Angle of Repose – What Are They and Why Do You Need to Know?

The angle of repose is the highest internal angle to the horizontal to which a powder can be heaped or piled without the material slumping or sliding. The angle of repose depends on the

• Material density,
• Surface area
• Particle shape
• Friction coefficient of the material.
• Gravity

Why is this angle of repose important in the study of powder rheology?

Angle of repose forms one of the commonly used methods for testing powder flow or the powder flow properties of powders. It is one of the two measurement tests known as the compendial methods, the other being:

Bulk density and tapped density measurements which help calculate the Carr’s compressibility index (CI%) and the Hausner Ratio (HR)

Compendial testing is a basic requirement for pharmaceuticals manufacture.

Measuring the angle of repose

The angle of repose is measured in the following way: A powder is heaped in the form of a cone by passing it through a funnel. The height of this cone and the radius of the base is measured. The angle of repose is given by the formula: Tan inverse of height/ radius.

Certain parameters of this angle are established for ranking the flowability of any powder as good, fair, passable or poor. Depending on the value of the angle this quality is determined as excellent flowability for less than 30 degrees, to poor flow for greater than 56degrees.

Non compendial methods

While we spoke about the compendial methods, there are the non-compendial methods which are also considered which are more modern and varied, using powder rheometers. Here the following can be measured:

• Cohesivity
• Caking ability
• Flow stability

Cohesion of particles is one of the characteristics of powder flow, and the use of lubricant alters this measurement and this is also studied.

The flow stability gives a numerical value to the stability of the powder during rigorous tests like the movement of the blade through the powder.

These are all important attributes to study in certain industries like the pharmaceutical industry.. Powder handling equipment also is designed according to these attributes. While making tablets, chemical powders are mixed at times and filled into the die They have to be of uniform weight and factors like humidity, moisture, have to be accounted for to determine the promised shelf life.

Compendial and non-compendial methods of powder measurement are used in the pharmaceutical industry to manufacture drugs that can meet international standards of acceptance.

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Need of Powder Flow Measurement Device

Rheology is the study of flow of materials under strain or in response to some applied force. Rheometers are instruments used to measure the way in which any material flows in response to applied forces. It is used for materials like powder, which cannot be defined by a single viscosity value, and therefore require more parameters to be set and measured. Hence a powder rheometer used for powder is called a powder rheometer.

There are two types of rheometers.  Rheometers that control the applied shear stress are called rotational or shear rheometers and those  that apply the extensional stress are called extensional rheometers.

The flowability of a powder can also be defined as the ability of a powder to flow in a desired manner within a specified piece of equipment. Hence any powder cannot be simply described as free flowing unless it refers to that piece of equipment it is within. The specific bulk characteristics of a powder that affect its flowability and that can be measured are called powder flow properties. Some of them are:

– Density
– Cohesive Strength
– Wall Friction

Such flow properties arise due to the action of collective forces like Van der Waals, electrostatic, friction and interlocking forces acting on individual particles. Flow properties should not be confused with flowability, as flow properties describe the powder alone and not in relation to the equipment carrying them. Flowability is a function of the flow properties and the powder handling equipment.

Bulk Density – is studied in order to study the impact of compaction or the compaction behavior of a powder when undergoing bulk transfer operations. There are many methods to measure bulk density incorporating various sized containers that are measured for volume after being loosely filled with a known mass of powder and then measured again after tapping (Tapped density). These measurements, though, cannot predict the behavior of the powder during actual compaction and bulk transfer operations. A study of the degree to which a powder compacts as a function of applied pressure with the results plotted on a graph would give a better estimation of the way a powder would react to bulk transfer operations.

An example of powder measurement is the study of cellulose powder mixed with water, using a mixer torque rheometer.  As the mixture becomes wetter, the torque increases upto a point when it becomes saturated and then further the torque reduces as the mixture becomes slurry.

Such studies and measurements have various implications in industry for storage, handling, blending, fluidization, transfer and other treatments carried out on powder material.

All About Powder Flow Properties And Powder Handling Equipment

Powders of various types are used in the pharmaceutical, cosmetics, construction and food industries among others. Hence, a thorough and in-depth understanding of powder characteristics, behavior and powder flow properties is crucial to the smooth functioning of these industries. Profitability, cost-effectiveness, efficiency and quality-control are all affected by the way in which powders are handled during the manufacturing process in these industries.

What Are Powders?

