Head Thickness For Shell External Pressure Check

[roca]

Morning All
Firstly apologies if this has been asked before but I would like some opinions on the following.
When checking a vessel for external pressure the dished end head stiffness is taken into account by increasing the shell shell by 2 x Head Depth / 3 - assuming 2 dished ends are fitted.
The sections of head added to the shell tan to tan length are considered as part of a cylinder.
As such the head thickness in these portions should be as a minimum the same thickness as the shell.
In cases where external pressure is governing I always specify my dished ends to be the same minimum thickness as that required by the shell - which may be a conservative approach.
However as buckling is obviously the worst case I feel that this is the correct way to go.
There seem to be different views on this - one being that the head can be thinner than the shell because it is obviously "inherently stronger" in the knuckle region, the knuckle area and straight flange will be thicker than the shell after forming, etc.
This is always an arguement point when dealing with vessel manufacturers.
Comments welcome please.

 

[Tankman650]

roca,

I agree with you. Also fig. UG-28.1 , note 1 says the same thing about the head having the min. thickness of the cylinder.

They have the option of providing a ring stiffener at the head junction. That will allow for a thinner head.

 

[roca]

Hi Tankman
Thanks for the response.
You will find however that most people will make the head thinner than the shell - even if external pressure rules - as when you check each component individually the head normally comes out thinner.
Thanks
roca

 

[prex]

Well, many people, as you say, perhaps do that, but this is not according to the rules. The note pointed to by Tankman650 is formal and prescriptive: you are not allowed to consider a line of support at one third the head depth if the head is thinner than the shell.
No discussion nor different views possible in my opinion. prex

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