A girth weld is a closed loop, and a flat film sees only a limited arc of it before the beam starts passing through the wall at too steep an angle. At an angle phi away from the perpendicular, the path length through the wall rises as t / cos(phi). Once that path exceeds the permitted ratio k relative to the nominal wall, the density falls outside the acceptable band and the IQI requirement fails at the edge of the film.

Setting 1/cos(phi) = k gives the usable half-angle phi = acos(1/k), so the arc covered per exposure is 2 x acos(1/k) and the number of exposures is ceil(360 / arc). With the common class B ratio of 1.1 that is about 49 degrees per shot and eight exposures around the pipe; relaxing to 1.2 gives 67 degrees and six.

The technique chosen changes the picture entirely. Panoramic – source centred inside the pipe, film wrapped all the way round – covers the whole circumference in a single shot with the beam perpendicular everywhere, which is why crawler radiography of pipelines is so productive. Single-wall single-image with the source inside against the wall images the far wall only, and double-wall single-image with the source outside penetrates both walls but only the film-side wall is interpreted; both need multiple indexed shots, with three as the practical minimum for full coverage. Double-wall double-image – the elliptical technique on small-bore pipe – superimposes both walls on one film and needs two exposures 90 degrees apart.

ASME BPVC Section V, Article 2, T-271.2 restricts the elliptical technique to pipe of 3.5 in outside diameter or less; above that, double-wall single-image with a minimum of three exposures is used. Coverage must also be proved by the film length actually available.

Worked example

Technique	dwsi
Pipe outside diameter	168.3 mm
Nominal wall thickness	12 mm
Permitted through-wall thickness ratio	1.1
Usable half-angle	24.62 deg
Arc covered per exposure	49.24 deg
Exposures from obliquity limit	8
Exposures required	8
Index between exposures	45 deg
Penetrated thickness	24 mm

A 1.1 thickness ratio allows cos(phi) = 1/1.1 = 0.9091, so phi = 24.62 degrees and each exposure covers 49.24 degrees. 360/49.24 = 7.31, rounded up to 8 exposures indexed at 45 degrees. As a double-wall single-image shot the beam penetrates 2 x 12 = 24 mm of steel, although only the film-side wall is interpreted.

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