Trying to be of help to Pipewelder 1999 I've gone through the following Codes and Standards:
ASME Code, Section VIII: Unfired Pressure Vessels
API Std. 510: Welded Steel Tanks for Oil Storage at Atmospheric Pressure
API Std. 5L: Specification for Line Pipe.
If someone doesn't know who ASME and API are, ASME is the American Society of Mechanical Engineers in New York City (www.asme.org) and API is the American Petroleum Institute in Washington DC (www.api.org).
I must confess that the results of my research are not brilliant.
ASME VIII. On Part UW (vessels made by welding) of the Code, the paragraph related to Repair of Welded Defects, says that the defects which are OPEN TO THE EXTERNAL SURFACE, such as cracks, EXCESSIVE POROSITY, lack of fusion ...............shall be removed by mechanical means.......... and the removed portion shall be rewelded. I couldn't find what the Code means by "excessive porosity".
API 510. The paragraph related to Inspection of Welds refers to the two types of welds which are present in a storage tank: butt (side of the tank) and fillet (often, bottom and roof).
For butt welds, it says that if the VISUAL INSPECTION of the weld is NOT SATISFACTORY, its acceptance or rejection shall be based on one of the following criteria: a) RT of the weld under consideration, b) sectioning of the weld and destructive test of the specimen.
For fillet welds, the Std. says that they shall be always subjected to visual inspection (of course, you can't RT a fillet weld), and that if it is NOT SATISFACTORY, the acceptance or rejection shall be based on sectioning of the weld and destructive test of the specimen.
I couldn't find in the Std. when and why the visual inspection is "not satisfactory".
API 5L. The paragraphs related to Visual Inspection, not only of the weld but of the entire pipe, under the title of OTHER DEFECTS, say that any imperfection (I understand that porosity is included) having a depth greater than 12,5 per cent of the specified wall thickness, measured from the surface of the pipe, shall be removed by any of the following methods: etc. etc.
Frankly I doubt these results will be of any help, but if they are, I'll feel quite happy.
Giovanni S. Crisi
Sao Paulo - Brazil
In Table 6.1 (Visual Inspection Acceptance Criteria) of D1.1-1998 & 2000. Note 8 (A), (B), & (C) give you the criteria.
It addresses piping porosity which according to Jefferson's Welding Encyclopedia is "A form of porosity having a length greater than its width that lies approximately perpendicular to the weld face."
Any porosity that is deeper than it is wide is considered piping porosity.
If it is open to the surface then it falls into the Acceptance Criteria of Table 6.1. If it is not open to the surface, then it is covered under 6.12.2.1 & 6.12.3.1.
I hope this helps because it is al there is and it is correct.
Ronald B.