Pipe stability in aerated silos

• The pipe stability criterion in aerated silos by Johanson (2004) is discussed.
• It has been extended beyond the calculation limits present in the original paper.
• A complete table and an empirical algebraic expression for G(ϕ, A) are proposed.
• A procedure to account for pipe consolidation due to aeration is proposed.
• Theory is verified with experimental results on a lab scale aerated silo.

The procedure to calculate the pipe stability criterion in aerated silos proposed by Johanson (2004) has been discussed and extended beyond the calculation limits present in the original paper. A table of the dimensionless function G(ϕ, A) = Dpgρb/fc has been calculated for angles of internal friction ϕ between 35 and 70° and dimensionless radial pressure gradients −A between 0 and 10. Values of G in this table have been fitted with an algebraic expression which is able to correlate values with an approximation not larger than 10% and generally smaller than 3%. Theoretical findings were compared with previous experiments. The comparison between the theory and the experimental results indicates that aeration can produce significant changes in the effective body force acting on the powder to such an extent that the horizontal gas pressure gradient is larger than gravity and, therefore, beyond the limits provided by Johanson (2004). The theoretical approach is able to provide results which correlate reasonably well with the experiments if the appropriate bulk solid consolidation is used. In particular, if aeration is started before pipe formation, it can produce bulk solid compaction that increases pipe stability and has to be accounted for so as to evaluate critical aeration conditions for pipe stability.

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