Silo Design Best Practices 1: Homogenizing and Blending Silos
The production of cement with reliable characteristics requires consistency. Throughout the years, preparation and storage of the blend of materials used to produce clinker has evolved. This includes the production of the raw material, storage, and transport to the kiln.
The homogenizing and blending silos of recent years differ from most storage spaces. Material placed in a silo is often in a static condition. It is placed inside and stored until it is ready to be reclaimed. With blending and homogenizing silos, the structures are expected to hold material as well as be a part of the cement production process. Material is placed inside and manipulated while in the storage space.
This blending and homogenizing process fluidizes the material in segments, resulting in the material taking on different properties in each segment. Depending on the process, material pressures in the activated sector will be less stiff and exert less pressure outward than the remaining static material (blending silos), or the opposite will occur (homogenizing silos). This difference in stiffness and pressure results in a bending in the silo walls, causing stresses that typically exceed those originally expected for uniform material pressure. This bending causes stress on the silo walls that exceed the silo wall capacity. This first manifests itself in vertical cracking, then delaminated and spalled concrete, and in some cases, a catastrophic failure.
Improving communication to reduce errors
Based on first hand observation, historical information, and discussions with those in the cement industry, miscommunication and a misunderstanding of silo design best practices are key contributors to blending and homogenizing silo challenges. Many times personnel responsible for determining needed air pressures and geometry to homogenize the material do not properly communicate the process to the team responsible for designing the silos. Or, often the team designing the structures did not fully understand the material flow and differing material pressures. This should be a warning to owners to be certain the engineers they trust with the design of their structures are competent and experienced.
Homogenizing and blending silo arrangements, trends, challenges and repairs
Limestone, iron ore, and other minerals are combined and heat is added to produce a chemical reaction that results in clinker, which is then ground and mixed with gypsum to produce cement. The uniformity of the material prior to having heat added to it is important for a consistent product. Blending and homogenizing silos provide a more uniform, consistent and reliable material for clinker production.
While many silos are used to store material, blending and homogenizing silos are actually part of the process of cement production. Material is manipulated inside these silos, not just placed inside then reclaimed when needed. The manipulation of the material when activated by mixing with air, as well as the geometry of the silos, is what typically causes challenges with these silos. There are several documented cases of catastrophic silo failures, as well as many cases of slow deterioration that required significant repair.
While blending and homogenizing silos are similar to one another, and many interchange the two, they are distinctly different from each other. Let’s walk through the differences.
Blending Silos
A blending silo is used to blend raw ground material prior to sending it to be made into clinker. It is typically taller and more slender than a standard homogenizing silo, has an inverted cone or a similar material flow deflection device, and has multiple withdrawal and reclaim openings around the silo perimeter near the inside face of the silo wall.
Material is typically fed into the silo either by pressurized air or by air slide and is dropped into the storage space from one of several points. The material filters into the storage space, and low pressure air, typically five to eight pounds per square inch (psi), is used to fluidize the bottom few feet of material to allow it to be reclaimed.
The multiple withdrawal points are activated either randomly or on a repeating basis. The number of reclaim openings around the perimeter varies by silo. Air pads transmit the low pressurized air to allow material flow to the individual reclaim openings. The intended result is a well-blended material producing consistent results at a lower cost than older homogenizing silos. This is newer technology than homogenizing silos.
Homogenizing Silos
Homogenizing silos are similar to blending silos. However, traditional homogenizing silos are shorter and have a larger diameter than blending silos. Typically, the storage space of homogenizing silos have aspect ratios that are 1.0 – 1.5.
There are several variations for homogenizing silos. Some have internal chambers that house aeration equipment to assist with the homogenization of material. Many have flat or shallow sloped floors, and some of these have troughs directing material to a reclaim opening, either in the center of the floor or elsewhere. Others have material reclaimed through an opening in the wall.
Many homogenizing silos work in tandem with another or multiple silos, where material is transported into the storage space and the material is aerated at much higher pressures (25 psi or greater) than what blending silos use in designated sectors of the space. Some of the more typical arrangements include separating the storage space into quarters or twelfths or other variations and aerating each sector one at a time until the entire storage space is aerated.
The differences in how the homogenization is performed is based on the process design. Additional factors include the amount of pressurized air needed and available and used. The intended purpose is to homogenize the entire cylinder of material in the storage space and provide consistent material properties.
Some homogenizing silos operate in a batch mode, where material is deposited into the storage space prior to any homogenization operations. Others use a continuous process, where material is dropped into the silo continuously as the material is homogenized.
Homogenizing silos vary in how material is reclaimed. Some have reclaim systems that transport directly to a preheater tower to start the clinker production process. Others have concentric reclaim systems that deposit the homogenized material into a separate storage space below it in the same silo, with an equipment floor in between. Others homogenize the material, which typically increases the volume of the material, until it overflows into a separate storage space adjacent to it through an opening in a common wall, where the material is kept fluidized until reclaimed.
Homogenizing silos became the norm when cement plants changed from a wet process to a dry process. This monumental shift in technology, which resulted in a greener production cycle where less power was required and less energy was wasted, started in the 1970s and continued into the 1990s. Blending silos have since largely replaced homogenizing silos. Blending silos have honed the energy needed for creating material with uniform properties, allowing cement producers to create a more environmentally friendly product. They have allowed cement producers to make sound economic decisions, save money, and use less energy.
This blog is the first of a three part series. Read part two, Silo Design Best Practices 2: Structural Challenges and part three, Silo Design Best Practices 3: Repair Options.
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