What is an air cannon?
An air cannon is a pressure vessel discharged through a fast valve, sending a powerful blast of air into the material. Used where vibration is not enough, typically against firm deposits on the wall and against bridges in large silos. OLI's series is called PG. Two things come with it: the cannon is a pressure vessel and must therefore be held up against the Pressure Equipment Directive, and it is loud, rated in the catalogue at 105 dB(A).
Also called: air cannon, air blaster, air knocker, air jet
What is the angle of repose?
The angle of repose is the angle a free heap of the material makes with the horizontal. One of the eight measurements in the Carr indices under ASTM D6393. Together with compressibility it is the quickest screening indicator of whether a powder will arch. It is easy to measure and therefore often used on its own, but it says nothing about cohesive strength after storage, and there the Jenike test is indispensable.
Also called: angle of repose
What is bridging?
Bridging is when the material forms an arch across the outlet and carries its own weight, so the silo is full but nothing comes out. The bridge may be mechanical, where particles wedge against one another in too small an opening, or cohesive, where the material has gained enough strength to stand on its own. The two have different causes and different remedies, which is why the same vibrator does not help everywhere.
Also called: bridging, arching, bridge, arch
What is bulk density?
Bulk density is the weight of the material per unit volume in the loose or packed state. It enters the sizing of any vibrator, because the centrifugal force required is bound up with the mass that has to be set in motion. The difference between loose and packed bulk density gives the compressibility, which is a direct indicator of arching risk.
Also called: bulk density, loose bulk density, packed bulk density
What is centrifugal force?
Centrifugal force is the force a rotary vibrator delivers, stated in kilograms or newtons in the manufacturer's data. Together with the frequency it is the main figure in sizing. A rule of thumb says the force should relate to the mass of material that has to be set in motion, not to the total contents of the silo. Excessive force is not merely wasteful; it feeds directly into the fatigue consideration EN 1991-4 requires.
Also called: centrifugal force, output force
What is cohesion?
Cohesion is the material's ability to hold together with itself, independently of external pressure. It is cohesion that lets a bridge stand. It grows with time under pressure, which is why a silo emptied every day can behave quite differently from the same silo after a weekend or a holiday. Measured in the Jenike test both during continuous flow and after storage.
Also called: cohesion, cohesive strength
What is EPL?
EPL, the Equipment Protection Level, states the risk of the equipment becoming an ignition source. For dust the levels are Da, Db and Dc, where Da is the highest protection and belongs to zone 20, Db to zone 21 and Dc to zone 22. It appears last in the marking, for example Ex h IIIC T125 C Db. It is another way of expressing the same thing as categories 1D, 2D and 3D, and the two systems often appear side by side on the same plate.
Also called: epl, equipment protection level, da, db, dc
What does Ex h mean?
Ex h means non-electrical explosion protection. The letter h on the nameplate covers protection types c, b and k defined in EN ISO 80079-37. For pneumatic vibrators it is almost always c, constructional safety, that is bearing clearances, material pairings that do not produce friction sparks, and control of surface temperature. It is at the same time the proof that a purely pneumatic vibrator is ATEX equipment in its own right and not merely an accessory.
Also called: ex h, non-electrical protection, constructional safety
What is fatigue?
Fatigue is degradation of the material under repeated alternating loads, at stresses far below what the structure can carry once. It is the central structural consequence of fitting a vibrator or an air cannon to a silo, and EN 1991-4 clause 3.4(9) names precisely vibrators, air cannons and gyrating live bottoms as sources that must enter the fatigue assessment. What is documented to happen is cracking and reduced fatigue life, not collapse.
Also called: fatigue, fatigue life, fatigue limit state
What is fluidisation?
Fluidisation is when a thin film of air is blown in along the wall so that the material lets go and begins to slide. OLI calls the product group aerators, that is I100 fluidisation pads and U fluidisation nozzles. Watch two things: the air must be dry, or it condenses and re-cakes the material, and on a fine powder fluidisation can trigger flushing rather than solve anything.
Also called: fluidisation, fluidization, aeration
Caveat: Note the Polish terminology. OLI's own line is napowietrzanie; the competitor CTMS uses aeracja. Both are in live use and split the search volume between them. The word fluidyzacja pulls towards fluid bed drying and therefore misses the target.
What is flushing?
Flushing is the opposite problem. A fine powder takes up air and behaves briefly like a liquid, so it runs uncontrollably out of the outlet and cannot be dosed. Cement is a documented example. Here more vibration or more air is precisely the wrong medicine, and fluidisation can bring the problem on directly. It is worth knowing, because the symptom is easily mistaken for a flow problem while the remedy is the opposite.
Also called: flushing, flooding, uncontrolled discharge
What is funnel flow?
Funnel flow is when only the material above the outlet moves, while the rest stands still along the wall. It is the cheapest hopper to build and the one most silos actually have. The price is dead zones, a risk of ratholing, and material that can stand long enough to harden or degrade. Most flow aids are sold into funnel flow silos, because that is where the problems arise.
Also called: funnel flow, core flow
What is mass flow?
Mass flow is when all the material in the silo moves at once during discharge. That gives first in, first out, no dead zones and no ratholes. In return it demands a steeper and smoother hopper, and the wall friction must be low enough. Mass flow is not free; it is a design choice that costs height and surface treatment. EN 1991-4 illustrates that air injection and air slides promote mass flow.
Also called: mass flow
What is ratholing?
Ratholing is a narrow tunnel running down through the material while the rest stands still along the wall. The outlet appears to work, but at a fraction of the capacity, and the oldest material never leaves the silo. It is a particularly awkward failure in food and pharmaceuticals, where it means first in rarely becomes first out. Ratholing is closely bound up with funnel flow.
Also called: ratholing, rathole, piping, core flow
What is segregation?
Segregation is when a mixture separates by particle size or density during filling or discharge, so the product no longer has the composition it had going in. Funnel flow makes it worse, because the material leaves the silo in a different order from the one it entered in. Vibration can in some cases also worsen segregation, and that is one of the few situations where more force does harm.
Also called: segregation, demixing
What do vpm and frequency mean?
vpm means vibrations per minute. High frequency with low amplitude works on fine, cohesive materials, while low frequency with high amplitude works on coarse and heavy ones. It is the main reason there are several vibrator families rather than just one in several sizes. On pneumatic vibrators the frequency changes with the air pressure, which makes them easier to adjust in service than electric units.
Also called: vpm, vibrations per minute, frequency
What is the wall friction angle?
The wall friction angle is the friction between the material and the hopper wall, measured against the specific wall material. Together with the hopper angle it decides whether you get mass flow or funnel flow, and it is the reason a polish or a different steel grade sometimes solves a flow problem without a single vibrator. Measured under ASTM D6128.
Also called: wall friction angle, wall friction
What are zones 20, 21 and 22?
Zones 20, 21 and 22 are the dust zones under Annex I of the ATEX workplace directive. Zone 20 is where an explosive dust atmosphere is present continuously or for long periods, typically inside the silo. Zone 21 is where it may occur occasionally in normal operation. Zone 22 is where it is not expected in normal operation, and then only briefly. The zone determines the category: zone 20 requires category 1, zone 21 category 1 or 2, zone 22 category 1, 2 or 3.
Also called: zone 20, zone 21, zone 22, area classification, dust zone