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If you’re handling flammable materials, think electrostatic risks

12 June 2018

Electrostatics consultant Dr Jeremy Smallwood outlines basic precautions and tells some cautionary tales.

In any process where there are flammable materials, it is essential to consider static electricity risks. Unfortunately, people often do not do so until they are surprised by an incident. Certainly, anywhere that is classified as a Hazard Zone should be carefully considered for electrostatic risks. But even in unclassified areas, static electricity can be the source of unexpected events.

One company had a series of unexplained flash fires in waste bins placed under their workstations. The area was not a hazard zone and the process did not handle appreciable amounts of flammable material. So what was going on? At the workstations, moulds were coated with release agent from an aerosol before a plastic part moulding process. The mould release agent contained a material that gave off a flammable vapour. At the same workstations, the plastic parts were split from the moulds and cleaned up, with highly charged waste material discarded in the bins. “Empty” metal cans of release agent were discarded in the same bins. Heavier than air flammable vapours collected in a pool in bottom of the impermeable bins, creating a flammable atmosphere. The highly charged plastic waste created a strong source of static electricity. Cans thrown in the bin would attain a high voltage due to the charged plastic material. Every now and then, two cans would make contact and a spark would occur, igniting the flammable atmosphere and causing a fire.

Electrostatic charge is in the atoms of all materials. In a neutral material it is present in a balance of equal numbers of positive and negative charges. When two materials touch, some charge moves from one to the other and a small imbalance forms. The separated charges will try to find their way back together by some route, but if they cannot, the imbalance remains as static electricity. Further contacts between materials may increase the imbalance resulting in higher charge and voltage levels.

Insulating materials stop charges moving around freely and so promote buildup of static electricity. If enough charge builds up, electrostatic discharges (ESD) can happen in various ways. Two surfaces in intimate contact like plastic in a mould charge very highly. The plastic and mould, both highly insulating materials, remained highly charged when separated.
Common insulating materials include ordinary plastics used in packaging, containers and engineering materials. Some liquids, including many common hydrocarbon liquids are insulators and also create a flammable atmosphere, and so have to be handled with care to avoid ignition risks.

Materials that allow the charge to move around (conductors) can prevent static charge build up by conducting it away to earth. Metals, water, people and some hydrocarbon materials such as alcohols and ketones are also good conductors. If a continuous electrically conductive path exists from a conductor to earth, it is said to be earthed or grounded. This path is often fortuitously provided by other conducting materials, but it is often deliberately done to provide a drain for charge as a way of controlling static electricity. One problem with insulating materials is that they can easily interrupt the ground path, providing a block in the drain, and causing charge to build up on a conductor. A charged conductor is a possible source of sparks that can easily ignite flammable vapours. Higher energy sparks can also ignite some flammable dust clouds or fuel mists.

Ignition of a flammable atmosphere requires a small amount of energy that can easily be delivered by an ESD. This is called the Minimum Ignition Energy (MIE) of the material. Typical hydrocarbons have MIE around 0.2 mJ. Dust clouds can have a wide range of MIE from milliJoules to Joules.

Some of the most common ESD are sparks, which occur between two conductors, and brush discharges, which occur between a highly charged insulator and a conductor. Sparks are an effective source of ignition for flammable materials, and can arise in unexpected ways. Any conductor that is not grounded is a possible spark source. Common examples are ungrounded personnel, a metal tool held in an insulating gloved hand, a metal handle on a plastic bucket, or a metal container placed on a plastic bund. A metal drinks or aerosol can charged to a few thousand volts can ignite a typical hydrocarbon vapour. Larger objects need lower voltage to provide the same ESD energy. A few thousand volts is easily generated by static electricity.

This is easily demonstrated by the fact that we often experience electrostatic shocks in daily life. To experience the shock, we must have about 2000 V or more on our body. Body voltages over 5000 V (5 kV) are not uncommon, especially in cold dry weather. Much higher voltages up to 10’s of kV can be generated where insulating solids or liquids are handled or processed. Any material contact or movement is likely to generate charge build up in these materials.

