API 620 umbrella roof design

[TXTank]

Are umbrella roofs allowed for API 620 and, if so, what equations should be used in their design?

 

[JStephen]

Look through the interpretations available for API-620. I believe there are a couple that relate to this question.

 

[TXTank]

Before I posted I found

http://mycommittees.api.org/standards/techinterp/refequip/Shared Documents/620ti.pdf

However, it only says the following with regards to umbrella roofs:

Edition: 9th - Feb. 1996
Section: 1.2.1
Inquiry No.: 620-I-10/00
Question 1: Does Section 1.2.1 prevent tanks with umbrella roofs and eccentric cones, from being marked as being in accordance with API Standard 620?
Reply 1: No.

Edition: 10th - Feb. 2002
Section: 5.1.1
Inquiry No.: 620-I-05/02
Question: Are umbrella roofs, eccentric cone roofs and bottoms, and bottoms sloped to one side, among those designs for which API 620 does not present design rules as noted in Section 5.1.1?
Reply: Yes.

Are there other technical interpretations available?

 

[JStephen]

That pretty well covers it, means you're on your own for analysis.
There was an older one that basically said you couldn't use umbrella roofs on an API-620 tank, and I think the item above is an update to it.

 

[Duwe6]

I'd look to Section VIII for guidance; what you are making is a dished head with pressure on the concave side.

 

[JStephen]

Not so, the API-620 standard covers a true dome (dished plates) but does not cover an umbrella (made from plates rolled only in one direction), and that's the issue at question. I'm not aware that Section VIII covers umbrella construction, which is not normally used in the vessel industry.