Bulk powder electrostatics hazards

Electrostatic risks to be considered in ATEX risk analysis, DSEAR

Electrostatic sparks, cone discharge, brush discharges, corona discharges, propagating brush discharges...etc...

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Section summary
Introduction
Spark discharges
Brush discharges, corona discharges and propagating brush discharges
Cone discharge
Lightning

Bulk Solids Handling Safety
# Dust explosion physical properties # MIT - Minimum Ignition Temperature of dust

1. Introduction

Electrostatics discharges are responsible for a large share of powder cloud explosions observed in the industry. Depending on the source, country, static electricity discharge accounts indeed for 10% to 25% in some area of industry (plastics for instance) [Glor] [Van Laar].

Understanding the hazards related to static electricity for bulk solids handling industries is therefore critical to ensure the safety of the process and people.

This page is reviewing the different mechanisms leading to potentially hazardous static electricity discharges that should be taken into consideration in risks analysis (ATEX, DSEAR).

The following table is summarizing those different phenomena and the energies involved :

Table 1 : Electrostatic discharges involved in dust explosion [Van Laar] [Glor]

Type of electrostatic discharge Typical energy involved Ignition hazard ?
Sparks
All levels Yes
Brush discharge
4-5 mJ [Van Laar]
1-3.6 mJ [Glor]
Yes (In certain conditions with a flammable hybrid mixture)
Propagating brush discharge
500-1000 mJ Yes
Corona
< 1 mJ Not proven yet
Cone
< 20 mJ in some sources, higher for some authors (possible calculation through a correlation) Yes

2. Sparks discharges

Sparks, obtained between 2 conductive materials at different potential is one of the most common and most dangerous electrostatic risks for industries processing powders.

In the industrial environment, if one isolated piece of equipment, for example a metal tube, is getting charged, then there is a risk of electrical sparks if it comes close to another piece of metal, or simply an operator, which is charged differently, the difference of potential triggering a current that creates a spark.

For this reason, for factories handling bulk solids, it is MANDATORY that ALL parts of the process be grounded. It is ensured thanks to grounding cable connected pieces together, with the whole system ultimately earthed. A process operator should measure the resistivity in between each process part and confirmed it is low, typically less than 10 ohms.

[Glor] is giving different examples of situation that can lead to electrostatics sparks :

- Metal part / pipe insulated by a flexible
- Metal part insulated by gaskets
- Part of a valve isolated by the gasket
- Metal drum on a trolley with non conductive wheels
- Instrument held by an operator using non conductive gloves
- People insulated because of the shoes of a non conductive floor / platform

The energy that can be expected from a spark coming from an insulated conductor can be calculated thanks to :

W = 0.5 * C * U2

Equation 1 : estimation of the energy of a spark from an insulated conductive element

With :

W =energy of the spark in mJ
C = capacitance of the isolated conductive element in pF
U = difference of potention in kV

[Laurent] gives a table allowing to have an idea of the orders of magnitude involved.

Table 2 : Energy that can be stored in some isolated conductors [Laurent]

Conductive element, isolated Capacitance pF Energy with  10 kV difference potential Energy with  30 kV difference potential
Bolt
1 0.05 0.45
Roll 100 mm
3 0.15 1.5
Flange 100 mm
10 0.5 4.5
Bucket
20 1 9
Cyclone
h=1 m
D=1 m
30-50 1.5-2.5 13.5-22.5
Drum 200 l
100-300 5-15 45-135
Operator, people
200 10 90
Rail silo
1000 50 450
Silo truck
5000 250 2250

With the values for railcars and silo trucks, the reader can realize how important it is to ground those equipment during filling and discharge !

3. Brush discharges, propagating brush discharges and corono discharges

3.1 Brush discharges

(Decharge en aigrette)

Brush discharges happen when an isolated material is getting charged, for example a flexible drop tube. The movement of the powder is charging the material but as it is not conductive, they cannot be released and keep building up. If now an earthed conductive material is approached to the charged part, a brush discharge can happen as the conductive material somehow plays the role of an electrode.

