Nozzle Reinforcement on ASME Vessel

[1310laurent]

Hi,
did some one know this situation:
I have to recalculate the reinforcement required for a 24" Nozzle on Head taking in account a new loads ( caused by piping loads), the vessel is already in shop,
the first calculation ( without new loads)require a pad
( 4" x3/8")as reinforcement, this pad is already welded.

Now when I take in accunt the new loads,this pad is not suffisant for reinforement.So I decided to added a other a same pad (but on internal side), in this case I can consider the local thickness of the head as (min thk head +thkof pad(3/8"), Am I right with this consideration?
that is my question.
So please can some one confirm me this?
Thanks for your help.
reagrds.

 

[doct9960]

Adding a repad on the internal side is a very bad idea. I don't think it's even allowed by the Code. Why don't you go back to Piping and request them to reduce the loads. It is better to mess around with the piping rather than a pressure vessel.

 

[doct9960]

Okay, I checked the Code and found out that you can place repads on the inside, but you need to meet the requirements of ASME VIII-1 UW-16(c)(2). In your calculations, the thickness of your head and the internal repad should be the corroded thickness.

 

[CodeRef]

Good Catch Doc!

There are three kinds of people in this world; those who can count and those who can't.

 

[doct9960]

1310laurent,

If you are still considering the internal repad, you better be using FEA (not WRC 107) in analyzing the local stresses.

 

[LSThill]

Over the years WRC 107 has been used for pad reinforced geometries since no other tools were available. Two analyses are typically made for pad reinforced nozzle geometries.

One is for the edge of the repad. The nozzle OD is increased to equal the pad OD and the WRC 107 analysis run with the larger nozzle. For WRC 107 cylinder-to-cylinder intersections the thickness of the nozzle does not enter into the calculation.

The second calculation is made with the actual nozzle OD and the increased local thickness of the vessel and pad. Parameter studies are under way to determine when this approach will produce the worst results, but large errors have been witnessed for certain geometries. This is not the fault of the WRC 107 bulletin. The bulletin has simply been extended beyond its intended range of usefulness by programmers needing to find solutions for problems in all parameter ranges.

Rectangular Attachments (WRC 107): As might be expected, WRC 107 for a rectangular attachment that has essentially the same dimensions in the longitudinal direction as the 8” pipe above produces essentially the same stress. The FEA model shows higher stresses around the corners of the geometry where the stress is concentrated. The FEA model also shows the beneficial effect of pads and the gross errors that can occur when WRC 107 is used for pad type attachment geometries.

 

[jte]

As doct pointed out, the better solution to this dilemma is to calculate your way out of it. Determine the spring rate of the head, provide that spring rate to the piping engineer and ask them to rerun their calc's using that spring rate. This typically lowers the reported nozzle loading. If that doesn't work, evaluate the loading using FEA. Only if that doesn't work should you consider a physical modification.

Although code legal, presuming this is ASME VIII Div. 1, I would strongly recommend against both an internal and external repad, and would veto one if such a proposal came across my desk. By putting repads on both sides of the head, you are most likely making the plate underneath the repads more susceptible to corrosion (crevice), and you are definitely making the plate under the repad impossible to inspect visually and via UT. By making part of the head impossible to inspect you are severely limiting the life of the vessel.

But I just hate to leave you without a workable solution…

First, consider cutting out the repad and replacing it with one which is adequate. Not pretty, but mistakes happen and it can be fixed.

Since you are using a 3/8" repad, I'll presume that your head thickness is similar. Consider adding your reinforcement via weld buildup instead of a repad. This buildup can be on either the head or repad to make either or both thicker. Try to keep both the head and (nominal + buildup) repad thickness about the same. If you add buildup to the repad alone, your inspection problems go away. If you add weld buildup to the head, you'll have to taper the transition in thickness per UW-9(c) but you will not be left with a crevice nor a surface which cannot be visually inspected or UT'd later in the vessel's life. Refer to UW-16(c)(1) and UW-42 for thoughts on weld buildup for nozzle reinforcement.

Consider adding a note in the remarks section of the U-1 form noting that the additional reinforcement on the 24" nozzle is for piping loads. That will save some head scratching 20 years from now...

jt

 

[1310laurent]

Hi,
Thanks for all your confirmation and propositions, it was very interesting.particularly this from doct9960:
"I checked the Code and found out that you can place repads on the inside, but you need to meet the requirements of ASME VIII-1 UW-16(c)(2)"

I think also that the FEA analysis is the best way to do this calculations.

Thanks.

 

[decode]

I think in this case you may also weld another pad over the externel one as for layered vessels. In this case you need to provide 2 additional vent holes because you need to weld the new pad in 2 sections. There is nothing in the Code prohibiting this overlap of reinforcements i may give you references to provide to your A.I. if required.

 

[doct9960]

decode,

UW-16(c)(2) says that you MAY add repads to the outside surface of the shell wall, the inside surface of the shell wall, or to both surfaces of the shell wall. ASME did not express permission on adding repads on another repad. Even if it is allowed, you will need a U-2(g) analysis to prove its integrity.

 

[JStephen]

One point you mentioned up there was about just adding the thicknesses together to get an equivalent thickness. Generally, this would be applicable to tension in the plate but not to bending. I think WRC 107 has recommendations in the discussion about how to handle this.