Dense Phase Conveying of Abrasive Minerals

Angular mineral particles remove metal. In a dilute phase line at twenty five metres a second, a bend carrying silica sand or alumina can need replacing within months, and the failure is rarely convenient: a bend wears through at height, in a pipe rack, with the plant running.

Erosion scales with velocity to a power well above one, which is the whole argument for dense phase. Moving the same tonnage at a quarter of the speed changes the wear rate by far more than a factor of four, and that is what turns bends from a recurring outage into a scheduled item.

But dense phase is not available for every material. Stable plug or moving bed flow depends on permeability and de aeration behaviour, and a mineral that does not form stable plugs will block a line designed on the assumption that it does. That is a design risk that only a conveying trial removes.

And the velocity is not uniform. Air expands towards the receiving end, so the last bends in the line are the fastest and wear first, regardless of what the design velocity says.

A system whose design velocity is derived from a wear target, with the resulting bend life known and the bends treated as scheduled consumables.

Confirmed dense phase behaviour for the actual material, from a trial, before the pipeline, the vessel and the compressor are sized around it.

Remaining impact points protected and measured, with wall thickness readings at fixed positions so a bend is changed before it opens.

With a fragile food product the limit is not wear but the velocity at the end of the line, which is higher than at the start and is where the breakage happens.

Conveying trials come first. They establish whether the material forms a stable plug or a fluidised bed, what pressure is needed for the rate and distance, and what happens when the line stops full. The last of those is often the most useful result, because restarting a blocked dense phase line full of mineral is the failure mode that costs a shift.

Velocity is then set from the wear target rather than from the pressure available. A stepped pipeline keeps the end of the line inside that target, and in abrasive service the step is not optional: without it the final bends carry several times the erosion of the first.

Bends are then detailed. Long radius bends in a hardened or ceramic lined construction at the impact points, or blind tees where the sacrificial pocket is acceptable, and every one of them given a fixed inspection position so wall thickness is a trend rather than a surprise.

The pressure vessel system is sized for the duty with the trial data, and the air injection arrangement is specified to match the flow mode: boosters along the line for plug flow, a controlled fluidising arrangement for fine materials.

• Conveying trials to confirm the flow mode, the pressure requirement and the restart behaviour.
• Velocity set from the wear target, with a stepped pipeline to hold it at the far end.
• Bends hardened, lined or designed as sacrificial, at every impact point.
• Fixed wall thickness measurement positions with a recorded trend.
• Air injection matched to the flow mode: boosters for plugs, fluidising for fine powders.
• Restart procedure for a line stopped full, established from the trial.

• Particle hardness, shape and size distribution, which together set the erosion rate at a given velocity.
• Required rate and distance, which decide the pressure and therefore the velocity profile along the line.
• Available air pressure and volume, since dense phase trades volume for pressure.
• Wear target and the inspection plan that will confirm it.
• Flow mode, measured by trial rather than assumed from the material name.
• Dust control at the receiving end, which is smaller than for dilute phase but still has to be sized.