NON FREE FLOWING MATERIALS:
Problems, Prevention and Solutions
Prof Mike Bradley
Director
The Wolfson Centre for Bulk Solids Handling Technology
University of Greenwich
Contents
• Why are some materials free flowing and
others not?
• The nature of non free flowing bulk materials
• Role of material characterisation in design
• How to use characterisation economically
• Other risks
• Interfacing
Effect of FLOW PROBLEMS?
Risks the plant
• Not working at all
• Not achieving throughput
• Not achieving satisfactory “up-time”
• Not delivering the product quality required
• Requiring excessive maintenance/troubleshooting
• Requiring excessive manning
Consequences
• Financial loss
• Loss of customer confidence
• Loss of staff morale
How well do solids processing
plants work?
• The Rand Report (1990)
•60%
of solids processing plants
never achieve satisfactory
operation!
How high is technical risk with
solids processing plants?
• The Rand Report (1990)
• Average cost over-run on novel
• solids processing systems is
110%!
• I.e. plants cost on average more than twice the
original estimate on which the business case for
building them was based!
Why is a material not free flowing?
Cohesive/adhesive
• Wet materials
• Very fine materials
“Nesting”
• Extreme shape particles that
“entangle”
• Mostly biomass and waste
materials
• Need special treatment – see
2012 and 2013 presentations
by Bradley
Examples of non free flowing bulk solids
Cohesive/adhesive
• Wet materials - examples:
– Clay, sand, soil
– Coal with fines
• Fine powders - examples
– Cement or gypsum powder
– Fine lime
• Fine and wet examples:
• FGD gypsum,
“Nesting”
•
•
•
•
Chopped straw
Wood chips
Flake materials
Waste containing shredded
sheet
1826
Flow problems
3036
Flow problems
1827
Discharge pattern: Core-Flow
Most common in silos and hoppers
• Central discharge channel
• Material flows from top
surface into central flow
channel
• Static or “dead” regions of
product
• “First in last out” discharge, so
gives poor stock rotation
• Erratic discharge caused by
product on product shear
during emptying
• Exaggerates segregation
effects of particles
• Hopper half angle shallow
Flow from Top
of
Static
Flow
fromMaterial
top of
material
Static
Material
Discharge
Through
Central
“Core”
Discharge pattern: Mass-Flow
Achieved only in specially-engineered hoppers
•
•
•
•
•
Steep hopper walls
All storage capacity is “live”
“First in, first out” discharge
Consistent discharge rate
encouraged by the reduced
levels of shear generated as
the product discharges against
relatively smooth wall material
- not static product
Degree of remixing during
discharge minimises
segregation effects
All Material in
Motion During
Discharge
Sliding on
Wall of
Converging
Section
Determination of the Mass-flow Limit –Conical
Hoppers
50
θ
B
Angle of wall friction φ w [deg]
delta j =30 deg
delta j =40 deg
40
delta j =50 deg
delta j =60 deg
30
delta j =70 deg
Core-flow
Mass-flow
20
10
0
0
10
20
30
40
Hopper half angle θ [deg]
50
60
Flow Stoppages Mechanical Arching
Problem for very large particles if outlets under sized (Outlet dim
>12x max pellet dimension
Flow Stoppages Cohesive Arching
Problem with cohesive or “nesting” materials
Flow Stoppages ‘Rat-hole’
Problem with cohesive or nesting materials
Ash
Baghouse
• Manual “therapy” to
persuade flow
• Manual digging out
of half a tonne of
ash from each of
eight hoppers
• Downtime of 40 MW
power plant = 0.5%
= £88,000 p.a.!
0000
Caking of silo contents
Some materials SET HARD in time –
special attention required
Why is technical risk so high with
solids handling equipment?
Why do so many plants fail to satisfy?
 Over-emphasis on capital cost
 Limited knowledge of particulate properties
 Reliance upon equipment designs or choices used
with “similar” products
 The “it’s always been done this way” approach
What is CHARACTERISATION and
how can it help?
The First Rule of Warfare:
KNOW YOUR ENEMY!
(Sun Tzu, China, 600 BC)



