Heavy Fuel Oil tank leakage 2

[magmath76]

Hi All,

We have recently constructed a 19mtr dia x 9 mtr high storage tank for the storage of heavy fuel oil with specific gravity of 0.985. It has a lap joint floor. The tank was hydrotested and water was there in the tank for morethan 2 months. When we started filling the tank with hfo the tank started leaking at the bottom at 1.2 mtr high fill. Luckily we got another tank hence we transferred the hfo. We yet to clean the tank and do the tests and locate the sopt where it was leaking. It is a bit surprise to us as nothing happened in hydrotest and the tank leaked after that. Has any one come across such peculiar issue ? or Any got an idea of what could have caused this failure?
Any feedback on this would be highly appreciated.

with thanks
Magesh

 

[fegenbush]

Were the floor lap joint seams vacuum box tested in accordance with API 650? That would be the method I would use to determine the location of the leak.

As for how it happened, it is possible that the leakage of water was not detected and the tank was leaking the whole time. What is the foundation type? Is there a leak detection system (flexible membrane liner or equivalent)? How are you detecting the leak currently?

 

[JStephen]

Was the fuel oil hotter than the test water? Minor thermal movement might have opened up a crack that wasn't otherwise noticeable.

 

[magmath76]

We will be using the vacuum box testing method to determine the location of the leak. Foundation were done by another contractor and attached is the arrangement also the photograph showing the leakage. I am not sure but I think with this arrangement if the tank got the leak during the hydro test we could have detected that.

on the temperature of the oil, I think it could be slightly hotter than the water we used

 

[magmath76]

more attachments

 

[magmath76]

more attachments

 

[chicopee]

I think that you'll find your leak in the center of the tank where there is a low point. As pointed out above, water may have leaked out and seeped into the ground without being noticed. Heavy fuel oil by contrast would not seep out infiltrate the ground quickly and it accumulated to the point where you found the leak.

 

[Kiwi2671]

magmath,
You are mentioning the tank having passed a hydrotest and you are unsure why it is leaking out of the bottom - you make no mention of other testing.
The hydrotest is only for checking the shell welds - API 7.3.5 & 7.3.6
Tank shell to bottom welds (7.2.4.1)require testing with one of four methods listed and Tank Bottom welds (7.33)mustbe inspected with one of three methods listed - the most common being vacuum boxing.
Are you the owner of these tanks or the fabricator / builder ?
Are they supposed to be fabricated in accordance with API 650 ?
Regards,
Kiwi

 

[magmath76]

Hi Chicopee,

I am not sure as the tank was holding water for more than two months (not in full capacity) ie the tank was filled gradually over six weeks due to water scarcity and pump issues. however no sign of leak or any decrease in water levels in the tank....at the same time your and JStephen's thinking could be true.

Hi Kiwi,
shell to floor welds are MPI tested and we used MPI to do the testing of the floor as well not vacuum box. I am the owner....not sure whose fault is tank contractor's blunder or foundation contractor's blunder.

 

[Kiwi2671]

Magmath,
Was the tank supposed to be constructed in accordance with API 650 ?
If so and the contractor did not vacuum box (or one of the other two options)it is a non code compliant tank and all the costs should be put back on the contractor - that is the erector, not the foundation contractor.
Regards,
Kiwi