Reinforcement of Large Openings in Rectangular Flat Heads.

[Sam35]

A question on Appendix 13, UG-36, UG-39 of BPV Code - ASME Section VIII Div.1.
I want to add a 3 NPS nozzle to a rectangular header box of an air-cooled heat exchanger. The opening meets the requirements of UG-36(c)(3. But the diameter of the opening exceeds one half of the shortest span of the head as mentioned in UG-39(c). In this case, are the openings exempted from reinforcement calcualtions? or reinforcement calculations are required to be performed per U-2(g)as mentioned under UG-39(c)(3). If U-2(g) is to be followed, are there any references to perform the calculations. See also Appendix-13(j). Under any case, I am interested to know if there any references if U-2(g) is required to be followed under a different scenarion as I might bump into this case in future.

 

[unclesyd]

How thick is your head?
Is there another 3" nozzle in the head?

 

[prex]

See UG-39(a): small openings are exempted from UG-39, so you don't need to consider the large opening issue.
Concerning your question on how to calculate a large opening in a rectangular flat head, I would be surprised if a general method for this situation existed in the literature. I would treat it as a circular head with diameter equal to shorter span: unless you have a very large or complex head, it's no use to go deeply into the calculation to save a fractional kg of base metal or weld.

prex

http://www.xcalcs.com

Online tools for structural design

 

[johnnymist2003]

Hi all,

just want to add my bit. I disagree a bit with Prex here....sorry mate...

1) Small openings are not automatically exempt from reinforcement calculations. They are only exempt if the reinforcement limits (and thus reinforcing area in shell/nozzle) are available in both the shell/nozzle necks. In the shell you must have at least a diameter equal to that required by Fig. UG-37.1 If this is not available, then nozzle reinforcement calcualtions must be carried out (regardless of opening size). This is why the reinforcement of multiple openings becomes important if the pitch between the openings is small (regardless of opening size).
2) I've tried to find references to the problem mentioned above and have not managed to find anything. In this particular case, I think you need to exercise a bit of engineering judgement as to what is the best method.
3) When designing header boxes with openings, I have always used this approach, which although conservative, and a bit time consuming, seems to work ok. Using the inner dimensions, calculate the required header box thicknesses for pressure loading only. Take the required thickness and calculate the required area of reinforcement (tr x ID nozzle - corroded). Assume that there is no area available in the box for reinforcement of the opening and design accordingly. Alternatively, if this results in excessively thick nozzle necks, then take the area available in the box as that across the length of the short side.
4) Concordia university generated a paper a while back with regards to the required thicknesses of header boxes with end flanges, although this did not address reinforcement of openings.

John

 

[Sam35]

The head is 3/8" thick. Further, I would like to know if there is any FEM software to perform the analysis of this kind.

 

[kstaylor]

John,
Do you have a copy of the Concordia paper that you would be willing to share via email or fax? If not able to share, can you provide more details (title, author, publication date, etc.) that would allow one to request it from other sources?

KST

 

[unclesyd]

A 3/8" thick head and a 3" nozzle are the break points under the NBIC as to whether a new nozzle is a repair or alteration. “johnnymist2003's” statement about pitch comes into consideration when reinforcement is a consideration and the new nozzle is less than 3 of its’ diameters from another nozzle. This applies when the new nozzle is the same as an exiting one.

These assumes that the head was properly design according to applicable code. We had some older refrigeration machines where the heads on the water boxes were not according to code.

 

[johnnymist2003]

Hi KST,

email me off-forum at johnfo@murrob.com....we'll make a plan.

john

 

[prex]

John,
It is my turn now to disagree (just a bit) with you.
The exemption from UG-39 for small openings per UG-36(c)(3) doesn't work of course for multiple openings, as stated in UG-36(c)(3)(c-d), but holds IMHO for large openings (John if you disagree please give the exact paragraph of code supporting your position, I might be wrong of course): so I insist, in the situation of Sam35's first post (assuming all the conditions of UG-36(c)(3) are met) there is no need to check the opening reinforcement.

Sam35,
don't think you'll find a FEM application ready to handle your situation. You can build your own of course, but its cost would be hard to justify.

prex

http://www.xcalcs.com

Online tools for structural design

 

[johnnymist2003]

Hi Prex,

this is gr8, debate across the world...

To my mind, if one reads the whole of UG-39(a), it only exempts small openings in accordance with UG-36(c)(3), provided that the openings do not exceed 1/4 of the head diameter or shortest span. Sam35 said that the opening exceeds 1/2 the shortest span, thus I reckon that one needs to check reinforcement. By specifying the 1/4 limit, the Code has ensured that the limit of reinforcement in the head as shown in Fig.UG-37.1, and specified in UG-40 has been maintained. Thus, by reverse logic, if the reinforcement limit is not present (ie. the length along the shell/head available for reinforcement is less than specified in Fig.UG-37.1/UG-40) then reinforcement should be checked.
I'd appreciate any comments, please.
John