What could cause white discharge

Appendix A: Charging and discharging processes in electrostatics

A1 Electrostatic charges

The most common cause of electrostatic charging is contact charging. Come z. B. two previously uncharged objects in contact, a charge transfer takes place at their common interface. During the subsequent separation of the surfaces, each surface carries a part of this charge, each with opposite polarity.

Contact charging can take place at all interfaces between solid and / or liquid phases. Gases cannot be charged, but solid particles or liquid droplets contained in a gas flow can.

Conductive objects can also be charged by influence when they are in an electric field. Another way of charging is by accumulating charged particles or ionized molecules.

A1.1 Charging liquids

Liquids are essentially charged using contact charging. Typical examples are the flow of a liquid along a solid wall, e.g. B. a pipe, a pump or a filter or the stirring, shaking, spraying or atomizing liquid. If the liquid contains at least one further phase, e.g. B. in the form of suspended solids or other dispersed liquids, the charging is considerably increased, since the expansion of the phase interfaces increases significantly. Normally, dangerous electrostatic charges occur only with liquids of low electrical conductivity.

If liquids are sprayed or atomized, see also Appendix A1.3.

In containers, brush discharges between fixtures and the surface of insulating liquids are not to be expected if the potential on the surface of the liquid does not exceed 25 kV. For hazardous substances of explosion group IIA, the discharges that occur are only ignitable from a surface potential of around 58 kV.

Note:
The surface potential is not a directly measurable quantity. With knowledge of the charge density and the physical properties of the liquid as well as the container geometry, it can be estimated by calculation.

A1.2 Liquids in pipes and filters

Turbulent flows generate more charges than laminar flows. In the case of laminar flow in single-phase liquids, the electrical current generated is almost proportional to the flow velocity, whereas in turbulence it is a quadratic. Turbulent flow processes are common in industrial processes. If uncharged liquid enters a pipe, the charge density in the liquid increases with the length of the pipe and - provided the pipe is sufficiently long - reaches a constant value.

For liquids of low conductivity, e.g. B. saturated hydrocarbons, the charge density can be estimated according to the following numerical equation:

ρ = 5 · v

With ρ = Charge density of the liquid in the infinitely long pipe (μC / m3)
v = Velocity of the liquid in the pipe (m / s)

A pipe can be viewed as infinitely long if

L. ≥ 3 · v · τ

With τ = εr · Ε0 / κ
L.= Length of pipe (m)
v= Velocity of the liquid in the pipe (m / s)
τ= Relaxation time of the liquid (s)
ε0= electric field constant (As / Vm)
εr= relative permittivity (dielectric constant) of the liquid
κ= electrical conductivity of the liquid (S / m)

The formulas can be used to estimate the charge density of a liquid containing e.g. B. emerges from the pipe when filling a tank.

A1.3 Spraying and blasting with liquids

The division of a liquid jet into small droplets can generate highly charged liquid jets or mist, regardless of the conductivity of the liquid. In general, the more conductive the liquid, the stronger the charge generation. A jet of water generates more charges than an oil jet. Even stronger charges cause multiphase mixtures, e.g. B. from oil and water.

A1.4 Loading of bulk goods

Contact charging is very common in bulk solids. The charging properties are determined both by the surface properties of the particles and by the chemical composition of the bulk material itself.

The amount of charge is usually difficult to predict. The level of charge depends on the amount of charges produced and the capacity of the arrangement. Charges are always to be expected when bulk goods with a medium to high specific resistance come into contact with a different surface. This is e.g. B. in mixing, grinding, sieving, pouring, micronizing and pneumatic transport the case. Examples of the amount of cargo that a bulk cargo can hold can be found in Table 12.

