Lap-Weleded Bottom Joints

[stjimm]

We are replacing a section of bottom plate in a 40 foot dia. and 32 foot tall tank which has a lap joint. API 650 describes this type of lap joint in section 3.1.5.4-Lap-Welded Bottom Joints and shows a sketch of this type of lap in figure 3-3B. This is the lap joint where the shell sits on the bottom plate.
Is this lap formed in the fabrication shop? Or is it field bent and fitted. . The bottom plate is 1/4" thick.
If it is shop fabricated, wondering if there is a detail that shows the shop what needs to be done. Or must that detail be drawn up for the fabricator. I cannot find any exisiting tank drawings that show this detail, other than referring to the figure 3-3B in API 650.

 

[FTank]

This joint is done on field erected tanks by the field erectors.

Shop tanks have butt welded joints.

 

[IFRs]

FTank is right - API 650 covers field erected tanks. The corner weld is done in the field. Other API documents cover shop built tanks where the corner weld (lap or butt) is done in the shop. What kind of tank is yours?

 

[jimmoore]

Since this is going to be a repair then it falls under API 653. This is going to be done in the field and will not be able to be bent since the tank is already in place. on 1/4" plate the breakdown (lap plate) is not to big and can easily be welded to fill the gap that will be present between the shell plate and the bottom plate. This is done on new erections to help close the gap between the bottom and shell plate to help minimize welding. The contractor will be welding the corner weld with fairly big rod and will be able to fill the gap with the 2 passes that he will be welding it with.

If you would like to make the breakdown then you could have them install the plates but before tacing the plates into place for welding , just have them tac the two together get all the laps right then slide the breakdown out from under the shell , make the breakdown , then slide it back under for final positioning , tacing , and welding.

The contractor if he is experianced in tanks should understand the process. although he will probably think you are crazy to ask him to make the breakdown.

Hope this helps.

 

[IFRs]

The breakdown detail is very commonly done in the field with hammers and strong wrists.