5% vs 10% radiography of butt welds in normal service

[SNORGY]

ASME B31.3 calls for 5% of all butt welds in a lot to be radiographed for 100% of their lengths. However, common industry practice - for decades - appears to have been 10%. Does anyone know what started that and why?

 

[moltenmetal]

5% is a code minimum. Not wanting to be outdone, people did 2x the code minimum. 5% full random with each welder's work represented is more than sufficient in our experience to warn the welders that "we're watching". We see no additional benefit from 10%.

 

[weldstan]

For the vast majority of B31.3 normal service pipe projects that I have worked on in the past 40+ years, 5% random RT has been required. There have been a number of projects that required 10% and even greater RT. Statistically, there is somewhat greater assurance of more welds meeting the workmanship acceptance criteria defined in the Code. It is also possible that additional repairs derived therefrom may be more detrimental than the discontinuity itself, if it were not found and left in place,

 

[racookpe1978]

Now, if you do 5-10% random, and find 20-30% of a single welder's work is bad, THEN you have specific and justifiable cause to require all of his/her be RT'ed.

And to fire that welder.

 

[BigInch]

So making the repairs defeats the purpose of making repairs. That kind of logic defeats the purpose of raidographing entirely. I doubt that making repairs would be so detrimental, at least assuming that you made repairs using a different welder and radiographed the repairs. 100% radiographing is often used on pipelines, offshore and in high consequence areas increasing the number of flaws found and to no observed dencrease in reliability that I'm aware of.

 

[weldstan]

The 5% RT requirement is primarily a check on welder workmanship. It s well known metallurgically and through fracture mechanics that some discontinuities, considered by Code as defects, are not injurious in Normal Fluid Service. Localized repairs are often more injurious from a metallurgical standpoint due to creation of unbalanced stresses at the repair location(s).

EPRI is currently investigating these types of issues for Pipe welds in the Power industry. In the late 1970's considerable work was done by the government as a result of the RT cheating on the Alaska Pipeline in this regard.

 

[BigInch]

Yet it appears that DOT stuck to its guns and required that ALL welds that were identified as unacceptable were repaired.

http://testimony.ost.dot.gov/test/pasttest/76test/Barnum5.PDF

 

[weldstan]

Not so much DOT but a Federal judge. The pipeline Owners promised 100% RT to the Feds and the judge made them do it. The work performed to e=valuate discontinuities did help in API 1104's decisions to include fracture mechanics in reducing acceptance standards and I was on the API 1104 Committee during this time frame. It is also noted that the operating parameters in cross country pipeline are considerably different than generally found in B31.3 Normal Fluid Service. Systems in Severe cyclic service require greater scrutiny because of the well documented adverse effect of weld discontinuities in fatigue service.

 

[EdStainless]

I was involved in a vessel job some years ago. When we did the RT (10%) it was with film, and they didn't have the usual size available so they shot with larger sheets. In 90% of the shots there were defects in the longitudinal welds just beyond where the usual film size would have reached. The owner went ballistic and demanded 100%. A new crew of welders was brought in for the repairs.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[weldstan]

One final observation; from my API 1104 Committee experience, it is next to impossible to reduce rejection criteria once established in a Code.

 

[Gator]

EdStainless, did you mean *smaller* sheets?

Piping Design Central

 

[BigInch]

If I compare NDT criteria B31.3 & 8, pipelines would appear to either have considerably worse service conditions, or a very much lower tolerance for faults.

B31.8
826.3 The following minimum number of field butt welds shall be selected on a random basis by the operating company from each day’s construction for examination.
(1) 10% of welds in Location Class 1
(2) 15% of welds in Location Class 2
(3) 40% of welds in Location Class 3
(4) 75% of welds in Location Class 4
(5) 100% of the welds in compressor stations, and
at major or navigable river crossings, major highway
crossings, and railroad crossings, if practical, but in no
case less than 90%. All tie-in welds not subjected to a
pressure proof test shall be examined.

A826.2.1 (Offshore)
90-100%

B31.4
Generally 10%, except that all welds in the following locations (or a min of 90% of the welds, if some are inaccessible) shall be completely inspected;
(a) within populated areas such as residential
subdivisions, shopping centers, and designated commercial
and industrial areas
(b) river, lake, and stream crossings within the
area subject to frequent inundation; and river, lake, and
stream crossings on bridges
(c) railroad or public highway rights-of-way,
including tunnels, bridges, and overhead railroad and
road crossings
(d) offshore and inland coastal waters
(e) old girth welds in used pipe
(f) tie-in girth welds not

 

[DekDee]

Big Inch,
Have a look at the differences between acceptance criteria's for B31.8 ( API 1104) and B31.3 ? - I do not have access to B31.4.
Night and day difference on what is acceptable irrespective of % required,
Cheers,
DD

 

[BigInch]

No argument with that, however a bad one, no matter how you define it, is statistically more likely to be found with the increased number of tests.