Question about Tank Repair 3

[SaCaZu]

Greetings
If I'm going to repair an oil tank who owns localized pitting in its first plate and I decided to install a plate on this one. where should I put this plate, in or outside of tank shell?

thank in advance.

 

[gr2vessels]

Sacazu,
The pitting is a sign of a particular type of corrosion;- if the pitting is inside the tank and you keep in the same fluid and you need to extend the pitting corrosion to a crevice corrosion, you weld the plate on the outside. However, if you want to control the internal corrosion and keep an eye on the next stage of metal loss, then weld it inside.
If the pitting is on the outside of the tank wall and is located on the first strake, then the environment is very corrosive, rain is splashing up on the tank wall and the bad paint job allows the corrosion to penetrate through the holidays. Then a better paint job could solve your problems, hence you weld the strake inside or outside by flicking a coin.
cheers,
gr2vessels

 

[unclesyd]

How large a tank are you talking about?

Where is the corrosion located, inside or outside?

Have you looked at the tank floor?

 

[SaCaZu]

Unclesyd,

It's a tank 500 bls, Height 5 mts, Diameter 4.57 mts. The pitting is located inside tank first wall.

Now I've one doubt, if I had a pitting (perforation) in the tank bottom near (aprox 2 cms) the joint between the bottom and shell. what should I do? to replace that plate or I could put a plate on this one, trying not to weld on the joint between both (bottom and shell)... ? can I weld on that welding?

thank in advance

 

[unclesyd]

I'm speaking from experience and not necessarily the latest codes. You can get some very good guidance from you codes and local jurisdiction.

Normally if we have a tank with corrosion on the first ring, say with in the bottom 6" inches and corrosion to the bottom plates we will put in a new bottom and shorten the tank by 6" or so inches. We have found that if we have internal corrosion to first ring and bottom we generally have external corrosion to the bottom plates especially if there is a leak. If you have lap welded bottom plates you have to be careful when removing the affected area so you have room to make a good weld

If you have to weld at the "T" joint there should be no problem. Saying that I've have run into a "T" that had a void caused by the two fillet welds. The void being full of oil caused a few welding problems.

 

[JoeTank]

I suggest that you get yourself a copy of API Standard 653, or API-RP 1R1 and read it carefully. There are significant restrictions on the use of lap patches for tank shell and bottom repairs. You should be aware of such rules before you attempt any repairs.

Joe Tank

 

[JoeTank]

My apologies about the error in the previous post. API-RP 1R1 should read API RP-12R1

Joe Tank

 

[packerboi2]

You fail to mention the code of construction for this tank. API,AWWA or ASME. Each have there own requirements for repair.

Assuming the tank is an API tank, then joetank is correct in directing you to API 653 for the repair. API 653 does have some restriction on lap patch repairs to the shell and floor plate. You could replace the shell/floor plate with new and do the required testing also.

 

[BoyTNDT99]

sacazu,
if localized is not big and API 653 is allow welding repair, the easy way is to weld fullfill buildup (groundflush finish).