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Hoppers & Silos


PET storage vessels showing wall buckling caused by silo quaking.


of 850 Kg/m³, will result in serious overloading of the silo. This can be over 40% more than the original design stress and can lead to the complete collapse of the discharge cone or skirt region of the silo.


The images here show two, one year old silos used for storing PET granules. The vessels had not been designed with any anti-honking / quaking protection and serious buckling damage can clearly be seen on the third and seventh panels of the left-hand silo and the seventh panel of the right-hand silo. These silos were replaced by Barton Fabrications’ PET storage silos and have operated successfully for over four years without re-occurrence of noise or structural damage.


Silo honking and quaking has


Close up of silo wall showing the extent of buckling


Vertical view showing the distortion from cylindrical shape caused by silo quaking


been recognised for many decades and


although there has been considerable research


carried out, and papers written, the exact mechanisms are still not fully understood. The phenomenon occurs with other materials, including


other plastics, however PET seems to be the only commonly used plastic where


BELOW: Table 1 - Density and Co-effi cient of friction values for common plastics [5], [6] & [7].


PLASTIC TYPE DENSITY g/cm³ COEFF OF FRICTION VALUE FORM POURED BULK DENSITY Kg/m³ PET (PETP)


PE (LDPE / HDPE) PVC PP


PA (Nylon) PTFE PEEK


1.37 – 1.45 0.91 – 0.96 1.30 – 1.58 0.57 – 0.92 1.11 – 1.29 2.10 – 3.90 1.31 – 1.51


0.18 – 0.22 0.08 – 0.22 0.20 – 0.30 0.10 – 0.30 0.16 – 0.35 0.06 – 0.35 0.38 – 0.43


x


Pellets Pellets Pellets Pellets x x


850 561 625 513 513 x x


www.solidsandbulk.co.uk


the author has seen problems reported. It is believed that key factors include materials with a relatively high poured bulk density and co- effi cient of friction. Table 1 shows the range of density, coeffi cients of friction and poured bulk densities for a number of common plastics. It can be seen that PET has relatively high friction and poured bulk density compared to the other polymers.


SOLVING THE PET PROBLEM


Although the exact cause and modelling of the problem is incomplete, there is suffi cient practical knowledge of the issue for silo manufacturers to be able to offer silo solutions which avoid honking, quaking and overloading problems. These involve changing the way material fl ows inside the silo, particularly increasing the friction between the material and the silo wall, combined with increasing the silo strength to be able to resist any high transient stresses and the higher bulk density of PET. In practice, it is very diffi cult to predict when silo honking or quaking will occur, but Barton Fabrications’ anti-honking ring silo design has been widely and successfully adopted, addressing every silo honking / quaking issue that the company has encountered. Barton’s anti-honking ring design can be specifi ed and fi tted in both new silos or retrofi tted to existing vessels which need to be used for PET granule storage. Rather than risk serious safety issues, or the high cost of a new silo installation, suitable silos can generally be upgraded to include anti-honking rings and stronger base sections offering a more cost effective solution for PET storage to the plant manager. n


For more information about PET storage silos contact Barton Fabrications on tel: +44 (0) 1275 845901 or e-mail: sales@bartonfabs.co.uk


References [1] PET Packaging market forecasted to grow to $57 billion by 2017, according to Smithers Pira https://www.smitherspira.com/market- reports/pet-packaging-market-forecasted- to-grow-to-57-billion-by-2017.aspx


[2] British Plastics Federation website http://www.bpf.co.uk/Sustainability/ PET_Plastic_Bottles_Facts_Not_Myths.aspx


[3] Silo quaking and silo honking: Springer-Verlag Berlin Heidelberg


http://link.springer.com/chapter/10.1007%2F978- 3-540-73768-1_14#page-1


[4] Silos: Fundamentals of Theory, Behaviour and Design - Chapter 3.6 Silo Quaking


http://books.google.co.uk/books?id=lOOuqGbAag wC&pg=PA171&lpg=PA171&dq=Silo+quaking,+also +known+as+silo+thumping&source=bl&ots=tRU6aKR N3a&sig=frWVmk8yBRiKwKmsIaRtjmg-izs&hl=en&sa =X&ei=Gh7SU8KyK9KV0QWqr4CwCg&ved=0CCA Q6AEwAA#v=onepage&q=Silo%20quaking%2C%20 also%20known%20as%20silo%20thumping&f=false


[5] [6] [7]


http://www.dotmar.com.au/co-effi cient-of-friction. html and http://www.dotmar.com.au/density.html


http://www.anval.net/Downloads/ Bulk%20Density%20Chart.pdf


http://www.stobec.com/documents/data/7844.pdf Solids and Bulk Handling September 2014 31


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