Difference betwen ellipsoidal and torispherical heads

[Fario]

Difference between ellipsoidal and torispherical heads

What is the difference between ellipsoidal and torispherical heads? since the point of view to utilization, fabrication, membrane stress distribution, others.


For example, in my case I have Pi = 1,7 MPa, nitrogen gas to -45 C, Di = 1.800 mm, SA-240 316L.

1 Ellipsoidal 2:1, then L = 0,9Di and r = 0,17Di

S = 149 MPa @ -45C
E = 1,0
K = 1,0 (Table 1-4.1)

t = PxDixK/(2xSxE - 0,2xP) = 10,2 mm


2 Torispherical wiht L = 0,9Di and r = 0,17Di (identical geometry to Ellipsoidal 2:1)

S = 138 MPa (see UG-32 d)
E = 1,0
M = 1/4x(3 + (L/r)^0,5) = 1,33

t = PxLxM/(2xSxE - 0,2xP) = 13,2 mm


Both geometries are identical, but the equations, procedures and S (admisible strees in tension) are differents, but even worse the minimal required thickness are differents.

Can someone help me with this?
In this case both heads are ellipsoidals or both torispherical or what?
If I ask to "vessel maker" Ellipsoidal or torispherical, like this case with identical geometry, what is the difference and why does the ASME VIII D1 to deliver different results for t to identical geometry?

 

[TGS4]

This is a well-known problem. There will be a paper (co-authored by yours truly) presented at the PVP2017 Conference on this subject. There is also an item before the relevant Code Committee to address this discrepancy (yours truly being the Technical Project Manager).

Note that a 90-17 torispherical head is NOT identical to a 2:1 ellipsoidal head. That equivalence is completely bunk. The two heads look somewhat the same, but are quite different.

Stay tuned. Once the draft paper goes through peer-review, and settles into its final state, then I will be able to discuss further. That will also drive the Code change - hopefully for the 2019 Edition (the window for making changes to the 2017 Edition having closed around December of last year).

 

[Christine74]

"That equivalence is completely bunk. The two heads look somewhat the same, but are quite different."

How can they be "quite different" if they're made from the same material and the geometry is virtually identical? In any case I don't think head manufacturers form actual ellipsoids, they just use torispherical approximations.


-Christine

 

[TGS4]

If you draw the geometries, you will see that they are similar, yet quite different in the knuckle. The stress pattern in the two are quite dissimilar.

The overall dimensions may be similar, but they are definitely not identical nor equivalent.

Our (the paper) survey of North American head manufacturers showed that half made true ellipsoidal heads.

 

[marty007]

I always wondered whether head forming companies made true ellipsoidal heads or used approximations, and actually asked one of Canada's largest producers just a few weeks ago. They came back saying that they follow the ASME guidance of 0.17D IKR / 0.9D ICR, so not true ellipsoidal shapes.

This post is timely, I'll watch to see how things change in the code...

 

[Fario]

Thanks a lot to everyone

I have had very luck with your answers and comments because I see that I have the best of two worlds, for a side a guy with the knowledgement of norm from inside (he says that the norm has a problem in this topic and should change on near future) and on the other side guys with comments from the manufacturing industry of vessels (market, reliability of industry, fabrication problem, others)...great !!

I live in Chile and here it's even harder to find vessel manufacturers that can to make really ellipsoidal heads, so it is better in my situation and case, make a design for torispherical head and to be conservative.

Trevor, do you have any paper or similar that to explain this problem between Ellipsoidal 2:1 and torispherical 90-17?

Cheers

 

[TGS4]

If everything goes well, it will be presented at the PVP Conference this summer (shameless plug... The conference is in Hawaii!!). It will be published by ASME shortly thereafter.