What would you expect to learn from vacuum box testing? Only that no pin hole leaks exist through the weld?
Dye pen test would tell you more actually. And then an RT.
Why RT? Is the geometry unsuitable for UT?
Oh, by the way. What material, what pressure, what fluid, what criteria (specification) are you building against? A toy? A spacecraft? A pressure vessel? An air conditioning assembly?
Are the tank bottom welds accessible for RT? I believe only Vacuum box testing is required per API 650 but potential for content leakage and cost of cleanup thereof may require further NDE. I would recommend wet fluorescent MT be done assuming the material is low carbon steel and thin (<or = 1/4 inch or 7 mm.
This type of welds should have backing strips and AFAIK , the mandatory tests are Visual testing ( VT) and , Vacuum box testing (VBT ) . Also , LPT is desirable from construction point .
Why would LPI be desirable from construction point of view? I'd start with the materials first. Ofcourse, for Chromium-alloyed steel, you're correct. But we're talking standard containers here.
If Standard austenitic stainless steel, LPI would be a waste of time.
If low carbon steel and relatively thin, LPI (and/or MT) would be a waste of time.
If you expect cracking in these materials, you should change the supplier for a better one, one that uses decent base materials and that follows good welding practices.
EDIT;
-The title of this thread ( NDT req for butt weld bottom plates where no annular plates small diameter API 650 tank..) The OP should clarify if the tank is Shop-Assembled Storage Tank Annex J. since proposed NDT methods may change.
- Liquid penetrant testing can locate any pinhole defects that might have been missed in other inspections and this test is not very expensive .
