Before choosing - RO-BO Mechanical Workshop

WHAT  SHOULD  KNOW AND   CONSIDER   BEFORE   CHOOSING A  BLOW-THROUGH  ROTARY VALVE

The proper functioning of such a valve is subject to several factors that we will examine below .  First you need to think that the valve becomes part of a circuit under pressure which must convey the material between   two or more points. This circuit is composed of several parts that, to obtain good results,  must be harmonized with each other.  Simplifying to the maximum, we can say that the components can be :  a compressor that provides power,  a pipe which conveys the material, a tank for drawing, an inlet valve,  a receiving unit or storage .
To well begin the work,  you should know :
a) the physical and chemical properties of the material to be controlled;
b) the pressure of the stream ;
c) required degree of sealing;
d) any operations such as splitting the flow;
e) overall dimensional limits, if any.
Characterization of Powders and Granular Materials
The conveying and handling characteristics of a solid material are characterized by such factors as: chemical-physical properties; rheological properties; time.
VOIDAGE FRACTION
All bulk solid materials have spaces between the particles, filled with air. The percentage volume relative to the total volume not occupied by particles, the fractional free volume, is known as voidage fraction e . The percentage volume actually occupied by the solid is (1- e ).
The value of this parameter varies greatly from one material to another, and largely depends on the shape of the particles. For spheres of uniform diameter e = 0.48.
DENSITY
This is generally indicated by the symbol r ; the density is defined as the ratio of the product mass and the volume it occupies. Bulk and vibrated density are only two values of a range that varies as a function of consolidating pressure.
PARTICLE-SIZE DISTRIBUTION
The methods for expressing the dimensions of the particles depend on the measuring device employed; the most common one uses standard screens. The data obtained from the granulometric analysis are usually presented as integral distribution curve - in X-co-ordinate the diameter of the particles and in ordinate the materials percentage in weight.
SHAPE OF THE PARTICLES
The shape factor most widely adopted for classifying particles is known as the sphericity factor y ; for spherical particles y =1.
FLOWABILITY
Flowability is the more or less distinct tendency of solid particles constituting the material to flow:
- with respect to one another, evaluated in terms of internal friction angle;
- with respect to a surface, evaluated in terms of wall friction angle.
The internal friction angle indicates the tendency of the material to flow on itself in the presence of a compression force applied perpendicularly to the shearing stress direction; usually, it is evaluated using the Jenike's test, and depends on various factors:
- pressure to which the material is subjected;
- humidity content of the material;
- temperature: the flow of many plastic materials tends to be reduced when heated;
- particles size and shape.
The wall friction angle indicates the tendency of the material to flow along a wall in the presence of compression forces applied perpendicularly to the shearing stress direction; it is evaluated using the Jenike's test and, in addition to the factors described above, It also depends on:  wall surface, mainly in terms of degree of roughness.
The flowability of a material is also influenced by the time factor: when a material remains stored for a long time in a silo without being subjected to movement, when it is unloaded from the silo it is often much less free-flowing than would be expected.
COHESION AND AGGLOMERATION OF THE PARTICLES
Cohesion may be defined as the strenght exerted by a bulk solid material against a shearing stress in the absence of compression forces applied perpendicolarly to the shearing stress direction. This resistance is generated by a complex set of mechanisms; important factors that influence the behaviour of the material in this way are:
- humidity content: usually, cohesion increases as humidity increases;
- dimensions and shape of the particles: there is no direct correlation between dimension, shape and cohesion; in spite of this, it has been found in many materials that the smaller the dimensions of the particles, the higher the tendency to cohesion.
- the presence of oil which, depending on the quantity, tends to reduce or to increase the phenomenon of particles' cohesiveness.
The tendency to agglomerate is mainly due to particles interactions caused by electrostatic forces, by van der Waals forces and is strongly affected to the presence of water in the form of humidity, which may even lead to the formation of bridges between the particles.
