How to Support the Inner Tank from the Outer of a 2-Skin Horizontal Storage Tank

Just out of curiosity, how would one make an outer tank that's smaller than an inner tank? A simple thought experiment: How can I make a 1100 liter tank fit inside a 1000 liter tank?

I think you misread the question. He wants to know if the outer tank should be 110% or 210% of the inner tank capacity.

Sorry, I have no answer to offer, it's not my line of expertise.

Thanks for the input but I didn't mean 110% or 210%. What I meant was that the the inner tank that sits inside the outer tank has a shell and heads made of steel of say 6mm - and that material - not the tank holding volume - has a volume. So my question is - does that volume of fabrication material have to be added to the code requirement of 110% - or can it be ignored / included in that 110% ?

Anyway, I am impressed that you guys got onto it so soon after it was posted.

Thanks.

I would say yes. If the requirement is volume, the volume of the outer tank must be equal to 10% of the volume of the inner tank plus the whole of the inner tank.

I would say it depends on the total volume of your tanks. You will have a small degree of variance. If the volume of your support structure is smaller than your accuracy, don't worry about it.

Also, if you are going to fill your annular space with insulation you might have to account for the volume of that.

My experience with cryogenic tanks is for liquid oxygen and nitrogen, both had pearlite insulation and we were not concerned about the volume of the annular space with regard to containing the volume of leaked product. Our annular space was a void for insulation purposes. The inner tank is supported by stainless steel because it conducts heat less and results in a cooler inner tank which is necessary for liquid nitrogen and oxygen. Also, we kept a good vacuum on the annular space to reduce thermal transfer to the product.

As I understand it LPG tanks don't always have a vacuum on any annular space because the product is not as cold as liquid nitrogen. If your tank is to keep liquid nitrogen you will need to manufacture it with an insulated vacuumed annular space.

The annular space will need to have large emergency vents in case of pressure buildup from inner tank leaks. To limit the number of holes in the outer tank our manhole access was also the emergency vent, it was hinged and designed to blow off in case of excess pressure. The inner tank was also fitted with a multiple vent valve manifold. The manual vent was used to prevent pressure buildup during periods of inactivity, a pressure relief vent in case of excess pressure and a burst disk vent for emergencies.

Hope all this helps. If you are making a cryogenic tank you have to keep in mind that it is a pressure vessel and therefor a potential bomb.

Drew

Many thanks for that.

We are actually ASME Certified but the ASME Code does not cover this double tank support business.

Are the separation supports in the form of circumferential tube or bar ? Or are they in the form of longitudinally placed tube or bar? Or are they in some other form - frames or saddles, etc. ?

The volume question only cropped up in the Double-skin Diesel Storage Tank where the double-skin design replaces the requirement for a "bund" wall. Perhaps I should have kept that question separate. But we still have the same question for these storage tanks, concerning the separation / support of the inner tank from the outer.

I am amazed that I cannot find any more info directly from the internet. Even in Wikipedia.

Thanks again.

You might find more by contacting a Dewar manufacturer.

All I can tell you is my intuition from the marks I see on the outside skins. It looked like there were linear ribs along the inside, I pictured them like skids on the bottom between the layers and more along the sides. The picture I have in my mind right now is a plastic tube I saw that was dual layer with the outer layer kept separate with strips between the two at right angles to the circumference of the cylinder.

This is a similar tank, but appears to have braces around the outside. The skid is just behind the toolbox and the pipe is for pressure buildup. The tanks I worked with most did not have the rib - like braces on the outside but did have weld marks where it seemed the internal supports are. I imagine they have holes cut to conserve weight along their length.

These tanks were designed to be transportable and probably more rugged than what you have to design.

Drew

Many thanks. Unfortunately I could not view the photo - only got a little red X in the corner.

But I shall try contacting the Dewar manufacturer.

Your help has been much appreciated. Thank you.

Good luck. These guys might help.

Drew

That's great. Thanking you.

No problem, I would appreciate if you let me know how you work it out in the end though.

Drew

Will do. Thanks again.

I have done an LPG cryo storage tank in the past where inner shell had circumferential stiffner rings spaced appropriately, not equidistant, may be depending upon load and shell course thicknesses at various levels. One of them was welded right at the top shell course and there was adjoining suspended deck whose dia was less than dia of ring formed by this stiffner ring. The bottom shell course was welded to tank bottom plate.

You must also find out and refer relevent API/BS/ASME codes for designing these stiffner rings which would be fabricated from strct steel.

sorry, what i said was for vertical tanks, you wanted for horz. tanks

That's great. Thank you. I can see the photo quite nicely now. We have gone for the longitudinal rods for separation but we haven't got the job yet so haven't seen how it pans out.

Thanks again.

Trevor N.

Thanks for the update, I look forward to hearing it all works out for you .

Drew