Wolfson Electrostatics
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Lightning strikes inside reactors and fuel tanks?
We know that incendiary electrostatic discharges in, say, a reactor containing solvent vapour are possible only if there are potential differences between materials inside the vessel or around the manway. This is because differences in potential give rise to electric fields which, if they attain the electrical breakdown strength of the air-vapour mixture, may promote discharges. Even if the reactor is entirely metallic and the product is also conducting, with everything being at zero potential through proper earthing at the beginning of the process, energetic electrostatic discharges are still possible in some operations. These are primarily operations in which a mist forms as a by-product in the vessel due to vigorous agitation of the liquid product or to vessel-washing actions in which high-velocity water or solvent jets impact on the internal surfaces. The mist droplets are inevitably charged electrostatically, due to the atomisation process. These charges set up potentials and electric fields throughout the mist. The strength of an electric field within a vessel increases radially, reaching a maximum value at the vessel walls. This has prompted several companies in the pharmaceutical and chemical industries to ask us in recent years whether large reactors containing both a mist and a flammable atmosphere are inherently unsafe because of the possibility that high electric fields at the internal surfaces could result in the onset of lightning-type discharges.
The possibility of lightning discharges inside vessels can be discounted. Work by Boshung, Maurer and others1,2 has shown that for lightning discharges to be possible, the electric field must satisfy two conditions: It must reach the breakdown strength of air to initiate the discharge. In addition, its mean strength over an extended distance needs to exceed some threshold value, possibly a few hundred kilovolts per metre, to propagate the discharge, as in large atmospheric storm clouds. It is also true that electric fields approaching the breakdown strength of air are unlikely to be present at the vessel walls in practice, because edges and surface irregularities would otherwise produce weak corona discharges, releasing ions that reduce the droplet charges and the resulting field values. Probably as a result of all these limiting factors, no lightning discharges have been reported in any industrial-scale activities.
Unfortunately, although a charged mist in a large vapour-filled reactor will not in itself produce incendiary discharges, the mist may give rise to an electrostatic hazard if ungrounded conductors are present. It is usually straightforward to ensure that all metalwork and any operators at an open manway are earthed. However, examples of conductors which are not continually grounded may be as follows:
- Spillages of liquid from structures near the vessel roof.
- The introduction of conducting intermediate product through the manway.
- Airborne fragments of water or solvent in vessel-washing operations arising from, for example, interruptions to the flow of liquid through the nozzle.
By suitable design, the possibility of spillages can be excluded. In addition, the pouring of conducting intermediate product through the centre-space of the vessel, where electrostatic shielding by earthed structures is least effective, can be avoided. However, the formation of airborne fragments of liquid carrying trapped charge, previously induced by the charged mist onto the jet, is more difficult to avoid. Much work has been performed by Walmsley and others 3,4, regarding the breakup of liquid jets, to determine the conditions under which a resulting discharge from a charged fragment could ignite the vapour. Based on this work, Wolfson Electrostatics has used a finite-element computer package, OPERA (see Applications page), in several investigations to assess the electrostatic risk when liquid fragments are a possibility 5. The on-site measurements required are simply the electric field at the boundary of the mist (at the manway) and the dimensions of the vessel. More recently, an analytical approach, involving the solutions of equations for the actual and safety-limiting electrostatic conditions, has been used with the same inputs to provide on-the-spot risk assessments 6.
References
- Boshung, P "An experimental contribution on the question of the existence of lightning-like discharges in dust clouds", J. Electrostatics 3 (4) 1977.
- Maurer, B "Discharges due to electrostatic charging in large storage silos", Chemie Ingenieur Technik, 51 (2) 1979.
- Walmsley, H "Electrostatic hazards from water slugs formed during the washing of ships' tanks: spark energy calculations", J. Phys D: Appl. Phys, 20 1987.
- Bustin, W American Petroleum Institute Statics Research Program: Part 2, 1973/4.
- Jones, R, Williams, T and Abu Sharkh, S "Assessment of industrial electrostatic hazards using finite-element field analysis", J. Electrostatics 40 & 41 1997.
- Williams, T and Jones, R "Modelling the electrostatic ignition hazards associated with the cleaning of tanks containing flammable atmospheres", Electrostatics '99 Conference.
For further information on electrostatic discharges contact Wolfson Electrostatics.