Dear members,

I'm confused with the joint categories given in table UW-12. I’ll highly appreciate if anybody could answer following queries

1.In type 1 joints, what exactly mean by “double welding”. Is it double V?
2.In which type the welds, using metal backing strips which remain in place, fall?

Thanking you in anticipation

Hello Mazaffar

Welds fall into categories and types. The category of the weld is its location within the pressure envelope, and the type is the geometry and method of welding the joint.

A type 1 joint is welded from both sides, or has a backing strip which can be removed. Such a backing strip is usually copper which does not bond with the two plates being welded. A type 1 is a joint in which full fusion and full penetration can be guaranteed. This can be achieved if the plates are welded from both sides, or the backing can be removed. The second side of the weld can be inspected easily. Inspection of the second side makes it a type 1 joint.

In the case of the type 2 joint, you have a backing strip that remains in place. The second side cannot be inspected to ensure full fusion and penetration of the root pass. Because the weld is unknown; in other words, full penetration and fusion cannot be guaranteed, you pay a price by having a lower joint efficiency.

I hope this makes matters a little clearer.

Dear Ray,

considering the option of joint 1, how do we select the final weld seam (A or B) for a vessel which when enclosed completely and nozzles of such size that full inspection of weld seam from the other side is not possible. moreover if the option for joint 1 is welding on both sides then I presume that for any vessel (having two heads elliposidal welded to shell) the closing seam has to be joint 2. how is it possible to have such vessels fully fabricated based on joint 1.

I would apprecaite you feedback/comments in this regard.

Dear Ray Delaforce,

Thank you so much for such a detailed explanation. Please answer the above query as well.

Hello Folks

In the case of pressure vessels that are not subject to internal corrosion they are often not provided with access opening or manways for internal inspection. In such a case, the final circumferential weld seam has to be welded from one side only. As I stated, a type 1 weld is one that is either welded from both sides, or where a removable backing strip is employed.

The definition of a type 1 weld also includes the wording that a weld as good as double welded can be treated as a type 1 weld. The root pass is the crux of the matter. In my opinion, if you can guarantee that the weld achieves full penetration and full fusion, it can be considered a type 1 weld even though it is welded from one side only, but this must be decided by the manufacturer and approved by the AI. You must have WPS and PQR that supports the fact that a one sided weld can achiever full penetration and full fusion.

One way of getting good results for the root pass is to employ GTAW (TIG) welding, but this is very slow. The other alternative is to treat the weld as a type 2 weld. Remember, a circumferential weld is a Category B weld. Calculations for the required thickness are based on Category A or longitudinal welds. The Category B welds are not generally considered except in the case of wind and seismic loading where the longitudinal bending stresses in the shell are considered.

I hope this clarifies matters a little more.