API 620 internal structure design 3

[EngFax]

Hello,
I'm designing a cylindrical tank using API 620, 96ft diameter, conical roof, 15 psig. The required thickness of the roof based on internal pressure (15 psig) is unreasonable ~ 7in.

How can I design internal structure (rafter, girders, columns ..) to reduce the thickness of the plates?

Thanks

 

[fegenbush]

The shallower the pitch of the roof, the more it approximates a flat plate. The bending stresses on flat plates are quite high for a given uniform pressure.

I recommend using a dome roof with a hefty compression member. The best bet is to contract with an experienced tank builder and let them perform the design work. It is quite easy to make large, expensive mistakes while working on large API 620 tanks.

 

[EngFax]

Thanks fegenbush,

Do you know of any reference books for designing internal structure in API 620?

Thanks

 

[fegenbush]

Just about any steel structures book can help with the design of a stiffened dome roof along with Roark's Formulas for Stress and Strain. Both will require copious use of engineering judgement and free body diagrams.

 

[JStephen]

You need to adjust your tank parameters to make the tank practical to build rather than adding stiffeners to the roof. A tank of that volume and that pressure would likely be built as a sphere or some similar shape.

You can reduce cone roof thickness by increasing the slope, using higher-strength material, using higher joint efficiency, using less corrosion allowance, or reducing the pressure.

A cone roof that size would normally have supporting structure below it as well.

Another potential problem will be the anchor bolts and foundation size required to anchor that uplift force.

 

[fegenbush]

Good point, JStephen, I hadn't considered the anchorage problem.

If EngFax is set on cylindrical tanks, a larger number of smaller diameter tanks would be a much easier proposition.

 

[MJCronin]

Your problem is in the choice of storage vessel type and your design pressure.

With the volume you seem to desire and the 15 psig design pressure, you are beyond the reasonable range for a flat bottomed tank design, in my opinion.

Yes, I realize that API-620 designs go nearly to this limit, but it is not always wise to choose this type of design.

I believe that your 15 psig design pressure may be an arbitrary choice and not indicative of the liquid's true vapor pressure..... you should press the process engineer on this because it has such a large effect on your design.

Ask experienced API-620 design firms what their dimensional/pressure limits are for their tanks.

A better and cheaper choice may be a series of horizontal "bullet" tanks, designed to ASME-VIII

What is the stored liquid and what is the design temperature ?

What is the total volume ?

 

[EngFax]

Thanks MJCronin,

The liquid stored is Raw Methanol, 150 F. I'll go with you guys suggestion (multiple small tanks). But do you know of any firm that design according to API-620 or any calculations example of a API-620 tank.

 

[fegenbush]

There are many fabricators and erectors that design and build to API 620. I would imagine that some members on this forum work for such companies. This forum is not the place to advertise such services, so I recommend an internet search.