Low Temp. Joint Type

[Shmulik]

Acc. to UW-2 (b) Low Temp. Welds using metal backing strips which remain in place are not permitted. My question is, what is the reason for that?
Is that refer to MDMT colder than –55 ºF or –20 ºF ?

 

[metengr]

Shmulik;
I am not seeing this reference in the 2004 Edition of ASME Section VIII, Div 1, Part UW. I might have missed it. The only reference to backing rings for impact testing is Part UCS-66 (a) in the second paragraph. Backing rings is specifically mentioned as part of components, which remain in place, and would be treated as a separate component for impact testing; material classification, thickness and design metal temperature.

 

[Shmulik]

metengr,
UW-2 (b): When vessels are to operate below certain temp. designated by part UCS (see UCS-68), or impact tests of the material or weld metal are required by part UHA, the various categories (see UW-3) shall be as follows:
(1) All joint of Category A shall be Type No. (1) of Table UW-12 except that for austenitic Cr-Ni …..
(2) All joint of Category B shall be Type No. (1) or No. (2) of Table UW-12

UCS-68 all along regards to MDMT of –55 °F.
UHA-23 regards to MDMT of –20 °F.
Which of them is the governing?

Best regards,
Shmulik

 

[ijzer]

At low temperature there is always the danger of brittle fracture , especialy initiated at notches. A backing ring always leaves a gap with could act as a crack starter.

 

[metengr]

Ah ha.. I believe I understand your question. First part, for lethal service restrictions in UW-2, all Category A weld joints shall be Type 1 weld joints with no backing strips remaining in place, per Table UW-12 because the welds must have surface finish requirements meeting UW-35 AND must be full penetration, as mentioned by ijzer. Service temperature has nothing to do with this requirement.

All Category B and C weld joints shall be either Type 1 or Type 2, per Table UW-12. Here you have flexibility for using backing strips.

For low temperature service per UW-2 (3) (b), wording remains the same, the Category A weld joints shall be Type 1 per Table UW-12. For Category B and C weld joints, these can be either Type 1 or Type 2.

The use of backing strips has nothing to do with minimum design metal temperature. Use of materials for low service temperature is governed by the requirements in UCS or UHA. So, you need to review these sections for your service conditions. Permitted use of backing rings is per Table UW-12.

 

[Shmulik]

metengr,
I’m afraid this time I disagree with your conclusion that “Service Temp. has nothing to do” with weld type requirement. You have suggested me to review the sections of service conditions, and so I really done.
What I have succeeded to dig is as follows:
1) UW-2 (b) is very obvious talking about vessels are to operate below certain temp. designated by part UCS.
2) In the guide book of “Pressure Vessels – the ASME Code simplified” by Robert Chuse & Stephen M. Eber (I’m looking at 6th edition, page 106) it explained that the vessels which demand special requirements listed in code par. UW-2 “Service Restrictions” are Vessels intended for lethal service; Vessels that are to operate below –20 ºF ; Unfired steam boilers with des. press. exceeding 50 psi.
3) In the “Pressure Vessel Handbook” by Eugene F. Megyesy (12th edition): Design of Welded Joints (page 175) paragraph 5 defines Vessels Operated Below –20 ºF for joints A Acc. to type No. (1) and joints B Acc. to type No. (1) or No. (2).

Is there anything that I maybe didn’t see or I misunderstand?

One additional point: as I now realize, only the joints of Category A (Longitudinal) are not permitted for Type No. 2 (with backing strip which remain in place). Joints of Category B (Circumferential) may be made acc. to Type No. 2.

Since the logic of the explanation suggested by ijzer should work at either category A & B, thus, why backing strip is permitted for the circumferential case?

 

[metengr]

Sorry about the confusion. The stresses in circumferential welds are approximately half of the primary (hoop) stresses in long seam welds, and as such, having a stress riser caused by a backing strip in a long seam weld can result in greater susceptibility to crack propagation regardless of low service temperature because you also have concerns related to fatigue or cycling loads in the pressure vessel. Also, if you review Table UW-12 the requirements are very clear of Category A welds – you must assure finish requirements and full penetration – you can’t do this with backing strips in place.

My comment is that the use of backing strips is dictated by the requirements in Table UW-12 as referenced for lethal service. I did not exclude or downplay low temperature service.