Vessel Vacuum Rating with Cylinder/Head Flange 1

[Doiscond]

I've got a ASME Div 1 pressure vessel that needs to be designed for relatively low pressures (+10 psig to -2 psig, @ 275 deg. F) and I have a question about a flange that separates the cylinder from the F&D Head. The image below shows the location, so they would be able to pull the whole top of the vessel off from the cylinder.

It's my understanding that I've got two choices for this flange, Appendix 2 or Appendix Y.

For Appendix 2, I need to apply 2-14 Rigidity calcs per the latest code requirements, and I get a flange thickness of over an inch to satisfy the rigidity at gasket seating calculations.

For Appendix Y, I can't show the vessel as being rated for any amount of external pressure per ASME Interpretation VIII-1-95-16 and Y-1(b). So, I can't have any type of vacuum rating on this vessel if I were to use this type of flange. It does allow for a thinner flange to meet the external pressure requirement since the calculation requirements are different.

My issue is that they want a thinner flange than what I calculate with Appendix 2, but I don't see how I can provide a vessel rated for vacuum pressure any other way.

Any thoughts or comments would be greatly appreciated. For now, I have told them they need to use the Appendix 2 flange with the calculated thickness.

 

[Christine74]

I'm not familiar with Appendix Y and I'm not aware of any shortcuts for meeting the flange Rigidity requirements, but with a design pressure that low your Appendix 2 flange calculation will almost certainly be governed by the Gasket Seating condition, so your options for reducing the required thickness of the flange boil down to 1) select a gasket with a lower m & y factors, 2) reduce the diameter of the vessel, 3) reduce the width of the gasket in order to minimize the bolt load needed to seat the gasket, and 4) place the gasket as close as possible to the bolt circle to reduce the bending moment in the flange.


-Christine

 

[Doiscond]

Thanks for the response Christine!

I see how I can adjust the gasket parameters to bring down the flange thickness. My calculations originally had a 1" wide 1/16" thick, PTFE flange, and I can see how decreasing the gasket width drops down the flange thickness. There's nothing I see stopping me from specifying a 1/4" gasket width, but there must be some realistic limit to how skinny a gasket can be right?

 

[EdStainless]

It could be an o-ring.
The real issue is if the flange is stiff enough to give adequate sealing.
I used to use small pressure vessels (up to 8" pipe) with o-ring seals.
We used 3/4" thick flanges with bolts every 2-3".
So our ones made from 8" pipe had 14 bolts to provide uniform pressure.
The 4" ones used 1/4" o-rings, but the larger ones all used 5/16" o-rings.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Christine74]

Here's a link to a previous thread on minimum gasket width:

https://www.eng-tips.com/viewthread.cfm?qid=440192#

I don't think a 1/4 inch wide PTFE gasket is practical. At a minimum I would comply with the Brownell & Young formula for minimum gasket width.


-Christine

 

[r6155]

More info: diameter, material, ....

Regards

 

[GD2]

OP, Do you want to use the rules of Para 4.16.7 ASME Sec VIII Div 2?

GDD
Canada