Vertical 20K Tank on Stilts

[cwigg66]

Simple question. Is it ever appropriate to take a 10' diameter vertical storage tank (34' tall)and weld I-beam stilts to the shell effectively raising the bottom of the tank 12-18" in the air. I have never seen this done. I believe the installer did it to prevent corrosion from standing water within the secondary containment area. The stilts are seam welded to the sheel and extend up the shell approximately 24". The stilts are also welded to anchor plates embedded in the floor of the containment area. I feel I need to at least check these tanks for stability / overturning issues, and possibly localized stress issues in the shell. Anyone run into this before? I have been asked to assess the suitability of the tanks, which are currently in service and used ot hold recycled fuel oil. Thanks.

Chris

 

[jte]

Supporting a vessel on legs is quite common. A 2' overlap of the leg to shell sounds to be in the ballpark. Yes, it's worthwhile checking the load path all the way from the baseplate to the leg to the shell.

You didn't mention the type of tank you're dealing with; if it has some sort of conical or formed bottom head you're probably in good shape. If it has a flat bottom you will soon have a formed head! So that little detail is worth checking also.

jt

 

[stookeyfpe]

You also must consider the NFPA 30 requirements for column fire protection. And, the additional surface area resulting from elevating the tank will impact the surface area of the tank, and therefore, the emergency venting of the tank.

While NFPA 30 is silent, the 2006 International Fire Code requires the column protection to meet ASTM E-1526.

Please don't tell me the columns are 1/2-inch L-shaped carbon steel columns. They lack sufficient mass to meet ASTM E-1526 or UL 1709.

If this is the case the tank most likely does not meet UL 142. Scrap the tank.

 

[chicopee]

Check out Bednar "Pressure Vessel Design Handboo" on support skirt design eventho you are dealing with support legs.

 

[JStephen]

In the design of tanks, some design procedures are clearly defined by different tank standards, but others are not. Design of leg supports, and of the shell for leg supports, is one issue that is not well defined. Consequently, there's not much of a way of knowing how the tanks were designed, or even if they were designed, for that configuration. And the configuration you describe probably doesn't meet any particular tank standard.

Note that in some cases, a leg attachment can be treated similarly to an anchor bolt chair.

 

[JoeTank]

This sounds like a disaster waiting to happen. A flat bottom tank is never suitable for elevating unless the bottom is designed for the entire weight of the product. I would expect to see conical boottom or a formed head used (such as ellipsoidal or torospherical).

Joe Tank

 

[cwigg66]

That was my first reaction this situation. The tank was most probably originally designed to be flat on the ground, although identification coudl be found on the tank which would indicate the method of construction (i.e UL, API, etc) The bottom was probably flat at one point, but with the lack of support it is very slightly deformed downwards at this point. It seems like this is alot of weight for the tank bottom welds to be supporting. I have not run calcs on it yet.

 

[cwigg66]

Could NOT be found.... sorry for the typo

 

[MJCronin]

cwigg...

Questions:

1) How many legs (stilts ?)are there and what size are they ?

2) Has the tank ever been filled or hydrotested ?

2) Was the tank originally flat-bottomed and what does it look like now ?

3) When "Cooter-and-the-boys-from-maintenance" change things around and "weld stuff up",is it then the job of engineering to justify the changes and take responsibility ? Does your boss have an MBA ?

4) While I am sure that your organization has discarded all design drawings and calculations, are there any markings, or API-type nameplates on the tank ? Is it possible to get the original fabricator involved ?

5) Where are you located and does your state have any periodic tank inspection program ?

- My opinion only

-MJC