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#73342 - 06/13/19 08:30 AM Using Thicker TEE to Reduce Stresses in B31.3
M Waheed Offline
Member

Registered: 03/08/07
Posts: 36
Loc: UK
Is it allowed in B31.3 to use the thicker TEE alone to reduce the stresses or do we have to use the thicker matching pipe for of some length with the TEE?

When calculating SIF in Appendix D Thickness of matching pipe is used for the TEE and Fitting thickness is used for the elbow and mean radius of the matching pipe is used for both elbows and Tees.

Appendix-D of B31.3 also says "(8) Stress Intensification Factors for branch connections are based on tests with at least two diameter of straight run pipe on each side of the branch"

Therefore it seems that thicker TEE alone cannot be used to reduce the stresses in B31.3
Similarly thicker elbow alone cannot be used as well.

Your opinion on this will be highly appreciated.

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#73343 - 06/13/19 03:33 PM Re: Using Thicker TEE to Reduce Stresses in B31.3 [Re: M Waheed]
Michael_Fletcher Offline
Member

Registered: 01/29/10
Posts: 710
Loc: Louisiana, US
I think what they're trying to get at is that there are no fittings attached to the tee to inhibit or promote ovalization in the tee, and that real world SIFs may vary if you attach, for example, flanges (to inhibit ovalization) or elbows (to promote ovalization).

It does not state "two diameters of piping with equal thickness to the tee."

Most forged tees are already thicker than the pipe, at least in certain parts, and we generally don't take credit for this fact, because exact thickness rarely is known in advance, unless you're on really good terms with a fittings manufacturer.

However, with that said, if you had a custom application with SCH 80 piping, and you said "make me a SCH 160 tee, instead," I'd absolutely input it as SCH 160.
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#73345 - 06/14/19 03:10 AM Re: Using Thicker TEE to Reduce Stresses in B31.3 [Re: M Waheed]
M Waheed Offline
Member

Registered: 03/08/07
Posts: 36
Loc: UK
Thanks Michael Fletcher,
You confirmed what I have been thinking and what I have used in a recent project.
Could you please shed some light on Elbows. The actual fitting thickness is used in SIF for elbows in B31.3. However we generally do not input the value of fitting in elbow input spreadsheet in CAESAR. So if we use a thicker elbow (Sch 80) with s Sch 40 pipe, do we have to input the thickness in the input spreadsheet or use element Sch 80 thickness for the the elbow element?

Thanks for your help

M. Waheed

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#73346 - 06/14/19 05:03 AM Re: Using Thicker TEE to Reduce Stresses in B31.3 [Re: M Waheed]
engineer001ch Offline
Member

Registered: 12/21/18
Posts: 11
Loc: China
I think you can enter Sch 80 bend wall thickness, but CAESAR II will still use mean diameter of connected pipe. You need to add 2 additional nodes on both bend ends and enter Sch 80 pipe wall thickness

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#73347 - 06/14/19 09:08 AM Re: Using Thicker TEE to Reduce Stresses in B31.3 [Re: M Waheed]
Michael_Fletcher Offline
Member

Registered: 01/29/10
Posts: 710
Loc: Louisiana, US
There's an option in the bend dialog to change elbow thickness. You may alternatively break the element "before" and "after" the bend so that you can have your elbow be a discrete element on its own, but that's tedious, but I suppose there are benefits:

1. You can visually review elbows (and therefore provide visual documentation) that the elbows are indeed different.
2. You can assign larger ODs and WTs, if this is the case. (Note that you'll also need to look into custom specifying elbow radius, too.)

There are also detriments:

1. More time to input. More risk of error.
2. More likely to encounter "do not specify node on bend" errors.
3. More node management.

In my experience, you shouldn't have to input SIFs (or flexibility factors) on bends in most normal cases. Exceptions include:

1. You have FEA results you want to incorporate, just for a more accurate answer.
2. You have a trunnion attachment, in which case you have to use FEA.
3. You have non-standard bends for your application that have their own SIFs and flexibility factors per the manufacturer.

The last one is a bit of an oddity. I've seen it in cases where manufacturers can't yet comply with new FRP standards (maybe they're working on it, maybe they aren't), and continue to re-purpose codes such as B31.3 for FRP.
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#73348 - 06/15/19 06:08 AM Re: Using Thicker TEE to Reduce Stresses in B31.3 [Re: M Waheed]
engineer001ch Offline
Member

Registered: 12/21/18
Posts: 11
Loc: China
Found this article about elbow wall thickness https://www.whatispiping.com/several-asm...engineer-part-3
It seems that elbow flexibility may be underestimated when using blank WT field

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#73350 - 06/18/19 12:17 PM Re: Using Thicker TEE to Reduce Stresses in B31.3 [Re: M Waheed]
Michael_Fletcher Offline
Member

Registered: 01/29/10
Posts: 710
Loc: Louisiana, US
The elbow ID needs to meet or exceed pipe ID. So, unfortunately, if you really wanted to model elbows with nth level precision, you'd need to specify OD and WT.

However, the vast majority of the time, piping stresses aren't going to be greatly affected by this.

Equipment nozzle loads likelier could be.
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