What is Silo Protection?

There are several misunderstandings surrounding silo over-pressurisation and silo protection, so this page addresses the most frequently asked questions.

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1. What is silo protection?

The definition of silo protection is the prevention of over-pressurisation of storage silos during powder deliveries. Silo protection techniques are required for handling any powdered product, including cement, lime, flour, GGBS, or plastics.
The risks associated with storage silos are outlined below, and they must be monitored and addressed 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.

2. What causes silo over-pressurisation?

To understand silo protection, you must first understand what creates excess pressure in a silo. Pressure build-up occurs during the pneumatic transfer of a powdered product from a vessel (for example, a road tanker, reclaim system or a baghouse) into a silo. If the filtration system allows the inward-flowing pressurised air to escape sufficiently quickly, there should be no problem.

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

3. What are the risks of silo pressure?

There is a common misconception that the most significant risk to silos comes from overfilling; this is not the case. The damage that can be caused by over-pressurising a silo is far greater than any of the problems arising from 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. Such an incident could result in critical injuries or even fatalities, as well as potential lawsuits and HSE investigations.
  • Environmental - powdered product blowing out of a silo under pressure means pollution and material 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 - additionally, there are potential slip, trip and fall risks arising from maintaining any existing safety equipment on top of a silo. 

4. How big a problem is this?

Put simply, the evidence gathered by Hycontrol and other organisations points to this being a huge problem globally. For example, based on an approximate number of 4,000 silos in the UK concrete industry, Hycontrol estimates (based on statistics recorded from a sample of 250 silos) that approximately 10,000 over-pressurisation events occur annually. Of course, these are not all of a destructive nature, but the scale of the problem far exceeds previous suspicions. Moreover, this problem is a global issue. 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 industries must address it to keep workers and the general public safe.

5. What are the essential components of a comprehensive silo protection system?

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. In addition, a system must have equipment test functionality and alarms and be able to stop the product inflow automatically if the pressure becomes dangerous. 

However, Hycontrol asserts that comprehensive protection requires more than just this basic functionality, 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.

6. What steps can I take to make sure my site is safe from silo pressure risks?

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 - do this in-house or engage an independent company, 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 - 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 practices.

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