Why does my tank have a bulge? Elephant's Foot Bulge??

[chels468]

Hi All,

We have a large tank which stores sugar syrup (around 30C). Last week we noticed what I think look like Elephant's foot bulge, I have attached a photo. The bulges all occur where the tank meets a leg support. The bulges aren't equal on each leg support.

Has anyone had experience with such an occurrence? This is a head scratcher for us. Most of the research I have done into this describes the cause as being one of the following:
• Seismic activity (which we have ruled out)
• Axial compression (including frictional drag)
• Transverse loading – generating compressive stresses on one side of the tank - Mostly due to winds or impact, and this tank is inside and has not been impacted.

http://books.google.com.au/books?id...page&q=pressuresilo axial compression&f=false

http://books.google.com.au/books?id...ge&q=bulge "elephant foot" -earthquake&f=true

Any ideas?? Any input would be appreciated.

 

[fegenbush]

Indoors, stainless steel makes me wonder if your tank has a mixer/agitator. If that is the case, was the tank designed for the loads imparted onto it by that piece of equipment? Also, is this an API 650 tank or designed to some other standard? I have experienced some odd behaviors in tanks not designed to any standard except the fabricator's experience. These tanks are often 12ga or thinner wall thickness and can be quite unstable.

 

[Duwe6]

Looks to me like fegenbush is probably right - my first thought is "too much vertical load on a very thin shell". Check the loading of the upper portions of the shell, including roof-mounted equipment, in the region above this bulge. There probably was some preexisting local distortion due to the welding of the circ. seam, and that misalignment of the thin shell caused the shell to bulge out due to a heavy vertical load from above this seam.

 

[LittleInch]

I'm curious about your phrase "where the tank meets a leg support" You might be able to see this or know what it is but I don't.

Have you any more details or section drawings of the tank, things like diam, height, wall thickness, material, design code, other things mounted on tank, what the seam distortion is that appears in part of the shot - was that always there or has it just arrived?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[JStephen]

If the tank is leg-supported, as the statement suggests, it looks like it is just excessive compressive stress at the top of the leg due to tank contents.

 

[chicopee]

About posting all the specification provided by the manufacturer on this tank.

 

[msquared48]

JStephen:

Very true. If this is at the location of a hard point in the support system of the exterior tank wall, this deformed shape could very likely occur. Modifying the support system, or lowering the level of the material in the tank, or a combi8nation of both, is warranted.

Mike McCann, PE, SE (WA)

 

[chels468]

Hi,

Thanks for your replies. I have attached a bigger picture of the tank, however in this shot you can only see one of the bulges (and the worst bulge isn't visible as it is next to the wall and hard to get into frame!).

This tank was installed in the year 2000, and the bulges were only identified about a week ago.
> Tank has an internal agitator and a wall thickness of 3mm (0.118")
> We have conducted wall thickness tests which confirm that there is no thinning of the walls
> We have confirmed that the vent valve on the top of the tank is working, and fully operational as per design
> Tank was designed to Australian Standards

The only thing (That we know of!) that has changed, is that the piece of equipment which has been determining the concentration of the incoming sugar syrup has been malfunctioning - so there have been variances in syrup density.

I will dig out some more details on the tank and post them today.

We have lowered the level in the tank for the time being until we can figure out what has created the distortion.

Thanks for your help on this! Appreciate all your comments.

 

[IFRs]

The appearance of such a bulge, as others have stated, is most likely due to excessive vertical load in the shell, confirmed by the location at a support. That said, we want to figure out why and eliminate future occurrences. Could the syrup be clinging to the shell? Could the overall weight of the contents be high enough to cause this local failure? This may be a force from above or a force from below. Could the legs be unevenly loaded, from floor conditions perhaps? Is heat involved? There have been seismic events in NJ recently, sometimes such events are not felt by people but your tank may have rocked and overloaded a support. Could a maintenance or cleaning crew jacked up one leg to clean under it?

 

[Compositepro]

Other possibilities include settlement under other legs or sloshing of tank contents. It seems to be an awfully large tank to not have a foundation. Perhaps there is one under the tile floor.

 

[bimr]

It appears that some condition inside the tank has caused the fluid to expand or push the tank walls outward. The bulges occurred because the tank legs were stiff enough to resist the expansion and caused the metal to deform.

Has the sugar solidified or thickened in the tank? Is the heat source on the bottom? What exactly is the sugar? What is the viscosity and how does it change with temperature?

 

[geesamand]

Don't skip the investigation of the agitator. Agitators have instable turbulence around the impeller(s). This instability causes transient spikes of high bending load on the tank flange. Combined with the dead weight, a bending load applied to the top flange can result in an "axial" force on one side of the tank wall. Many agitated tanks are not design with the agitator bending loads in mind.

Furthermore, the amount of instability in an agitator varies widely with it's placement in the vessel. If the agitator has been updated/moved/repositioned recently, that's certainly something to consider.

David