ASME VIII UG-42 Multiple Openings

[loilfan]

I am having some trouble understanding how to approach a reinforcement limit problem that I am having and hope that someone understands the intent of the Code:

In UG-42(a) it states "When any two openings are spaced such that their limits of reinforcement overlap [see Fig. UG-42 sketch (a)], the two openings shall be reinforced in the plane connecting the centers, in accordance with the rules of UG-37, UG-38, UG-40 and UG-41 with a combined reinforcement that has an area not less than the sum of the areas required for each opening. No portion of the cross section is to be considered as applying to more than one opening, nor to be considered more than once in a combined area."

In UG-37 it states "Not less than half the required reinforcement shall be on each side of the centerline of single openings."

Do the limits of reinforcement need to be 50% on each side of a nozzle when it is part of a multiple opening?

For example, say I have an NPS 4 and an NPS 6 nozzle close enough so that their limits of reinforcement overlap by 30mm. The area that is shared would then be distributed based on the size of the openings as per UG-42 (a)(1) where NPS 4 gets 40% (or 12mm) and NPS 6 gets 60% (or 18mm). Say the reinforcement limit is 100 mm for NPS 4, would one side have a limit of 100 mm while the other overlapped side is 82mm? Would I be able to add those two areas, or would they both need to be 82mm?

My thought is that I could add them up since the quote from UG-37 above refers to "single openings".

 

[SnTMan]

loilfan, the application of UG-42 and its relation to UG-37 etc. is, at least to me, not straight forward.

In your example, I think it would be permitted to assign limits of reinforcement differing values on either side of the opening. However, this would be true only for the plane containing the two openings.

So, imagine the following cases (for cylinders only):

1) Two radial nozzles with overlapping limits, placed on an axial common centerline. In other words, contained in the longitudinal plane.

Limits between the openings are apportioned per UG-42(a)(1), limits on opposite sides are per UG-40(b). Determine the area required for each opening, the areas available per UG-40(b) and UG-42(a)(1), add it all up. Simple, hey?

2) Two radial nozzles with overlapping limits, placed on a circumferential plane. Now, the plane containing the two nozzles is possibly not the governing plane, quoting UG-37 "the plane containing the axis of the shell is the plane of greatest loading due to pressure".

You should then check the longitudinal plane, setting the limits per UG-40(b) and then check the circumferential plane setting the limits between the openings per UG-42(a)(1) and the limits on opposite sides per UG040(b) again. Add 'em all up. Not so simple now, but do-able I suppose.

3) Two radial nozzles on a skew plane. Check the longitudinal plane, UG-40(a), check the circumferential plane, again UG-40(a). Now check the skew plane. UG-42(a)(1), UG-40(a), perhaps you can take advantage of Fig UG-37, perhaps not, depends on the detail. Yuk.

4) Two hillside nozzles on a skew plane... Oh, forget about it.

Not even talking about openings in heads or cones, non-circular openings, etc, etc.

Makes a lot more sense to me to restrict the limits of reinforcement such that they are tangent to each other, apportioned per UG-42 if desired, and use the same on both sides of the opening(s).

Besides the plain simplicity of this approach I have yet to see any canned software that can do it any other way.

Regards,

Mike

 

[GenB]

You can distribuit or allocate But per ug37, you make sure that each allowance covers not las than half when calculated individually.

 

[loilfan]

GenB,

But that's my question - I don't think that the requirement in UG-37 applies since it refers to single openings.

 

[prex]

loilfan, what the code says is clear to me: when you have two openings with overlapping limits, then you are required to reinforce each one as a single opening insuring that no area is used twice. After all every opening in a cluster is first of all a single opening by its own. What the statement "...with a combined reinforcement that has an area not less than the sum of the areas..." means, is that it leaves to you the choice on how to distribute the reinforcement between the two openings.
Think of another situation: you have a single opening that's close to a discontinuity (a flange, a head), so that you are forced to take a reduced limit of reinforcement: would you treat this case differently from yours?

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[loilfan]

prex, how does it leave us the choice on how to distribute the reinforcement between the two openings when UG-42(a)(1) says "The available areas of the head or shell between openings having an overlap area shall be proportioned between the two openings by the ratio of their diameters." It tells me how I need to treat the shared areas of the openings.

When I think about the design philosophy of the reinforcement calculations I look at the area replacement method. Even if I have added a repad that has 60% reinforcement on one side and 40% on the other (for example), they would have enough material to satisfy the resistance to tensile forces from the area replacement method. However, the 40% side would have higher stresses than the 60% side since there is less material to resist the same amount of forces (from internal pressure). I understand why the Code would still want us to keep 50% on either side, but the phrasing of "...with a combined reinforcement that has an area not less than the sum of the areas..." and UG-37 specifically referring to "single openings" indicated to me that we might be allowed to have unequal reinforcement.


I have seen repads that overlap. Is that because of fabrication or to avoid stress risers? Because if we know the limits of reinforcement between two openings, I see no reason why they should overlap unless they had been able to claim material from the other side of the opening by making a larger radius repad.

For example, if there are two identical openings 10" apart, and both the limits of reinforcement are 6" per UG-37, they would each have maximum limits of reinforcement of 5" per UG-42. What would be the reason for making either repad any larger than 5"?

 

[prex]

OK loilfan, for what UG-42(a)(1) states. However you must make a subtle distinction (not apparent in code wording) between the maximum limits of reinforcement and the chosen limits of reinforcement. The maximum limits are per UG-40, but if you don't need to go to those maximum values (e.g. because you have sufficient material in the nozzle neck), then you can choose a lower limit in the shell. In this way you could share differently the shell material from what is required in UG-42(a)(1) without violating code rules.
However it is obvious that for two equal openings (both far from any discontinuity) the limit of reinforcement will be halfway.

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