Bulk materials conveying: Six questions

March 1, 2015

System developers will need these questions answered to dimension, design and assemble a materials-handling system that suits.

Anyone considering installation of a vacuum conveying system for granular bulk materials should review the questions below. System developers will need these questions answered to dimension, design and assemble a bulk materials conveying system that suits.

What’s the material?

The material is the key. The designer must know the material bulk-density of the material, i.e. how much it weighs in kilograms per liter (kg/l). It is also important to establish an approximate size for individual particles in the material, measured in micrometers (µm).

Determining how sticky the material is requires a more hands-on approach. Stickiness, or fluidity, as it is more correctly referred to, is different from other parameters in that it is difficult to measure; hence there is no single unit or test to apply.

However, there are methods that will help determine the fluidity. First, there is the angle of repose. This is the internal angle of the conical pile, formed when a material is poured onto a horizontal surface. It represents the steepest angle of descent relative to the horizontal plane to which a granular material can be piled without slumping. This means that the material on the slope is on the verge of sliding at this angle.

Density dependent

The angle of repose is dependent on the material’s density and the coefficient of friction, as well as the surface area and the shape of individual particles. However, it has also been shown to be gravity-dependent. Nevertheless, it provides a guide for how the material will perform when handled in a conveying system.

Next, there is the "squeezing-it-in-your-hand" test. This involves squeezing a handful of the material in the palm of your hand. Some materials, for instance milk powder, will form a ridge in the hand, whereas others, such as granulated sugar, will not. If, in the next step, the ridge stays intact when prodded, the system developer might need to include functionality to improve the fluidity.

However, as a rule of thumb, any material that forms a ridge which falls apart when prodded, as milk powder would, can be handled by standard equipment.

How clean need it be?

To choose the right type conveying system, the developer needs to know what’s required in terms of cleanliness of the transported material. It goes without saying that a mining company, a manufacturer of foodstuff and a pharmaceutical provider have very different perspectives on cleanliness. Therefore, conveyor systems are often offered in different designs targeted at specific industries or purposes. However, there may be special circumstances in which the norms or standards need to be adjusted to suit a specific application.

How far must it travel?

Spatial data regarding the transportation system itself is also vital. The developer must know how far the material needs to be transported. However, simply measuring the distance in meters is not enough. Accurate measurements for horizontal versus vertical distances are necessary, as these greatly impact system design.

The transportation route will determine the number of pipe bends needed in the system, and this will in turn influence the choice of vacuum pump used. For vertical lifts, a rule of thumb is that any material can be transported eight meters straight up without any particular problem.

However, lifting beyond eight meters may require installation of a pipe emptying unit towards the end of the vertical pipe. Such units use air to help "push" the material further along the pipe. A conveying system equipped with a pipe emptying unit can transport materials vertically up to 25 meters.

What capacity’s required?

The capacity, throughput, or flow rate of the system is next on the designer’s list of questions. The capacity is measured in tons per hour and, together with the respective vertical and horizontal distances, it provides the designer with a figure that is used to select a pump for the system.

The material’s bulk density will also help determine the dimensions of the pipes used in the system in order to achieve the required capacity. The size of the individual particles will determine which type of filter is needed.

What are the risks involved?

To avoid dust explosions, specialized equipment may be required for materials prone to developing static electricity. Compliance with industrial standards included in regulations such as the EU ATEX Directive is an essential part of the design work.

High pressure and high temperature are other potential risk factors that might need to be considered. Examples include overpressure in connecting equipment and heat sources that increase the temperature near the conveyor. However, these problems are usually relatively easily overcome by using equipment of the right specifications.

What testing is involved?

Experienced designer have no problem dimensioning and building a vacuum-based conveying system for common materials. However, faced with unfamiliar materials or perhaps extreme spatial constraints, the designer may decide to test the system in-house before it is implemented at the client’s premises. Having access to such test facilities is a clear advantage, as well as reassuring for clients.

Having ticked off all the questions on the checklist, the designer is ready to set to work building a system fit for purpose. Furnished with all the necessary information regarding: the fluidity, bulk density and particle size of the material; the horizontal and vertical distances in the available space; the desired capacity; any special requirements concerning cleanliness or potential risks; and results from a potential trial run, the designer can choose the right vacuum conveyor for the job.

Thomas Dahl is in strategic sales OEM at the material handling division of Piab AB.

Established in 1951, Piab designs innovative vacuum solutions that improve the energy-efficiency, productivity, and working environments of vacuum users around the world. Piab is a worldwide organization with subsidiaries and distributors in more than 50 countries. Its headquarters are in Sweden. For more information about Piab vacuum solutions for a diverse range of applications, visit piab.com.

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