How to estimate pressure against a wall of a non-liquid like wood pellets?

Question

For the creation of a wood pellet silo, I need to calculate the required resistance of the walls to support the lateral pressure:

Let suppose a silo of $W * L * H$ full of wood pellets.
Let suppose standards wood pellets of $600kg/m³$ (considering the space between them, usually the compressed wood itself approximate $1000Kg/m³$).

For a fluid, I would assume wall pressure at height h from the ground to be:

$P=(H-h) * g * 600$

e.g. For a silo of $3m$ Height, the pressure at the bottom of the silo would be some $17.6kN/m²$

Fortunately, wood pellets are not a fluid, and I would expect the pressure to be significantly lower than for an equivalent fluid. However, I still expect some pressure against the lateral walls.

How to estimate pressure against a wall of a non-fluid like wood pellets?

Answer 1

There are complex methods to estimate pressure on silo walls, and some simplified ones for specific cases (e.g. fresh concrete on coffers). However it's quite easy to see the way that pressure deviates from hydro-static pressure - the one you already know how to calculate - to decide whether those effects will be significant.

The reduction of pressure is mainly caused by:

• Internal friction, which is highly dependent on angle of repose, and I'm afraid that angle of repose for pellets must be quite small.
• Silo effect: If the recipient is narrow and high, some vertical load will be transferred to the walls as friction and by arch effect, and that load won't increase pressure below. Therefore, this source of reduction is only expected in a narrow high recipient - like the coffer of a column or an actual cylindrical silo.

Then, if the container you are building isn't significantly higher than wide, I wouldn't count on any significant reduction compared with hydro-static pressure and would estimate pressure as if pellets were a fluid with the same bulk density.

Additionally, Eurocode 1 part 4 about loads on silos and tanks may be a good source for an exact answer.

Answer 2

If the pellets were like bricks then the wall pressure could be taken as zero as the bricks form a separate internal structure when stacked.

But, for that scenario, as the pellets can move over each other, then I would assume that the pressure is increasing as the depth of pellets increases.

Based on the density of the pellets then I would go with what you show, as that will be the worst case scenario.

However, as many of the pellet suppliers provide a rapid pellet store of wood frame supporting a hanging bag then you could investigate the strength of the bag material used for a more relevant answer.