Replacement/double bottom tank questions

[robsalv]

Hi ya folks.

We have an API650 tank which has a GRP lining on the tank floor and extending several feet up the walls.

The GRP lining is over 20yrs old and the top gel coat is pretty much gone. There are also some areas of exposed fibres.

Due to the non uniform thickness of the lining, we've not been able to do any floor thickness scanning over the years and have no feel for the recent under floor corrosion situation.

A bunch of planning options are being considered for this tank, but I'd like your thoughts/advice regarding the secondary/replacement tank bottom line of thinking. (API653 is a bit light on in this area)

* Would it be better to keep the original GRP lining (degraded as it is) and have it provide additional seperation between the old and new tank bottoms?

* Before installing a new tank bottom, would it be prudent to conduct some kind of annular ring inspection and bring the annular ring (assuming excessive corrosion) back to code allowable condition for structural integrity purposes?

* Is it acceptable to remove the old GRP lining and install a much thicker GRP lining in lieu of doing tank bottom repairs?

Your thoughts are appreciated.

Thanks.

Rob

 

[robsalv]

...hmmm, no replies.

This issue will be discussed with the specialist contractor in due course, but current thinking is:

*Strip the lining.

*Repair the annular ring where required for structural integrity.

*Balzona (metal putty) any floor holes.

*Put some kind of fill in between the old and new floor.


Happy to take advice or comments.

Thanks folks.

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"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."

 

[khardy]

First, where is the tank? Does the excising bottom have a secondary containment liner? If not, will you have to add secondary containment to comply with local laws?

If you are installing a new bottom on top of the old bottom / GRP combination I would leave the GRP liner alone. There is no reason to remove it and there could be significant disposal costs if you do. If it is intact you can use it for secondary containment. If not you may need to add a liner under the clean sand.

It is common to use clean sand or grout between the old and new bottom. If you use sand you’ll want to put cathodic protection in the sand to protect the underside of the new bottom. You shouldn’t have to worry about the integrity of the old corner (shell to bottom) weld or the old annular plate. The new bottom corner area will assume all hydrostatic loads. Keep in mind if you put the new bottom over the old you may need to modify the shell nozzles to keep bottom to centerline dimensions required by API-650.

I can’t recommend a new GRP liner. They always end up costing more in the long run. They won’t last as long as a properly maintained steel bottom and when they do fail it’s always difficult to deal with the fuel that’s between the liner and the steel bottom.

 

[robsalv]

Thanks for your reply khardy. Food for thought. Cheers.

- - - - - - - - - - - - - - -
"Life! No one get's out of it alive."
"The trick is to grow up without growing old..."