API 653 - Max sizes for Patch Repair Plates on Tank Shell. 1

[sajk14]

Hello All,

Looking at the method of using patch repair plates on the tank shell.........

In API 653 clause 9.3.1.7 it clearly states the maximum size of lapped patch plates is 48inches in height and 72 inches in length.

It also states in clause 9.3.1.10 that patch repair plates shall not be lapped onto other patch repair plates.
However it doesn't say in the code that you cant 'Butt weld' patch plates to one another? Therefore can you get a patch plate 48in x 72in and butt weld it to another patch repair plate of the same size?

My opinion is that you CANNOT butt weld patch plates to one another as by doing so you in effect make it one bigger patch plate, and therefore break the max allowable patch plate rule. Otherwise you would be allowed to use butt welded patch repairs right around the complete circumference of a tank or right up the tank shell height, which of course is a big NO NO!?

What is everyone elses thoughts on this as the code is not clear or is poorly worded?

Also are you allowed to carry out patch repairs to the inside of the tank shell? I have looked in API 653 and cannot see any details to say you can. All patch repairs are written in the context of being applied to the outside of the tank shell only?

Appreciate your comments/thoughts on the above.

 

[leonant]

I have a question to add to yours!

API 653 allows the use of lapped patch repair, but is not well stated the code to use. I mean, if we a had a look to

the cluase 9.3.4.2, it states:

"The repair plate is designed to withstand the hydrostatic pressure load between the repair plate and the
shell assuming a hole exists in the shell using a joint efficiency of 0.35"


My question is: How to perform a calculation? I don't think you have to use the formulas in 4.3.2 / 4.3.3, because

the stability of the tank is already assured by 9.3.4.1.

Any of you have already had this issue??

Thanks in advance

 

[IFRs]

I'd use 4.3.3.1 (b) using E=0.35 because you are calculating the required thickness of the lap patch plate for hydrostatic load. If you cut out the bad area and use the appropriate lap configuration, use continuous weld on both sides then you are using 9.3.2.3 and can have an E=0.70.

 

[sajk14]

I agree with IFRs to use 4.3.3.1b for locally corroded areas on the tank shell. Also keep in mind that the requirements of clause 9.3.4.3 are met.

Do any of you have any comments or feedback with regards to the original post?

 

[IFRs]

9.3.1.6 mentions patches on the inside of the shell. If the repairs are so extensive that you are considering butt welding lap patches together, I'd recommend a butt welded patch for the entire area.

 

[sajk14]

Thanks for the clause reference IFRs.

Just to clarify in your last response.....
"If the repairs are so extensive that you are considering butt welding lap patches together, I'd recommend a butt welded patch for the entire area."

Do you actually mean if the repairs are so extensive you would recommend a butt welded a shell insert plate replacement for the entire area?
As i mentioned the code only allows a max patch size of 48in x 72in and you cannot butt weld 2 patches of this size together?

 

[IFRs]

What I meant to say is if there is that if the bad shell is that extensive, I would strongly consider replacement shell plates using butt welds all around. I don't think API intended that lap patches be used as major shell repairs although it does not specifically say this. If you were to have multiple lap patches I would maintain weld spacing between them and not butt them together.

 

[sajk14]

Nice one IFRs,

This was my thoughts also but is nice to get a second opinion as the code is not clear on this subject.

Thanks for your feedback.

 

[IFRs]

The committee is considering a revision to this portion of the code, essentially allowing the use of alternative repair details if an experienced engineer has reviewed and approved it. Time will tell if this revision is passed. In the meantime, and always of course, you must use your best judgment and stay within the code. The code does not address multiple lap patch spacing but does allow lap patches and gives guidance on weld spacing. You could stay within the code by using lap patches that meet the code and space them in accordance with the code. But, does it pass the red face test? Would you be uncomfortable defending the repair method in front of your peers or another tank engineer? Would you put your seal on the drawings? Are you really saving any money long term doing extensive lap repairs as opposed to butt welded replacement plates? The final answer depends a lot on the specific situation - tank size, location, operating condition, contents, liquid level, corrosion rates, shell thicknesses, public risk, etc. Good luck and be safe.

 

[PMCap]

Perhaps restoration by encapsulation would be appropriate - all attachment welds would be full penetration and there would be no size limits on the repair - encapsulation hardware can be installed either on the inside or on the outside and the restoration remains code compliant - refer to NBIC Interpretation 04-01