There is a common misconception that the biggest risk to silos comes from overfilling. This is simply not the case. This confusion may arise because LAPPC (Local Authority Industrial Pollution Prevention and Control) permits focus on level when the real issue is pressure.  The damage that can be caused by over-pressurising a silo far outstrips the problems of overfilling. Overfilling will not cause a silo filter unit to blow off the top of the silo – pressure build-up will.

To understand silo protection you must first understand what creates excess pressure in a silo. Pressure occurs where product is pneumatically transferred from a vessel (for example a road tanker, reclaim system or a bag house) into a silo.


If the filtration system allows the pressurised air 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 escaping or venting from the silo,
as shown in the graphics below:
A safe silo, where air in equals air going out.


This is an example of a healthy delivery: if air can enter a silo and exit via its venting unit without restriction then there will be no over-pressurisation issues. Silo over-pressurisation only occurs when the volume of air entering the silo exceeds the volume air that is able to escape, as shown in the examples that follow.

An unsafe silo where air is trapped inside the silo.


Here, airflow out of the filter unit is restricted, leading to a pressure build-up during filling. Failure of the filtration system to vent air efficiently could be caused by inadequate maintenance of safety equipment, or by filter blinding due to over-filling.  An increase of air pressure above 1 psi inside a silo can cause serious damage.

An unsafe silo where air is inserted at a faster rate than it can escape.


Here, more air is being blown in to the silo than the system can exhaust. This is usually caused by uncontrolled discharge from the tanker during filling. A tanker can offload product at 13,000 m³ per hour, but a typical silo filter can only discharge air at a maximum rate of 1,700 m³ per hour. Even a new filter cannot cope with this! Evidence suggests that this problem is far more common than previously thought, posing a severe safety risk.

Over-pressurisation can occur because of several factors, either individually or combined. These include:
  • Failure of the pressure sensor
  • Failure of the filtration system to vent air efficiently
  • Failure of the level measuring equipment
  • Uncontrolled discharge from the tanker
  • Filter-blinding caused by over-filling
  • Inadequate maintenance of safety equipment
  • Faulty or damaged inlet control valves

As the majority of silos are not pressure rated vessels they can rupture at pressures as low as 1 or 2 psi. Without adequate safety systems in place sites risk fatal injury to staff, damage to equipment, product loss and environmental pollution – not to mention hefty Health & Safety fines. Therefore silo protection is essential for avoiding the risks of over-pressurisation and overfilling. Guidelines from the Mineral Product Association (MPA), the HSE and DEFRA all make clear the importance of a reliable, testable safety system that is kept in optimum condition through regular servicing.

However, even with a protection system in place there is still risk: testing safety equipment at the top of a silo brings additional hazards involving working at height. This also means it is unlikely sites will test their safety equipment before every fill.

Hycontrol’s advanced SHIELD Lite silo protection system is not only the most advanced on the market, but it is also the only system that meets and even exceeds current safety guidelines. The key components of a typical SHIELD Lite SPS installation are shown below.

Components of the SHIELD silo protection system

Hycontrol’s silo protection systems have received widespread industry recognition and several awards. They have several key advantages over competing systems:

  • Hycontrol’s systems are the first to eliminate working-at-height risks with the introduction of a complete ground-level test (GLT), enabling all essential safety components (including the pressure relief valve and pressure sensor) to be checked before each and every fill. The entire system is checked in just six seconds before allowing the fill. If any component is not functioning correctly the system will alert the user and the valve will not open – it’s that simple.
  • The patented FLEX pressure sensors are the only such device custom-designed for silo protection. They are the only pressure sensors that are fully and accurately testable over the entire working range, ensuring correct functionality as part of the GLT. They are self-cleaning and control the inlet valve throughout the filling process. If the pressure level becomes dangerous, the sensors trigger an alarm and close the inlet valve whilst the situation is addressed.
  • The PRV test facility is precisely calibrated to ensure it will only lift at the correct pressure. Previously this was only possible by removing the unit and bench-testing it. This has the added bonus of cleaning the valve of any accumulated product.
  • The system uses DP Series vibrating high-level sensors, which (unlike old-fashioned paddle switches) have no mechanical parts to wear out and break. The system integrates with Hycontrol’s Hyvent, a highly efficient, self-cleaning filter unit.
  • Crucially, the systems record high pressure, high level and PRV lift events with a time and date stamp, and count the number of times an alarm condition is reached. This is essential for preventative maintenance purposes as it will allow engineers to address issues in the system before they become acute problems.

As with all safety and protection systems, regular servicing and maintenance is essential to ensuring continued optimum performance.
For further information please read the datasheet here:

Silo protection panels at a fill point Hycontrol silo protection system on a silo Panels at the fill point

HYCONTROL LTD Larchwood House, Orchard St, Redditch B98 7DP UK. Tel: +44 (0)1527 406800 Email: sales@hycontrol.com