ABRASIVENESS AND CORROSION
The abrasiveness of granular materials depends on the shape, dimensions, hardness (measured with the Mohs scale) and specific weight of particles.
Corrosion may be defined as the process through which a metal tends to degrade, passing from the elementary state to the state of oxidized material, caused by environment conditions. The tendency to corrosion of many metals is influenced by the pH of the substance with which it is in contact. Also temperature influences the corrosion process: an increase in temperature increases the speed of the phenomenon.
STATIC  ELECTRICITY
All substances are electrified to a greater or lesser degree when placed in contact with substances of a different nature; this phenomenon, known as tribo-electricity, is caused by the migration of a certain number of free electrons from one body to another during the contact. The electrified material thus acquires an energy state governed by the laws of static electricity. When the voltage of the electrified material exceeds a certain value, an electrical discharge may occur between the material and the nearest conductor having less electric potential. The discharge gives rise to the dissipation, in the form of heat, of most of the electro-static energy and such heat, released at an extremely high temperature may, in the presence of flammable gaseous mixtures, trigger very dangerous explosions or fires.
HYGROSCOPICITY
This is the tendency of a solid material to chemically or physically absorb on its surface the water vapour present in the air or in gaseous currents.
DECOMPOSITION AND CONTAMINATION
As regards decomposition, this property depends on the intrinsic biological characteristics of the product. It may be identified as the tendency to form mould, germs and bacteria that give rise to decomposition of the substance. The determining factor is the residence time.
As regards contamination, this property identifies the risk of contamination of the product due to fluids from the handling system's mechanical parts, or from contact with the handling system's surfaces that are not properly protected or finished.
DEGRADABILITY
This expression describes the risk of fragmenting and breaking the original granulometry of the material due to the conveyor's mechanical actions (i.e.: slipping). This effect causes an increase of the finest granulometric fractions, which may affect the production processes downstream.
VOLATILITY AND FLUIDITY
These terms describe the tendency of a solid material in granule or powder form to remain in suspension if struck by a gaseous flow. These properties can be evaluated by experimentally estimating the permeability of the material to the passage of a gaseous stream.
POWDERINESS
Powderiness is the macroscopic result of Brownian motion, which mainly concerns the materials finest fractions, determined by collision of the solid particles with the gas molecules which are in turn subject to disorderly motion of thermal agitation.
FLAMMABILITY AND EXPLOSIVENESS
A powdered material in dense form, if primed, may give rise to a fire.
If a cloud of powder is primed, an explosion may occur which, unlike a fire, is characterised by a sudden increase in pressure and by the instantaneous formation of intense heat.
The lower limit of flammability is defined as the minimum concentration of powder (g/m³) capable of exploding in air when provided with sufficient priming energy.
Handling materials in a controlled environment or in nitrogen represents a valid solution for reducing the risk of fire or explosion.
TEMPERATURE
In some processes it is necessary to handle products at high temperature; this condition influences both the choice of material of construction and the type of handling system to be adopted.
TOXICITY CHARACTERISTICS
Another factor which requires considerable attention is the toxicity of the vapour or gas that may be generated by the material; awareness of this characteristic may considerably influence the type of material handling system to be adopted, in order to ensure that the working environment is as safe as possible.
INTERACTION BETWEEN HANDLING PARTICLES AND THE WALL
The behaviour of a solid in granular or powder form with reference to the material that contains it may be characterized by several phenomena such as: the stratification of the particles on the walls of the circuit; the formation of a crust resistant to mechanical cleaning to varying extents; and the tendency of some materials to plasticize. These phenomena are subject to studies by the branch of science called rheology.

