API 650 storage tank roof selection criteria

[Mech2325]

Hi everyone,

Please tell a thumb rule for the selection of a roof type (self supported, rafter supported, rafter+column and dome) based on the diameter of a storage tank being designed in accordance with API 650

 

[JStephen]

If there are no other requirements:
Small tank- self-supported cone roof
Large tank- supported cone roof (rafters with column(s))
The break point between small and large will vary with the fabricator- but around 25' to 30' diameter. If there are no other requirements, let the fabricator quote the most economical.

Self-supporting cones, umbrellas, or domes may be desirable due to pressure requirements or corrosion considerations.
Rafter-supported roofs may be desirable for easier roof access.

 

[Mech2325]

JStephen

Thanks for your reply.
Can you be a bit more specific in terms of definitions of a small and large tanks?
As i have read, the storage tank guide book by Bob Long gives the following criteria for the the roof selection of the tanks based on diameters

For tanks upto 8 metres in dia: Self supporting cone roof

For tanks upto 15 m in diameter: Rafter supported roof

For tanks greater than 15 m: Go for rafters+columns or truss structure

 

[LittleInch]

This really belongs in the storage tank forum. ... The reply is good. There are no such definitions only guidance.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JStephen]

Mech, if there is not s specific reason to use one or the other style, then fabricators will quote whichever is cheapest, and that varies depending on how they do the work and on how they do their estimating; that's not uniform across the industry or around the world. It'll vary to some extent based on tank height, roof thickness, as well.

 

[DSB123]

Mech 2325,
Do not beleive everything you read in the BL book.There are a number of items which are debateable. This is basically what one fabricator used also. The real answer is - the most economical that can meet the design parameters and the process conditions.

 

[Mech2325]

JStephen and DSB123

Thank you all for your replies.

As an EPC contractor, how do u know that the fabricator is giving you the cheapest option. I mean if you don't have proper knowledge, how would you analyze properly what the fabricator is proposing. The vendor may, in order to earn more money, ask you to go for a rafter supported cone roof (for example), whereas a self supported cone may have served the purpose. My purpose of asking the question was, therefore, a rough idea into the matter and a knowledge as to which type of roof generally serves the purpose based on the diameter of the tank.

 

[LittleInch]

Ask three fabricators for prices and compare....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MJCronin]

Dome roof discussion:

http://www.eng-tips.com/viewthread.cfm?qid=369101

http://www.eng-tips.com/viewthread.cfm?qid=73622

MJCronin
Sr. Process Engineer

 

[Mech2325]

Thank you all for your replies