OBROUND NOZZLE REINFORCEMENT CALCULATIONS

[XGI_451]

Has ASME every provided an Interpretation or Code Case for alternate rules for reinforcement of non-circular nozzles?

To my understanding, for ASME BPVC Sec. VIII, Div.1 (2017) or (2021), UG-37(b) covers the reinforcement area requirement for any non-circular nozzle opening such as an obround nozzle. this is stated immediately below for convenience.

UG-37(b): Reinforcement shall be provided in amount and distribution such that the area requirements for reinforcement
are satisfied for all planes through the center of the opening and normal to the vessel surface.

What I am being told is that for obround nozzle openings, ASME BPVC allows you to convert the obround opening area into an equivalent circular area and then calculate the obround nozzle reinforcement as if it was a regular circular nozzle. ASME does have rules for non-circular flanges using equivalent circular flanges but, to my knowledge. not for reinforcement.

Can anyone provide any background as to where this equivalent circular opening reinforcement idea came from? Is it a misinterpretation of ASME? Did old versions of the ASME Code allow this? Is there a Code Case or Interpretation which covers this topic?

 

[TGS4]

Interpretation VIII-1-86-23, in the third question addresses obround nozzle. The question and answer are:

Question (3): 13-11(b) states, "For any other type of reinforced obround cross section vessel, see U-2 ." Does this apply to an obround nozzle?

Reply (3): Yes.

Note that UG-36(a)(1) does permit obround openings (with geometric restrictions). UG-41(c) also provide some additional requirements for obround openings. And obround openings are permitted for inspection openings, per UG-46(g)(1). So, in general, I would see this as a U-2(g) subject, where general industry practice has been to follow the rules that you noted in your OP.

 

[Cobra17]

In Canada, the provincial jurisdictions let you do oblong firetube flanges according to BS 5500 per U-2(g), but for the nozzle itself its required that you calculated as an opening based on the larger diameter, not an equivalent area since reinforcement is taken as a cross section. You can also assume the reinforcement follows the actual opening, not a giant circle, so if the reinforcement goes 3" past the long side OD, it can be 3" past the short side OD when placing additional nozzles.

 

[XGI_451]

Thank you TGS4 & Cobra17:

Without current access to a copy of ASME BPVC - 1986, it is difficult to interpret the response above regarding Interpretation VIII-1-86-23.

Cobra17 - You may find it interesting that TGS4 is also located in Canada.

TGS4 - When you say "...general industry practice has been to follow the rules that you noted...", do you mean it is Standard Industry Practice to use the Equivalent Circular Nozzle Removed Area Reinforcement Method? If this is Standard Industry Practice why hasn't ASME BPVC clarified it and codified obround nozzle rules? It is not as if obround nozzles are uncommon.

Finally, how does ASME BPVC treat Interpretations? Are they only applicable to the Code Edition/Year which was responsible for the initial question or are they somehow grandfathered into later editions so long as they do not conflict with any of the rules of the later editions? Thanks Again!