The energy involved is, according to data found in litterature [Glor] very limited, in the range of 1-3.6 mJ and in usual circumstances are not able to initiate a dust explosion if the cloud is in suspension in air alone. However, this energy is sufficient to initiate a dust cloud if a flammable gas is present, which is modifying a lot the MIE of the mixture. Brush discharges must therefore be systematically checked in dust explosion risks analysis.

Some examples of activities that can lead to brush discharges are given below :

- Bringing a plastic bag just discharged close to a metal part
- Approaching a finger close to a charged isolated part, like a plastic drop tube, flexible...
- Sampling of powder just being charged in a silo with a conductive and earthed tool
- Having internal fittings in a charged dust cloud, that can play the role of electrodes

3.2 Propagating brush discharge

(Decharge en aigrette propageante)

Propagating brush discharges happen when a the 2 sides of a non conductive material layer are charged with opposite polarity. When a short circuit happen in between the 2 highly charged layers, a propagating brush discharge can be triggered, leading to a very high energy release.

Some examples of materials that can be charged differently on both sides of a non conductive sheet are given below :

- Insulated pipe in pneumatic conveying lines
- Conductive pipe with an internal insulating internal coating, typically in pneumatic conveying lines
- Sight glass in pneumatic transfer lines
- High speed movement / impact of powder, for example in a drop tube, on an insulated surface (insulated pipe or pipe with a coating)
- Filling of Big Bags or vessels with insulating layers

In order to avoid propagating brush discharges, only conductive, earthed material should be used, or, when having to use a non conductive material (flexible connection, big bag), the breakdown voltage against the layer of material should be less than 4 kV, which prevents propagating brush discharges to happen [Glor].

These discharges can also take the name of Lichtenberg discharges [Laurent]

3.3 Corona discharges

Corona discharges have some similarities with brush discharges but generate much lower levels of energy, typically 0.2 mJ. They are normally not hazardous for pure dust cloud, but again any operator performing a risk analysis should verify there is no circumstances in the process, like the presence of highly flammable gas mixed with the bulk solids, that could change significantly the MIE and actually make corona discharges hazardous.

4. Cone discharge

(Maurer discharges)

Cone discharges constitute a very important risk in bulk solids handling, although it is often overlooked or not well understood. This kind of discharge is initiated when a poorly conductive bulk solid is stored in a hopper after an operation that can charge it, like pneumatic transport or gravity chute over long distance.

As the material has a high resistivity, it can build-up charges during the filling up to the point that the difference of potential with the vessel is high enough to trigger a cone discharge. The risk is all the more important that the diameter of the hopper / silo / flexible container is bigger and that the particle size diameter of the bulk solid is coarser.

[Glor] is giving a correlation to estimate the energy released during a cone discharge :

WAe = 5.22 * D3.36 * d1.462

Equation 2 : estimation of the cone discharge energy

This correlation is valid for : 0.5 m < D < 3 m and  0.1 mm < d < 3 mm

With :

WAe = equivalent energy of the cone discharge in mJ
D = conductive, grounded silo diameter in m
d = median particle diameter in mm

According to the literature reports, cone discharges are likely to happen for powders having a resistivity > 1010 Ω.m. Note that the risk is quite significative with polymers granules which have a very bad conductivity.

It is very important to calculate the energy expected from a cone discharge for vessels filled after an operation that can generate high charges of the powder (typically high speed transport like pneumatic conveying) and if found higher than the MIE, to review the design by considering explosion protection measures.

5. Lightning

When having a highly charged powder in suspension in the air, it could be in principle possible that lightning happen, like it happens in nature. However, tests have shown that this risk is not probable for silos less than 60 m3 and less than 3 m in diameter. For bigger silos, the risk should at least be mentioned in the risk analysis although so far it seems that this risk is also not probable for larger vessel capacity [Glor]

Note that lightning from the external environment during a storm is a risk to take into account, especially for external silos.




Sources

# [Glor] Ignition hazard due to static electricity in particulate processes, Martin Glor, Powder Technology, 135-136 2003
[Van Laar] Ir. Van Laar. Static electricity as a hazard in industry. Journal de Physique III, EDP Sciences, 1991,1 (8), pp.1377-1387.
# [Laurent] Securite des procedes chimiques, Andre Laurent, Tec et Doc, 2003, page 332




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