(198c) Bonded, Grounded, and Burned to a Crisp: Electrostatic Ignition of Flammable Gases

Authors: 
Cox, B. L., Exponent, Inc.
Bishop, J. A., Exponent, Inc.
Ogle, R., Exponent, Inc.
Traina, N. A., Exponent, Inc.
Ignition by electrostatic discharge of clean flammable gas, such as natural gas, being released to the atmosphere during blowdown operations is unlikely. However, the presence of foreign material in the form of solid particles or liquid droplets changes the risk level significantly. The discharge of a flammable gas stream with a cloud of foreign material poses a potential electrostatic hazard, even if the piping system and associated equipment is otherwise bonded and grounded. Foreign material may be solid debris originally present in the system and subsequently suspended and transported by a gas stream. Alternatively, the foreign material may be aerosolized liquid, entrained in the gas stream due to impingement on pooled liquid or condensation from the vapor phase. In either case, the foreign material may be capable of accumulating sufficient electrostatic potential to ignite the flammable gas if an oxidant is present.

There have been several documented incidents where ignition has occurred during a flammable gas blowdown. In some instances, gas lines were being cleaned, while in others, the blowdown was completed as a safety measure for another process. Here, we focus on case studies where a stream of flammable gas was ignited during a blowdown and an obvious ignition source (other than electrostatic discharge) was not present, but foreign material was observed before, during, or after the incident. The potential influence of particle size and material properties is presented. We also discuss guidance offered by the available literature on the control of electrostatic ignition hazards in flammable gas streams.

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