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Hoppers & Silos


In this report written for S&BH, Mark Barton, MD of Barton Fabrications details how growth in the use of PET plastics has led to an increase in noise and structural problems reported in storage silos. The properties of the material, particularly its relatively high poured bulk density and co-efficient of friction, can result in silo overloading and phenomena known as honking and quaking. In the worst case this can cause complete silo collapse when storing PET granules. These issues are outlined by the head of the UK’s largest aluminium silo manufacturer who concludes with details of how these problems can be overcome.


Addressing PET storage problems in silos


P


olyethylene terephthalate or PET is one of the most common types of plastic in use. It is popular as a general engineering plastic and,


in particular, for packaging. It is predicted that the global consumption of PET packaging will grow to almost 19.1 million tonnes by 2017, with an annual growth rate of 5.2% between 2012 and 2017 [1]. In the UK, 70% of soft drinks containers use PET plastic bottles and nearly 35% of PET plastic bottles in the household waste stream are now being collected for recycling, in 2001 this figure was just 3% [2]. In summary, there is a considerable and growing tonnage of both raw and recycled PET plastic being processed.


The increase in the use of PET feedstock has 28 September 2014 Solids and Bulk Handling


led to greater storage of the material both in greenfield manufacturing facilities dedicated to PET and in the conversion of plant from other plastic types. Unfortunately, the material properties of PET are quite different to many commonly used plastics and three main problems can arise when storing/discharging PET: silo honking, silo quaking and silo overloading. 1) Silo honking, also known as silo noise or music, occurs when the PET material is moving within the silo and generates a low frequency (>20Hz) sound. This noise is intermittent (with intervals between seconds or hours), unpredictable and typically sounds like a truck horn [3]. The volume of this noise is sufficiently loud to be


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of concern where silos are installed close to environmentally sensitive or residential areas and has led to noise abatement orders being issued.


2) Silo quaking, also known as silo thumping, again is related to the movement of PET granules within the silo but, in this case, the movement and rapid deceleration of a greater mass of material can cause buckling of the silo wall or even catastrophic failure of the silo’s structure.


3) Silo overloading: If silos have been designed to suit most common plastics, stress calculations are likely to have been made using a bulk poured density of around 600 Kg/m³. Filling them with PET, which has a bulk poured density


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