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What is it and why have it?                                         TECHNICAL ARTICLE

Many industries such as food, pharmaceutical and quarry,  handle and transport millions of tonnes of product every year. This product comes in many different shapes, sizes and densities, but a great deal is shipped in powder and particulate form. These products are shipped from site to site, mainly by road tanker, and are then discharged from the tanker to the site's silos by fluidising the powder and blowing it into the silo.

This pneumatic conveying operation uses air to carry the product into the silo and this air must then be vented via a suitable filter. Silo protection is necessary to prevent:

1. Over-pressurisation.
   If the air is not vented then the silo can become pressurised and most silos are not tested as pressure vessels. Indeed, only a small increase (such as 1 psi) may be sufficient to either rupture the silo or blow the filter element off the silo roof.
2. Over-filling.
   Another common problem is over-filling the silo when either the level probe has failed or it is the wrong unit for the application. Both of these conditions necessitate the need for a silo protection system.

Without protection what is likely to happen?

The effect of over-pressurising a silo can be disastrous, resulting in damage to the silo and an unquantifiable risk to personnel and the public. Over-filling a silo, whilst less onerous, also presents a real risk to the local environment.

1. Damage to silo:
   Resulting in loss of production and expensive repair or replacement of the silo or filters, as well as costly clean up operations.
2. Dangerous situation:
   Operator safety is at risk from heavy filter units falling from great heights. This has serious health and safety implications.
3. Overspill of product:
   Emissions to atmosphere could have a serious effect on the environment especially if the product is corrosive or hazardous and may result in large fines.

The risks associated with the above can be avoided if a basic Silo Safety System is in place but it is essential that the sensing elements of this system can be tested in-situ prior to each and every pressurised fill. All safety systems for pneumatically filled silos should have two sensing elements and a pressure relief valve.

1. PRESSURE SENSOR
2. LEVEL SENSOR
3. PRESSURE RELIEF VALVE

These three elements together provide warnings of approaching danger and ultimately provide relief of dangerous pressure, if the sensors are ignored. Monitoring the level and pressure allow other control functions to be activated such as:

• High level alarm
• High pressure alarm
• Tanker fill pipe shut-off
• Activate filter alarms
• Alarm on a PRV event pressure

AS WITH ALL SAFETY AND PROTECTION SYSTEMS, MAINTENANCE AND REGULAR TESTING OF THE SYSTEM IS ESSENTIAL.

EDITORIAL                       SILO PROTECTION - AN OVERVIEW