Head Joint Efficiency UW-12(d)

[RPRad]

Folks

I know this has been discussed in different themes in the forum before but I would like to get a clear interpretation on it if possible....so a couple of questions on this issue.

a) When using a standard seamless pipe cap or seamless formed head it has always been my understanding that the joint efficiency used in the head thickness calculations is 0.85 unless the spot radiography requirements of UW-11(a)(5)(b)have been met on the circumferential weld in which case a joint efficiency of 1.0 can be used.

I have been recently told by a pressure vessel manufacture that the joint efficiency of the seamless portion of the head is always 1.0 and that the 0.85 is referring only to the calculation of the thickness of the seamless straight flange portion of the head (ie effectively calculating the thickness of the straight flange as section of the shell )...I have never heard an interpretation like this before....is such an interpretation possible? Does ASME have an official interpretation on this point?

b) Assuming the pressure vessel manufacture is not correct, it appears that under UW-12(d) that if the seamless head is welded to the shell using a Type 3 weld (single V)that you cannot use a joint efficiency higher than 0.85...regardless of whether you spot radiograph the joint or not

If you fully radiograph the joint can you use a joint efficiency of 1.0 for the head thickness calculations or are you always limited to 0.85 when a Type 3 Circ Weld is used?

R

 

[JStephen]

When you calculate the minimum head thickness, that governing thickness is actually in the knuckle area, and would use the specified joint efficiency.

You are allowed to calculate a reduced thickness in the crown part of the head for use in reinforcing calculations, and that reduced thickness could be based on a joint efficiency of 1 for a seamless or fully radiograhed head, if I remember right, even if head thickness was based on 0.85. That should be spelled out in the reinforcing sections, not as an interpretation.

 

[CodeRef]

I believe this interpretation may help:

Interpretation: VIII-1-98-109
Subject: Section VIII, Division 1 (1998 Edition, 1999 Addenda); UW-11(a)(5)(b)
Date Issued: March 22, 2000
File: BC00-081

Question: The thickness of a two-piece flanged-and-dished head with a straight flange is calculated using a joint efficiency of E = 1 for the Category A weld. Calculation permits an E = 0.70 to be used for the thickness of the straight flange, when the flange is calculated as a cylindrical shell. The shell of the vessel is calculated using an E = 0.70 for the Category A and B welds.
Is the spot radiography requirement of UW-11(a)(5)(b) in Section VIII, Division 1 required on the circumferential weld connecting the head to the shell when per UW-11(a)(5), the design of the straight flange is not "based on a joint efficiency permitted by UW-12(a)"?
Reply: Yes.

If you fully radiograph the joint can you use a joint efficiency of 1.0 for the head thickness calculations or are you always limited to 0.85 when a Type 3 Circ Weld is used?

You are always limited to 0.85 when a type 3 Circ weld is used. Please reference the last sentence of UW-12(d) which states the following:
E = 0.85 when the spot radiography requirements of UW-11(a)(5)(b) are not met, or when the category A or B welds connecting the seamless vessel sections or heads are Type no 3, 4, 5 or 6 of Table UW-12.
This is supported by the answer to the 3rd question in
Interpretation: VIII-1-04-101
Subject: Section VIII, Division 1 (2004 Edition, 2005 Addenda); UW-11(a) and UW- 12(d)
Date Issued: June 20, 2006
File: BC05-1574


FAQ731-376

 

[RPRad]

Folks

In the case of seamless hemispherical head the circ weld attaching the head to the shell is a category A weld and the Code is clear in that it is considered part of the head, and therefore follows the joint efficiency (based on radiography performed) applied in Table UW-12

Where it becomes less clear (at least as far as the PV manufacturer is concerned) is what joint efficiency you use when you are calculating the min required thickness of a single piece seamless ellipsoidal or torispherical head which is welded to the shell (Category B Circ). Most manufacturers (and standard pipe caps) provide some amount of straight flange in order to make it easier to weld the head to the shell.

In my mind, as the Code has clearly gone out of its way to indicate that the joint efficiency used in the calculation depends upon amount of radiography performed on the (Category B) circ weld, then that is the joint efficiency that should be used in calculating the min head thickness ....the fact that the head has been manufactured with a portion of straight flange should have no bearing on the min head thickness calculation (ie provided it is manufactured with approximately the same thickness as the main body of the head). I dont see that having a straight flange allows you to use a joint eff of 1.0 in the min head calculations instead of 0.85 when the spot radiography requirements on the circ weld are not met. The example calculations in Appendix L appear to support this (See in particular examples L-1.5.2 and L-1.5.3 in Table L-1.5.3).

The danger in misinterpreting this is that you use a thinner head (or end up with less corrosion allowance) than specified by code because of the difference in joint efficiency.

I agree that it appears if you were to fully radiograph the Type 3 Cat B weld that you still must use a joint efficiency of 0.85 in the min head thickness calcs

R

 

[SnTMan]

RPRad, I would agree with your interpretation and disagree with your PV manufacturers'. I think UW-12(d) is pretty clear.

Is the manufacturers' straight flange thinner than the knuckle?

Regards,

Mike

 

[doct9960]

RPRad,

I have been recently told by a pressure vessel manufacture that the joint efficiency of the seamless portion of the head is always 1.0 and that the 0.85 is referring only to the calculation of the thickness of the seamless straight flange portion of the head

Scary coming from a pressure vessel manufacturer. : )

There's an article in the National Board Bulletin (Fall 2008) about common misconceptions when applying Code rules. One of the topics is joint efficiencies in seamless shells and heads.

http://www.nationalboard.org/SiteDocuments/E-Publications/fall08web.pdf

 

[RPRad]

doct9960

That article pretty much nails it, actually that issue has some other good articles in it as well

I agree, I think the PV manufacturer is trying to "split hairs" and maybe somewhere down the way he was able to convince somebody on the regulatory side of it,but clearly you cant change the joint efficiency because it has a straight flange (I suppose you could conceive of a situation where for some reason you may want to make the straight flange thinner than the main body and want to try and make an argument for a separate evaluation of the straight flange...but the min thickness of the head would still be dependent upon inspection of the circ weld.)

R

 

[CodeRef]

There must be some miscommunication somewhere. Disagreements can always be solved by referencing a Code paragraph. I cannot see any interpretations or Code paragraphs allowing what you say the manufacturer is stating. It just does not make any sense at all. my $0.02

FAQ731-376

 

[RPRad]

No miscommuication, I pointed him to the examples in Appendix L...and the response was ASME gets those wrong all the time! His basic premise was that inspection performed on the circ weld should have no bearing on the thickness of a seamless head with regard to pressure (I think he is a finite element guy). While I can see his point, the rules indicate otherwise.

Anyway I forwarded the article Doct9960 sent across to him....hopefully he spends the day checking his past calcs...

R