Backing strips? 1

[RonJeremy]

Yes, before you ask: I HAVE searched this subject on Eng-Tips.
For the last couple of decades I have worked in an industry that prohibits the use of backing strips in Pressure Vessels. Why?
OK, I know what backing strips are for, and when they are appropriate, but I do not understand why there is such a 'downer' on them in Oil/Gas.
A few years ago I visited a first class German shipbuilder (not Oil & Gas, of course) who welded, bombed, sliced and macroed a small diameter weld with permanent backing strip for me - perfect fusion and no inconsistencies in the RT. I had a similar experience at a very reputable Italian pressure vessel manufacturer, also with a weld which was inaccessible for back-gouging for the 2nd side.
So why do people have such a problem with backing strips? I could NOT fault the RT versus the macro from that shipbuilder. The radiograph was PERFECT (IMO). The backing strip was fully attached to the pipe, both by fusion with the weld itself, and by the continuous fillet that attached it to one part of the pipe. Absolutely perfect welding. Do people assume that GTAW roots are necessarily essential when the second side is not accessible for back-chipping?
Mech Engr, not welding engineer asking ......

 

[metengr]

Backing strips reduce joint efficiency per design rules in ASME Section VIII Div 1. Second, backing strips can be sources of crevice corrosion, SCC and corrosion fatigue. Third, backing strips can interfere with radiographic examination interpretation.

 

[RonJeremy]

Thank You metengr. I should have said that I was aware of your points 1 and 2 (why didn't I say that in the OP????). I did NOT know that they contribute to SCC and corrosion fatigue. I have seen RTs that show very good correlation with the actual weld, when macroed (see OP). Under what circumstances do backing strips mask interfere with the RT?
Thank You again.

 

[metengr]

An example, Section I of ASME B&PV Code does not permit backing strips to remain for longitudinal welds because they must be proven as full penetration welds. There is no joint efficiency factor to rely on to compensate for not having a full penetration weld.

 

[dik]

Thanks metengr... there's at least another guy that didn't know.

Dik

 

[weldstan]

One more point. If improperly installed (not in intimate contact with the base metals) cracking can occur.

 

[jzmx]

Further to corrosion and erosion, and the acceptance of weld by RT, there is a concern of crack of the weld attaching the backing material after years operation, and the loose material become a risk of system blockage.

 

[david339933]

Backing strips should be attached by tacking IN the weld joint...and subsequently incorporated into the weld...not on the inside surface of the vessel/head.