API 620 F&D Bottom:

[Outlaw500]

I'm starting to get involved heavily in the design of API 620/650 tanks.

A tank that we are designing has a F&D bottom that is unsupported. In reviewing the API code I'm not really finding anything that looks like a fit. In reviewing the E-tank software they use the ASME VIII Div 1 formulas for calculating the F&D head. Which I am familiar with.

Is there an API 620 code for this?

Is there a good source for interpreting the API code and where to apply it?

It seems that both the nozzle calculations and the domed head calcs are very similar to the ASME calcs only the API is extremely vague and confusing.

Help a confused Engineer.

 

[JStephen]

API-620 does cover it. They give general equations for membrane structures. Then they give simplified results for domes, for cones, etc. The dome equations will work fine for the dome part of the F&D head. You have to go back to the general equations for the knuckle part, though. In times past, when I did this, I came up with unreasonable knuckle thickness, as compared to ASME. Using a very large knuckle helps. It's less of a cookbook approach because it's a more general approach.

ASME equations could be used, and are generally considered conservative; however, API-620 doesn't include provision for using them, so you don't necessarily have an API-620 design when you get done. Note that ASME has provisions for very thin heads that don't normally apply to pressure vessels, but could come into play here.

 

[gr2vessels]

Outlaw500,

You are again to break the law, by applying a legitimate law (standard) to an illegal application. The API 650 / 620 applies only to tanks with fully supported bottoms, it does not apply to your tank suspended somehow on legs or other means. I'm not familiar with the E-tank software, but I doubt it they use the pressure vessel code for the design of tank components, it is a different world altogether.
The design af silos and the like storage equipment is not part of API standard and you should look for structural design software covering your type of equipment. Please come back with more details of your work, should you require further guidance.
Cheers,
gr32vessels

 

[Outlaw500]

The tank is an above ground girder ring suppored vessel.

About 9ft in dia x 12 ft tall. The bottom has a nozzle centered in it. The top is flat plate with 4 beams across it for roof support and an Agitator.

Design Pressure: +/- 1.5 psi. The negative pressure is outside of API 620 rules.

The bottom is currently spec'd by the customer as an F&D head with what appears to be a 6% knuckle radius.

The tank is completely filled with contents at S.G. of 1.2

In doing my calcs I found that the walls should be the minimum of 3/16", the bottom per ASME could be 3/16", The top Per API would be 1/4".

Manway in the side of the tank 24" (Needs reinforcement).

My thoughts are:

Calculate the bottom & Sides to ASME VIII Div 1, Look at the top as a API minimum thickness and check the stress in the beams on top and top plate membranes using Roark's formulas.

Any comments?

 

[JStephen]

API-620 is also applicable to tanks with suspended bottoms. Some requirements for support structure are given in App. D.