Good ENgineering practice - Tank API vs ASME 1

[Idrus]

Hi Guys,
i really need some help here, my contractor is designing a caustic scrubber vessel, SS316L material, design pressure 85kpa, design temperature 400 deg C max (process gas coming in at 400 deg C, assuming all solution evaporated, tank needs to withstand 400 deg C.
The problem is the design criteria does not fit into any API code. API 620 is limited to about 120 deg C whilst API 650 is limited to about 17.2 kpa.

How should i move forward?

Contractor is suggesting side wall and head to use ASME VIII whilst bottom plate to use API 620. They dont want to turn the tank into all ASME because it would cost more, i guess.

How do i make sure that the bottom plate can withstand 400 deg C, whilst API 620 limit the design to 120 deg C?
Shed some light, please.

Thanks

 

[metengr]

Design and use materials under Section VIII, Div 1 with no Code stamping required. Stick with one code of construction.

 

[JStephen]

Is it a flat-bottom on grade? If so, that's not covered by ASME. If it's not too big, use a suspended F&D bottom and then use ASME.

 

[Idrus]

Hi Stephen, the diameter is 8meter, with transition from 8m to 4.6m at 9m from grade. Overall height is 29.5m. Contractor is defining the whole design as per best engineering practice. Hybrid bottom plate API 620 and upper portion to be ASME VIII.
Im inclining towards metengr, API 620 said that code only applies to tanks that have single vertical axis of revolution which this tank is not.

Any other ideas is most welcome, at the moment, i will go for full ASME VIII.....

 

[JStephen]

What I was getting at, is that ASME simply doesn't have provision for a grade-supported bottom as used on API tanks. The only provisions are for flat heads, which would make it unreasonably thick if used for the design. So if that's the construction, then "full ASME" won't work unless you take exceptions to parts of it, or modify the geometry.

In looking through this a while back, the ASME requirements for cones are not bad for steep cones, but for flatter cones, it appears to be a rather involved analysis, as compared to API-620.

I'm not sure about the single axis of revolution item. If it is a vertical concentric cone section with a vertical shell above it, that is a single vertical axis of revolution. Something like an eccentric cone or wye shape is not.

 

[Idrus]

Hi Stephen, thanks for the reply. This caustic scrubber has a barrel for process gas inlet not at the axis and it has process activity inside the tank/vessel i.e scrubbing, demister, can it still be qualified to be called as a tank?

 

[mandarkardekar]

This is API620 tank and not ASME vessel for sure. However as design temperature is beyond API limits need additional thermal analysis (FEA).

 

[Idrus]

Hi Mandarkardekar,
I went through API 620, and it said there that "Flat bottoms of cylindrical tanks that are uniformly supported on a ringwall, grade, or concrete-slab foundation are pressure-resisting membranes but are considered nonstressed because of support from the foundation"

If it is considered nonstressed, should we need thermal analysis on the bottom plate? (Currently Stainless Steel 6mm).

Thanks in advance.

 

[mandarkardekar]

Hi Idrus,

In my opinion if FE Analysis to be carried out, it is better to be performed for entire tank.
I am considering design standard as, API620 (no ASME consideration at all)for entire equipment.

API 620 temperature limit is applicable for entire tank and hence to design same tank for higher temperature entire tank shall be considered for thermal analysis.