Silo Design & Manufacture

Silo specification

The starting point for any silo installation is detailing the storage materials and where the vessel, or vessels, are to be sited. The nature of the material to be stored must be assessed in relation to the explosion risk it presents and how that risk can be controlled / mitigated. The legislation covering this falls within ATEX and is implemented in Great Britain through the DSEAR and EPS regulations.

ATEX, DSEAR and EPS

ATEX is the name commonly given to the two European Directives for controlling explosive atmospheres:

a) Directive 99/92/EC (also known as 'ATEX 137 or the 'ATEX Workplace Directive')
b) Directive 94/9/EC (also known as 'ATEX 95' or 'the ATEX Equipment Directive').

In Great Britain, the requirements of Directive 99/92/EC have been put into effect through regulations 7 and 11 of the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR). The requirements of Directive 94/9/EC have been implemented through the Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996 (SI 1996/192) (EPS). {i}

KST and Pmax: Design to avoid destruction

The basic design philosophy is to avoid ignition of the material being stored however, should an explosion occur, construction should be such that the vessel is strong enough to withstand explosive damage and burst panels are necessary to allow the safe release of the excess pressure. Of particular importance in silo design and explosion venting to prevent silo rupture in the event of an explosion are the materials explosion indices KST and Pmax. KST indicates the maximum rate of explosion pressure rise and Pmax is the maximum pressure generated during an explosion. By using these values, which are determined by laboratory testing, the silo explosion safety vent area can be determined. This is calculated to ensure that the pressure rise within the silo in the event of an explosion will not exceed the maximum silo design pressure and therefore prevent catastrophic rupture of the vessel. Table 1 shows typical values for flour and sugar. Barton’s silo design, which incorporates ‘thick wall’ construction with stiffening, allows the safe adoption of a minimum vent area to maximise working space at the top of the silo. In addition, vertical silo fabrication minimises residual wall stresses normally associated with horizontal construction and silo ovality.

CE Marking and the latest legislation

The latest legislation to affect silos is compliance with the latest Construction Products Regulations. The now mandatory Construction Products Regulations replace the Construction Products Directive and dictate that any steel or aluminium construction product, which includes silos, are CE Marked to the harmonised European Standard EN 1090. The standard gives customers the confidence that the silos, pipework, gantries and ladders, which are all considered structural products, have traceability and conform to mandatory structural requirements. As part of this standard the design, manufacture, testing and documentation must all be clearly defined within a factory production control document and, specifically, fabrication must be carried out by ‘Coded Welders’ who have been approved to weld the relevant material grades, gauges and joint configurations.

Other factors

In addition to explosion risk and structural considerations, material bulk density and flow characteristics must also be reviewed to ensure that the silo is strong enough for the required static and dynamic loads, and that the stored material will flow smoothly within the silo under gravity. Future proofing is also a sensible consideration at this stage. Example questions for consideration include: whether there is a possibility that the process may require a different feedstock in future and likely changes in usage volumes. For instance, a silo designed for polypropylene granules will have a different construction from one to be used for PET with its higher bulk density and different flow characteristics. If the extra loading is not considered, then a silo can buckle as shown in Image 3 or, in the worst case scenario, suffer complete collapse. Raw material batch traceability is also an increasing consideration and sometimes mandatory. Within the automotive industry, full batch traceability is the norm requiring a single silo for each tanker load which therefore determines how many silos are needed.

Design for maintenance and safe working

Consideration should also be given to maintenance, with adequate provision for access ladders, gantries and railings.

Here again, legislation covers how this provision should be provided and the standards required.

Silo access is designed in accordance with the current editions of BS EN 4211 and BS EN 14122-1, 1 & 2 for platforms. Guard rails are designed in accordance with the current edition of BS EN 14122-3.

Barton, for example, offers the ultimate in ‘safe working at height’ with their full or half penthouse designs which offer a fully enclosed environment at the top of the silo. See Image 4.