Follow this article, we would like to share you about Ug determination and calculation the distance from Source to Object.

First thing, I just ask you about the different between Ug and IQI selection? Yes, some guy may say that it is the same selection which are based on the single of base metal wall thickness.

I think you may wrongly understand about Ug determination with IQI selection.

IQI Selection

Based on nominal single-wall material thickness plus the actual weld reinforcement thickness estimated.

Although the radiographic beams penetrate through 2 walls.

Ug Recommendations

 Applicable to both single-wall and double-wall radiographic techniques. So we are easy understand this is a shape not a wall thickness.

if OD < 50mm, Ug = 0.51 mm

If OD up to 75 mm, Ug = 0.76 mm

Second thing, Ug determination accordance with ASME BPVC Sec V Article 2 (Snap shot for reference only)

Ok, Lets check with us about it.

If pipe NSP 2" 5.54 mm wall thickness subject to RT by DWDI technique. We have look to below example:

  • Ug = 0.51 = Fd/D  => D = 3.16 * (60.3+1.5*2) / 0.51 = 373 mm.
  • Ug = 0.76 = Fd/D  => D = 3.16 * (60.3+1.5*2) / 0.76 = 263 mm.
  • d = 60.3 + 1.5*2, 1.5 mm is an estimated reinforcement at one side. So, both sides are 1.5*2 = 3mm.
  • F = 1*1 + 3*3 = 10 mm2,  => The square root of 10 is 3.16 mm.

Follow the above formulas, If we reduce D dimensions, It will lead to decrease the exposure time.

and vice versa. 

So, all NDE-subcontractors like it and improvement the productivity (refer below the formula  for clearance)

Third thing, to clear above the question, you need to visit an ASME Interpretation portal and verify it at the interpretation number V-13-04 here.

Questions and Answers

To clear the questions which we posted on the fanpage of Welding Inspection from BCK055 or Welding Info.

OK, follow this question, just a photo will make you clear.

Ug = 0.76 mm => The max. OD dimension will show in the RT film, L = 60.3 + 2*0.76 = 61.82 mm (if film wrapped to cover weld seam). 

But, the actual RT film placed as plane and perpendicular with radiography beam. So, the OD of pipe will show in RT film is 68 mm.

OK, Come back the above photo, this RT film is acceptable?

So, If you are a Client, Rep's Client must place yourself at them's scopes due to make a RT film at site is not easy like you review it at the office. If you do you need to point at the Ug, so many findings are found and Subcontractor say good bye.

    In my opinion

    If OD of NPS 2" (60.3mm) pipe' RT image was measure over 70 mm, it should consider re-shoot. The welding inspector should keep calm and double check the root of causes. With me for the first time, you just make a soft remind to them for avoid happen again and preventive actions.

    If OD of NPS 2" pipe' RT image was measure less than 68 mm,  it should accept for RT film. Why you are considered to accept the RT film?

  • The OD of pipe may be distorted or deformed, or ovality after welding?
  • This weld joint may fix at restricted locations and was it really hard to access the source.
  • If RT by X-ray, so it is very hard to adjust the X-Ray machine.


I have reviewed many RT films like that from the subcontractor and vendor, and I have never rejected any RT films. It just make a soft remind them, but I always focus at the other points such as IQI visible and sensitivity, film class, technique, etc. 

You must support them to catch up the fabricating schedule and productivity, please don't point at an unnecessary works.

Are you agree with me? Please leave your comments here.

Nguyen Duy Bien

Enjoy learning and researching, reading book. Knowledge welding inspection painting inspection.

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Maria Gracia
Maria Gracia

Thanks for sharing such a nice post which is very informative. Welding is a fabrication or sculptural process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool causing fusion. Welding is distinct from lower temperature metal-joining techniques such as brazing and soldering, which do not melt the base metal.