The consideration of conductivity in flammable and potentially explosive atmospheres

The consideration of conductivity in flammable and potentially explosive atmospheres

This article by Amy Rigby, Technical Service Manager at Innospec, looks at the challenges associated with electrostatic build-up in liquids and at potential measures to reduce the risk of spark discharge.

Wherever there is a flow of liquid, there is the potential to generate static. As fluids are pumped, stirred or mixed, solids dissolved or crystalised charge is generated at interfaces.

The rate of flow, the conductivity of the liquid, and the diameter of the vessel or pipe have a dramatic effect on the electrostatic buildup. In non-conductive fluids this charge can build and accumulate even when the system is grounded and bonded. If static charge is generated more quickly than it can be taken away, there is potential for static discharge. If this charge discharges as a spark it can lead to a number of problems in terms of process manufacture from pitting of the vessel to explosion.

Electrostatic charge

The electrostatic hazards of liquids are not always well understood. An electrostatic charge can build up within a liquid, particularly those with low conductivity such as hydrocarbons. Even with the pipe or vessel being earthed, a charge can remain within the liquid for a period of time (1, 2). Accumulated charge can also give rise to electrostatic discharges from the liquid surface sufficiently energetic to ignite a flammable atmosphere. This flammable atmosphere may be evolved from the liquid itself if the liquid is flammable or combustible and it is above its flash point temperature, or in the form of a spray or mist.

Electrostatic charge is generated when two dissimilar surfaces – such as the liquid and the walls of the vessel or piping- come into contact. The greater the area of the interface between the liquid and the surfaces and the higher the flow velocity, the greater is the rate of charging. The charge is carried with the liquid to the receiving vessels where they can accumulate (figure 1). The electrical properties of the solvents play a major role in determining both charge generation and relaxation. Static electric charge on a liquid in a grounded conductive container will dissipate at a rate that depends on the conductivity of the liquid. (2)

Conductivity

The conductivity of a liquid affects its charging ability. The conductivity is expressed in terms of siemens per meter (S/m) or more commonly picosiemens per metre (pS/m). According to NFPA-77, the US consensus practice on static electricity, liquids can be divided into three classes; conductive (>10,000pS/m), semi-conductive (50-10,000pS/m) and non-conductive (<50pS/m) (Table 1). For conductive liquids any static generated within the liquid can be conducted to the pipe/vessel and be dissipated safely via the grounding. For semi-conductive liquids, the rate of charge generation is critical, i.e. when charge generation is rapid, there may not be time for the charge to be dissipated. Low conductivity liquids are unable to dissipate the static charge. Static buildup can occur even if the vessel is earthed. Conductivity is a factor of temperature; hence the conductivity of a liquid will be lower when it is cold. Therefore it is important in a manufacturing process to measure the conductivity of a solvent when it is at its lowest temperature.

Prevention of Static Build-up

All conductive equipment associated with processing of flammable liquids should be grounded in order to prevent the accumulation of static charge.

Limiting the liquid velocity during vessel and container filling operations helps to limit electrostatic charge increase. In pipes, buildup of static is limited by reducing flow velocity. The recommended maximum flow for a low conductivity solvent is 1m/s where a solid or second liquid could be present. Where this is not the case, a maximum limit of 7m/s is suggested.(3)

Increasing Conductivity

The electrostatic hazard posed by non-conductive liquids such as hydrocarbons, aromatic solvents, and insulating oils to name but a few, can be decreased dramatically by increasing their electrical conductivity. The conductivity of an insulating liquid can be increased by addition of an anti-static additive. The increased conductivity enables charge to be more readily dissipated from the liquid. The use of antistatic additives, also known as conductivity improvers, to render a solvent ‘conductive’ and enable static electric charge dissipation is becoming a recognised practice, particularly in the solvents and coatings industry and is described in the industry guidelines.(2-5) Static dissipater additives increase the conductivity of solvents to render them conductive (>10,000pS/m), mitigate the build of static charge and its resulting consequences.

The latest such additives have the advantage of very low dose rates, requiring the addition of only a few parts-per-million, and can increase the conductivity of an insulating liquid by several orders of magnitude, as well as being available in a food contact approved version. The low treat rates can offer a cost effective solution to static electricity in fluids, when used in conjunction with appropriate grounding of equipment, for use in solvents, and coatings applications.

Handheld conductivity meters allow the conductivity to quickly and easily be checked to ensure target conductivity continues to be maintained after a period of time or downstream in the process. This conductivity consideration gives an extra layer of protection against static electricity, helping to keep your plant, people, processes and business safe, and also helping to achieve ATEX compliance.

ATEX 137/ DSEAR

Where flammable and potentially explosive atmospheres exist the ATEX 137 European directive, implemented as DSEAR in the UK, places a mandatory obligation on employers to: “consider and eliminate possible sources of static electricity”. (6,7) One source that is often overlooked is the flammable liquid itself. Use of best handling practices in conjunction with anti-static additives can be effective at minimising the risks associated with generation of static electricity in moving liquids.

Conclusion

The role of conductivity in static generation of flammable liquids is often overlooked. The conductivity of a liquid determines the rate at which generated static can be dissipated via grounding. Rendering a liquid ‘conductive’ with a conductivity of >10,000pS/m by the use of an antistatic agent reduces electrostatic hazards in a variety of applications. This is a cost effective solution to reduce the risk of static discharge and fire, and also helps to achieve ATEX compliance when used in conjunction with other safety methods such as appropriate grounding of equipment and reduced flow rates.

Liquid

Conductive Liquids

Conductivity (pS/m)

(>10,000pS/m)

Relaxation Times Constant (s)

Ethyl Alcohol (25 oC)

1.35 x 105

1.6 x 10-3

Isopropyl Alcohol (25 oC)

3.5 x 108

5 x 10-7

Water, distilled

~1 x 109

7.1 x 10-4

Semi-Conductive Liquids

(50-104 pS/m)

 

Methylene Chloride

4300

1.8 x 10-2

Pentachloroethane

100

0.3

Non-Conductive Liquids

(<50pS/m)

 

Heptane

3 x 10-2

~100

Hexane

1 x 10-5

~100

Toluene

<1

21

Xylene

0.1

~100

References
1. Walmsey, H. L., J. Electrostat. 1992, 27, Nos. 1 and 2
2. National Fire Protection Agency document, NFPA-77, ‘Recommended Practice on Static Electricity’, (section 7.4.3).
3. British Standard 5958, control of undesirable static electricity, (1991)
4. American Coatings Association, Generation and Control of Static Electricity in Coatings Operations, 2010
5. European Solvents Industry Group, Best Practice Guidelines, Flammability: A safety guide for users No.4, V3, 2013
6. European Directive 99/92/EC (‘ATEX 137’)
7. DSEAR (the Dangerous Substances and Explosive Atmospheres Regulations) 2002