Plug Conveying

Plug conveying is a specific dense phase behavior. Material forms discrete columns that occupy much of the pipe cross section. Gas flows through or around the solids and drives each plug forward. The regime can be gentle, but only when the material forms stable plugs and the system has enough pressure to overcome their resistance.

How plugs form and move

Coarse, permeable particles can allow gas to pass through a plug while maintaining a pressure difference across it. The plug collects particles at its front and can leave material behind at its tail. Plug length and speed change along the route as gas compresses, leaks through solids, and interacts with bends or vertical sections.

Fine powders with strong air retention may favor fluidized dense flow rather than discrete plugs. Cohesive material can compact and form a blockage instead of a moving plug. Classification charts can support early screening, but permeability and deaeration data improve prediction.

Suitable material characteristics

• Particles are sufficiently permeable for controlled gas passage.
• The material forms repeatable plugs without permanent compaction.
• Moisture and fine content remain within a known range.
• Particle quality benefits from lower conveying velocity.
• Representative material is available for pilot trials.

Research on flow mode prediction identifies plug flow and fluidized dense flow as distinct categories. Basic particle size and density charts have useful regions, but air related properties such as permeability and deaeration can provide better discrimination.

Potential advantages

Lower particle velocity can reduce erosion at bends and attrition of coarse or fragile solids. A University of Surrey study describes vertical plug conveying as a useful option for coarse particles where leaner flow could create erosion or particle damage.

The solids concentration can reduce the required gas volume. This does not guarantee lower energy because the pressure difference can be high. The complete compressor duty and throughput determine efficiency.

Feeding and plug generation

A pressure vessel can introduce a controlled batch into the line. Other systems use specialized continuous feeders. Stable feed quantity and pressure determine the initial plug structure. Surges can create a plug longer than the available pressure can move.

Some systems generate or divide plugs with controlled air injection. Injection points can prevent excessive plug length and help restart flow. Poorly controlled air can increase velocity, disturb plug structure, or move the system toward another regime.

Pipeline geometry and pressure

Each plug creates friction and requires a pressure difference. Multiple plugs share the available pressure along the line. Vertical lift adds the weight of solids, while bends can compact or break plugs. The design must consider the complete route and the likely number of plugs.

Pipe diameter influences plug mass, gas passage, and velocity. A change in diameter changes the flow structure. Route modifications made after testing require new evaluation because a different bend or lift can alter the stable window.

Instability and blockage

Dense conveying is unsteady by nature. Research using high speed imaging and electrical capacitance tomography shows discrete slugs and plugs as important flow structures. Pressure fluctuation is therefore expected, but its pattern should remain within the tested operating envelope.

A plug can stop when pressure is insufficient, material compacts, the feed surges, or the route changes. Adding pressure without understanding equipment limits is unsafe. Recovery needs an engineered sequence that identifies stored pressure and material before access.

Instrumentation and control

• Pressure at the sender and selected line points.
• Conveying time and pressure signature for each cycle.
• Vessel level and valve position.
• Secondary air flow and pressure where used.
• Receiver filter differential pressure.

Changes in pressure shape or cycle time can reveal different material, leakage, filter restriction, buildup, or wear. Monitoring should support diagnosis rather than only a final high pressure trip.

Testing and scaleup

Pilot trials should reproduce important bends, lift, pipe diameter, feed method, and material condition. Useful outputs include pressure gradient, plug speed, capacity, gas use, product damage, restart performance, and sensitivity to material variation.

Scaleup should document how plug length, number, pressure, and route were transferred to the plant design. A successful short horizontal trial does not alone prove a long route with vertical lift.

Safety review

Plug conveying can use significant pressure and transport combustible dust. Pressure rating, relief, depressurization, grounding, ignition prevention, explosion isolation, and receiver protection require formal review. A blocked line can retain both material and pressure, so safe access procedures are essential.

Sources and further reading

• Particuology, Predicting the mode of flow in pneumatic conveying systems
• Powder Technology, Investigations of flow instabilities within dense pneumatic conveying
• University of Surrey, Vertical plug flow pneumatic conveying from a fluidised bed
• Canadian Journal of Chemical Engineering, A one dimensional model of plug flow pneumatic conveying
• OSHA Technical Manual, Combustible Dusts

• Define the powder or bulk solid and its relevant physical and safety data.
• State the required transfer rate, operating schedule and acceptable residual material.
• Map horizontal distance, vertical lift, bends, pickup points and destinations.
• Define product degradation, wear, contamination and containment limits.
• Record available utilities, filtration, controls, cleaning and maintenance access.