All bulk solids are very different
To get the material to do what you want it to,
you have to understand how it behaves and
reacts to what effects you apply to it
E.g. does it flow readily, segregate easily,
fluidise with air, go hard in store, pick up
moisture etc
The critical importance of
CHARACTERISATION
Often quoted properties:
Angle of repose
 Median particle size

Neither of the above are of
any real use;
– They do not relate to
flow properties of the
material
– Misleading at best
Information required on:
Bulk density
 Internal flow properties
 Wall friction
 Time dependency
 Particle size distribution
 Segregability, Friability
 Caking tendencies
 Susceptibility to moisture,
reaction, storage
temperature etc.
 Pneumatic conveying
properties


etc
Example of use of handling
properties for system design
HOPPER DESIGN
 Objective: reliable discharge
(Failure of silos and hoppers to discharge
reliably is one of the most common practical
failures on bulk solids handling plants)
What is bulk solid
“FLOWABILITY” and how is it
measured?
0000
“Flow function” of a bulk solid
Idealised Flow Function Test
Unconfined failure test
σ1
σ1=σc
Diagonal
failure plane
σ3=0
1. Consolidation
stage
2. Failure stage
Broken sample
3041
Flow functions for different products
3044
Wall friction tester
1835
Wall friction measurement
1802
Mass flow limits
Brookfield Powder Flow Tester
• Economically priced
~ £12,000
• Software controlled
• 4 basic tests
• 2 – 30 minute test time
• Powder volume
required
– 263cc standard cell
– 30cc pharma cell
– “Micro” cell under
development
Brookfield Shear Cell
Wall friction lid
Trough filling tools
Flow function lid
Trough
Standard cell lid and trough, 150mm od
3041
Flow functions for different products
Very cohesive
Cohesive
Free-flowing
Diagrammatic representation of the “flow, no-flow” criterion in a hopper
3053
Flow / No Flow criteria
3406
bmin
σ crit H (α )
=
ρb g
Where
bmin = smallest outlet size for reliable discharge
σcrit = critical stress in arch from flow / no flow criterion
H(α) = dimensionless factor accounting for shape of
converging section
= bulk density of bulk solid (kg/m3)
ρb
Calculation of hopper minimum outlet dimension
0000
Parameters determined by design procedure
1842
The importance of good interfacing
Good and bad practice in belt and screw interfacing
No hammer
marks!
Key characterisations

Hoppers and silos:
Arching/rat-hole dimension (from Flow Function)
 Wall friction
 Time dependency of above
 Caking tendency


Pneumatic conveying


Chutes


Impact adhesion
Impact adhesion and wall friction
Belt conveyors

Surface adhesion
Key to successful design
Understand the need for measurements of
the bulk solids properties
 Identify the properties which are likely to be
important in this application
 Bulk solids are all different!
 Measure properties (or get them measured)
 Use sound design procedure

Understand the contractual problem:


Property measurements cost money!
Who pays?



Many buyers look only at price



At tender stage, no money for measurements;
Once contract let, price fixed so no opportunity for
changing design!
But design without proper information will lead to
expensive problems!
Need to educate buyer of value of proper design
Cost of bulk solid property measurements through


Separate contract for design
Factor costs of measurement into tender
Conclusions (I)
• All bulk solids are unique
• Success must be based on knowing the
behaviour of the material
• Need for knowledge of bulk solid
behaviour
Conclusions (II)
• Many solids handling projects have
substantial problems
• Reasons are linked to buyer’s
procurement process and lack of
technical judgement
• The above means even good suppliers are
often constrained to tender poor designs
Means of minimising technical risk
Key point 1:
• Encourage the buyer to exercise close technical
judgement on suitability of design
Key point 2:
• Contract must ensure the study of behaviour of
the materials to be handled – and allow the final
spec to take account of this
3017
Free flowing materials – easy (???)
THE WOLFSON CENTRE
for Bulk Solids Handling Technology
University of Greenwich
www.bulksolids.com