Dear friends,

I had designed a vessel referring to good engineering practice. Vessel having a bottom cone of half apex angle 75 degree & then a cylindrical shell & top cover plate.

Now if half apex angle exceeds 60 degree then ASME (UG-33) provides guidelines to design it as a flat head. However API 620 also provides the guidelines for design of cone as per 5.10 & 5.12.

Which is the correct method to be used for design of cone for external pressure when half apex angle is more than 60 degree? (Note: - If I refer ASME then THK becomes twice than API 620)

Or, if you have any different way to design this cone please provide me the guidelines.

_________________
Makrand Pandurang Sakpal,
Static Equipment Design,
makrands9@gmail.com,
Ph no:- 9967949656.

Dear Makrand,

well, from my point of view the ASME VIII-1 is in this case clear. As your angle is greater than 60deg the thickness of the cone shall be the same as the required thickness for a flat head under external pressure.

In this case the ASME Code makes it a bit easier to calculate with the disadvantage of using more material.

But if you need or want to use the ASME Code VIII-1 I can imagine to use stiffeners. If this would be thinkable for you I can provide a sketch, the formulas to calculate and the link to the corresponding sections of the ASME code. Even flat heads can be reinforced with stiffeners.

So long,
Claus

Dear Claus,

Thanks for your reply.......

Yes, it is very clear from ASME that above 60 deg angle cone shall be designed as per flat head. But it needs to be stiifened by external stiffeners (as per roark methods for flat plate) which seems to be odd for conical elements.

However, API 620 aslo provides the formulas for determining the THK of cones for external pressure (clause 5.10 & 5.12) which gives very less THK (less than required THK for internal pressure).

I am confused why there is a huge amount of difference in both the THK determined by two different codes?

_________________
Makrand Pandurang Sakpal,
Static Equipment Design,
makrands9@gmail.com,
Ph no:- 9967949656.

Hello Makrand,
Now I'm confused.
To be honest I must admit that I'm not familiar with the API 620.
But I can't believe that the required thickness for external pressure is less than for internal pressure as long as you don't use stiffeners.
So long,
Claus

I'm not familiar with API 620 either, but I can make a suggestion as to internal/external. ASME is normally very concerned with fatigue failures, which only happens to material under tension. Heads with pressure on the convex side essentially discount fatigue failure and assume a joint efficiency of 1. That can result in a lower required thickness than for internal pressure if not much NDE is done, or castings are used, etc.

But once you go over 60 degrees and ASME forces you to assume a flat head, there's not much difference between internal or external pressure. You must use a joint efficiency factor, although it may be hidden inside the C factor for a corner joint. Maybe API 620 is less concerned about fatigue failure, being a low-pressure code, and allows a high joint efficiency even at 75 degrees? Just a guess.

Can anybody please elaborate what could be the effect of external pressure on the cone whose half apex angle is more than 60 degree (behaves like a flat plate)?

I would like to know the behavior of this kind of cones for external pressure.

Please note that as the angle becomes more than 60 degree for a cone it behaves like a flat plate for which there is no any philosophy of internal or external pressure, pressure will be the maximum value of either side.

_________________
Makrand Pandurang Sakpal,
Static Equipment Design,
makrands9@gmail.com,
Ph no:- 9967949656.