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The classic problem when working with powders is their failure to discharge reliably from bins, hoppers, silos, feeders, dosing machines, packing machines, etc. Their unpredictable nature causes interruptions in the production process and at times requires complete plant shutdown in order to correct flow restrictions and stoppages. Failure to flow reliably also leads to variations in pack weight, mixture, performance, and sensory properties of powder products.

Quality Control Departments handle raw materials from multiple suppliers in powder form. The variability in particle size and distribution, moisture content, and basic ingredients requires a battery of incoming inspection tests, none of which assure that proper flow will take place when the powder is loaded into the plant equipment.

R&D

R&D Departments adjust formulations of powder products to satisfy customer demand for improved properties, such as better coating action for paints, enhanced taste for spices, rapid dissolving of chemicals, etc. The unpredictable flow of new formulations causes production problems when the process is scaled up to a high volume.

How are Flow Problems Eliminated?

A Powder Flow Tester eliminates flow problems with a Flow Function test; a proven scientific method used to analyze powder flow behavior. ASTM D6128 describes the Flow Function test procedure, which consists of compressing and shearing powder samples in a defined annular shear cell, using an established well-defined methodology. The Flow Function test enables you to:

  • Calculate the critical outlet dimensions for feeders, hoppers, etc., through which powders flow.
  • Compare powders against one another for flowability, ranking, and benchmarking purposes.
  • Determine whether a powder will flow successfully through an existing plant or process.
  • Determine the required geometry for potential new solids handling equipment before purchasing.

In addition to measuring the flow function, the powder flow tester also measures the angle of internal friction, angle of wall friction, cohesive strength, and bulk density all of which help to eliminate potential flow problems.

 

Key Differences Between Powders and Fluids

For Newtonian fluids, the resistance to shear (viscosity) is independent of the normal pressure but dependent on the shear rate.  In powders, the effect of these factors is reversed so that the shear stress of a powder is strongly dependent on the normal stress but independent of the shear rate.  Hence, when characterizing powders, tests are undertaken at a single speed but over a range of normal stresses.  The other key difference is that powders are anisotropic so the stresses are not equal in all directions and are frictional so that they can generate shear stresses at wall boundaries. 

Application of Powder Flow Analysis

Benchmarking

Measure flow properties on all raw powders and blends to determine if there are differences in their flowability and whether these correspond with plant experience

New materials

Test new ingredients/blends versus existing ingredients /blends to determine whether the alternative material is likely to be easier or more difficult to handle. This potential material handling cost can be factored into the purchasing decision

Reverse engineering

If you have plant experience with powders on a given process line, you can use the Powder Flow Tester to determine the flow properties of each powder and use this as a flowability reference for future batches

Design

Design the geometry (converging angle and outlet size) of new hoppers/silos for reliable flow

Related Products

Brookfield Powder Flow Tester
The PFT Powder Flow Tester delivers quick and easy analysis of powder flow behavior in industrial processing equipment.

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