Increase of allowable stress via xray in welded pipe 1

[montec]

Given the scarcity of SEAMLESS pipe in 304-304L 16-inch sch.80, the question I have is: IF the longitudinal weld applied at the mill on WELDED pipe is given 100% x-ray inspection successfully, then can the allowable stress on the welded pipe be raised to ASME code value of 18,000 psi as in seamless pipe?

Thanks for your consideration.

 

[metengr]

If you review the allowable stress values in Section II, Part D, you will find the welded pipe has lower values because of the long seam weld. The only exception is for tubing seam welded using ERW or autogenous in a boiler setting where the tube is protected should the weld fail in service. Otherwise, no.

 

[impeller1]

But even though there is a long seam weld, isn't the weld itself typically stronger than the parent material? So provided the weld is 100% acceptable as per RT, why can't the higher stress value be used?

 

[metengr]

Because welds are cast structures, and the Code deals with cast structures by assigning a 0.85 penalty factor.

 

[moltenmetal]

Any chance of building the article as an ASME VIII vessel? Joint efficiency of 100% could then be taken with 100% full RT examination, no?

 

[metengr]

Joint efficiency and allowable stresses are not the same. Yes, a thinner wall can be used because the joint efficiency is increased to 1. However, that does not change the allowable stress value as published in Table 1A for welded versus seamless product forms.

 

[montec]

It is interesting to note that the reduction ratio in allowable stress for seamless vs. welded pipe, in this case from 18,800 to 16,000 psi, is 0.85. This is same efficiency value given for spot xray inspection vs, 100% x-ray which is given an efficiency of 1.0 per table UW-12.

It is understood that welded pipe inspection at the mill is spot type.

Of course, efficiency and allowable stress are two different parameters, but they appear to be numerically proportional as the highest weld efficiency of 1.0 must match that of the parent metal. Since there is no welding present in seamless pipe, for example, it can be said the efficiency is also 1.0. If this is not the case, then what other physical parameter is the efficiency number measuring, especially in non heat treatable alloy like 304 stainless?

 

[SnTMan]

montec, et. al, not knowing anything about the intended application, Code of construction (if any) etc, etc, I hesitate to join the discussion. But:

It appears to me, based on Sec II, Part D, Table 1A notes W12, W13, W14 there are some bases for using seamless allowable stresses for welded pipe. Detailed information about how the pipe is welded is needed, and may not be practical to get.

For most ordinary classes of work, I'd say, not worth pursuing.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[racookpe1978]

Buy a thicker pipe. It is MUCH cheaper than X-ray of the pipe, and x-ray of every field weld.

 

[r6155]

Option, see SA 312 6.1.5
.........pipe of a size larger than NPS 14 shall have a single longitudinal weld or shall be produced by forming and welding
two longitudinal sections of flat stock when approved by the purchaser. All weld tests, examinations, inspections, or treatments shall be performed on each weld seam.

Regards
r6155