Often bulk bags require inner liners, usually made of polyethylene, to protect the ingredient.  Care must be taken when discharging lined bulk bags to prevent entanglement with downstream equipment and to ensure 100% product yield.

Liners come in many forms: loose tubular, loose fitted, sewn-in, glued-in.

Loose means that the liner is not in any way attached to the outer bulk bag.

Tubular means that the liner is basically a cylinder of polyethylene film that is stuffed in the bag.

Form fit means that the liner is constructured with sides, top and bottom and usually inlet and outlet spouts - it looks just like a bulk bag only it is made from polyethylene film.

Loose Bulk Bag Liners

Whether tubular or form fit, a loose liner requires some sort of physical means to prevent it from discharging with the ingredient and becoming entangled in downstream equipment.  A device called a liner tensioner is typically attached to the bulk bag lifting frame or rigging frame to prevent this from happening.

Liner tensioners vary in their operation, but the most basic simply hold the liner in place preventing it from moving out of the bag.  More sophisticated tensioners wind up a portion or all of the liner as it is emptied.

Before the bulk bag is lifted into the discharger the inlet spout of the bag is untied to expose the tied liner inlet.  The liner neck is then wound onto the liner tensioner spool piece, which is then either fixed in place or rotated to take up the slack in the liner by activating its actuator (typically pnemuatic).

Attached Liners

Even though liners that are attached to the outer bulk bag are designed not to discharge with the ingredient, it sometimes happens anyway.  Often a basic liner tensioner that holds the liner in place is used to guarantee that the liner cannot move.

Liner Tensioner Conisderations

1. Liners 'grow'. Loose liners and liners attached to the top of the outer bulk bag will extend out of the bulk bag outlet spout during ingredient discharge.  Particularly in the case of loose tubular liners, care must be taken to ensure that even if a liner tensioner is used, the liner cannot extend or grow out of the outlet spout enough to foul downstream equipment.
2. Too much liner winding.  If a liner tensioner that winds continuously is used typically the liner outlet is clamped to prevent it from winding all the way up onto the spool piece.  If this is allowed to happen, and the ingredient is at all dusty, dust will be sprayed into the surrounding area when the empty liner is rapidly wound up through the empty bulk bag and then spins around the spool piece.
3. Liner length.  When a liner tensioner is used the liner inlet must be long enough to reach the tensioner spool piece.
4. Ensuring 100% yield.  A liner tensioner that winds up a loose liner - partially or completely - can assist with ensuring that all of the ingredient is dishcarged from the liner.  The act of winding up the liner helps to get the last pounds of ingredient out of the bag.