With construction output in Ireland now at its highest point since the crash of 2008, concrete production has risen substantially. This means that there is a more demand for new cement storage facilities, or to prolong the life of some cement silos in operation.
If you are currently in the process of tendering for new silos, one of the most important aspects of the discussion should be the Silo Protection System. In the first of a two-part series, we look at the silo filtration system and what it’s purpose is.
Most cement silos are now filled from a pressurised road tanker by blowing the powder through the fill pipe (pneumatic conveying) which runs the length of the silo. The silo filtration system is very important at this stage of the process as the air that carries the cement powder is vented through the top of the silo.
While venting, it is important that the air is filtered of powder which was previously suspended while transitioning into the silo. There have been where silo filters have not been maintained and suspended powder has freely emitted into the atmosphere via the top of the silo. While it will be impossible to separate all dust from the air passing through the filter; the purpose is to bring the amounts down to levels allowed through permitted laws.
Types of Silo Filter
There are two main silo filter types; where each has its own purpose in filtering powders:
Bagged Filter Systems:
Filtering Bags are manufactured of cloth in a cylindrical shape where the filtering surface is calculated by multiplying their diameters by their height. The filter bag material can be sourced in numerous fabrics which will suit different applications of the Construction, Food & Chemical industries.
Cartridge Filter Systems:
The cartridge filter system is the most used version of filters on cement silos. They offer substantial filter surface area. Each cartridge is a cylinder covered in a pleated fabric similar to that of the silo filter bags; except due to the pleating of fabric, you can expect the filtering surface to be 3 (and up to 4) times higher than that of the filter bags. Cartridges are excellent options for all types of powders except perishable food powders.
Silo Filter Cleaning Systems
Reverse Jet Cleaning:
A buffer tank containing dehumidified compressed air shoots the air through a group of ‘impulse valves’. A programmable sequencer (Electro Pneumatic Sequencer), complete with timer opens the ‘impulse valves’ to control flow from the buffer tank towards the filter elements (Cartridges/Bags). Reverse Air Jet Cleaning impulse valves removes dust from the surface of the filter element.
Vibration Cleaning Method:
Vibration cycles are carried out by an electro vibrator installed above the bag/cartridge holder. The vibration of the filtering unit will cause the displacement of dust from the surface of the bag or filter.
Removal of Filtering Elements
Like most things that comes into contact with abrasive and corrosive materials; as many powders are, there is a lifespan on filter elements. Due to this, the lifespan differs drastically due to filter utilisation but also the characteristics of the powder material. Depending on the type of cleaning systems above the lifespan of each element can be prolonged. For instance, the Reverse Air Jet Cleaning system is more expensive than the Electro Vibration system; however, much more efficient.
Most silo filters have the capabilities of being opened either from the top of the unit, or from a side door. It is best practice to access and remove each element individually for cleaning as there is downtime on the plant.