Powders are basically dry, bulk granular materials which exist in the form of numerous particles. These fine particles have the capability to flow freely, somewhat similar to liquids, when shaken or tilted. The grain sizes of powders may differ and this also has an impact on their ability to clump together while flowing. Finer grain sizes tend to clump naturally, while larger ones clump usually when wet.

Typical examples of powders include talcum-powders and cosmetics, flours, sugar, coffee, gun-powder, powdered grains, pharmaceutical powders etc. Naturally-occurring powders are found in the form of volcanic ash, dust, sand, snow, etc.

Powder Characteristics

Powders can be compacted and loosened into different ranges of bulk density unlike larger grained materials. Bulk density refers to the weight of a unit volume of loose powder to the same volume of water. Powders behave differently based on the force they’re subjected to. For instance, sprinkling fine powder over a surface gives a light and fluffy appearance, while compressing it makes it more dense and less able to flow. Segregation, jamming or unjamming, fragility, frictional-shearing etc are some of the other characteristics.

Powder behavior is generally quite varied, and depends largely on many factors including composition, viscosity, moisture content, etc.

Why Is Knowledge of Powder Flow Important?

Knowledge and analysis of powder flow properties and selecting the right powder handling equipment is a keystone in any manufacturing process that uses powders as raw material. Powders behave differently based on their particle-size. They’re able to form piles and clumps because of the shape of each individual particle and the angle of repose relative to the surface on which they’re placed.

A powder rheometer can be used for powder measurement and powder flow measurement. The commonly used methods for testing powder flow are:

• Angle of repose
• Compressibility index (Hausner ratio)
• Flow rate through orifice
• Shear cell

Powder Flow Property Tests

Properties like cohesive strength, wall/internal friction, compressibility, permeability, loose and tap density, chute angles etc of the equipment are also taken into account.

Pressure, temperature, humidity and ambient atmosphere are other important aspects that have to be controlled to ensure smooth flow of powders.

These are critical factors used to design the powder-handling system most suited to the powder characteristics and the manufacturing process involved.

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Features Of Metal Powder Flow Instrument

One of the most interesting questions posed about materials in nature is the nature of sand.  Is sand a solid? Is it a liquid? Is it even a fluid? It could be a solid, because it has solid particles. It could be a liquid because it flows like one. It could be a fluid because of the same property. But the fact is what you see is not what you get as far as sand or any other granular material is concerned.

Granular material is a collection of microscopic particles that look like they have the property of liquids or fluids, being able to flow easily and offering no resistance to pressure, but the fact is that they behave differently and are primarily characterized by a loss of frictional energy when they rub against each other and the side of the container, as they flow.

There are other observations such as the smaller the particles, the more difficult their gravitational flow is to predict, unlike larger particles. Hence it is the smaller particles that determine how the powder will flow.

Another observation made is how they behave after a period of resting. How is their flow then?

They behave differently in different types of flows too, which may be mass flow and core or funnel flow. These observations are important, because they are real occurrences in high volume manufacturing processes using powders.

Hence there are instruments called flow instruments or flow measurement devices to analyze the changes in the rate of flow of granular materials, by carefully studying the effect of all these factors and those of humidity, temperature etc. on powder flows. This finds several applications in the food industry and the pharmaceutical industries. This study is called granular mechanics.

A powder flowability tester is one such device. It can predict on a small scale what a powder would actually do say in a hopper during an important manufacturing process. Then the necessary adjustments can be made to deal with the situation on a macro level. It can give a comparison and ranking of flow functions of different powders.

A powder flow tester consists of a trough which is filled with the powder. A shear cell technique is employed to compress and then shear the material until it flows against its self. Using the software provided, the user selects the type of test they wish to run. Typically it will be a flow function test. During this fully automated flow function test, a curve is generated showing how cohesive the powder has become. The other tests are:

• Time consolidation test,
• Bulk density test
• Wall friction test

Thus Powder Flowability Tester take the guess work out of anticipating how different powders will behave in a manufacturing process, and thus avoid potential stoppages and downtime. All their tests provide observations according to which powder handling equipment on a large scale can be designed.

How Can the Bulk Density of A Dry Semi-Solid Material Be Determined?

Bulk density is a property of any granular material, including powders, which is not intrinsic to the material. It can change depending on how the material is handled. Bulk density gives the powder or granular material the driving force for gravity flow.

Powders are granular but their individual particles are very tiny, changing their properties from those of other granular materials where the grains are bigger. Powders are considered to be dry semi-solid material.

