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Chubb J.,Infostatic Ltd. | Walmsley H.,Harold Walmsley Electrostatics Ltd
Journal of Electrostatics | Year: 2010

Studies on materials with long charge decay times (notably pharmaceutical powders) have shown that the local charge decay time constant comes to increase linearly with time during the progress of charge decay. This is an empirical result. This linear increase provides the basis for calculating the time to 1/e and/or to 10% from much shorter periods of observation than the times to achieve these decays. The approach involves simple numerical processing of the rate of increase in the local charge decay time constant with time and the intercept of this at some selected time. This analysis is useful for assessing and comparing materials that have decay times over say 104s. © 2010 Elsevier B.V. Source


Walmsley H.L.,Harold Walmsley Electrostatics Ltd
Journal of Electrostatics | Year: 2011

We recently reported calculations of the influence of induced-charge errors on brush discharge charge-transfer measurement for electrostatic hazard assessment. We concluded, tentatively, that, provided concave charged systems were avoided, the use of a correction factor of ×2 to ×2.5 when interpreting charge-transfer measurements made with fast, unshielded probes would avoid the underestimation of hazards at a 60 nC threshold level. However, our calculations applied directly only to brush discharges with a restricted range of geometries and consequently, it was not certain that the factors were necessarily sufficient in all circumstances. This paper presents numerical calculations that extend our results to a wider variety of geometries and include sparks as well as brush discharges. For brush discharges, the results reinforce the conclusions of the original work. For sparks they suggest that a similar worst-case correction factor (×2.2) is applicable. © 2010 Elsevier B.V. Source


Walmsley H.L.,Harold Walmsley Electrostatics Ltd
Journal of Loss Prevention in the Process Industries | Year: 2012

Occasional electrostatic ignitions have been associated with the use of buried, non-conductive (insulating), plastic fuel pipes at petrol stations. The incidents are rare and normally involve only minor flash fires but there is scope for escalation and it is prudent to prevent them. This paper analyses the electrostatic processes associated with the use of non-conductive plastic pipes at petrol stations to establish what ignition mechanisms are plausible. To do this it considers details of the reported incidents alongside the results of published voltage measurements, observations of the typical spread of streaming currents recorded in gasoline handling and theoretical estimates of the voltages on non-conductive pipes. The analysis suggests that the greatest risk is posed by incendive sparks from unbonded conductors but that incendive brush discharges from insulating pipe surfaces are also possible. In all cases the charging mechanism was by fuel flow. Careful earthing of all conductive items would eliminate incendive sparks from unbonded conductors but the hazard from incendive brush discharges would remain. By using plastic pipes with earthed conductive or dissipative inner linings the hazards from both incendive sparks and brush discharges arising from liquid flow charging can be prevented. Hazards arising from charging mechanisms other than liquid flow can be controlled by applying suitable operational procedures whenever flammable atmospheres might be present. © 2011 Elsevier Ltd. Source


Walmsley H.L.,Harold Walmsley Electrostatics Ltd
Journal of Physics: Conference Series | Year: 2011

The paper analyses some ignition incidents that have been reported with insulating, (non-conductive) underground plastic pipes in retail petrol stations. The occurrence of the incidents is compared with voltage measurements, observations of the typical spread of streaming currents recorded in gasoline handling and theoretical estimates of the voltages on the pipes. The comparisons suggest that neither incendive sparks from unbonded conductors nor incendive brush discharges from insulating pipe surfaces can be ruled out although both are expected to be rare. The hazards can be prevented by using pipes with earthed conductive or dissipative inner linings. Source


Walmsley H.L.,Harold Walmsley Electrostatics Ltd
Journal of Electrostatics | Year: 2013

A Bessel function expression is developed for the voltages produced when annular tanks (vertical axis cylindrical tanks with central conductors) are filled with liquids of uniform charge density. The expression is used to calculate the maximum surface voltage and this is compared with the maximum voltage predicted for tanks without a central conductor. Previous estimates of the percentage voltage reduction produced by a central conductor during tank filling have indicated a reduction of about 42% for practical tank dimensions. The new results, which are obtained with a more realistic model geometry, suggest a reduction of only about 29%. © 2013. Source

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