Corroded (Reduced Thickness) of Fuel Oil tank Roof

[mech471]

We have 03 Nos Fuel oil tanks, brief data of tanks and products is as follows:
Nominal Capacity : 12760 m3
Tank Inside Diameter: 33.300 m
Tank Height: 13.710 m
Service: Fuel Oil
Storage Temperature: 85 oC
Specific Gravity of Fuel Oil: 0.96
Design Code: API Std. 650 9th Edition July 1993 With Addendums Variable design point method
Roof Type: Cone Roof Tank (Supported roof on columns and rafters)
Roof thickness: 4.76 mm (corrosion allowance 0.00 mm)
Roof material: ASTM A283 Gr. C
Insulation Shell: Yes 65.00 mm thick
Roof Insulation: No


About a month ago, cracks were developed on tank roofs, when thickness was monitored it was found 2.0 to 2.8 mm around crack. As a temporary solution we want to reinforce the corroded area with the same plate by applying some adhesive material to join existing roof and new plate. Reinforcement shall be applied such that total thickness shall be 6.00 mm.

My quires are as follows:
1) Your advice regarding the addition of minimum corrosion allowance in roof
2) Effect of increase in thickness up to 6.00 mm as weight of roof will increase on Strength of supporting structure columns, rafters (specially) and girders.
3) References, websites in this regard
4) Any other alternate solutions or commenst

Regards,

Mech471

 

[chicopee]

about the thicknesses in the areas where there are no cracks-any significant reduction? Also do you think that the rafters are in good shape? You may be able to seal weld outside patches if the intent is to keep rain away from product but definetely induce a nitrogen or carbon dioxide atmosphere in the tank before seal welding. and check with your insurance company and local fire department if the fire marshall approves of the method.

 

[jay165]

Hopefully, SteveBraune will read this post and respond, but in my opinion your repair method won't do jack as far as arresting the cracks or maintaining roof integrity. Your post talks about cracks and corrosion as the same - which they are not. Are the cracks a result of SCC or improper welding? And how do you "add a minimum corrosion allowance"? Sounds like yours is already used up!

 

[JoeTank]

mech471,
Responding to your questons in order...

1) Your advice regarding the addition of minimum corrosion allowance in roof.

Seems that you lost about one-half of your roof plate in the last ten years or so. Seems obvious that you need to address that problem by adding a corrosion allowance or use an internal coating. Coatings would be a real challenge as it is difficult to get everything properly coated. Sounds like CA would be more appropriate. If you are loosing 2.5mm in ten years... you would need to add 5mm to get twenty years of service.

Oh, by the way, I'd be loosing sleep over your situation. If the roof plate is corroding, you can be sure that the roof rafters are probvably in worse shape. This also represents a safety concern for anyone going onto the roof. When adding a CA for the roof, remember that two surfaces are being corroded.

2) Effect of increase in thickness up to 6.00 mm as weight of roof will increase on Strength of supporting structure columns, rafters (specially) and girders.

Yes, the extra thickness adds weight, so your design loads on the roof structure will go up. That combined with the CA on the roof strutural components will probably require much heavier roof structure.

3) References, websites in this regard.

Brownell and Young's book has some pretty detailed stuff on roof structure design methods. Can't think of any websites other than this one.

4) Any other alternate solutions or comments.

That's a very high corrosion rate in the vapor space for fuel oil service. Is there anything else that you ocould share with the group about the service?

Hope this helps.

A self supprting roof with external framing, such as a dome roof, may be something to consider.




Steve Braune
Tank Industry Consultants

http://www.tankindustry.com