Table 12: Charging of bulk solids with medium or high specific resistance
processSpecific charge μC / kg
seven10-5 until 10-3
Pour10-3 until 1
Transport with screw conveyor10-2 until 1
Grind10-1 until 1
Micronizing10-1 until 102
Pneumatic transport10-1 until 103

A2 Charge accumulation

Charges that do not recombine, drain to earth, or otherwise dissipate remain on the surface of the charged material. Charges on insulating materials are retained due to the resistance. Charges on conductive or dissipative materials and objects are only retained when there is no contact with earth. Under normal conditions, pure gases are insulators. They isolate dust particles and droplets so that clouds and fog retain their charge over a longer period of time.

In technical processes, a balance is often reached between the relaxation of charges and their continuous generation. For example, the electrical potential of an insulated metal container into which a charged liquid or a charged bulk material is placed is determined by the speeds at which the charge is fed and discharged. The resulting potential difference (voltage) is calculated by:

U = I R (1 - e-t / τ)

With U = electrical voltage of the container (V)
I. = "" Electrostatic "" charging current (A)
R. = Resistance to earth (Ω)
t = Time (s)
τ = Relaxation time (s)

To assess a dangerous electrostatic charge, the maximum voltage is used, which is reached with long times according to the above formula:

UMax = I * R

Leak resistance and capacitance can often be measured. The product

τ = R * C

with C = capacity (F)

can be used to assess the charge level.

A2.1 Charge relaxation in liquids

The relaxation of charges in a conductive or dissipative container with liquid depends essentially on the electrical conductivity of the liquid. If no charge is generated, the following applies for the relaxation time:

τ = εr · Ε0 / κ

With τ = Relaxation time of the liquid (s)
ε0 = electric field constant (As / Vm)
εr = relative permittivity (dielectric constant) of the liquid
κ = electrical conductivity of the liquid (S / m)

For example, the relaxation time is τ = 18 s for a hydrocarbon with a conductivity of κ = 1 pS / m. Experience shows that even with low conductivities and very high charge densities, dwell times of 100 s are sufficient to dissipate dangerous charges.

A2.2 Charge relaxation in bulk solids

Experience has shown that the electrical potential at the limit of a dust cloud in air is at most 3 · 106 V. The cause is the charging of the bulk material.

The charge accumulates on a bulk material when the speed of charge generation exceeds that of charge discharge. The relaxation time is determined by:

τ = εr · Ε0 · Ρ

With τ = Relaxation time of the bulk material (s)
ε0 = electric field constant (As / Vm)
εr = relative permittivity (dielectric constant) of the bulk material
ρ = specific resistance of the bulk material (Ωm)

For bulk goods with ρ = 1010 Ωm and the permittivity of 2 · 8.85 · 10-12 F / m is the relaxation time τ, in which 2/3 of the accumulated charge is diverted to the earth, 0.2 s.If a bulk material causes a cloud of dust, then considerably longer relaxation times can be assumed, which cannot be calculated.

A3 Types of discharge in electrostatics

The various types of electrostatic discharge differ considerably in their ability to ignite an explosive atmosphere.

A3.1 Spark discharge

A spark is a discharge between two conductors with a well-defined luminous discharge channel through which a high density current flows. The gas is ionized throughout the canal. The discharge takes place very quickly and is usually clearly noticeable. It occurs when the field strength between the conductors exceeds the electrical breakdown field strength of the atmosphere. The required potential difference depends on the shape and the distance between the conductors. The standard value for the breakdown field strength is 3 · 106 V / m assumed. Experience has shown that this value applies to flat surfaces or surfaces with a large radius in air and a minimum distance of 10 mm. The breakdown field strength increases with decreasing distance.

Note:
Example 14 shows a schematic representation of the spark discharge.

The energy of the spark between a conductive and a conductive, earthed object is calculated:

W = 1/2 Q * U = 1/2 C * U2

With W. = maximum converted energy [J]
Q = Amount of charge on the ladder [C]
U = Potential difference (voltage) [V]
C. = Capacity [F]

Typical values ​​for the capacitance of conductive objects are shown in Table 13.

