Pressure Design

[JoseAcevedo]

In a API 650 tank design.

In a cone roof supported tank with an internal design pressure bigger than the roof plate weight. Can I weld the roof plate to the structrure?

 

[JStephen]

You could, but it's not necessarily a good idea.

In a normal pressurized cone roof, you assume that all the pressure is handled by the roof plate in membrane tension. If you attach the roof plate to the rafters, then some or all of that load will be transferred to the rafters. You then need to design the rafters to resist uplift which becomes a problem. You also get reversed bending in the rafters which makes the bottom unsupported flange the compression flange. Loading in the roof plate itself would be an indeterminate combination of bending and membrane stresses.

 

[MJCronin]

jose...

Your question actually begs the question:

"For an API-650 tank, what are the practical design usage limits for a cone roof ?"

This, of course, is a function of the tank diameter, pressure and roof weight under API-650.

Some companies have a policy that limits cone roofs to tanks with design pressures of less tha 10-12 IWG...

API-620 covers the rules for tanks operating at higher pressures ( up to ~14 psig as I recall..)

Anyone out there have company (or any other type of) guidelines on roof selection based on pressure ?

It would be useful to compare a cone roof evaluation under the rules of API-650 and 620.

You may want to re-post under the API Engineering forum to get more responses..

-MJC

 

[JoeTank]

Despite advice offered by others, the correct answer is NO. The design procedures included in API-650, Appendix F are based on the roof plate alone being used to resist uplift pressures due to internal pressure.

Joe Tank

 

[FILGUEIRAS]

If your tank is already built per API 650, Joe Tank said the last word.

If not, API 620 would be more appropriated

Anyway,there's a forum on tanks:

http://www.ast-forum.com/index.asp

 

[JoeTank]

FILGUEIRAS,
JoeTank doesn't make the rules, API does. The design equations in API-620 are also based on simple membrane action and do not accouint for the structural effect of welding the plate to a roof structure. A storage tank engineer would have to address this approach as "non-code" and apply only the applicable rules.

Joe Tank