Internal welding on roof after hydrotest (API 650 tank)

[MaruthiKK]

Continued from thread404-448069...

I am working on a project which consists of fixed cone roof newly built API 650 tanks, passed the hydro test requires some welding to be performed.

In the current condition, As per Fig.5.3a roof-plate joint, lap joints of roof are externally fillet welded. Our design considered the same and test has been completed. As per its service requirement, the tank needs to be internally coated inclusive of its roof and accordingly the support structure was built as external.

In order to pass the holiday testing after internal coating on the roof, the interior side open lap joints shall be sealed. Hence, My contractor is planning to do one pass welding on the roof joints to close the gap and also to avoid any crevice corrosion in the open area of the roof joint.

Since the tank has been already hydro tested, In accordance with 7.3.6.1 of 12th Edition of API 650, only non structural small attachments may be allowed to weld please see the extract below. I need expert advice to achieve the same. Please help.

7.3.6.1 This hydrostatic test of the tank shall be conducted before permanent external piping is connected to the
tank. Attachments to the shell defined in 5.8.1.1, located at least 1 m 3 ft above the water level, and roof
appurtenances may be welded during the filling of the tank. After completion of the hydro-test, only non-structural
small attachments may be welded to the tank in accordance with 7.2.1.12.

 

[IFRs]

7.3.6 says: After the entire tank and roof structure is completed, the shell (except for the shell of tanks designed in accordance with Annex F) shall be strength-tested and the foundation initially loaded.

At first I thought you would be OK since only the roof structure is required to be complete and the hydrotest is testing the shell and foundation only and not the roof plates. But then I looked at your drawing and from the wedge being cut out this looks like it may be a self-supported cone roof. If so, the roof plates are structural and you are going to have a harder time.

 

[MaruthiKK]

Hello Mike, thank you so much for your response.
Yes the roof is self supporting type. The roof joints are already welded from outer side (External platform, structural are erected. Hydro test completed but only for the shell).
As you see from the typical lap joint of the roof of the attached sketch, the interior weld is marked by me. This is the planned activity.
But, it is not a full fillet weld (only a single pass to close the opening of lap).
Even though the activity is minor, carefully performed with no issues, the acceptability of the code to perform this task is my concern.

Thank you so much again.

 

[IFRs]

The code is meant to be read literally - each word, phrase, sentence and punctuation mark is there for a reason. Since the roof plates are in this case part of the roof structure you are technically violating section 7.3.6 of the standard by putting a lot of weld on the roof structure post-hydro. I think if you send in a technical inquiry you will get the same answer. It will be harder for an owner to sign off on your plan than if the cone roof plates were not structural. I still think you have a good chance of getting a variance but you'll probably have to jump through some hoops. Please remember that this is the opinion of one person you found on the internet, and keep us informed of your progress!

 

[MaruthiKK]

Thanks again for your response. My issue is the same; In accordance to the code and per my understanding, welding on to the tested tank is a violation because it is allowing only insulation cleats/pins..etc. only as per 7.2.1.12.

My purpose of welding is to seal the gap of the interior side of the roof lap joint which is useful to make the holiday test (after internal lining) pass.

May be sealant can serve the purpose; but, it will last soon and can't be permanent to avoid any crevices between the open joint.

PLEASE SEE THE ATTACHED SKETCH.

 

[IFRs]

I agree with your conclusion that this welding is a violation. However, I would not feel too bad asking the owner for an exemption or if I were the owner, granting the exemption, perhaps with additional NDE as you have read on the AST forum.

 

[MaruthiKK]

Thank you everyone. Exemption taken and completed the welding (just a single pass closure of the lap joint). It went fine; only a small repair on the exterior side paint repair (still minor).