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Powder Question Challenge

10/20/2014 - My powder is highly compressible. The conventional “Tap Test” shows that the volume in the cylinder shrinks by almost one-half. How can I correlate this information with the Brookfield Powder Flow Tester? I realize that the PFT gives flow behavior information, but all of our historical data is based on the Tap Test. I need to show a correlation between data from the PFT and the Tap Test.

The most popular test that you can run with the PFT is the “Flow Function”. During this test, the PFT measures the change in volume of the powder as the shear cell lid presses down on the powder sample in the trough. Part of the data output from the test is a density curve that shows how much the powder compresses together at each consolidating stress. There will be a point on this density curve that correlates with the data obtained in the Tap Test.

An important observation is that the density curve from the Flow Function test is highly repeatable, perhaps more so than the Tap Test.

The PFT also has a standalone function called the “Density Test” that can be run in under 3 minutes. This gives a very quick method for providing data that will correlate with the Tap Test.


Brookfield Responds: Q&A

Powder Question Challenge

Customer Challenge: The Flow Function that we measure with the Powder Flow Tester on our product appears to be shifting upwards. Our powder is a special flavoring that we blend for a new brand of low calorie corn chips. The production staff tells us that the powder seems to come out of the hoppers ok, but we hear grumbling that the flow can become erratic at times during the run. We are not sure if the problem is with the ingredients in our blended powder or the PFT. How do we know whether our PFT is measuring correctly?

Brookfield Answers: Checking the PFT for proper calibration has two possibilities. Use specially calibrated powder known as BCR-116 limestone, which is available fromBrookfield(Part Number PFT-BCR-116), and run the Flow Function in accordance with our published test procedure for performing a calibration check. The measured values for Unconfined Failure Strength must lie within established min/max limits for the test to pass.

The second approach is to use a special calibration kit (CalKit) which checks the sensors in your instrument that measure torque, axial load, and position of lid. If the sensors are out of calibration, the CalKit will identify the problem and recalibrate the sensors. You will also receive a new Certificate of Calibration for your PFT.

Concerning your blended powder, consider performing the Flow Function test on the ingredient that is the primary carrier for the mixture. It has the greatest influence on ‘flowability”. This powder may constitute the greatest percentage by volume. The objective of the Flow Function test is to find out whether this material is changing by performing a QC check during incoming inspection.

Powder Question Challenge

Customer Challenge: We use a Flodex Tester to make a quick assessment of our powder’s flow behavior before we start processing operations by opening the discharge on our main hopper. It’s a relatively inexpensive device and easy to operate. However, it only gives a “go” or “no go” indication of whether we are likely to experience problems. And it doesn’t always agree with what actually happens once we start operations. Is there a better way to measure the flow behavior of our powder?

Brookfield Answers: There are several ways to make measurements of powder flow behavior. Some are popular, because they have been around for a long time, like the Flodex Tester. If your material is relatively “free flowing” like granulated sugar or crystalline salt, this may be a satisfactory method.

Powder Question Challenge

03/20/2014 - Our powder sometimes has a sticky quality that increases the flow problems that we experience in processing. We think that it could be related to humidity. But we also have suspicion that one or two ingredients from different suppliers may be contributing to the stickiness. Does the Brookfield shear cell give you the ability to detect this type of characteristic before it becomes a problem out on the production floor?

If your material has a granular free-flowing nature like sugar, but changes due to humidity/temperature, then the Powder Flow Tester is designed to characterize this property and quantify it for you in terms of a reduced flow capability. This is exactly the type of application the PFT is used for; any type of powder that shows cohesive behavior and is being used in gravity feed systems and the like are what the PFT can measure.

Brookfield has a Powder Laboratory where the relative humidity can be elevated from 30% to 60% and the effect on flow behavior of customer powder samples is measured with short turnaround time. The testing is free for a limited number of samples from each customer.

Powder Question Challenge

03/04/2014 - The application on your website called “Solid Dosage – Stacking Sacks of Powder for Processing” is relevant to one of the problems in our company. We didn’t know how to determine the recommended “stacking height vs. time” in order to avoid caking of the powder. Your article gives us a clear way to make this calculation by running the time consolidated flow function test with the Brookfield Powder Flow Tester. The formula for calculating the arching dimension of the powder is:
Arching Dimension = 2 x Critical consolidating stress x 1000
Density of powder x gravity

What do the factors 2 and 1000 represent?

The answer to your question is easier than you may think. The important note is that the Powder Flow Pro software, which runs the Time Consolidated Flow Function test, does the calculation for you.

1000 is for conversion from units of kPa to Pa.

2 is an arch shape factor for a circular outlet in a conical hopper. This is a simplification of the Jenike H(θ) function which is approximately 2.

Note that for a wedge or plane hopper with a slot outlet the arch shape factor is 1, which is again a simiplifcation of Jenike H(θ) function which is approximately 1.

Once you know the arching dimension related to the time period that the sacks will be stacked, you can determine a maximum height to avoid caking.

Powder Question Challenge

07/09/2013 - Why do processors of powders and bulk solids try to measure powder flow behavior?

Variability in raw ingredients and the need to produce various blends of finished product for different applications constitute the major challenge for processing plants. Flowability of the powders is the recurring problem that can lead toinconsistent blends on the one hand and total plant stoppages on the other.

Powder Question Challenge

07/09/2013 - What are examples of inexpensive traditional methods used to predict powder flow?

Typical methods include the Flodex Cup and the Angle of Repose test. Although simple and easy, they fall short of what's needed. Both procedures evaluate loosely consolidated powder which is not a true reflection of the powder's condition when sitting in a storage vessel on the plant floor awaiting discharge. Once the bin is full, consolidation begins automatically due to the powder's self weight. This natural compaction process affects discharge through the hopper opening. Therefore, flowability is not being assessed correctly.

Powder Question Challenge

07/09/2013 - How does a shear cell test powder flowability?

Shear cell testers compact the test sample, then shear the powder against itself to measure the amount of interparticle friction that will cause resistance to movement. This type of test gives several pieces of information that can be useful, such as flow function and density vs. consolidating stress.

Powder Question Challenge

07/09/2013 - What does the expression "flow function" mean?

Flow Function is a graph which plots the strength of the powder (the force needed to cause the powder particles to slide against each other) against the consolidating stress (gravity's effect on the powder which causes compaction). At low consolidation stress the powders can become "highly cohesive" which might explain why they will not completely discharge from a hopper.

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