Risk Management for Empty Steel Tank

[pepe1234]

Is anyone aware of best pracitces, publications, standars, or anything that addresses risk managment strategies against wind damage for large welded steel tanks that are sitting empty (or near empty). We are in an area that produces a lot of ethanol and current market conditions have been such that ethanol production is down. Way down. So we have many customers that have large welded steel storage tanks for ethanol that are currenlty sitting empty or nearly empty. One of out customers actually sustained damage to the sidewall of one of their large welded steel tanks yesterday during a windstorm (tank is 2 million gal capacity, 120' diameter, 9 years old). The wind pushed in the sidewall.

I know little about how these large welded steel tanks are typically designed. Does the floating roof mechanism in these sort of tanks normally have a device that keeps the floating roof from going lower than 1/2 the sidewall height to help stiffen the sidewall of an empty tank?

Also, if anyone would like to additionally comment of methods to repair tanks with sidewall damage, I'd appreciate your thoughts on that too.

 

[Soln]

If the internal floating roof (IFR) is a suspended IFR, the customer may be able to pull the roof to the desired elevation and hold it there. I'm hesitant to go into recommended procedures for fixing a tank that's blown-in without knowing the extent to which to the side wall failed. Buckle? Collapse? What shape is the roof in? Do you have pictures?

 

[pepe1234]

Here is a pic of the damaged tank. Currently only have one picture. Hopefully I'll get additional pics in the ocming days showing the roof...

 

[IFRs]

Given that there is a fairly compliant seal between the tank and the floating roof, the floating roof can move laterally, etc, I would not count on the floating roof to prevent a buckle from forming. If the tank buckles near the floating roof you may have real trouble getting the floating roof to move past it, complicating repairs. The tank should have been designed to resist a declared wind load and if it is sustaining damage then the wind was under-estimated or the actual is greater than the design. Can you fill the tanks with water? If you see a real need to add strength to the tank, you will need to determine a new wind load and review the tank design for this new load. Intermediate wind girders or tie-offs may be in order. Good luck, keep us informed if you can.

 

[Soln]

Great points (IFRs). I've seen buckles such as the one in the picture pulled out using strategically placed hitch lugs, and a back hoe. There are probably two kinks (permanently deformed sections) in the lower portion of the forth ring- primarily cosmetic, I venture, however; more susceptible to future buckling if not reinforced, or the shell plate replaced in those sections.

 

[IFRs]

This tank does not look that bad, if I had to guess I'd say a 6 to 8 inch dent. Pull it out and go home. Find something else to fill the tank with. If the OP really wants to make the tank wind-proof, it is not rocket science.

 

[pepe1234]

Thanks for the replies thus far guys! I've finally got some additional information on the tank. 3 million gallon cpacity, 120' in diameter, and 36' in height. Constructed with 1/2", 3/8", and 1/4" steel plates. Original design documents state it was designed according to API 650, 10th Ed.

I'm looking thorugh API 650 now and the requirements for when a tank is required to have stiffening rings (this tank does not have them installed). Section 5.9.7 speaks to maximum heights of unstiffened shells. If I'm applying the equation at 5.9.7.1 correctly, I'm getting a maximum height of about 25' with t=1/4", D=120', and V=90 mph.

Anyone agrre/disagree that this tank should have been designed to include wind girders? Or are there additional criteria within this code that also apply to the requirment of stiffening rings?

Thanks!

 

[pepe1234]

I should probably mention that this is not an open top tank. There is 3/16" steel plate roof structure on this tank.

 

[Soln]

Your right; about 25' with 1/4" top course. Designers often increase the thickness of the top course to avoid intermediate wind girders, i.e., 5/16" top course course increases the allowable unstiffened height to 44'.