Dense Phase Conveying of Plastic Pellets

Angel hair has one cause: a pellet in sliding contact with the pipe wall, moving fast enough that friction melts its surface and draws the melt into a thread. Everything else, bends, materials, coatings, changes how much of it happens, but velocity decides whether it happens at all.

Once formed, the threads behave badly. They collect at bends, at screens, at the throat of a dryer or an extruder, and they build into mats that block a line that was conveying perfectly a month earlier. Removing them means opening the line.

Dilute phase conveying is fast by definition, which is why plants that fight angel hair for years often fight it with the wrong variable. Dense phase moves the same tonnage with the pellets travelling as plugs at a fraction of the speed, and that is the difference.

Pellets also charge heavily. A conveying line is a very effective electrostatic generator, and the fines the line itself creates are explosible for most polymers, so the static and the dust belong to the same safety case.

In plastics processing the conveying line is also a surface treatment, and the treatment it applies is the one nobody wants.

A line whose velocity stays below the smearing threshold from end to end, with the increase in velocity along the pipeline designed out rather than tolerated.

Plug flow established for the actual pellet shape and size, with the air injection arrangement that keeps plugs short enough to move.

A fully bonded and grounded system with measured continuity, and the polymer dust explosion case assessed on the fines the plant actually generates.

In a building materials plant the same line is designed from a pressure profile rather than from rate and distance alone.

Pellets are highly permeable, which makes them well suited to plug flow but also means plugs can grow and stall. Air boosters or a bypass line along the pipeline keep the plugs short, and the arrangement is specified from a trial rather than from a rule of thumb, because pellet shape, from lentil to cylinder to irregular regrind, changes the behaviour considerably.

The velocity limit is then held along the whole line. Air expands as pressure falls, so a single diameter pipeline runs much faster at the receiving end, and that is exactly where plants find their streamers. A stepped pipeline is the normal solution.

Bends are chosen for smearing rather than for erosion. Long radius bends in a smooth bore keep the sliding contact gentle; blind tees are poor here because the pocket holds pellets that are then dragged out.

Grounding is designed as a system and tested, with every flange, liner, sight glass and flexible connection bonded, and the continuity measured at commissioning and periodically afterwards. The fines are tested for explosibility on plant material, not taken from the polymer grade.

• Velocity limit established from a trial on the actual pellet shape.
• Stepped pipeline so the limit holds at the receiving end.
• Air boosters or a bypass line to keep plugs short enough to move.
• Long radius smooth bends; blind tees avoided in pellet service.
• Full bonding and grounding, measured at commissioning and re measured.
• Explosibility of the fines measured on plant material.

• Pellet shape and size, which decide plug behaviour far more than the polymer type does.
• Smearing threshold velocity, established by trial for the specific material.
• Required rate and distance, which set the pressure and the velocity profile.
• Downstream sensitivity to streamers, which is what the whole design is buying.
• Static behaviour and the grounding concept for the complete system.
• Regrind content, since angular irregular material conveys differently from virgin pellets.