Thursday 18th Aug, 2022

The differences between dry powder silos

Mario Scutti, Managing Director of Italian silo manufacturer Scutti, explains the advantages and disadvantages of various bulk silos.

Mario Scutti, Managing Director of Italian silo manufacturer Scutti, explains the advantages and disadvantages of various bulk silos.

The sheer range of bulk solids that can be stored in a silo is enormous, ranging from grain or cement to ultrafine powders used in the processing industry.

Similarly, the number of different constitutive models for bulk solids is extensive. Most have been designed by professional engineers, using the experience they have gained in their specific fields.

It is important to consider that the market consists of several types of steel silos made by specialised companies, such as Scutti. These silos are already developed in sizes, for different storage capacities that can be adapted to the specific needs of the customer.

In the past few years, the demand for small and medium storage plants has increased, mostly due to the development of emerging markets across the globe. In this scenario, a common question that almost all these businesses ask themselves before starting a new storage project is, “what kind of storage silo should I choose?”

There are four types of silos available on the market today: concrete silos, onsite fabricated silos, steel spiral silos and steel bolted silos. There are many factors that can be taken into consideration when it comes to silo selection – including storage capacity, location, ease of transportation and budget.

In general, all of these silos are good for storage, but what are the advantages and disadvantages?

Concrete silos

These silos tend to have the highest storage capacity of the four. Its structure is composed of concrete reinforced by steel bars. The silo is erected from the ground, step by step, pouring concrete continuously in moving forms.

Concrete silos are mainly used in large storage facilities, exceeding 15m in diameter. The manufacturing process usually comes with a high cost and long start-up time, but the volume of product stored is very high.

Onsite fabricated silos

For these silos, the metal sheets of the silo body are rolled and welded at the construction site, before being treated and painted. Everything is done by the manufacturer using machinery that has been transported onsite.

In general, the manufacturing process for the welded silos can create quality and timing problems, due to the weather and location. On the other hand, this kind of manufacturing allows the owner to fully customise the dimensions of the silos and its structure.

Spiral silos

This silo design is based on a method for the construction of silos onsite. Steel coils, machines and accessories are transported to the construction plant where the silo is being built. A special machine unites the layers of the strips, which are automatically joined up as the silo grows in an upward spiral.

The diameter of these silos can be adapted to a customer’s needs and, in general, are used for light products such as cereals or wheat. Weather conditions on site can cause delays in the manufacturing process.

Steel bolted silos

This type of silo is often the best compromise between cost, erection time, and storage capacity. There are two main types of bolted silos available on the market, overlapped panel silos and flanged panels.

Overlapped panel silos consist of drilled steel panels that are rolled and painted by the manufacturer in the workshop. The panels are bolted overlapping their perimetric parts, which are already drilled directly on the metal sheet.

Then, the bolts are sealed inside and out using a liquid sealant. This kind of silo can be installed completely from the ground using a jack lifting system, but the structural design makes it unsuitable for heavy-duty applications.

Flanged silos consist of high thickness flanged panels that are already manufactured and painted in the workshop. The panels are bolted on the ground and lifted using a crane. The high thickness flanges and additional reinforcements make them robust, easy to install and suitable for heavy duty applications. Support structures can be customised to suit the client’s specific needs together with specially designed discharge cones for differing product problems. Transportation is not an issue as all the panels are designed to fit onto steel pallets for road transport and containerization to any place in the world

Case study

Scutti and its partner Silos Solutions NZ Ltd designed, built, and installed a 3000 tonne externally flanged steel silo in Wellington, New Zealand. A seismic fault line was located only 200m away from the silo, where potential seismic acceleration could reach 0.4g.

The planed silo site is located at the Port of Wellington, where neighbouring cement storage manufacturers in the close vicinity caused site difficulties. These included coordination work schedules to operate cranes and closing operations when dangerous activity was occurring. The limited space provided for installation was also a challenge that needed to be handled.

Silo Solutions engineers had to do a peer review for the calculations by adjusting the Eurocode to meet the Australian/New Zealand standard for engineering for the wind and seismic requirements for Wellington.

The foundations of the silo also had to be redesigned after the events of the November 2016 earthquake in New Zealand, which badly damaged the port area. The foundation piles were drilled 28m into the ground to provide the stability needed to withstand a similar earthquake.

The silo height was also restricted to 27m, due to the local government’s laws. This made the engineer’s job difficult, as they had to allocate 3000 tonnes of cement storage within the restricted area. To accommodate the regulations, Scutti suggested that the standard cone design be changed to a truncated cone with airslides this allowed the standard structure to be lowered by 4.0m, solving the potential problem. 

The silo was assembled by Scutti’s partners, Silos and Conveyor Systems Ltd, and with a very limited storage space on site a just in time management system for deliveries was implemented to maximise the storage and assemble on a very restricted site. A total of 10 containers featured all the componentry to build this silo. 

A tower crane was used for the sub assembly of the structure and a 350-tonne mobile crane was used to lift the silo sections into place. The total time taken to complete the project including weather delays was 10 weeks. Scutti and its partners are proud of the result, as there were many challenges from design through to installation.