Dense Phase Conveying in Feed and Grain Plants

Pellet durability is tested and sold. A feed pellet that leaves the press with a good durability index arrives at the farm with a lower one, and the difference was made by the handling in between: every drop, every bend and every metre of fast conveying.

Fines are not only a quality loss. They segregate in the bin, they dust at loading, and in a medicated feed they carry a different additive concentration from the pellets, which turns a handling issue into a composition one.

Dilute phase conveying is the fastest damage mechanism in the plant, which is why dense phase is attractive for finished feed. The constraint is that feed pellets are large, permeable and variable, so plug behaviour has to be established rather than assumed, and a plant that switches between mash and pellets is asking the same line to carry two very different materials.

Moisture is the practical hazard. Feed pellets pick up moisture, feed meals contain fat, and a dense phase line that stops full with damp material in it is difficult to restart.

What a grain and feed plant asks of a conveying line is unusual: the product carries a measured durability figure, and the line is allowed to spend part of it.

A conveying system whose contribution to fines is measured, with durability tested before and after the line so the loss is attributed rather than argued about.

Velocity held below the damage threshold along the whole pipeline, with the increase towards the receiving end designed out.

A line that can be emptied and restarted, with a procedure for the case where it stops full of damp material.

In a pet food plant the velocity limit belongs to the coated kibble and not to the bare one, because a coating that survived the press breaks at a velocity the uncoated product tolerated.

The design starts with a measurement that most plants do not have: durability in and durability out for the existing handling route. That number tells you whether the conveying is worth changing and gives a target for the new system.

Conveying trials then establish the flow mode and the damage threshold for the actual pellet, which varies with diameter, formulation and press conditions. A mash duty on the same line is trialled separately, because a fine fibrous meal behaves nothing like a pellet.

The pipeline is stepped so the velocity limit holds at the far end, and the bends are long radius and smooth, since a pellet striking a sharp bend breaks rather than abrades.

Then the line is detailed for the awkward realities: a drain and a restart arrangement for a line stopped full, access at the low points, and attention to condensation where a line runs outside and cools. The dust explosion case is assessed on the fines the line creates, not on the pellet.

• Durability measured before and after the existing route to establish a target.
• Conveying trials for the pellet and, separately, for any mash duty on the same line.
• Stepped pipeline so the damage threshold velocity holds at the far end.
• Long radius smooth bends, since pellets break rather than abrade at a sharp change of direction.
• Drain points and a restart procedure for a line stopped full.
• Dust explosion case assessed on the fines generated, not on the intact pellet.

• Pellet durability specification and the loss the plant can afford across handling.
• Pellet diameter and formulation, which change both plug behaviour and fragility.
• Whether mash shares the line, which is effectively a second, different duty.
• Required rate and distance to the silos or the load out.
• Moisture and fat content, which decide the blockage and restart risk.
• Medicated feed carryover, since the line is part of the flushed route.