Short Radius Elbow vs Long Radius Elbow Allowable Stress' 1

[OscarAb]

I recall a number of years ago, when using short radius elbows for ASME Section VIII, Division I vessels there was a reduction to the allowable stress compared to that of a long radius elbow. Does anyone in the community remember what the reduction was and if so what was the document that directed the designer to do this?


Thanks,

Trevor

 

[C2it]

Allowable stress for bends is material dependent rather than radius. That said, an SR bend will have a higher stress intesnification factgor than an LR bend and thus will effectively have a 'lower allowable stress' for a given bending moment.

 

[OscarAb]

I should have clarified myself, I am talking about ASME B16.9 butt welded fittings.

For example:
ASME B16.9
Short Radius Elbow = 1 x NPS
Long Radius Elbow = 1.5 x NPS

Lets take an 18"-SCH XH seamless long radius elbow, Material: SA-234-WP11 a Design Temperature of 400°F, ASME Section VIII, Division I (Table IID) has an allowable stress of 16800 psi.

C2it, effectively I am looking for what you call the "stress intensification factor".

Using a B16.9 long radius elbow the allowable stress would be 16800psi. I recall there was a reduction in the allowable stress of (A) * 16800. What I am looking for is what the value of (A) is and the referenced code/paragraph.

Thanks,

Trevor

 

[C2it]

B16.9 is essentially a dimensional standard. Regarding allowable stress / strength it says ...

2 PRESSURE RATINGS
2.1 Basis of Ratings
The allowable pressure ratings for fittings designed
in accordance with this Standard may be calculated as
for straight seamless pipe of equivalent material .....

Clearly a bend of whatever radius will have higher allowable if made from duplex stainless, compared to say, 316L. The radius does not come into allowble stress at all.

My reference to s.i.f may be checked out in Appendix D of ASME B31.3.

 

[OscarAb]

I have found what I am looking for and indeed there was a reduction in allowable stress, I just could not remember the referenced code. The applicable code and paragraph was B31.3 table 326.1, Note 5 and B16.28 and ASME Section VIII, Division I.

"The pressure-temperature ratings for ASME/ANSI B16.28 Wrought Steel Buttwelding Short Radius Elbows and Returns, have changed as of the 1994 edition of this standard..."

"It is important to note that the potential increase in pressure-temperature rating only applies to items manufactured to the 1994 edition of B16.28, as stated in the Cautionary Note number 5 in table 326.1 of B31.3 "Pressure rating of components manufactured in accordance with editions prior to the 1994 edition of this standard were derated to 80% of equivalent seamless pipe. This derating is no longer required for components manufactured in accourdance with the 1994 edition."

Reference The Pressure News, Volume 5, Issue 3, September 2000.

Thank-you for your help C2it.


Trevor

 

[C2it]

Trevor,

This troubles me a bit .... my 2010 edition of B31.3 Table 326.1 Note 5 says ...

(5) MSS SP-119 includes three classes of fittings: MP, MARINE, and CR. Only the MP class fittings are considered a “Listed Component” for
the purpose of this Code. Cautionary Note: See MSS SP-119 (Section 6) for special provisions concerning ratings. (In accordance with
MSS SP-119, the pressure ratings for MP class fittings are 87.5% of those calculated for straight seamless pipe of minimum wall
thickness.)

It only applies to MP fittings from MSS SP-119 and I don't see any other notes relating to B16.28 or other ASME standards.

My concern is that, with th exception of the higher SIF, I do not recall a specific clause that requires a reduction in allowable stress for SR bends.

 

[OscarAb]

C2it,

I do not have a copy of the 2010 edition of B31.3 but in the copy I have on my desk (2004) the reference to Note 5 says to see also Appendix A and the reference to MSS-SP-119 in ASME B31.3 (2004) is note 4.

However note 3 in ASME B31.3 (2004) has the cautionary note which removes the derating requirement if manufactured to the 1994 edition.

Thanks,

Trevor