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Technical Discussions
cln
cln
16:44 Dec-06-2016
Magnetic Particle Inspection

Ok so, i have a 3" wide weld that is ground smooth (66" O.D. Vessel) and i am performing MPI. I am going to straddle the weld on the 1st inspection then for my second inspection i will be rotating my yoke 90 degrees which will place it directly on the weld. Is it acceptable to place the yoke on the weld as long as i am getting full coverage with my overlap? If that is unacceptable, where in any code could i find this because it is not stated in ASME Sec. V

    
 
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Nigel Armstrong
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel Armstrong

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
14:00 Dec-07-2016
Re: Magnetic Particle Inspection
In Reply to cln at 16:44 Dec-06-2016 (Opening).

Determine the maximum pole spacing at which the yoke can lift the 4,4 kg block. If ıts at least 100mm then, as the weld cap is ground flush, you can position the yoke obliquely to the weld centre line (making sure the poles clear the HAZ +6mm on either side of the weld) and make your first shot. Repeat at 90 degrees to the first energisation to cover the same area of weld with the magnetic flux perpendicular to the first shot.. In this manner any orientation of discontinuity will always be at between 45 and 90 degrees to the magnetic flux.

You dont need to place the pole pieces on the weld metal at all. ASME V Article 7 may not specifically state that the yoke feet should not rest on the weld metal because usually this is not viable with unground weld caps.

    
 
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Dent
Consultant, NDE Manager NDELevel III/3
NDT Consultant, Canada, Joined Nov 1998, 248

Dent

Consultant, NDE Manager NDELevel III/3
NDT Consultant,
Canada,
Joined Nov 1998
248
21:18 Dec-07-2016
Re: Magnetic Particle Inspection
In Reply to cln at 16:44 Dec-06-2016 (Opening).

There is nothing wrong with your technique. Nothing prohibits placing the yoke leg on the weld provided you achieve the overlap as you say you do.
I actually prefer your technique since the most likely direction of flaws is parallel to the weld axis, and in some cases (transverse cracks or backing bar joints not removed) right angles to the weld axis. Your technique will optimize the indications from such flaws. Several NDT schools teach the x technique, but for the very best chance of finding axial indications I prefer your way.

    
 
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