the limit, the shot simply cannot be made panoramically with that source, no matter how the exposure is set.

For single-wall single-image with the source inside against the bore, the far wall is imaged at OFD = wall with SOD = ID. For double-wall techniques the source is outside and the source-side wall sits a full outside diameter from the film, so OFD = OD and the required standoff jumps accordingly – which is why double-wall shots on even modest pipe need a metre or more of clearance.

The limit itself comes from ASME BPVC Section V, Article 2, T-274.2, keyed to nominal single-wall thickness. Meeting it is a precondition for the shot, not something that can be traded against exposure time – a blurred radiograph fails on IQI regardless of how good the density is.

Worked example

Technique	dwsi
Pipe outside diameter	168.3 mm
Nominal wall thickness	12 mm
Focal spot / source size	2.4 mm
Override Ug limit (0 = use ASME)	0 mm
Pipe inside diameter	144.3 mm
Unsharpness limit	0.508 mm
Object-to-film distance for this technique	168.3 mm
Minimum SOD	795.12 mm
Minimum SFD	963.42 mm
SOD used for the check	795.12 mm
Ug at that geometry	0.508 mm

For a double-wall shot the source-side weld is one full outside diameter from the film, so OFD = 168.3 mm. At 12 mm nominal wall the ASME limit is 0.508 mm, giving SOD_min = 2.4 x 168.3 / 0.508 = 403.92 / 0.508 = 795.12 mm. SFD_min = 795.12 + 168.30 = 963.42 mm.

Use at your own risk — verify before you act

These calculators support, and never replace, the judgement of qualified NDT and engineering personnel. Results are provided as is, without warranty of any kind, express or implied, and must be independently verified against the governing code edition named in your contract before being used in any inspection, acceptance, rejection, radiation-safety or fitness-for-service decision. By using them you accept full responsibility for how the results are applied; NDT Inspect, its owners and contributors accept no liability for any loss, damage, injury or death arising from their use or from reliance on them. If a result matters to safety, check it by hand and have it reviewed by a competent person.