What is Silo Protection?

Silo protection is defined as the prevention of over-pressurisation of storage silos during powder deliveries. This applies equally to any powdered product, including cement, lime, flour, GGBS, or plastics.
The risks associated with storage silos are outlined in brief below, and they must be monitored and addressed in order to ensure site safety and equipment integrity. A silo protection system (SPS) is an integrated safety system for monitoring and controlling both level and (crucially) pressure within a powder or solids silo. The system must be failsafe and able to automatically stop the product inflow before the pressure reaches a dangerous level.

To understand silo protection you must first understand what creates excess pressure in a silo. Pressure build-up occurs where a powdered product is pneumatically transferred from a vessel (for example a road tanker, reclaim system or a baghouse) into a silo. If the filtration system allows the pressurised air that is being blown in to escape sufficiently quickly then there should be no problem.

Silo over-pressurisation occurs when the volume of air entering the silo exceeds the volume air that is able to escape or vent from the silo. This happens due to one of two possible reasons: either the airflow out of the silo is restricted (for example by a filter blockage) or the airflow in is excessive (likely due to an uncontrolled tanker discharge). It is important to remember that the vast majority of storage silos are not pressure-rated vessels and have a zero pressure rating. This means that it is possible to cause critical damage with a pressure build-up of as little as 70 millibars (1 psi).

There is a common misconception that the biggest risk to silos comes from overfilling. This is simply not the case. The damage that can be caused by over-pressurising a silo far outstrips the problems of overfilling. Overfilling will not cause a silo to rupture or the filter unit to blow off the top – pressure build-up will!

The key risks associated with silo over-pressurisation are as follows:

• Silo failure - potentially splitting the silo or ejecting the filter unit from the roof. This could result in critical injuries or even fatalities, as well as potential lawsuits and HSE investigations.
• Environmental - including pollution and product loss, leading to damage to reputation, fines and potential site closure. With some products there is also a considerable health risk from leakage.
• Working at height - potential slip, trip and fall risks arising from maintaining any existing safety equipment on top of a silo. 

Put simply, the evidence gathered by Hycontrol and other organisations points to this being a very big problem globally. For example, based on an approximate number of 4,000 silos in the UK concrete industry, Hycontrol estimate (based on statistics recorded from a sample of 250 silos) that something in the region of 10,000 over-pressurisation events occur every year. Obviously these are not all of a catastrophic nature, but the scale of the problem far exceeds what has been previously suspected. The same situation is repeated across the world. A Google search will soon turn up stories of ruptured silos, blown filters, pollution, injuries and worse - and these are only the events that get reported.

This problem is real, it is substantial, and it must be addressed in order to keep workers and the general public safe.

The Mineral Products Association (MPA) has published guidelines covering essential SPS components. It must have an accurate and reliable pressure sensor, self-cleaning filter system, sufficiently-rated pressure relief valve and a level probe to prevent filter blinding. A system must have equipment test functionality, alarms, and be able to automatically stop the product inflow if the pressure becomes dangerous. 

However, Hycontrol asserts that comprehensive protection requires more than just this basic functionality, and so our systems are the only ones to exceed MPA guidance. Please refer to our literature, videos and other pages on this website for further information.

In the UK, safety guidelines for silo storage are available from the Department for Environment, Food and Rural Affairs (DEFRA) and the Mineral Products Association (MPA) and the with additional relevant guidance from the Health & Safety Executive (HSE). Similar organisations in other territories will likely have similar materials available. Hycontrol recommends that site managers, health & safety officers and service personnel be fully conversant with these essential guideline documents. Additionally, Hycontrol recommends five key steps that all sites, management and personnel should be following to ensure site safety:

1. Follow the industry best practice guidelines and issue comprehensive written standards for your company - this ensures all parts of the business understand the issues and most importantly the correct solutions.

2. Regularly audit your silos - this can be done independently or in-house, but maintain standards of practice and equipment test results.

3. Competent servicing - any silo safety equipment must be regularly examined or tested by qualified, experienced, competent engineers. Ask for confirmation of the critical pressure set points.

4. Educate at all levels - all site personnel must be aware of the risks involved with silos, including pressure safety and working-at-height. Know your responsibilities and how to meet best practice.

5. Utilise the best available technology - modern silo protection systems will test safety equipment from ground level (reducing working-at-height risk) and ensure auto shut-off systems are fully operational.