So how can we effectively control electrostatic risks? IEC 60079-32-1 gives many recommendations specific to a wide range of industries and circumstances. Only some general precautions can be given here. The first task is to realistically identify where flammable atmospheres may occur, and know the lowest likely MIE material present. Within these areas ESD sources should be controlled or eliminated. All conductors within the likely flammable atmosphere must be reliably grounded to eliminate spark discharges. But, connection must never be made to an ungrounded conductor within a flammable atmosphere, as this could generate a spark to ignite the atmosphere. Grounding must be accomplished before the flammable atmosphere is present. One very significant conductor is a person working in the atmosphere. Personnel working in Zone 1 or Zone 0 must always be grounded through “antistatic” footwear according to ISO 20345 and a conductive floor. Other special Personal Protective Equipment such as outer clothing or gloves may be needed for personnel working in these Zones. The PD CEN/TR 16832 document gives advice on this aspect.

The risk of brush discharges is controlled by eliminating or restricting use of unnecessary insulators wherever possible. The maximum allowed area of an insulator may need to be restricted to 100 cm2 (about the size of a CD case) or in some cases less. It is especially important to ensure that use of an insulating material cannot result in ungrounded conductor (e.g. a metal handle on a plastic bucket). Plastic containers used for flammable liquids must be no larger than 5 litres (IEC 60079-32-1).

Safety in a Zone 2 area relies on a low risk of a flammable atmosphere, combined with a low risk of ignition source occurring. If spillage or escape of a flammable material occurs, a flammable atmosphere is certain and precautions to avoid ignition sources must be observed until it is cleared. In some industry areas such as petrochemical or gas, this may occur when normally sealed plant is opened for maintenance. A recent incident resulted from unexpected emergence of flammable liquid from plant in this way. The ignition source was thought to be a metal tool held in an insulating gloved hand, used to turn an earthed metal isolator valve covered in the spilled material. The glove was an additional item worn due to cold weather.
Ignition occurred when the tool touched the valve in the presence of the flammable material.

If the glove had not been worn the tool would have been grounded via a correctly specified under glove to the operator’s body, which was grounded through antistatic footwear and flooring. It had not been recognised that the additional glove blocked the grounding path that normally prevented the tool becoming an ungrounded conductor and possible source of ESD. Fortunately, damage was limited by the small amount of spilled flammable material present.

In the electronics industry, static control is managed according to established electrostatic discharge control standards. These require the user to document an ESD program  plan covering their ESD control precautions, an ESD training plan, an ESD control product qualification plan (to make sure ESD control products work as intended) and a checks and audit plan (to make sure they continue to work and pick up any failures in equipment or procedures). This is a very effective approach that goes a long way towards preventing unexpected ESD problems. Organisations handling flammable materials would do well to emulate this approach in establishing reliable electrostatic control procedures.

References

*International Electrotechnical Commission. (2013) Explosive atmospheres Part 32-1. Electrostatic hazards, guidance. PD/IEC TS 60079-32-1 ISBN 978-2-8322-1055-0
*International Organisation for Standardisation (ISO) (2011) Personal protective equipment -- Safety footwear. ISO 20345:2011 ISBN 978 0 580 86012 6
* CEN (2015) Selection, use, care and maintenance of personal protective equipment for preventing electrostatic risks in hazardous areas (explosion risks) PD CEN/CLC/TR 16832:2015 ISBN 978 0 580 97674 2

About the author

Dr Jeremy Smallwood is an ESD and Electrostatics Consultant and Trainer. He works in standardisation with BSI and IEC and was Chairman of IEC TC101 Electrostatics from April 2000 until April 2012. Jeremy is a Fellow of the Institute of Physics and Member of the Institution of Engineering and Technology (IET).


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