Dense Phase Conveying in Pet Food Plants

Coated kibble is the most fragile finished product in the bulk solids industries. It is porous, it is brittle, and it carries a fat and palatant coating that is both the value of the product and the reason it sticks to everything.

A dilute phase line destroys it. At twenty metres a second a bend is an impact that shatters pieces, and the fines go to the screen rather than the bag. On a line running several tonnes an hour the loss is a continuous cost that often exceeds the energy cost of the conveying by a wide margin.

Dense phase solves the speed and introduces the fat. Material moving as a slow bed rubs along the pipe wall, and a fat coated product leaves a layer behind. The layer narrows the bore, changes the conveying behaviour and becomes a cleaning problem, particularly where a line is shared between recipes with different claims.

And the meal side upstream is a different material entirely: fine, fibrous, dusty, with its own conveying behaviour and its own carryover requirement.

A line whose velocity is below the breakage threshold measured on coated product, with the increase towards the receiving end designed out so the final bends do not undo the rest.

A pipeline that can be cleaned of a fat layer, with access and a method, and whose behaviour is checked as the layer builds rather than after it blocks.

A documented place in the changeover procedure, since the line is a carryover path between recipes with different claims.

For abrasive minerals the deciding step is a conveying trial: hardness, shape and size distribution settle whether dense phase wears the line out before it pays for itself.

The breakage threshold is measured on the real product, coated, because an uncoated trial gives a flattering answer. The trial gives both the achievable rate and the velocity at which damage starts, and the second becomes the design limit.

The pipeline is stepped to hold that limit at the far end. Bends are long radius and smooth, and their number is minimised, since on a fragile product each bend is a measurable cost rather than a pressure drop entry.

Fat build up is then planned for. Access points for inspection and cleaning, a route a cleaning head or a pig can take where that is appropriate, and a check on conveying pressure as an indicator: a rising pressure at constant rate is the layer announcing itself.

Changeover is designed into the line. Drain and blow through arrangements, an emptying sequence, and the line listed in the changeover procedure with the recipes whose claims depend on it. The meal duty, if it shares the line, is trialled and specified separately.

• Breakage threshold measured on coated product, and adopted as the velocity limit.
• Stepped pipeline so the limit holds at the receiving end.
• Long radius smooth bends, with the number of bends minimised.
• Inspection and cleaning access for the fat layer, with a defined method.
• Conveying pressure trended as the early indicator of build up.
• Line named in the changeover procedure, with the meal duty specified separately.

• Breakage threshold of coated kibble, measured rather than estimated from the uncoated piece.
• Fat and palatant level, which decides the build up rate and the cleaning interval.
• Route length and bend count, since each bend has a measurable product cost.
• Claim sensitivity of the recipes sharing the line.
• Whether meal shares the line, which is a second duty with its own trial.
• Dust explosion characteristics of the fines and the meal dust.