Junk sub is an effective drilling tool for catching the heavy scrap iron or metal debris which are too heavy to be completely circulated out of the hole in normal drilling operation. Its cup OD is big, smaller annular gap with the wellbore, but the mandrel is small at the rim of cup, larger annular gap with wellbore. So the fluid speed dropped promptly and form a vortex at the rim of cup, meanwhile, the carry ability of fluid also dropped. So the heavy objects in the fluid will fall into the cup and carried out in the trip process.
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