TANK REPAIR AN ALTERATION

[lganga]

I have to repair an API tank 650 appendix F.
we want to do some lapped plates in the internal shell.
I am reading API 653 and I couldn not find if the lapped patch can we welded in the internal shell.

We want to do it in that way, just to avoid crevice corrosion for in case the patch is welded externally.

We read in the 9.3.1.6 a limitation in the internal shell patch of 6" distance from the shell to bottom weld.

Is correct to weld internally following the figure 9-1.?
Regards
Luis

 

[Duwe6]

Internal patches are not addressed by API-650/653, thus they are technically allowed [by being not disallowed], providing "good engineering judgement and practices" are used. If you follow 653-9.3 you would be using both. That is how I would procede.

Fig 9.1 would indeed apply.

 

[IFRs]

I take it the opposite way - if API is silent on an issue it is not allowed. Only that which is expressly described is in compliance with the code. If interior lap patches are not described then a tank repair that used interior lap patches would not be repaired in compliance with API 653. That's not to say that good engineering judgment can not be used to make a good repair, just that it would not be an API compliant repair. The owner would assume any risk or increased risk for a non-code repair.

 

[IFRs]

Lap patches are included in API 653:

9.3.1.6 Repair plates may extend to and intersect with the external shell-to-bottom joint if the vertical sides intersect the tank bottom at a 90° angle and the shell-to-bottom weld is in conformance with Figure 9.6. Repair plates
positioned on the shell interior shall be located such that the toe-to-toe weld clearances are a minimum of 6 in. to the
shell-to-bottom weld.

 

[lganga]

IFRs

As I see in the API paragraph you sent, it is allowed to do an internal shell lap patch.

After sandblasting and inspection of the internall wall we will decide what type of lapped patch we will apply.

The problem with the lapped patched when covering a hole on the shell is that ,it is not possible to pneumatically test the lapped patch.

thanks IFR and Duwe for your answers

regards
Luis

 

[IFRs]

Lap patches need to be welded on their outer perimeter and on the inside of the hole. The patch can then be tested.

 

[fegenbush]

IFRs,

I think what he was saying is that they are pro-actively placing this patch before there is a hole. You are correct, they need to cut a hole in the center of the patch and weld it there and drill and tap a telltale hole in the shell (for an internal patch) for pneumatic testing.

 

[lganga]

IFRs-Fegendush
I am sorry for my delay

I have done an inspection of all the tanks we have in the field and for my surprise none of the lap patch repair (external diameter) has been internally sealed and tested.

Of course i found that a lot of crevice corrosion appeared in the lap patch weld after that.

Nobody at our site has payed attention about sealing the leak after welding the lap patch and of course no pressure test was done.

Of course this procedure gives a dependable repair but in case of leak sealing with a patch I will prefer to weld a butt weld patch instead of it.

Thanks a lot
I will appreciate any other comment
Luis