Adding stiffening ring due to change of service

[waltzt]

Is is possible to add a stiffening ring/s to increase the fill height of a change of service tank? I would like to change service of a storage tank, but based on API653 calcs and the new specific gravity, the tank level will have to be significantly reduced (original S.G.=1.0, new S.G.=1.21). Is there any option to increase the fill height allowed by 653?

 

[JStephen]

Generally, not. And I'm not aware of a good solution for your problem, either.

Theoretically, you could add hoops to a tank and increase the strength of it and thus the fill level. However, it's not normally done, and so is not included in the design codes. (The hoops don't actually have to be stiffeners, they just need to act in tension.)

 

[IFRs]

The shell stress is directly proportional to the specific gravity so your entire tank is under 21% greater stress now than it was designed for. A stiffening ring only helps in a limited zone. To strengthen the entire tank, you would have lots of stiffeners and it would be extremely costly and look silly. I wold rather cut off the top ring, jack the tank and add a thicker bottom ring. Run the calculations for the original tank, see where you are and what you could do. Possibilities that would help are: 1-fractional thicknesses were used giving you some extra thickness for 1.0 density, 2-a sizable corrosion allowance was used and you can tolerate less or non now, 3-a low joint efficiency was used and you can NDE the seams to increase this, 4-the tank was heated and now it is not. Good luck, keep us posted!

 

[JStephen]

Is there actually any provision in the standards for increasing joint efficiency after the fact? I was thinking that was another situation where in theory it could be done, but wasn't supported by the standards. A similar situation would be going from the 21,000 psi of App. A to the higher stresses of the main body of the code.

If you start using excess thickness from the shell due to higher specific gravity, you'd also need to recheck reinforcing area at repads. And there might be some increase in manway flange and cover thicknesses.

 

[LittleInch]

Don't forget the tank floor and foundations which will also experience a 20% increase in load at full height, especially the ring beam, if you have one, if you start adding any meat to the shell.

My motto: Learn something new every day

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

 

[fegenbush]

I am in agreement with JStephen. Increasing the joint efficiency is a bad idea. If you do take additional shots, and they fail, repairs will be necessary and tracer shots may be necessary. Then you are in the position of welding on at tank that has been in service with potential weld contaminants, etc.

The other ideas presented (corrosion allowance, temperature, actual vs. calculated thickness, tank jacking, etc.) are all excellent, but only within the restrictions of the foundation and soil as LittleInch mentioned. There are ways to increase the foundation if necessary, but the added expense may be out of your budget.

 

[LittleInch]

You say tank level would need to be "significantly reduced". Surely this is only 5/6 of the original tank level - a reduction of 17%, but surely not too significant especially as it would probably be pretty rare for the original tank to be filled to it's max level. Significant to me would be 40% or more...

My motto: Learn something new every day

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