Table 13: Capacities of selected bodies with exemplary charging

Charged bodyCapacity (pF)Potential (kV)Energy (mJ)
flange10100,5
small metal objects, e.g. B. shovel, hose nozzle10 – 20100,5 – 1
bucket10100,5
Small containers up to 50 l50 – 10082 – 3
Metal containers from 200 l to 500 l50 – 3002010 – 60
person100 – 200127 – 15
large parts of the system, immediately surrounded by a grounded structure100 – 1 0001511 – 120

Calculation example:

A non-earthed metal barrel is filled with bulk material. The charging current I can be 10-7 A and the leakage resistance R. of the barrel to earth 1011 Ω and its capacitance are 50 pF.

After that, the barrel has a maximum potential of

UMax = I · R = 10 kV,

a maximum charge stored on the barrel QMax of

QMax = C · UMax = 500 nC

and a maximum energy W.Max of the discharge spark of

W.Max = 1/2 C * U2Max = 2.5 mJ

to be expected.

W.Max is to be compared with the minimum ignition energy of the bulk material. The transferred charge can also be used to assess the ignition effectiveness of sparks Q can be used.

Note: substance-related values ​​for MZE and MZQ see also Table 18 in Appendix G.


Example 14: Spark discharges (see Appendix A3.1)

A3.2 Corona discharge

Corona discharges occur on the surfaces of conductive objects with a small radius of curvature, e.g. B. at sharp corners or peaks when local field strengths of over 3 MV / m are reached. Since the electric field decreases rapidly with increasing distance, the range for the corona discharge is not very wide. Corona discharges are severe and often only recognizable in the dark.

Note:
Example 15 shows a schematic representation of the corona discharge.

Their energy density is much lower than that of the sparks and, as a rule, they are not effective in ignition. When handling large quantities of bulk material with medium or high specific resistance, corona discharges cannot be avoided.

A3.3 Clump discharge

Brush discharges can occur when earthed conductors are moved towards charged insulating objects, e.g. B. between a person's finger and a plastic surface or between a metal object and the surface of the liquid in a tank. They cannot be avoided when handling large quantities of bulk material with medium or high specific resistance. Brush discharges are short-lived compared to corona discharges and can be visible and audible.

Note:
Example 15 shows a schematic representation of the brush discharge.

Although brush discharges usually have only a fraction of the energy of a spark discharge, they can ignite most flammable gases and vapors. According to the current state of knowledge, no dust is ignited by clump discharges as long as there are no flammable gases or vapors. The ignition efficiency of brush discharges can be determined by measuring the charge transferred Q be assessed. Brush discharges are not ignitable if the transferred charge Q less than the minimum ignition charge MZQ is.

Note 1:
In a homogeneous electric field below a field strength of 100 kV / m, even with the introduction of field-distorting devices, the triggering of brush discharges is not to be expected.

Note 2:
Substance-related values ​​for MZE and MZQ see also Table 18 in Appendix G.


Example 15: Brush discharges (left, see Appendix A3.3) and corona discharges (right, see Appendix A3.2)

A3.4 sliding stem tuft unloading

Gliding stalk bundle discharges are generally ignitable for flammable gases, vapors and dusts and have energies of up to 1 J or more. Experience has shown that the high energy densities required for the discharges of sliding stalk tufts occur under the following special conditions:

- thin insulating objects or layers of material,
Note:
Discharges from sliding stalk tufts are observed on insulating plates, foils or coatings because these objects can store charges on both sides.
- high breakdown voltage of a material,
Note:
The breakdown voltage has a significant influence on the charge density on the surfaces.
- existing strong charge-generating processes,
Note:
Strongly charge-generating processes are z. B. pneumatic transport, high-speed drive belts.
- little spraying of loads.
Note:
Charges can spray off at points, corners and edges.

The sliding stem tuft discharge often has a brightly shining, tree-like structure and is accompanied by a loud bang. It can be used with bipolar charged layers that are free in space, e.g. B. packaging films, as well as with coatings of a conductive base body.