Components for Pneumatic Conveying Systems
In this type of conveying technology, the solid is carried by a gaseous stream, which imposes a pressure gradient along the conveyor line.

Depending on the materials chemical-physical properties (hygroscopic behaviour, pyrophoric behaviour, etc.), the conveying fluid may be air, de-humidified air or inert gas.

A pneumatic conveying system basically comprises the following elements:
- a machine for generating a gaseous flow;
- a feed system (Venturi-type ejector, rotary valves, powder pumps, etc.) that can insert the solid in the gaseous stream, while preventing passage of the gas into the storage equipment;
- a conveyor line, both the main conduct (the pipe) in which the material is carried, and the accessories (flanged couplings, curves, flow dividers etc.) of which it is made up;
- a solid-gas separation system (in the form of the classical centrifugal separators or sophisticated reverse jet bag filters type)

The most frequently used pneumatic solid movement systems are:
- negative-pressure system;
- positive-pressure system;
- combined positive- an negative-pressure system.

In negative-pressure systems the material moves in a gaseous stream with a pressure lower than atmospheric pressure. The advantages of this type of system are that all the pumping system energy is used for moving the product and that the product is sucked into the line without rotary valves or other sealing devices to prevent infiltration of gas at the pick-up point.
They are suitable for low rates of solids and for paths not excessively long, so as to avoid using machines with a high vacuum level. This type of movement is suitable for pneumatic circuits with pressure drops not greater than 300 mm Hg.
This type is used for picking up material from several points and feeding it to a single point.
It is ideal for toxic material, where even a single hole in the duct could lead to dangerous leaks.

In positive-pressure systems, the material falls through a rotary sealing valve into a positive pressure gaseous stream. The speed of the gaseous stream keeps the material in suspension until it reaches the apparatus provided to contain it, where it is separated from the carrying gas by a filter or a centrifugal separator.

Depending on the surface velocity of the conveying gas, two different flow regimes are possible, generally described as dense phase flow (solid mass/gas ratio higher than 10) or dilute phase flow (solid mass/gas ratio lower than 5).
Compared with the negative-pressure system, the pressure drops along the circuit may be higher; generally up to 1 bar for dilute phase flow and about 6 to 7 bar for dense phase.
Generally, dense phase flow (which requires a considerably lower gas flow rate) is much more efficient than the dilute phase; this means, however, that it is necessary to construct more expensive circuits, as the systems are working in pressure and require high pressure compressors.

The gas velocities in dilute phase are about 25 m/s, compared with 1 m/s for dense phase systems.

High velocities give rise to high degradation of the solids, due to collision between the particles and, above all, between the particles and the circuit walls; this may lead to considerable abrasion on the conveying line; therefore, it is often necessary to provide an anti-wear coating.

The combination of positive- and negative-pressure systems provides the best technological solution for many processes.

It is common practise, for example, to employ a vacuum system for unloading solids in powder form from trains, which cannot be pressurised, and convey the product to a filter fitted with a collector. Subsequently, a positive-pressure system can be adopted to convey the product from the collector filter to the storage point.

ADVANTAGES OF PNEUMATIC CONVEYING
The advantages of a pneumatic system compared with a mechanical conveyor system are:
- absence of dust dispersal in the environment;
- geometric flexibility of the conveyor system;
- simplicity of designing circuits that have several pick-up and unloading points;
- low maintenance costs.

DISADVANTAGES OF PNEUMATIC CONVEYING
- Makes it necessary to purchase a suitable machine, with high power consumption for achieving the necessary pressure gradient;
- solid conveying capacity lower than that of mechanical systems;
- limitated distance along which the product can be conveyed;
- possible wear along the circuit in case of dilute phase flow;
and - since complex fluid-dynamic phenomena are involved in pneumatic conveying systems, require very careful design of the conveyor circuit.

Designing a conveyor circuit is quite complex and requires a theoretic-experimental approach based mainly on the adoption of a pilot plant with which to carry out the flow tests. A positive-pressure conveying system, in dense or dilute phase, is generally selected by evaluating the characteristics of the material to be conveyed, and the head provided by the blowing machine (compressor or fan) .