Minimum SOD for pipe

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.

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