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picasa (Mechanical)
30 Jun 05 14:17
What are the advantages of ellip. head against hemispherical head? I guess with hemi head you get more volume and lesser thickness of head. Is it easier to fabricate ellip. heads as against hemi heads? Also with hemi heads you lose more volume in  the skirt. Is that an impotant consideration?

Are ellip. heads flatter than 2:1 ratio common? I mean higher D/2h ratio.

JStephen (Mechanical)
30 Jun 05 17:08
Price?  IE, smaller blank, easier to form.
PEHasan (Mechanical)
3 Jul 05 19:37
As per UG-32(a) of ASME Sec VIII Div 1, thickness of ellipsoidal or torispherical head shall in no case be less than the required thickness of a seamless hemispherical head divided by the efficiency of the head-to-shell joint.

The inside spherical or crown radius of Ellipsoidal Heads is 0.9D. The inside spherical or crown radius of Hemispherical Heads is 1.0D.

The Ellipsoidal Head has to have a major-to-minor axis ratio of 2:1. The Hemispherical Head has to have a hemispherical shape.
PEHasan (Mechanical)
3 Jul 05 19:47
The advantages of Ellipsoidal Head over Hemispherical Head are as follows: -
·    The blank diameter of Ellipsoidal Head is smaller as compared to the blank diameter of Hemispherical Head
·    Ellipsoidal Head is easy to form.

There is one disadvantage with Ellipsoidal Head. We cannot weld any nozzle on the Knuckle of Ellipsoidal Head. Whereas for Hemispherical Head, there is no knuckle.
codeeng (Petroleum)
4 Jul 05 12:07
The choice is usually an economical one in that its just not practical to form a very thick 2:1 head when you can use half the thickness on a hemispherical head. Basically, for any given diameter, there will be an economical thickness range for a 2:1 head versus hemi. High pressure reactors generally use hemi heads and the additional space is used for internals (distributors, catalyst, etc). Other types of vessels may not need additional volume and you should look at each head type with respect to economics. I don't see any loss in skirt volume as a concern. 2:1 heads are most common as they have the same thickness as the shell. Flatter heads will be thicker.    
MJCronin (Mechanical)
12 Jul 05 12:37
PEHasan....

I do not agree with your statement about nozzles in the knuckle region of F&D or elliptical heads.....at least not for vesels designed under ASME VIII

Whereas it is "bad practice" to locate a nozzle in this region of the head, ASME VIII does not prohibit this.

Review this thread:

http://www.eng-tips.com/viewthread.cfm?qid=67661

You should note that the British Pressure Vessel Standard, BS 5500, puts limits on nozzle locations, but ASME VIII does not.

MJC

  

PEHasan (Mechanical)
13 Jul 05 19:05
MJCronin,

I agree with your point.

However, we should try to avoid welding any nozzle to the knuckle of ellipsoidal/torispherical head.

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