Powder flow properties like clumping when pouring, frictional shearing, compaction, segregation, loss of kinetic energy, stratification, jamming and unassuming are various behaviors exhibited by powders.  The study of all these behaviors affecting the flow of the powders is done with the help of machines called powder rheometers.

Important powder measurements taken by the rheometer

1. The powder’s resistance to flow – This is measured while the powder is in motion
2. The powder’s shear strength – which it measures with a shear cell. Shear strength is of a powder is its inherent quality which resists any deformation of its bulk.
3. The powder’s wall friction – which determines the resistance to slip on contact with the wall of the powder handling equipment.

Bulk properties such as the following are also calculated by the rheometer

1. Density
2. Compressibility
3. Permeability

Measurement of Bulk Density without rheometer

The bulk density / tapped density can also be measured manually without a rheometer, by using the formula density = mass/volume, or in pounds per cubic foot.

The easiest way is to find the weight of one cubic foot of the powder, so that the denominator in the above equation is 1. It would be difficult to get such a sample container. So using a smaller sample container, whose volume when multiplied by a correction factor, will bring it up to one cubic foot is possible.

One cc of pure distilled water is considered to weigh one gram. Using this relation and measuring the weights of the empty sample container and then with one full measure of water,  we can find out the volume of water in one measure of the sample container in cubic feet. (Using conversion tables for cc->cubic feet and grams->ounce)

When we take the reciprocal of this quantity, it gives us the number of times that the container should be filled to make up one cubic foot. This is the correction factor.

Next, we take the weight of one full measure of the  powder sample in the same container. We must remember to subtract the weight of the container to get the weight of the sample alone. We multiply this by the correction factor. This will give us the density in ounces per cubic foot. If we divide this value by 16, we will get the density in pounds per cubic foot.

Two measurements of densities are calculated, one being ‘freely settled’ density (after flowing freely) and ‘tapped’ density, (into which it settles after compaction, usually achieved by tapping the container). When calculating the inventory of any powder or granular material, the average of these two measures of density is used.

Know About Powder Flow Properties And Powder Handling Equipment

Particulate matters are dry, bulk solids that can flow when subjected to shaking, tilting, pressure etc. Finer grain sizes are called powders, while larger and coarser ones are known as granular materials. Powders tend to clump or cake because of their finer texture unlike granular materials which would do so only when they’re wet.

Powder Flow Properties

It’s important to know about powder flow properties / electrostatic properties since it helps in designing powder handling equipment. This helps to eliminate problems like flow obstructions, blocks, separation/segregation, irregular flow, flooding, caking, etc. In the pharmaceutical, food and cosmetics industries, different powders of varying size and composition are blended and it’s crucial to have the correct quantitative information regarding their flowability. This information helps in monitoring and regulating powder flow and in ensuring quality control systems for the final product.

One of the issue with powders is that they may not discharge correctly from bins, silos, hoppers etc into packaging, feeders, dosage machines etc. causing interruptions in the work-schedule and production targets.

Powders can be classified according to two broad distinguishing characteristics:

• Primary or fundamental characteristics that concern their basic material properties, particulate size, shape and surface area
• Secondary or derived characteristics that arise from behavioral properties like flow, bulk, trapped density, compactibility, lubricity etc

Other important properties of powders include:

• Powders have a very high surface to volume ratio.
• Unlike liquids, powder have a structure that allows them to form piles.
• Unlike liquids, powders do not change their flow behavior when subjected to tilting or variable shear rates.
• Pressure controls the strength of powders and they flow less easily when subjected to pressure.
• Powder flow depends on friction and wall-friction angle (inside hopper walls)
• Particle size, distribution, shape, chemical composition, moisture and temperature easily affect powder flow.
• Can be compacted or loosened into different bulk densities.
• Powders can be dispersed by air, water, sprinkling, dusting, application of force.
• They may increase in volume with addition of moisture or lose volume when dry.

Measurement and Handling Equipment

Powder measurement depends on the shape, size, material characteristics of the powder and the vessel it is contained in, filling and discharge rates, bulk density variations, pollutants etc

A rheometer is an instrument used to measure the flowability of liquids, powders or slurry in response to applied forces. There are two types: Rotational meters that control applied shear stress and Extensional meters that apply extensional stress.

Tapped density refers to the reduced density of powders when tap forces are applied, forcing the individual grains to get more compacted. Measuring it is important while packaging powders.

