Electrostatic ignition hazards in road tanker loading: Part 2. Statistical analysis for standard conditions

Abstract

This paper is the second in a series of three on the electrostatic ignition hazards that arise when road tankers are loaded with hydrocarbon fuels. The middle distillate loading data presented in the first paper are used to estimate, for a standard loading condition (top loading at 5 m s −1 through a 100 mm line), the risk per compartment fill of exceeding the maximum safe (threshold) tank inlet charge density. The corresponding ignition risk is then very approximately deduced and compared with an estimate of the actual frequency of ignitions taken from an accident survey. The third paper of the series will examine how the risks vary with the loading conditions. For the standard condition, the risk of exceeding the threshold inlet charge density is estimated as approximately 10 −7 per fill which, with typical working practices, corresponds to about 2 × 10 −4 per man year. The risk of ignition is likely to be at least an order of magnitude lower than the risk of exceeding the threshold charge density because, in addition to a hazardous potential, there must also be an electrode for a discharge and a flammable atmosphere. The actual rate of ignition estimated from the accident survey is about 3 × 10 −5 per man year for loading operations carried out in accordance with established procedures. This is slightly higher than the rate estimated by combining our estimate of the risk of exceeding the threshold charge density with an allowance for the occurrence of suitable electrodes and flammable atmospheres. Agreement between the actual accident rate and the estimated risks is probably as good as can be expected given the uncertainties in both estimates.