The Safety Effects of Erratic Flow Behavior in Bulk Solids
Greg Mehos, PhD and Eric Maynard, MSME, Jenike & Johanson, Inc.

Flow Problems

Compared to chemical processes that handle liquids or gases, those that involve bulk solids can present a number of unique hazards.

Dangers range from poorly designed storage vessels that suddenly collapse to dust explosions and fires.  If solids are reactive, erratic discharge or, in extreme cases, no flow at all can cause downstream processes to become uncontrollable.  Even materials that are considered benign and non-reactive can be dangerous, should they become dispersed in air.

The most prevalent cause of problems in solids processing plants is the improper design of bulk solids handling equipment, specifically, bins, hoppers, and feeders.  These problems can be accentuated when the units are used to heat, cool, or condition a bulk material.  Solids handling problems that can lead to hazardous process conditions include:

Segregation:  Mixtures composed of fluids tend to remain homogeneous.  Because concentration differences in gases or liquids provide a driving force for diffusion, any heterogeneity is apt to disappear.  On the other hand, solid mixtures are prone to segregation due to differences in particle size, shape, or density. (see photo above)

For example, sifting occurs when smaller particles move through a mixture of larger particles, causing fines to accumulate in the center of a storage vessel and coarser material to roll towards the periphery, as shown in the picture to the right.  As a consequence, potentially unsafe operating conditions may result since (unless zero order) the reaction kinetics can depend on the specific surface area of the reactive components.  If the reaction is exothermic and the rate of heat generation becomes greater than its rate of removal, a runaway reaction may take place.

Dusting:  If a sufficient amount of material becomes dispersed in air, a sequence of events leading to a violent explosion or flash fire can result.  Should the bulk material’s minimum explosible concentration (MEC) become exceeded and if a source of ignition is present, combustion can occur rapidly, liberating heat and releasing gases.  The result is a sudden rise in temperature and pressure, i.e., a dust explosion.

Flooding:  Usually, a feeder such as a rotary valve, screw, or belt controls the discharge rate from a storage vessel.  When a fine powder becomes entrained in air, the solids can sometimes exhibit fluid-like behavior.  Because most feeders are designed to handle solids and not fluids, the result is uncontrollable discharge and generation of dust.  In extreme cases, the hopper may empty completely.

Non-uniform flow:  An improperly designed hopper or feeder can cause discharge rates to become unpredictable.  Flow may become erratic or even cease entirely.  If solids discharged from a hopper are reactive or act as a diluent, a disruption of flow can lead to unsafe operating conditions in downstream processes.  In addition, dryers and kilns rely on a steady flow of solids to maintain desired temperature profiles.  When manual intervention is required to reinitiate flow, contact between hazardous solids and operators is more likely.

The severity of segregation, the tendency of solids to become dispersed in air, and the ability to predict and control the discharge of bulk materials from a vessel depend on the flow pattern within the vessel.  Hence, an understanding of the materials’ fundamental flow properties and how they affect solids flow patterns is essential to be able to operate solids handling processes safely.

Vessel Flow Patterns

The flow pattern inside a vessel has a major influence on the frequency of flow problems.  In general, there are two types of flow: mass flow and funnel flow, which are illustrated below.

Mass Flow:  In a mass flow bin, the hopper is sufficiently steep and low enough in wall friction to cause flow of all the solids without stagnant regions whenever any solids are withdrawn.

In general, mass flow is preferred over funnel flow.  Flow is uniform, and the feed density is practically independent of the level of solids in the bin.  This frequently allows steady discharge rates even with volumetric feeders.

Since stagnant regions are eliminated, low level indicators work reliably.  Even though the solids may segregate by sifting at the point of charge into the bin, segregation of the discharge is minimized by the beneficial first-in-first-out sequence that also ensures uniform residence time and deaeration of fine powders.

Funnel Flow:  Funnel flow occurs when the hopper is not sufficiently steep and smooth to force material to slide along the walls.  Funnel flow may also be a consequence of an improperly designed feeder.

When a funnel flow pattern exists, solids flow toward the outlet through a channel that is surrounded by stagnant material.  When the bin discharge rate is greater than the fill rate, the level of solids within the channel drops, causing layers to slough off the top of the stagnant mass and fall into the channel.  If the solids are cohesive, they may pack upon impact and plug the outlet.

Under some circumstances, a rathole (to right) can form when the stagnant regions surrounding the flow channel remain for an extended period of time.  When fine powders are charged directly into the flow channel and are withdrawn at the same rate, they lack sufficient time to deaerate and will remain fluidized.  Flooding through the feeder becomes inevitable.

Preventing Solids Flow Problems

The best way to prevent flow problems is to ensure that the vessel from which bulk solids are discharged is a mass flow vessel.  In mass flow, all material is in motion, and hence ratholes do not form.  Since ratholes are eliminated, trouble-free operation of feeders is more likely, and the frequency of potentially dangerous dispersions of fine particles in air is drastically reduced.

To achieve mass flow, two criteria must be met: the walls of the hopper must be steep enough and wall friction must be low enough to allow solids to flow along the walls, and the hopper outlet must be large enough to prevent a stable arch from forming.  The hopper angle for a given wall material and outlet size and shape are determined by first testing the bulk solids and the wall material.

Wall Friction:  Wall friction is measured by following ASTM method D6128-00 in which a small sample of the bulk solid is placed in a cell and sheared along a coupon of wall material.  A variety of weights are applied to the solids, and the shear force required to move them along the coupon is measured for each normal load.  A procedure described by Jenike is then used to determine the hopper angle that ensures that the bulk solid will slide along the wall and allow mass flow.  A number of factors influence angle of wall friction, such as wall material, time at rest, corrosion, moisture content, and temperature.

Outlet Dimension:  Cohesive arching results whenever the cohesive strength of the bulk material that develops in a hopper is greater than the stresses imparted onto it at the outlet.  Cohesive strength is measured using a Jenike direct shear cell tester following the ASTM method referenced earlier.  Once the cohesive strength of the bulk material is known, minimum outlet dimensions that prevent formation of a cohesive arch can be determined using Jenike’s proven procedure. 

As with wall friction, a number of similar factors affect the minimum outlet dimension required to prevent cohesive arching.  When designing storage vessels for materials with a wide range of physical characteristics, it is wise to determine the worst-case circumstances and perform testing under those conditions.

Concluding Remarks

Chemical process engineers possess the education and experience to safely handle liquids and gases.  Their training on the fundamental principles that describe the flow of bulk solids, however, is sometimes lacking in comparison.  By knowing the flow properties of a bulk material and understanding how equipment design affects flow patterns, processes that involve solids can be implemented to prevent dust explosions, feed starvations, and runaway reactions.

Contact our authors

If you have any questions regarding this article or any other bulk solids handling or conveying need, please contact our authors:

Greg Mehos, PhD, PE
Project Engineer
gregmehos@jenike.com

Eric Maynard, MSME
Senior Project Engineer/Education Coordinator
epmaynard@jenike.com

Jenike & Johanson, Inc.
400 Business Park Drive
Tyngsborough, MA  01879
Phone: (978) 649-3300
Fax: (978) 649-3399
Web site:  http://www.jenike.com/

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