AMETEK Brookfield Page 59
Manual No. M09-1200-F1016
Outputs of the wall friction test
While the results of the wall friction test can be displayed graphically in the form of a wall friction
locus as shown in Figure A-8a (representing the limiting shear stress the powder can support
at a wall), or the form of a wall friction angle function as shown in Figure A-8b (representing
how the wall friction angle changes with reducing stress), one of the following four flow indices
derived from the maximum wall friction locus are usually adequate. These wall friction properties
are:
•
θ
c
,
θ
p
The maximum mass-flow hopper half angle (measured to the vertical) for conical or
planar hoppers.
•
ϕ
w
The maximum wall friction angle to determine the minimum chute angle for gravity
flow (see Figure A-8b).
•
Grad
The maximum wall friction angle displayed as a coefficient.
•
c
w
The wall cohesion shear stress in kPa that can be supported at the wall under zero
normal stress (see Figure A-8a). This determines the ‘stickiness’, i.e. whether powder is
likely to stick to wall surface under close to zero stress. i.e. will powder build up on the
walls of the chutes around discharge/transfer points.
Normal Stress
σ
N
Wall Friction Locus (WFL)
Shear str
ess
8
τ
c
w
W
all fr
ic
tion angle
ϕ
w
Normal Stress
σ
N
ϕ
w Maximum wall friction angle
90
0
Figure A-8a: Wall friction locus
Figure A-8b: Wall friction function
An extended wall friction test allows the wall sample to be subject to large shear displacements
(on the order of 30 meters) to establish whether long term powder build up on the wall would
be expected.
Bulk density test
It is the self-weight of the powder, its bulk density, that controls the stresses acting on the powder
when flowing or when static in processing lines/ silos etc. The bulk density is measured during
the course of the flow function test (and is required to calculate the critical outlet dimensions)
and the wall friction test, but it can also be measured in a separate single test for bulk density
alone.
Compacted bulk
density (
ρ
comp
)
