Horizontal & Vertical Joint API 650

[hamidun]

Dear All,

I am studying the welding design of storage tank per API 650, the tank is 7-courses shell with thickness as below:

#1, 2, 3, 4, 5, 6, 7 shell course: 21, 18, 15, 13, 10, 8, 8 mm

In the drawing, it is shown that the horizontal joint is single beveled and for vertical joint is single V butt joint for all plates.

As per API 650 it is not stated that the type of welding should be used for a specific plate thickness where in AWS D1.1 figure 3.3 if the welding is single bevel (for horizontal joint), the base metal thickness is unlimited for SMAW and SAW and if the welding is single V (for vertical joint), the base metal thickness is unlimited for SMAW. Therefore, I assume there is no problem with the welding design.

But a person in the company argue that if plate 21 mm thick to be welded with 18 mm thick, there will be buckling between the plates and he suggest to change the welding type to be double beveled (horizontal joint) for plate thickness between 21 mm to 18 mm and 18 mm to 15 mm and for vertical joint, the welding type should be double V for plate thickness of 21, 18, 15 mm.

Here is the questions, is the person right?

If the person right, instead of changing the welding design, can we use strong back/clamp to restraint the plate in order to avoid buckling? Or is there any other method to avoid the buckling? Can we counter the buckling with some sort of calculation?

What would be the best solution for this case?

Please help, Thanks.

 

[IFRs]

Your weld procedures should clarify when bevels are needed. You need to know more about the process - are you stick welding, MIG, Sub-Arc, welding by hand or using single- or double-head automatics.

For hand welds and single-sided automatics I generally see no bevel up to 0.37", single bevels for 0.371" to 0.75" and double bevels for over 0.75" for both horizontal and vertical welds.

Experienced and skilled welders are the most important factor in getting distortion free joints, along with proper weld procedures and appropriate joint preparation. And of course accurately cut and squared plates.

You can't calculate or legislate it and in my opinion no amount of bracing will make up for poor plate or joint preparation, improper joint alignment or unskilled workers. It's a team effort that needs to be coordinated - design, drafting, purchasing, fabrication, supervision, erection and testing all play important roles.

If possible, find the "old guy" and pick his or her brain.