Flexible tank stress analysis

[denysy1]

Hi,

I am looking for a method to analyze stress within a flexible fuel tank, akin to the one in the picture:

http://image.made-in-china.com/2f0j00pBiEqWDJnVbe/Flexible-Fuel-Tank-KSD-009-.jpg

So far I have not been able to find a text that would discuss something similar. Simple hydrostatics do not apply if the tank is filled to a point where there is pressure head at the top of the tank.
I would also like to be able to derive the cross section geometry of the tank, but that is not necessary.

I would very much appreciate any pointers to any useful reading material.

Thanks!

 

[BigInch]

Consider the ovalized shape as a rectangle and your problems will basically go away.

With no membrane stretching, you will have a constant "circumference". If you consider that as a rectangle, its equal to 2 x the width when its flat.

Now start filling it up. As the height increases, the width decreases, because of the constant circumference.

The tallest this tank could ever be is when its tipped on its side, width = height; who said it had to stay horizontal (a carrier vehicle with this tank strapped on might turn over, so I think you wouldn't want the tank to burst then).

So the maximum stress in the membrane will be when the tank is the tallest, since hydrostatic pressure is then a maximum.

The maximum stress occurs on the bottom membrane of the tank at the centerline, which is flat, so your gometric problem is not eliminated. We can analyze this as a series of rectangles of constant perimeter and arrive at a conservative result. See attached,

It should be a simple matter for you to work out the precise geometry, given a few minor simplifications about how much it actually ovalizes. Personally I wouldn't care so much about that, as the rectangle assumption can tell you pretty much all you need to know about this tank from an engineering perspective.



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[saplanti]

I guess you can visualize how the hot water bottle works. It is almost flat; after taking the water in it becomes oval.

But, you case does not stop there. The hot water bottle gets cooler with the time and you can not feel the change in the shape due to almost incompressiblity of the water.

In your case the store the liquid fuel at atmospheric temperature, and if the fuel gets wormer/hotter during the storage it may vaporise and introduce internal pressure depending on the storing temperature.
If the internal pressure exists the shape is going to change to oval and ultimately to circular (cylindrical or spherical) by introducing membrane stresses. Therefore you need to check the process and the storage temperature issues all together not to be disappointed with your design.

Hope it helps

Ibrahim Demir