The connection joint of shell and girth flange

[mechengineer]

Hi all,
Shell material: SA-240 304. Allowable stress: 113 MPa (plate notes: G5)
Body flange material SA-182F304 Allowable stress: 83.3 MPa (without G5)
DP=3.25 MPa
DT=335 deg.C
Shell thickness is 20 mm (the required is 18.4 mm)
Body FLG hub small end g0 is 30 mm (the required is 30 mm)
Shell ID=1350 mm and TL to TL=2500 mm.
Due to there is a larger difference in the allowable stress that makes the required thicknesses of shell and g0 of the hub with a big difference.
Question:
How to make the connection of the shell and the hub of the flange?
1. Increase the shell thickness will be over design for the shell and waste material cost.
2. Add a short transition of shell may be more costly because need to purchase two different thicknesses of plates.
3. A better solution may find a common SS flange material which allowable stress closed to the allowable of the shell at 335 deg C.
Anyone can share the experience with the above issue?

Thanks.

 

[SnTMan]

mechengineer, I seem to recall some language in Sec VIII, Div 1 addressing the g0 thickness, but I have not been to find it this morning. I'd offer the following:

1) Confirm the language does not prohibit using the higher allowable for the g0 thickness due to pressure (simply as a cylinder0.
2) Confirm the said calculation is being performed using the correct efficiency, 1.

Failing the above then: Change of materials would be unusual, but perhaps possible. More practical might be to form the required 1:3 taper with weld metal, as permitted in UW-9. This is going to be a pretty large weld, and in stainless materials. It would be nice if the flange face could be machined afterwards, but not always possible.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[mechengineer]

Mike, thanks.
Theoretically speaking, a stiffness of flange is only related to elastic modulus and section properties, but not to allowable stress. And g0 calculate by GU-27 which is a membrane stress only. And the hub is far away from the sealing surface of the flange. It should use a higher allowable stress G5 for g0. Unfortunately, in current, PVElite does not provide the option for one flange with two allowable stresses, one is for g0 and another for the flange body.

 

[SnTMan]

mechengineer, software is just software. You are the engineer

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[BJI]

Why are you not making the hub longer so that the thickness at the weld is the same? There are high tangential stresses in the hub small end caused by flange rotation, which is why you need the longer hub. Theoretically saying they are different materials is meaningless, if your hub is too short then part of your pipe is acting as the hub. There is also benefit from removing welds from the high stressed regions of the flange. If you already have the flange then a pup piece would meet compliance, typically also used for code breaks, etc. I would go with the increased forging cost rather than a pup piece or weld build-up though.

 

[SnTMan]

BJI. the hub length has no effect on the gO thickness due to internal pressure calculated per UG-27.

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[BJI]

The lower material properties are to mitigate the effects of increased rotation on flanged joints, this is of most importance in the regions of the flange that are exceeding yield. If the flange has been sized for the lower allowable strength and the hub length is extended at the same taper ratio to match the shell thickness, you have the same design as weld build up or a pup piece (better actually). The G5 note is not mandatory, the UG-27 check doesn't fall under the precautions noted in G5. So, you can use the upper strength in you code check to demonstrate compliance, and this still serves to meet the intent of the recommendation.

 

[SnTMan]

BJI, please re-read the OP's posts. Why do you suppose the PVElite programmers implemented this calculation?

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[BJI]

SnTMan, I have reread it, no change to my previous reply. What is your objection? If I had to guess, I would say inability to apply suitable engineering judgment as a default setting? I haven't used PVE in sometime but previously the user had the option to select the material.

 

[SnTMan]

Ok, lets back up. First, I believe the origin of the OP's problem is found in Appendix 2-1(a). The statement is made "The hub thickness of weld neck flanges designed to this Appendix shall also comply with the minimum thickness requirements in Subsection A of this division." Specifically, UG-27 in this case.

This requirement is implemented in PVElite and other programs as well, using the flange allowables as selected by the user. The use of the lower allowables is generally considered a best practice and is mandated by some client specs. Obviously, when the flange allowables are lower than the adjacent cylinder allowables the UG-27 thickness at the hub small end will be larger than that of the adjacent cylinder. These programs typically do not have the intelligence to say. "Oh, the hub is thicker than the plate, I must back up and use the higher allowable for the hub small end thickness and then use the lower allowable for the balance of the flange design."

This is the origin of the OP's problem and I have run into this myself on occasion, although I have to say not to this degree. I offered a few suggestions to the OP, see my post 4-Dec-20 16:53. The first of these was to confirm the language did not prohibit use of the higher allowable for said calculation. I made a following suggestion (5-Dec-20) that the OP disregard any software error messages, based on the OP's own engineering judgement.

I had also previously stated that failing all else to form the required taper with weld metal rather than introduce an additional cylinder.

So, to recap, as I see it the OP has essentially two choices: 1) accept the software calculation and deal with that in one way or another, or 2) ignore the error message based on engineering judgement. I do not propose to select one course or the other for the OP. I don't know enough about his total situation.

I will say that if it were my design and I did chose to ignore the error message and set the g0 thickness equal to the shell thickness, I would not lengthen the hub to do so. Instead, assuming the hub was properly proportioned in the first place. Instead I would simply deduct the 10mm (in this case ) from the g0, and g1 dimensions. This would take 20 mm out of the flange OD.

EDIT: I don't know that PVElite is showing an error message, I might have better said discrepancy.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

@ mechengineer
I said several times on this forum that I do not use computer programs made by others. I don't trust that. I have my own programs and I can explain each step to my clients.
Try the old-fashioned manual calculations.

Regards

 

[BJI]

Mike, I wasn't disagreeing with you, just adding another option not discussed. I was addressing your question in my last reply, I think it makes perfect sense to exclude the G5 note by default, this would typically be the recommended approach but can't cover every situation, as you mentioned, engineering judgement is required. At least for this case the materials are the same, it is not a high strength line pipe mating with a lower strength MSS SP-44 flange, or similar, so it comes down the assessing the intent of the recommendation.

The hub is proportioned to be compliant with the lower strength material, so by extending the hub length (adding additional flange material), it would be harder to argue that the design is not acceptable under all conditions. Extending the length moves the thinner section further from the high stressed region of the flange, further ensuring that the region designed using the higher allowable will remain below 90% of yield at temperature. Using a thinner hub will only increase flange flexibility and resulting strains.

I don't have any details of the flange geometry but an MSS SP-44 flange around this size and class has a hub length of 100mm, so the increased total length would be around 130mm, which is only a rough increase in the overall forging size of 12%. Not an excessive additional expense considering the other options.

 

[SnTMan]

Hard to generalize, but custom HX body flanges typically have hubs much shorter than 100 mm, often under 50. Hub dimensions are what they are, meeting the allowable is what counts

EDIT: BJI, sorry to be so long-winded, I sort of felt we were talking at cross purposes...

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[BJI]

It was only an example to highlight the relative increase in forging size. It doesn't actually matter what the designed hub length is, the relative effect on changing the geometry will be the same.

..meeting the allowable is what counts

yep, it's someone else's problem to make it seal during operation

 

[SnTMan]

Well, I wouldn't put it like that exactly. Proper flange design practices must still be followed, whatever dimensions are selected.

The problem with sloppy work is that the supply FAR EXCEEDS the demand