AMETEK Brookfield Page 53
Manual No. M09-1200-F1016
Appendix A: Overview of Powder Flow Properties
Problems with Powder that relate to Gravity Flow Behavior
A typical industrial powder processing line will include several storage vessels (e.g. bins, bunkers,
silos, hoppers, Intermediate Bulk Containers (IBCs), sacks etc), feeding or handling steps (e.g.
belt conveyor, screw conveyor, pneumatic conveyor, gravity chutes etc) and processing steps
(e.g. milling, mixing, drying, bagging etc). A major industrial problem is getting the powder to
discharge reliably from storage into the next process step. Therefore to understand the application
of powder flow measurements, it is useful to have some background knowledge of the flow patterns
and flow obstructions that can occur inside the storage vessels on a processing line.
What are the powder flow patterns that can occur in a process storage vessel?
Principally there are two different flow patterns that can occur:
Core-flow
(shown in Figure A-1a) can be considered the default flow pattern and is characterized
by powder discharge through a preferential flow channel above the draw down point of the outlet.
Powder is drawn into the flow channel from the top free surface of the inventory. This gives a
first-in last-out discharge regime and, if operated on a continuous (rather than batch) mode, the
powder around walls in the lower section will remain static in the vessel until the time that it is
drained down to empty.
Mass-flow
(shown in Figure A-1b) is the desirable flow pattern for powders that are poor flowing
or time sensitive, but must be specifically designed for. Here the entire contents of the vessel
are ‘live’, giving a first-in first-out discharge regime. To achieve this, the hopper walls must
be sufficiently steep and smooth. For a given wall material/converging angle, the powder wall
friction must be below a critical value. Also, the product discharge must be controlled by a valve
or feeder that allows powder to flow through the entire cross sectional area of the outlet. (It is
this final point that prevents many vessels from operating in mass-flow.)
A wall friction test will be able to give an approximate assessment of whether a given hopper
geometry will support mass-flow (with the proviso that the outlet area is fully active). For an
exact calculation of the maximum mass-flow hopper half angle, both wall friction and flow
function tests must be undertaken.
What are the powder obstructions that can occur to prevent flow?
Principally there are two different flow obstructions that can occur:
‘Rat-holing’
(shown in Figure A-2a) is the principle flow obstruction in a core-flow vessel, where
the powder in the flow-channel above the outlet discharges leaving a stable internal structure.
‘Arching’
(shown in Figure A-2b) is the flow obstruction in a mass-flow vessel, where a stable
powder arch forms across the outlet or converging walls of the hopper, thereby preventing
flow.
For a given powder there is a critical outlet dimension that must be exceeded to ensure reliable
discharge of a core-flow or mass-flow vessel. These are the critical rat-hole diameter
D
RH
and
the critical arching diameter D
c
or D
p
(depending on the hopper geometry). The Brookfield
Powder Flow Tester (PFT) can calculate these critical dimensions following a flow function
