1) Drilling technology 
A flow line, used on a drilling rig, is a large diameter pipe (typically a section of casing) that is connected to the bell nipple (under the drill floor) and extends to the possum belly (on the mud tanks) and acts as a return line, (for the drilling fluid as it comes out of the hole), to the mud tanks. ==Flow Line Components== The flow line will ...
See Fall Line, the same thing but a nicer, less scary term...
the surface pipe through which oil travels from a well to processing equipment or to storage. 
A mark on a molded piece made by the meeting of two flow fronts during molding. Also called weld line.
A mark on a molded piece made by the meeting of two flow fronts during molding. Also called weld line.