Pneumatic Conveying at 18 m/s: How Much Granule Survives the Fifth Elbow

Two identical batches of 30/60 mesh granules went into the same silo system from different ends of the plant. One arrived with 1.2 percent fines below 150 microns and flowed perfectly. The other arrived with 6.8 percent fines, dusted the bag dump station, bridged in the feeder and produced gel balls that took twice as long to wet out. Same product, same silo, 90 metres of dilute-phase line at 18 m/s and five elbows between them.

Attrition is the quiet cost of moving granules with air. This article explains where the breakage happens, how to measure what you lost, and the three design changes that keep a fragile grade intact from silo to hopper.

Attrition Maths: Three Percent Fines per 100 Metres

Dilute-phase conveying at 18 m/s hits a granule with a kinetic energy that scales with the square of velocity. A 500 micron particle at 18 m/s carries enough energy on impact to chip a corner, and repeated impacts round it into a sphere or split it. In a 90 degree elbow with a short radius, the impact angle concentrates that energy on one face and the breakage rate roughly doubles compared with a long-radius bend.

Field measurements on polyacrylate granules typically show 1 to 2 percent fines generated per 100 m of line at 18 m/s, plus 1.5 to 2 percent per short-radius elbow. Five elbows and 100 m therefore cost 5 to 9 percent of the mass, converted from saleable granule into dust that behaves completely differently in the next process.

Elbow Radius, Velocity and the Dilute Phase Trade-Off

Velocity is the variable you can actually change. Dropping from 18 to 12 m/s cuts impact energy by 55 percent and typically halves the fines generated, but it also risks saltation: below the pickup velocity the granules drop out of the airstream and block the line. For polyacrylate at 500 micron and a bulk density of 0.65, the pickup velocity is around 9 to 10 m/s, so 12 to 13 m/s gives a safe margin.

Elbow design matters nearly as much. A long-radius bend at 8 to 10 times the pipe diameter spreads the impact over a longer arc and reduces peak stress. Replacing five short elbows with long-radius ones on one line cut fines from 6.1 to 2.4 percent over a six-month trial, with no change to the polymer.

Measuring What You Lost With a 150 Micron Sieve

You cannot manage attrition without a before-and-after number. Take a 200 g sample at the silo inlet and another at the hopper, sieve both for 5 minutes on a 150 micron screen with a 20 cm diameter stack, and weigh the fines. The difference is what the line cost you, and it should be under 2 percent for a well-designed system.

Sample at the same point every time and record the humidity, because a damp day adds cohesion that makes fines stick to the coarse particles and hides the problem. If the difference exceeds 4 percent, look at velocity first, elbows second, and the rotary valve third: a valve with 0.3 mm clearance shears granules on every revolution and can generate 1 percent fines on its own.

Dense Phase at 6 m/s for Fragile Grades

Where the product is genuinely fragile, dense-phase conveying at 4 to 7 m/s with a bypass or pulse system moves the material as a slow plug rather than a cloud of particles. Fines generation drops to 0.3 to 0.6 percent over 100 m, and elbow wear falls with it. The trade-off is higher air pressure, a pressure-rated line and a higher capital cost, typically 30 to 50 percent more than a dilute-phase system.

That premium is often justified. If your granule sells at 2,400 dollars a tonne and the line handles 400 tonnes a year, 4 percent attrition is 9,600 dollars a year in lost product, plus the dust handling and the quality complaints. A dense-phase retrofit at 40,000 dollars pays back in four years, and faster if the fine fraction is causing gel balls downstream.

If attrition is already a problem on an existing line, a coarser, tougher cut will survive better than a fine one. The coarse polyacrylate granule grades are specified for exactly this kind of handling.