STORAGE
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silo safety
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A comprehensive overview on silo safety guidelines
by Portasilo, UK
ilos that are filled directly from a pressurised tanker need to incorporate suitable mechanisms to allow displaced air within the silo to escape during filling. If these mechanisms are insufficient for the pressures created within the silo, or if the equipment is poorly maintained and becomes inefficient, dangerously high pressures can build up within the silo. This can lead to a catastrophic blow out, damaging plant and equipment, dispersing possibly toxic silo contents over a wide area and endangering the lives of employees. In this report, we bring together all of the guidance and recommendations into a single, easy-to-understand guide to safeguarding your plant against silo over-pressurisation. At the same time, we highlight the implications of the HSE and Defra guidance for silo safety equipment.
Prepare for peak airflow
One of the major changes to existing assumptions outlined in the HSE document was the need to specify silo safety equipment to cope with a peak airflow from the silo of 13,000m³/hour. The previous assumption had been that peak airflow caused by over-pressurisation during tanker filling could only reach around 2,000m³/hour. To test the new assumption, Portasilo built its own test rig and modelled a variety of over-pressurisation incidents. The company found that it was possible to reach peak airflow from the silos of almost 13,000m³/hour as a result of over-pressurisation. The next 38 | Milling and Grain
step was to investigate the ability of existing safety equipment to cope with this increased airflow and, where necessary, redesign it.
Silo filters – the first line of defence
All silos that receive pressurised tanker deliveries must be fitted with a filter in the roof. This allows displaced air from within the silo to escape to the atmosphere. Portasilo tests found that filter sizes and specifications did not need to be changed in response to the new peak airflow assumptions in the HSE guidelines. If properly sized and maintained, existing filters could already cope with an airflow of 13,000m³/hour and it was our assumed minimum filter velocity that had changed not the requirement for more filter area. However, regular filter maintenance is essential to sustain the required levels of performance. Silo filters incorporate filter bags to prevent dust escaping into the atmosphere. These bags must be regularly cleaned to prevent clogging which could inhibit the free outflow of air from the silo. A number of automated filter cleaning mechanisms are available, but the most efficient and effective are reverse jet-cleaned filters. These systems use jets of air to blow the dust from inside the filter bags. It’s also important that the air supply to the filter is clean and dry. Moist air can quickly exacerbate the clogging of filter bags, particularly with particulate powder silos.
Pressure relief valves – the last line of defence
If pressures within a silo build up to such an extent during filling that the filter cannot release enough air to maintain silo pressure