Pneumatic Conveying in Building Materials Plants

Two costs dominate and both are decided at design. The first is wear. Fine mineral powder at dilute phase velocity erodes bends, valve seats and pipe walls, and because the pressure differentials are modest the first symptom is often a leak rather than a failure. Erosion scales steeply with velocity, which makes velocity the design variable rather than an output of the calculation.

The second is moisture. Cement and lime hydrate, and a conveying line that admits damp air builds a deposit on the pipe wall that narrows the bore, changes the conveying behaviour and eventually sets. Compressed air that has not been aftercooled, dried and drained will do this, and so will condensation in a line that runs outdoors and cools.

Distance makes both worse, because these plants convey a long way: from a mill to a silo battery, from a precipitator to an ash silo, from a silo to a load out. A long line means a large pressure drop, and a large pressure drop means the air expands considerably and the far end runs much faster than the design figure suggests.

A velocity derived from a wear target, with dense phase chosen wherever the rate and distance allow it, and the bend life known rather than discovered.

Air dried to a specified dew point with drains and aftercooling, treated as part of the process rather than as a utility detail.

A stepped pipeline so the velocity at the receiving end stays within the target, with protected and measured bends at the fast end.

In food production the conveying air is part of the product specification: air specified by pressure and volume only, with nothing said about dew point or oil, delivers both into the line.

Flow mode is settled first from measured material properties: permeability and de aeration rate determine whether fluidised dense phase is available, and for cement and fly ash it generally is, which is why these plants convey long distances at low velocity.

The pressure profile is then calculated along the line, and the pipeline stepped from it. This is where the real velocities appear: a constant diameter line that is correct at the vessel can be running several times faster at the silo, and that is where the bends wear through.

Air treatment is specified with a dew point. Aftercooling, drying and drain management keep water out of a system where water is the one thing that cannot be admitted, and insulation or tracing addresses condensation in a line that runs outside.

Bends are then detailed at the fast end: long radius with a hardened or ceramic lining, fixed wall thickness measurement positions, and a recorded trend so a bend is changed in a shutdown rather than found by the hole it makes.

• Flow mode established from measured permeability and de aeration rate.
• Pressure profile calculated and the pipeline stepped from it.
• Velocity derived from a wear target rather than from available pressure.
• Air dried to a specified dew point, with aftercooling and drain management.
• Insulation or tracing where condensation in an outdoor line is credible.
• Bends hardened or lined at the fast end, with fixed wall thickness measurement positions.

• Distance and rate, which set the pressure case and the velocity profile.
• Abrasiveness and fineness, which decide the wear rate at a given velocity.
• Permeability and de aeration rate, measured, which decide whether dense phase works.
• Air dew point required to prevent hydration in the line.
• Outdoor route exposure and the condensation risk along it.
• Wall thickness inspection plan, which converts wear from a surprise into a schedule.