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Concentric Tank

Concentric Tank

Concentric Tank

I have a query related to storage tank design (related to hydrocarbon storage tanks of 3000 to 5000 KL capacity in Refinery complex) If we construct concentric storage tank with annular space in between.. does that allow us to go away with Dyke/Bund where there is not enough space available? Means outer shell covering the main tank from bottom to top. Is there any Guidlines or Standards applicable for concentric tanks design? If yes, please let me know.

RE: Concentric Tank


What you're referring to is double skinned tanks.

I've only seen them on relatively small horizontal buried tanks (<100 m3) where there is no bund, but for larger vertical tanks the issue is that the inner tank needs to be able to withstand external pressure without collapsing and as you get bigger this gets harder and harder and needs thicker and thicker steel.

I've seen concrete wall type bunds where the wall was quite high ( 5-6m) in very congested locations, but then you run into separation distances per NFPA which limits how close you can get the tanks.

Hence why I don't think there are many examples or standards.

What sizes and lack of space are we talking about here?

I've added my previous response here as I'm going to red flag the other two posts you've listed so ther eis only one - that's the house rules

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Concentric Tank

For all of our SPCC PE stamped plans for new power plants, we had double wall fuel tanks with a containment (dyke). Those requirements may vary based on physical location of the facility.

RE: Concentric Tank

It's not at all unusual to have a circular steel dike with a storage tank inside, particularly in and around large dense cities where land is very expensive. I worked on a 150 foot diameter tank inside a circular steel dike and several smaller one. Because the dikes are larger in diameter they are seldom the same height as the storage tank.

RE: Concentric Tank

IFRS. I bow to your greater knowledge and experience and if you have any photos that would great. Is this though no real difference to any other bunded area just made of steel and higher than an earth or concrete bund?

The OP seems to imply another tank wall within 2-3 ft from the main tank wall.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Concentric Tank

Here is one - a 150 foot diameter tank inside a steel dike.

I'll look for others...

RE: Concentric Tank

Wow, that's a nice tank.

My point is that this is, in principle, no different to a "standard" bunding arrangement, just made of steel and much higher than "normal"

The OP seems to be looking for a large (3 to 5,000 m3) vertical double skinned tank. Now that IMHO, is pretty unusual...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Concentric Tank

what is the purpose of double wall if containment dyke is also there for those fuel tanks? Am I missing something here?

Thanks for providing the photo, like to know the service fluid of the tank.

As LittleInch mentioned I am looking for any example or design standard/guideline in Petroleum refining sector, where inner tank (Max Capcity 5000 KL) is surrounded by an outer one with about one meter space between two tank walls. And in that case can we treat the outer tank as containment to get rid off another secondary containment (dyke).

But in case of fire in the inner tank the possibility of damaging outer tank is also very high in above mentioned arrangement. That's why like to have more info on this. If there is any Guidelines which covers all concerns and allow double walled tank.

RE: Concentric Tank

It's not a double wall tank. It's a tank inside a steel containment dike. There are no rules specific to this type of dike construction that I know of but I was not involved in the design or construction of this one. There is no need for an earthen dike or bund around the tanks. I don't think the fire risk is higher for this type of containment than a much larger but lower wall around the tank. Surface area is less which might help. Its purpose is not to stop fires but to contain a leak (or a fire) so it does not spread to neighboring areas. That tank was in ethanol / gasoline service, my job was to put an aluminum internal floating roof inside. Most of the tanks I've seen had much less distance between the tanks. I believe NFPA says the dike must be at least 5 feet from the tank, I have worked on tanks inside concrete dikes where this minimum space was required. There are some piping issues that may require a larger space between the tanks, especially if the thermal expansion of the piping is significant. Thermal expansion in the piping can transmit large forces, tank nozzles are ill equipped to resist large forces and moments. The outer steel dike is essentially just another tank and is designed using the same codes and calculations as the inner tank. Most of the steel containment dikes I have seen are open top, a few have a sloped roof to shed water and snow but then the space between the tanks has different conditions and risks. I have put floating roofs inside actual double wall tanks - one inside the inner tank and one in the donut shaped space between the tanks, they were surrounded by a containment system, I think an earthen dike.

RE: Concentric Tank

The biggest issue I see is the risk that any leak, or gathering of fluid, rain water, fire water, whatever in the annulus becomes higher than the internal fluid and creates a positive external pressure on the inner tank.

Positive external pressure on large atmospheric storage vessels such as a 5,000m3 tank leads to collapse usually or very thick tanks. The level at which this becomes a problems is low inches of water.

The smaller the distance between skins the smaller the volume required to allow this to happen.

Add in the essentially double cost of the steel walls and it rapidly starts to become more expensive to do this than any gain.

I know of no such guidelines other than the NFPA guidelines which provide various distances and separation.

what usually happens is that you group tanks together of the same substance and then your bund only needs to accommodate the largest tank within the set so separation can come down and also height of the bund. Bunding large individual tanks is the worse scenario.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Concentric Tank

Quote (nd222)

what is the purpose of double wall if containment dyke is also there for those fuel tanks? Am I missing something here?

Some states (and SPCC certifying engineers) want both because they argue that the tank could still be overfilled. The double wall protects against leaks and the containment protects against faulty operation.

RE: Concentric Tank

If a leak isn't a "faulty operation" then I don't know what is....

I've never seen a double skin tank AND a containment bund. Buried DS tanks yes and occasionally below ground DS tanks inside a concrete pit, but not above ground.

The pit issue was about not having any leaks creating a hazardous atmosphere, but was still overkill in my opinion.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Concentric Tank

Quote (LittleInch)

If a leak isn't a "faulty operation" then I don't know what is....

I'd say a leak is faulty maintenance and/or engineering inspection program. You could overfill the tank at any time after the tank goes in service, but the leak will most likely occur after the tank is in service for many years.

Not saying I agree with this philosophy, it's just what I have seen from some state's DEQs.

RE: Concentric Tank

The main purpose of many double-walled tanks is for leak detection prior to environmental contamination.

RE: Concentric Tank

Given the size differential, it may be impossible for liquid in the outer tank from a leak from the inside tank becoming higher than what remains in the inner tank. At least, I'd make it that way if at all possible. Rain water is drained out just like a normal dike, as expeditiously as possible.

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