Wolfson Electrostatics
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Outdoor test facility

The Wolfson Electrostatics outdoor test site has been the scene of a great deal of activity recently, with some large-scale electrostatic hazard investigations being undertaken.

The Wolfson test site is situated within the Marchwood Industrial Park, about 12 km to the south west of Southampton University. The site is ideal for tests involving the relatively large-scale handling of flammable materials, such as fuels, solvents and combustible powders.

Two recent projects of note have involved quantifying the electrostatic ignition risk associated with the flow of petrol through non-metallic pipelines and the electrostatic charge generation associated with a suspended fine mist of aviation fuel.

The latter investigation was a collaborative programme run by Dr D K Davies of Markab aimed at establishing a possible ignition source for several cases of unexplained explosions of aircraft fuel tanks during flight.

It is now well known that some vehicle fires have been caused by pinhole leaks in high-pressure hydraulic lines. The very high pressures involved create a fine aerosol with each droplet carrying a minute imbalance of electrical charge. The cumulative effect of all the droplets in the resultant mist can produce hazardous levels of static electricity, particularly if the charge is able to accumulate on an insulated metal component such as a metal clip or pipe section. In such cases, electrostatic spark discharges can occur which may ignite the aerosol directly or any residual flammable vapours.

This investigation is continuing.

Footnote facts:

• Non-flammable liquid mists or aerosols can give rise to an electrostatic ignition hazard when present in large enclosures or vessels if sensitive flammable vapours are also present. The mist could be an inert material of high conductivity (eg water).¹
  
• Many liquid mists or aerosols can be ignited by an electrostatic discharge even if the liquid is below its flashpoint temperature.²
  
• The presence of low concentrations of flammable vapours and gases can significantly reduce the minimum ignition energy of a dust cloud. This applies even if the vapour or gas concentration is below its lower explosive limit (LEL).³

References

1. BS 5958 Parts 1 and 2, "Control of Undesirable Static Electricity", 1991.



2. Lewis & Von Elbe, "Combustion, Flames and Explosions in Gases", Academic Press Inc. (1987) ISBN 0-12-446751.



3. ANSI/NFPA 68 (1987) "Venting of Deflagrations", NFPA, 1 Batterymarch Park, PO Box 9101, Quincy, MA 02269-9101.

For further information on your electrostatic testing requirements contact Wolfson Electrostatics.