In pneumatic conveying, the air molecules pass the material particles and when they touch, the combination of the 2 exchange electrons until electrostatic equilibration is reached.

When the air and material part from each other, the exchanged electrical charges remain also separated and leave the particles electro statically charged.

In the example of pvc-resin, the air becomes positively charged and the pvc-resin becomes negatively charged. (see tribo electric series)

This phenomenon is called the tribo-electro effect.

The electrostatic charge of a particle increases with the number of contacts between air molecules and the particles until the maximum possible charge exchange is reached.

The maximum possible charge exchange is limited by the ratio between product particles and air particles. (In fact this is the Solid Loading Ratio).

In the example of PVC-resin, the pvc particles collect electrons up to a maximum.

This maximum absorption of electrons depends on the availability of air molecules, supplying these electrons.

A low SLR implies that there are more air molecules available per product particle, resulting in a maximum charged voltage.

Electrostatic charging is therefore stronger at low SLR’s

Electrostatic charging:

Relative rate of electrostatic charging is represented by:

Triboelectric series:

Most positively charged

+

Air

Human skin

Leather

Rabbit’s fur

Glass

Quartz

Mica

Human hair

Nylon

Wool

Lead

Cat’s fur

Silk

Aluminum

Paper (Small positive charge)

Cotton (No charge)

0

Steel (No charge)

Wood (Small negative charge)

Lucite

Amber

Sealing wax

Acrylic

Polystyrene

Rubber balloon

Resins

Hard rubber

Nickel, Copper

Sulfur

Brass, Silver

Gold, Platinum

Acetate, Rayon

Synthetic rubber

Polyester

Styrene (Styrofoam)

Orlon

Plastic wrap

Polyurethane

Polyethylene (like Scotch tape)

Polypropylene

Vinyl (PVC)

Silicon

Teflon

Silicone rubber

Ebonite

−

Most negatively charged

Because of the electrostatic charge, the material particles gain extra pneumatic conveying properties.

Electrostatic forces:

The material particles, which are charged unipolar (charged at equal polarity) reject each other.

This causes the flow pattern to move towards the pipe wall, where the conveying air exerts less drag force on the material particles.

The result of less drag force on the particles is a lower material velocity.

In the computer calculation, this is represented by a high product loss factor.

(The product loss factor is in fact a velocity loss factor)

Charged particles, which collide with the earthed pipe wall, equalize the electrostatic equilibrium to a new level. Especially, when the material of the particles is an isolator the remaining, now attracting, electrostatic forces cause the material particles to stick to the pipe wall, forming a deposited layer.

The formation of deposited layers can increase until the pressure drop of the pneumatic conveying system becomes too high and that the pipeline chokes.

The electrostatic force is given by:

Charge1 * Charge2

F_es = Coulombs constant * —————————-

Distance²

The drag force by the conveying air is given by :

Fdrag = 1/2 * cw * rho_air * (v_air – vproduct)^2 * Ï€/4 * d^2

At short distances (possible with small particle sizes) the electrostatic force (F_es) becomes bigger and the drag force (F_drag) becomes smaller.

These effects are stronger, close to the conveying pipe wall. The short distance between the electrostatic charge of the particle and the electrostatic charge of the wall increases the electrostatic force.

Close to the conveying pipe wall, the conveying air velocity decreases to relatively low values, due to friction. (Boundary layer)

The above-described effects become more relevant with small particles that easily are electro statically charged.

It is also possible that particles are charged bipolar.

F.i. small particles acquire a negative charge and bigger particle acquire appositive charge.

In this case the smaller and bigger particles attract each other and form agglomerations of material. These formed agglomerations have a bigger virtual particle size and therefore a higher suspension velocity.

In the calculation program, a higher suspension velocity leads to lower material velocities and the calculated product loss factor from a field test then turns out higher.

(The too small-assumed particle size is then compensated for in the computer calculation by a higher product loss factor).

Flow of electric charge:

The flowing of charged particles (electric current = charge/time) induce magnetic fields in ferromagnetic pipe walls (steel)

As the particles are moving, the magnetic field is also moving through the pipe wall.

A steel pipe wall is also a conductor and the moving magnetic field induces a voltage in the pipe wall, causing an electric current that counter acts the moving particle.

Again, the velocity of an electrically charged particle is therefore lower and the consequence is a higher product loss factor.

