Electrostatic Charge Control in Explosion Risk Zones

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Electrostatic discharges

Static electricity can be found practically everywhere. In everyday life, we perceive electrostatic discharges as painful but harmless nuisance. However, in the industrial environment, where flammable and explosive atmospheres occur, their effects can be devastating. Each year, numerous fires and explosions are caused by electrostatic discharges.

In order to understand how serious this threat is and to be able to control it in the right way, one needs to learn the principles of formation of electrostatic charges.

Every industrial process accompanied by movement, direct contact and separation of various types of materials leads to the phenomenon of electrization, i.e. accumulation of an excess of charges with the same polarity mark. Electrization can be caused e.g. by liquid flowing in a pipeline, powder sliding down a chute, a mixing process, and even by a person walking up and down a floor. When the electrized object has a good enough contact with the ground, the charge will be dispersed the moment it is generated. However, if the object is isolated from the ground by e.g. paint coating, laminate, rubber gaskets, or the soles of shoes, it will accumulate charges and its electrostatic potential and will grow. The moment when another subject of a lower potential occurs in its vicinity, an electrostatic discharge may come about – a phenomenon characterised by high voltage as able to ignite many types of atmospheres.

How an electrostatic discharge occurs?

Electric charging of objects grows when the resistance of the path of charged object to the ground prevents a discharge. When two objects with different electrical potentials are near to each other, an electric field forms between them. In the case when its value exceeds the breakthrough strength of the atmosphere between the two bodies, a spark discharge may occur. The potential energy of an electrostatic spark discharge in [mJ] can be calculated when the capacity of the object on which charges accumulate (C) [pF], and the potential of the object generated by the accumulation of charges V [kV] are known:

W = ½ CV2

Example:

One can calculate from the above formula that in consequence of the so called electrization, a man with a capacity of 200 pF may be charged electrically to the level of 30 kV [IchemE]. As a result, there is a possibility of an electrostatic discharge with energy reaching the value of 90 mJ.

The processes most endangered with the an explosion resulting from an electrostatic discharge include:

  • loading and unloading of road and rail tankers,
  • filling and emptying of the various types of tanks, kegs and barrels,
  • pouring of a liquid from one vessel into another, dosing and mixing.

The above industrial processes occur in virtually every production plant, therefore, it is essential that their users should be aware of the hazard and have knowledge on controlling the phenomenon of electrostatic discharges.

Table 1. Potential energy in the typical elements of installations. Source: UK IChemE.

Object

Capacity (pF)

Accumulated
energy
with a voltage of 10 kV (mJ)

Accumulated
energy
with a voltage of 30 kV (mJ)

Road tanker

5000

250

2250

Man

200

10

90

Steel bucket

20

1

9

Flange 100 mm

10

0,5

4,5

Minimum ignition energy (Polish: MEZ, English: MIE) the lowest energy sufficient for causing ignition of the most inflammable explosive atmosphere in specific test conditions.

Typical values of MIE vary, depending on whether the inflammatory atmosphere contains fumes, dust or gas, with many commonly used solvents showing the MIE values of below 1miliJ.

Table 2. Minimum ignition energy (MIE) for vapours and dusts. Data source: UK IChemE.

Liquid vapour

MIE (mJ)

Powder cloud

MIE (mJ)

Propanol

0,65

Wheat flour

50

Ethyl acetate

0,46

Sugar

30

Methane

0,28

Aluminium

10

Hexane

0,24

Epoxy resin

9

Methanol

0,14

Zirconium

5

Carbon disulphide

0,01

Some pharmaceutical half-finished products

1

Electrostatic earthing systems

Earthing of fixed elements of installations, such as piping, silos, pumps, etc., is a relatively easy and well recognized task. However, the situation becomes complicated in the case of the facilities which are not permanently linked with the installation. The facilities in questions include road and rail tankers, barrels, IBC containers or mobile vats of mixers. In such situations, it is necessary to use specially designed devices for a temporary connection and earthing of different types of facilities.

In practice, wires fitted with special clamps are applied for earthing mobile components of process installations. The free end of the wire is permanently attached to a verified earthing point (e.g. hoop-iron), while the clamp is fastened on the earthed object.

There are also solutions that are not only used for connecting and earthing of objects, but can also verify that the connection has been made properly and block the process when a negative response is obtain. This type of electronic systems is referred to as ground controllers.

Regardless of whether the grounding was done with the use a simple cable with a clamp, or a system with a controller, its resistance must not exceed 10 Ohms – this is a normative value.

The main cause of the accidents known from the industry as caused by electrostatic discharges is the objects that are isolated from the ground and able to collect static charges. In the literature, they are referred to as insulated conductors. In practice, these are most frequently metal flanges, fittings or valves on pipelines, barrels, containers, hoses, road or rail tankers, as well as people. In the above cases, a barrier to the free movement of electrons to the ground can be provided various types of gaskets, vehicle tires, footwear soles, paints and protective coatings, non-conducting structure components, impurities and many others.

Classification of earthing systems in respect of their characteristics

The table below presents the most important features of each type of the earthing systems offered by WOLFF GROUP.

Table 3. Characteristics and benefits for the user of various types of earthing systems.

Characteristic feature of the earthing system

Earth-Rite®

Bond-Rite®

Cen-Stat™

Volt-free contacts to control external devices, e.g. a pump, light or sound signalling

Continuous monitoring of the earthing connection status

Built-in light indicator that indicates to the operator (a red/green light) the current state of the earthing connection

Clamps with the ATEX/FM approval, with the sharp teeth of tungsten carbide as able to break through a layer of paint, laminate, pollution, etc.

Mechanically and chemically resistant cables protected with resistant Hytrel sheath

Electrostatic safety rules

  1. Always use approved grounding terminals, cables and equipment.
  2. Check out all properties of the earthing and in places where a higher security level is required, use systems for checking a proper earthing.
  3. Make sure that the personnel working in the endangered area understand the risk of ignition and observe safety procedures.
  4. Ensure an appropriate maintenance programme aimed at maintaining proper earthing.

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