What to do when UG-39 doesn't apply

[KLee777]

I was asked to analyze two 6" S/40 nozzles (for instruments) that will be attached to a 20" 150# blind flange. This entire assembly will be bolted onto an existing nozzle. (I think the point of this was to avoid any welding on the existing vessel and have this blind flange/nozzle assembly built in a code shop and just bolted on in the field.)

The spacing of the two nozzles is too close to use the rules of UG-39(b)(2). Thus, the code refers me to U-2(g) (my favorite!).

I guess my question is, what's a good starting place? What do I need to make sure to include in my analysis?

Any help would be greatly appreciated, as always.

Regards,
Kelly

 

[jte]

Kelly-

Would you consider replacing the blind flange with a 2:1 SE on a weld neck flange? Seems that that might simplify the reinforcement a bit... Otherwise, FEA might do the trick. I'd lean towards a solid model.

jt

 

[prex]

Can you specify the position of the two openings?
I suppose that the limit you can't satisfy is

...no pair having an average diameter greather than one-quarter the head diameter...

However consider the head diameter for a bolted flange is the average diameter of the gasket, so it is close to 22" in your case. A small reduction in openings inner diameter could allow you to satisfy that limit (if that is your only problem).
As a last chance (that will for sure cost less than a FEM analysis) you could encircle the two openings with a circle (hopefully centered) and treat the flange as a reverse flange per App.14 (as also provided for in UG-39(c)(1))

prex

http://www.xcalcs.com

Online tools for structural design

 

[KLee777]

Prex,

Yes, that is my problem. I may try your suggestion. My designer insists that this is a "simple" problem and I should calculate it as a single 12" nozzle (approximately removing the same amount of material). I heartily disagree. If I remember my strength of materials correctly, it matters very much if it's two holes or one in the flat plate, and where they're placed.

I'm about to hand the designer a pencil and calculator, tell him to do it and then have the PE check it...we'll see what happens then!

JT-I'm not sure I'm picturing what you're suggesting. You mean welding the head onto the WN flange, then attaching the two nozzles through that head? Sounds like an awful lot of extra material. But it's a thought.

Thanks for the suggestions.

 

[prex]

Well Klee777,
can't fully agree with you.
It's true that it matters where the holes are placed, but if you remove some material to obtain one large hole from two smaller ones, then you are always on the safe side.
A general rule I follow when dealing with primary stresses (load related stresses, it's different for secondary or deformation related stresses): if you can positively check a part with some material removed, then the original part is also OK.

prex

http://www.xcalcs.com

Online tools for structural design

 

[jte]

Kelly-

Yup, that's what I meant.

jt

 

[mafa]

Hi there,
Is it a common solution anyway? to disable an existing nozzle in order to avoid opening a new hole in the vessel? In my case I am being asked to disable a Manhole, and it makes the distance between the next two manholes be 15 meters (49 feet). In this particular case the fact that inspection will be too complicated without this manhole makes me think that it is not a good solution. But leaving this factor behind, it is totally normal to turn to this solution of using a blind flange to make a new process nozzle pass through it?
Thanks

 

[jte]

mafa-

No, it is not. Your feeling uneasy about inspection and maintenance access is very well justified. Unless there's a good reason not to (and the project engineer's budget does not qualify as a good reason), just install a new nozzle as an alteration (or repair under certain circumstances).

jt