Electric charge equalization by sparks:

A charged particle that moves towards a non-charged particle or wall induces an electric field between the charged particle and the pipe wall.

Depending on the charge and the distance between the particle and another particle or pipe wall, this electric field can become strong enough to cause ionization of the air.

In that case, is a spark transfers the electric charge.

A spark of sufficient energy is capable of melting and or bonding particles (especially plastics). The formation of scales (streamers or angle hairs) is believed to be a result of these phenomena.

In straight pipelines, the electro statically charged particles are mainly in suspension.

Therefore, only the particles, impacting on the wall, will equalize there electrostatic charge and scale formation is minimal and eventual scale is removed by the impact of new particles.

In a bend, all particles collide with the outer wall of the bend and at that location, the major part of the built-up electrostatic energy is equalized.

The formation of scales is then also located in the outer bend.

For some materials that are explosive, the occurrence of static electricity and the resulting spark discharges are a potential hazard for the installation and personnel.

Formation of scales by converting kinetic particle energy into heat:

In case the full kinetic energy is converted into a temperature rise, the following applies:

E(kin) = ½ * m * vel^2

and

E(heat) = h(spec) * m * dT

h(spec) = specific heat content of PVC = 0.9 kJ/kg.degrC

Equalization results in:

E(kin) = E(heat)

½ * m * vel^2 = h(spec) * m * dT

dT = ½ * vel^2 / h(spec)

Example:

vel = 40 m/sec

dT = ½ * 40^2 / (0.9 * 10^3) = 0.9 degrC

In this example, the velocity of 40 m/sec is taken extremely high and yet the temperature rise is only 0.9 degrC.

In a bend, the velocity of the full product flow is reduced and all the converted kinetic energy is dissipated in to an instantaneous temperature rise.

In a straight pipe, only the outer particles are loosing energy through wall friction and (%-elastic) particle-wall collisions and the particles in suspension are loosing energy in inter particle (%-elastic) collisions. This heat generation is equalized with the surroundings in the following pipe sections.

Conclusion:

Temperature rise as a result of converting kinetic energy into heat is too low to cause melting and bonding of PVC in pneumatic conveying, providing that the starting temperatures of the PVC and the conveying air are sufficient low.

Influencing factors on the build up of electrostatic charge:

-          Air Relative Humidity:

A higher RH causes a decrease in the build up of electrostatic charge

RH control by adding water is a way to minimize the build up of electrostatic charge.

However, the conveyed materials require often, dry air and the same applies to cold (freezing) environments.

-          Anti static agents

Anti static agents are preventing the build up of electric charge between particles.

However, the required purity of the conveyed product does not allow the adding of any different product.

-          Material choice of pipe wall

The electrostatic charging by the conveying air is not prevented by an appropriate choice of the pipe wall material.

A pipe wall material, which is equal to the conveyed material acquires the same charge and therefore, will eliminate the occurrence of sticking.

Touching this pipe wall directly causes a electric charge equilibration with the person, who feels the immediate electric shock.

A pipe wall liner of the same material as the conveyed material in an earthed pipe is an option that postpones the static electric discharge to the end of the pipeline with the risk of extra formation of sparks.

In addition, this choice can cause extra wear problems.

-          Earthen of conveying pipe.

Earthen of the conveying pipe (mostly steel) only prevents the hazard of electrical shocks.

The electrostatic charge of the floating particles is not removed and in case of non-conductive conveyed materials, the electro static charge still causes the sticking behavior.

-          Higher or lower air velocities.

Higher air velocities increase the drag forces.

However, the higher air velocities also increase the electrostatic charging, because of  the increased number of air to product contacts and thereby the electrostatic forces and certainly the occurrence of discharging sparks

The computer program calculates as a conveying parameter the energy loss per ton because of the particle loss factor (therefore also including static electric effects) and the lost kinetic energy in a bend.

A high energy because of particle losses compared to other products or installations indicates the existence of more than one influence. (Particle breakage is one of them)

Recalculating a number of PVC pneumatic conveying installations (one of which showed no formation of scaling) showed no correlation whatsoever between the pneumatic conveying parameters and the formation of scales.

F.i the installation with the lowest conveying velocities showed scaling and the installation with the medium conveying velocities did show no scaling.

Whether the above-described hypothesis of electrostatic charging is a cause for the formation of scales, streamers or angel hairs in pneumatic conveying is certainly a subject that is worthwhile to investigate.