Cone Roof Tank Design with External Pressure

Is the tank to be partially submerged? and is the tank freely vented?

Hire an engineer.

If API doesn't address it, pressure vessel design codes typically have a section for vessels with external pressure.

Note also that the buoyancy of the inner shell may be quite high.

Hire an engineer.

Then, if when the internal tank floats up and takes the roof off, you can keep your job.

Start with a drawing.

Can you explain this more clearly please: Outer tank product static head= inner tank external pressure

Inner tank buoyancy may be an issue depending on its contents...

What's this for?

Our tank has the product of mixture of Crude oil,Water & gas. The purpose of tank is to separate the oil ,water & gas by gravity method. Inner tank has shell and it will be welded with bottom plate. So there will not be issue of buoyancy.Outer tank shell aslo welded with bottom plate [ Bottom plate is common for inner and outer tank and the gap b/w inner & outer tank is 3m].Inlet manifold is connected with top shell course of outer tank and will enter and dropdown in between inner tank and outer tank. There will be opening at the bottom shell course of inner tank where as the product will enter into the inner tank and the water & oil where separated at inner tank. Incase the product is blocked at the bottom opening in inner shell, the product will be filled in between inner and outer tank. So the inner tank inside is in empty condition and inner tank out side will be filled with product which has the static head pressure. The static head pressure between inner and outer tank will be considered as external pressure of inner tank. How to design the inner tank shell considering external pressure?. Hope i have given the clear picture for easy under standing. Also note that the tank has an internal pressure of 20mbarg. So it doesn't have open vent.

Ah! So it's a settlement tank!

It sounds as though a Process Engineer is required before the tank is designed. After all, with no process data to hand, how on earth can the tank be sized properly (rhetorical question)?

Hope i have given the clear picture for easy under standing.

Could not fully understand the description. You could have simply copied from your process description instead of righting in your own words. A spell check would have also helped. A simple outline drawing is also better to explain how process fluids enter, gets separated and exit.

You can design inner tank like a vacuum vessel (ext. pr. 10332 Pa abs & minimum int. pr. zero abs). For your case external pressure is static head of process fluid (max. hight in m x density in kg/Cu.m) and minimum internal pressure of zero (both in gauge).