Bulk Density 0.68: Setting a Feeder and a Silo That Agree With It

The feeder was set for 38 kg/h and the batch sheet said 38 kg/h. The operator checked the screw speed twice and the belt scale once. The actual dose was 43 kg/h, and the only thing that had changed was the delivery: the new lot arrived at 0.63 g/cm3 loose instead of 0.68. Nothing on the certificate was out of specification. The screw was turning at the same speed, so it moved the same volume, and a lighter volume weighs less per litre and flows faster per revolution.

Bulk density is the number that connects a feeder setting to a dose, and it is the number most often left off the incoming test. It is also the number that decides whether your silo empties or bridges, which is a much more expensive failure than a 12 percent dose error.

Two densities, and the gap between them

Loose bulk density is measured by pouring the powder into a known volume without settling. Tapped density is measured after a defined number of taps or vibration. The ratio between them tells you how much the material compacts, and compaction is the first clue about flow behaviour.

A typical 30/60 mesh polyacrylate sits at 0.65 to 0.75 g/cm3 loose and 0.80 to 0.88 tapped, a ratio of about 1.25. A fine powder might show 0.55 loose and 0.80 tapped, a ratio above 1.4, and that material will compact under its own weight in a silo, form a cohesive arch, and stop flowing without any warning.

Measure both. A single loose figure tells you how to set the feeder today; the ratio tells you what the silo will do next month, after the material has sat under three metres of its own weight over a holiday weekend.

Bridging, ratholing and the angle that fixes both

A cohesive arch forms when the strength of the powder at the outlet is greater than the stress trying to break it. Coarse granules rarely arch; they rathole, which is different and easier to miss. A rathole is a stable vertical channel above the outlet, with material sitting motionless around it. The feeder runs, the level sensor says there is product, and the silo is effectively empty while appearing full.

Hopper angle is the usual fix, and the usual mistake is to assume steeper is always better. For a mass-flow design you need the walls steep enough and smooth enough that material slides at the wall rather than over itself. With a 30/60 granule and a mild steel hopper, that often lands between 60 and 70 degrees from horizontal; with polished stainless and a low wall friction, 55 can work. A 45 degree cone is a funnel-flow design by accident, and funnel flow is where ratholes live.

Outlet size matters more than angle for arching. A rule of thumb of eight to ten times the largest particle works for free-flowing granules; cohesive powders need an outlet several times larger, or a mechanical discharger. If you are running a fine grade in a silo designed for a coarse one, the outlet is probably the problem, not the angle.

Vibration can make it worse

This is the one that surprises maintenance teams. A vibrator on the hopper wall is the standard response to poor flow, and on a compactible powder it often makes things worse. Vibration settles the material, raises its density, increases the cohesive strength at the outlet, and produces a harder arch than the one you started with. The silo then flows in slugs, which is worse than not flowing at all because it arrives at the feeder in a rush and overloads it.

If you must use vibration, use it in short bursts at the moment of discharge, not continuously. Better options are a mechanical agitator in the outlet section, air cannons fired on a sequence, or a properly sized screw that withdraws material across the full outlet area rather than from the centre.

Setting the feeder to the real density

Volumetric feeders should be calibrated on the actual product, not on a generic figure. Fill the hopper, run for two minutes into a tared container, weigh, and calculate the mass per revolution. Do this at three speeds, because the fill factor changes with screw speed and a single point calibration hides it.

Then re-check on every delivery. A 5 percent density change is normal between lots; 10 percent is a signal that the mesh or the moisture has moved. Either way, the feeder setpoint has to move with it or the dose drifts silently.

Loss-in-weight feeding removes most of this problem because it controls mass, not volume, but only if the refill cycle is not too frequent. A small refill hopper that refills every 90 seconds spends a lot of its time not weighing, and the dose error in that window is larger than the density error you were trying to eliminate.

What to write on the incoming sheet

Loose bulk density, tapped density, and the ratio, measured on the same sample as the sieve test. Add the moisture, because a 0.5 percent moisture change moves density by about 1 percent and cohesion by much more. Keep the numbers in a log against lot number.

When a lot arrives at 0.63 instead of 0.68, you have a choice: adjust the feeder, or reject. For most plants the right answer is to adjust, note it, and ask why. Two consecutive lots below 0.65 usually mean a change in the crusher or the screening cut, and that is worth a call before the third container. The SAP grades by bulk density page is a useful reference for what normal looks like, but your own log will beat it within six months.