Why Dilute-Phase Conveying Lines Block — and How to Prove Which Cause You Have
A blocked line means the balance between feed, air and velocity has been lost. The fastest route to the fix is identifying which side moved — here is the diagnostic order that works.
Blockage is an energy imbalance: the air can no longer keep the solids moving.
Ask 'what changed?' first — material, rate, moisture, air supply or route.
Start-up blockages usually mean residual product or feeder starting before the blower.
Mid-run blockages usually mean feed surges or a badly placed bend.
Filter blinding and air leakage mimic blower failure but are cheaper to fix.
Technical Overview
What a blockage actually is
In pneumatic conveying there is really only one process upset that stops the line immediately, and that is a pipe blockage. It happens when the energy supplied by the conveying air is no longer enough to keep the particles moving. They settle, the free cross-section narrows, the local velocity rises, and then the plug is complete; everything upstream and downstream stops, and the plug can extend both ways as material continues to arrive. Understanding it as an energy imbalance rather than a mechanical jam is what makes the diagnosis systematic.
The first question is always 'what changed?'
When a line that has run reliably for months starts plugging, the equipment has not usually changed — something around it has. Production rate went up. A different raw material lot arrived with more fines. Ambient humidity rose with the season and the powder now packs. A feeder speed was adjusted. An air line is leaking or the air supply pressure has drifted down. Or a modification moved a bend. Making that list before touching a setting prevents the common outcome of a parameter change that masks the real cause for a week and then returns.
Velocity and the saltation limit
The line must hold velocity above the saltation velocity at every point, most critically at the pickup and at every change of pipe diameter. Because air expands along the line, velocity rises with distance, which is why long lines are often stepped to a larger diameter — controlling velocity rather than letting it run away. The practical setting is a margin above the saltation velocity, typically tens of percent; more than that only buys air consumption, bend wear and product degradation.
Feed surges and bridging
Mid-run blockages are frequently a feeding problem rather than an air problem. A rotary valve discharging in pulses, a hopper that bridges and then releases, a screw feeder that floods — each sends a slug of solids into the line that the available air cannot carry. Smoothing the feed is often a mechanical change: a hopper agitator or bin activator for cohesive product, a different rotor, a surge hopper. It is also worth checking that the receiver's discharge cycle is not driving the surge.
Bends, layout and dead zones
Every bend disturbs the flow and slows the solids; the sharp, short-radius type is the worst offender. Consecutive bends with no straight run between them are a recurring source of trouble, and a bend placed immediately after a vertical lift is a classic. Long-radius bends, or blind-tee arrangements in abrasive service, plus a few metres of straight pipe after the pickup before the first bend, resolve a surprising share of chronic blockages. Inclined runs are best avoided where a horizontal-plus-vertical arrangement is possible.
Moisture, coating and condensation
Sticky, fatty or hygroscopic products coat the pipe wall progressively, and warm humid air condenses where the pipe passes through a cold zone. The result is a slow build-up that narrows the bore until it plugs. Drying or cooling the conveying air, insulating or tracing the pipe where it enters a cold area, and specifying smooth internal surfaces all help. Where the product is genuinely adhesive, a mechanical conveyor may be the more honest answer.
Filters, leaks and the false diagnosis
A loaded filter adds pressure drop the blower was not sized to overcome. A small air leak changes the velocity profile and can drop it below the minimum at some point in the line. Both present as a conveying rate that falls over time, and both are frequently misdiagnosed as blower wear. Check the filter differential pressure and leak-test the line before ordering a major component.
Getting the powder out, and staying out
Most lines are cleared by isolating the feed, purging with air in a controlled sequence and, where the design allows, using the access points built into the line for that purpose. Designed-in clean-out ports at predictable trouble spots turn a shift-long stoppage into a short intervention. The better answer is prevention: automate the high-pressure or high-vacuum threshold that stops the feed before the plug forms, and keep the cycle-time log that shows the drift arriving.
