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.
The possum belly is used to slow the flow of returning drilling fluid before it hits the shale shakers.
This enables the shale shaker to clean the cuttings out of the drilling fluid before it is returned to the pits for circulation.
This is a heavy duty rubber hose that is inserted at the end of the flow line and at the other end emplaced into the sample box itself.
The box is typically equipped with a raising door that allows the water and cuttings to escape after a sample is collected.