After charging has taken place, a sliding stem tuft discharge can be triggered by

- mechanical piercing of the surface,
- an electrical breakdown inside the material,
- Simultaneous approach of both surfaces via two electrically connected electrodes, e.g. B. for thickness measurements,
- Touching the exposed surface with a grounded conductor when the other is grounded, e.g. B. by touching the surface by a person.

The high energy of the sliding stem tuft discharge comes from the bipolar charged surfaces, which are discharged in the event of an electrical breakdown.

Experience has shown that the following prerequisites are necessary for unloading a sliding stem tuft:

- layer thickness D. <9 mm and
- surface charge density σ> 2.5 · 10-4 Cm2 and
- breakdown voltage UD. > 4 kV or UD. > 6 kV for textile fabrics, e.g. B. at FIBC.

Note:
Example 16 shows a schematic representation of sliding stem tuft discharge.

Discharges from sliding stalk clusters can ignite an explosive atmosphere consisting of gases, vapors or dusts. Their energy can be calculated arithmetically as follows:

W.GBE = (A D σ2)/(2 · εr · ε0)

With W.GBE = maximum energy to be expected from the sliding stem tuft discharge (J)
A. = Area (m2)
D. = Layer thickness (m)
σ = Surface charge density (C / m2)
ε0 = electric field constant (As / Vm)
εr = relative permittivity of the layer

Normally, no sliding stalk tuft discharges occur on thin layers of paint.


Example 16: Discharge of sliding stalk tufts (see Appendix A3.4)

A3.5 Thunderstorm-like discharge

In principle, thunderstorm-like discharges can occur in large clouds of dust; they have been observed in ash clouds during volcanic eruptions, but have not yet been detected in industrial processes.In experimental investigations, such discharges could be in silos with a volume V. <100 m3 cannot be determined. Also in containers of any height with a diameter d Thunderstorm-like discharges are not to be expected at <3 m. Theoretical considerations suggest that lightning-like discharges can occur in larger silos or containers at field strengths above 500 kV / m.

A3.6 discharge cone

If highly charged insulating bulk material is filled into silos or large containers, it creates areas within the bulk with a very high charge density and leads to strong electrical fields in the upper part of the bulk. Cone discharges can occur in this area. They typically occur in conductive, earthed containers and run radially along the surface of the bed as soon as the field strength on the inner wall of the container exceeds 3 MV / m.

Note:
Example 17 shows a schematic representation of the pouring cone discharge. Influencing factors for cone discharges are:

- the specific resistance of the bulk material,
- the supplied mass flow,
- the volume and geometry of the container,
- the grain size of the bulk material (median value)
- the bulk density of the bulk material,
- the specific load of the bulk material,
- the relative permittivity (dielectric constant) of the bed.

Based on the space charge density distribution in the container and taking these influencing factors into account, the field strength on the container wall can be calculated in model calculations, e.g. B. with the help of a finite element method.

For metallic containers with a diameter between 0.5 and 3 m and bulk goods with grain sizes between 0.1 and 3.0 mm, the energy of a pouring cone discharge can be calculated by:

W.SKE = 5,22 · d3,36 · G1,46

WithW.SKE = maximum expected equivalent energy of the cone discharge (mJ)
d = Container diameter (m)
G = Median value of the grain size (mm)

With increasing median value of the grain size, e.g. B. with granulate, the energy for cone discharges increases.

Cone discharges can also occur in containers made of insulating materials. In this case is in place of the container diameter d twice the value (2 d) to be used.

Particularly dangerous are situations in which the ignition energy for cone discharges is generated by coarse grain and at the same time fine parts of the bulk material, e.g. B. abrasion, with a low minimum ignition energy.

Cone discharges can ignite flammable gas and vapor / air mixtures as well as dust / air mixtures.


Example 17: cone discharges (see Appendix A3.6)