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About Best Powder Flow Instruments or Devices

The use of powders in the pharmaceutical, food and cosmetics industry has created the need for special instruments or devices like powder handling equipment, flow measurement devices, flow instruments etc. These devices ensure that granular material or material in powder form flow in the right way and that the exact formulations that are required can be transported and conveyed.

Flow Properties

Those in the pharmaceutical, cosmetics, agricultural, food and cosmetics industry etc require a thorough understanding of the flow properties of bulk materials. This ensures that staff and technicians can quickly identify and locate the cause of poor flow, flooding, segregation or lack of product uniformity. Hence flowability tester devices are used to ensure proper flow.

Understanding flow properties is also important when designing and installing new silos, bins, hoppers, stock-piles, chutes, feeders, conveyors and other materials handling equipments. If this is absent there could be problems in ensuring that:

– Powder flows smoothly and consistently into packaging containers
– Powders flow accurately, evenly and repetitively into tableting dies so that the correct dosage of drug is filled,
– It does not cake or granulate during flow to ensure that products like face-powder remain in the right consistency,
– The particle size remains the same during mixing, blending or handling,
– Remain in the same consistency if it has to/should not fluidize, should not form clumps when flowing out of silos, etc.

Flow Property Instruments or Devices

There are several different methods of measuring flowability. The units of measurement for flow of liquids and gases are the same, such as liters/kilograms per second related to the powder density. Different flow measurement devices may be mechanical, pressure based or optical flow meters, open channel flow measurement devices, thermal mass flow meters, vortex flow meters or ultrasonic, electromagnetic or coriolis flow meters. Modern devices use laser doppler flow measurement.

It’s important that all the powder handling equipments are correctly calibrated and the settings are appropriately modified to suit the kind of material being handled.

Important Variables and Factors Affecting Flow

Granular materials, fine powders, powders that tend to clump or cake must be handled differently and the flow rate has to be determined according to their consistency. Flow rate may vary according to the particle or the process. Often, the powder must flow through an orifice or a container to reach the die or packaging. It’s crucial to ensure smooth flow through these, otherwise the entire product could be rejected at quality control.

This is where powder flowability testers have a vital role to play. Constant and regular testing ensures the uninterrupted flow of powders and keeps the manufacturing process functioning correctly.

Types of Flow Measuring Instruments and Its Features

The market offers a wide range of flow instruments which could be used to measure the accurate flow of gases and liquids. These measuring instruments could be manipulated to control the flow of steam although industrial plants tend to measure liquid flow more commonly.

The flow rate in any flow measurement device is determined primarily by the change in liquid velocity or kinetic energy. These devices are also known as flowability tester which could be categorized as:

1) Flumes
2) Weirs

Choice of Device

The choice of device depends on the desired minimum and maximum flow rates in the specific area. Weirs are designed using aluminum or fiberglass built across an open channel with the liquid flowing through. This device is widely used for its simple structure and easy maintenance besides being more affordable than other options.

Flumes alter the channel area readily as there are specific channel restrictions; the alteration causes the fluid level and velocity to increase.

Secondary devices are utilized with the primary devices to measure fluid level variations. These include the capacitance probe and floats. The former device determines liquid variations depending on electrical conductivity while floats function similarly at a more affordable price besides being more accessible.

More accurate devices are available today due to the progress in technologies. Ultrasonic sensors are growing in demand in measuring liquid flow more accurately.

Types of Device

1) Velocity Flow Meters

This type of flow measurement device is designed to measure the flow speed of gas and liquid rather than granular material in various parts of a system such as household applications.

2) Mass Flow Meter

The mass flow meter could measure granular material mass flow without obstruction to the flow path. These devices are available as coriolis or thermal flow meter.

3) Positive Displacement Flow Meter

These flow meters with rotors are not powder flowability testers as they measure liquid flow in high precision. The strong liquid flow turns the rotors whose speed is measured with the quantum of liquid passing at every rotor turn.

4) Differential Pressure Flow Meters
This differential pressure flow meter computes the flow based on the liquid pressure drop when the liquid passes through the inserted obstruction. This is a highly accurate measurement for high pressure systems. It is not a powder flow tester but for air and gases.

Conclusion

Besides liquid flow measuring instruments, companies in the production and supply industries may wish to purchase powder handling equipment and powder flow tester devices where granular material could be measured precisely before it is converted into liquid or gas forms.

The types of processes involved in the production assembly depend on the area of specialization by the company to supply the right types of end products to the market.

