Shell Radiography and joint efficiency 3

[Paulettaa]

Dear All,

(For vessels fabricated as per ASME BPVC VIII-1)I have seen many datasheets indicating joint efficiency for a cylindrical shell to be 0.85 corresponding to spot radiography. What is the reason for that? I mean is radiography so expensive that increasing shell thickness is a preferred option? Even if it is the cost due to radiography why not only do full radiography for LW's and do spot radiography for CW's? since in most cases the LW's are the more important ones in determining the thickness of the shells.

Warm Regards

 

[marty007]

This all boils down to the quality and experience of the welders...

Probably about 2/3 of the vessels our shop builds use RT-3 (Spot), with very few defects being found. Many of our welders have 20+ years of experience, and take a lot of pride in their work.

 

[bpv66]

UT in lieu of RT is very expensive, especially when used with ASME Section VIII, Div. 1.

Section VIII requires an auto or semi-auto method of UT (typically phased array or TOFD) and a fracture mechanics based acceptance criteria which means compliance to ASME Section V, Article IV, Appendix VIII and IX. Procedure qualification is required on special blocks which contain defective welds. These blocks cost a lot of $$$ and the NDE firm is going to pass this cost on to the Certificate Holder. UT will pick up everything in the weld, so you may see significant rework.

There are advantages and disadvantages of using UT in lieu of RT that must be weighed by the Certificate Holder and the customer. If you have a 7" thick vessel then UT may be the way to go.