Importance of Powder Flowability Tester

Reputed analytical contract service laboratories in the market could indulge in various granular material productions with a specialization in material testing and powder size requests from clients of various industries. One essential service offered is powder flowability testing using a reliable and precise powder flow measurement device known as a flowability tester.

Role and Objective

The preferred powder flowability tester is used to ensure the quality and accuracy of the powder to be processed into the required end products requested by customers. A good powder flow tester could have characteristics directly related to the physical material properties and specific processing conditions that are required for an effective and efficient handling system.

The powder flow tester is instrumental in handling this complex processing system that monitors the precise flow of granular material powder in terms of particle size, distribution, surface area and density.

Technicians understand the fluid changes of powder flow properties when the material is moved between environments. Changes in environmental conditions at any stage of the process could affect the powder flow critically. These changes include temperature, humidity, time and electrostatic charges which require well calibrated powder handling equipment in every stage of the production process.

Moreover, multiple testing conditions must be exercised to analyze the flow of powder in any processing environment.

Industry Approved Testers

Good flow instruments must be industry approved according to the stringent standards set to ensure the expected outcomes. A dynamic flow measurement device that is appropriate as an approved powder flow tester like the FT4 Powder Rheometer must be a universal powder tester for measuring bulk and shear granular material and wall friction measurements.

Different testing techniques of powder measurement permit the testing of powder in various states; namely, static, dynamic or under stress. The appropriate flowability tester must be apt in measuring the required energy to produce the required flow conditions.

Powder flowability is highly influenced by air flow or compacting the powder sample in a container. Established powder handling equipment manipulates reliable testing methodologies incorporating proven techniques for accurate evaluations while controlling various conditions during the process.

The inclusion of a dynamic powder flowability tester in the measurement procedures helps reduce the handling factor caused by the operator. This brings on better results which are more accurate.

A standard powder flowability process would have the granular material tested with graphical and numerical results generated for further analysis. The reports would reveal the flow characteristics of the powder in aerated, conditioned and consolidated packing states which offer greater insight in the flow of powder in various processing environments.

The testing includes checking on the stability, variable flow rate, aeration and de-aeration, compaction, permeability and compressibility of the powder to form desired end products.

What Exactly Are Powder Handling Equipment?

The manufacturing industry is likely to indulge in many chemicals which require powerful powder handling equipment and versatile flow tools that are capable of controlling the flow and volume of granular material required in batch packing for sales and market distribution.

Sophisticated Innovation

Many dynamic manufacturers in chemical-related industries require innovative flow tools which include precise powder rheometer and versatile powder flow tester to handle granule packing and production.

An innovative chemical manufacturer manipulates the best of powder handling equipment such as the Powder Pump which is a dynamic apparatus for conveying granules or powder from equipment to equipment via dense phase flow principle. There are proven concepts using state-of-the-art technologies that enhance powder handling processes.

Advanced technologies could boost further innovation with customized flow tools that could move the granules more smoothly and efficiently while maintaining product integrity and safety during operations. Industry users need to define their application specs precisely especially on their preferred starting point and ending point for an innovative powder flow tester to be designed and integrated into a dynamic solution that satisfies business goals and objectives.

Available Market Options

The market offers a host of powder handling equipment to handle or control granular material flow rate effectively and efficiently. There is the Granular material flow meter which is a semi-automated measuring tool on the flow rate of powder via apertures.

The principle is very simple with a quick measure of the granular material mass flow in accurate precision via various diameter holes. The flow rate is analyzed to determine the optimal flow diameter for the specific granular material.

There are dynamic turnkey engineered systems for effective powder (powder density) handling using standard modular solutions which could be installed in a plant efficiently. Custom powder handling equipment could be manufactured to fit the plant’s specific process needs.

Professional Manufacturers and Support

As the demand grows for sophisticated powder flow tester systems, qualified and skilled manufacturers with a strong track record are desired to provide the best of flow tools and granular material handling facilitation.

Such manufacturers and support services must be certified to execute the apt Factory and Site Acceptance Tests on any powder handling equipment before routine processing begins operation.

These manufacturers would have sufficient replacement components and spare parts for minimal downtime of operation as skilled service technicians and engineers are always available on call through the nation. This ensures prompt maintenance and repairs where necessary to maintain the smooth operations of the plant.

Prompt and skilled technical support is assured from reputed flow tools manufacturers and experienced support service providers in the installation of the powder handling process. There would always be the essential testing options to confirm the performance of specific process components from powder rheometer to powder flow tester.