Static electricity: New guidance for storage tank loading rates

Abstract

This article describes how the IEC TC31/101 JWG29 committee adapted a new theoretical model for practical use in the forthcoming IEC Technical Specification 60079-32-1, "Explosive Atmospheres - Part 32-1: Electrostatic hazards, Guidance". The model, by Britton and Smith, addressed flow rate limits for the transfer of single-phase, static-accumulating flammable liquids into vertical cylindrical tanks containing no significant sediment or water bottom. Practical adaptation involved a technical evaluation of the model, optimization of variables in the empirical expression relating charging to flow velocity, plus selection of applicability limits in terms of tank size. From the results we present easy-to-use expressions for calculating the flow velocity and flow rate limits, plus tables of example solutions. We also show how to apply the flow velocity limits to piping systems that involve a multiplicity of flow velocities in different pipe segments. During the adaptation process, the model was extended to liquids such as simple ethers, whose dielectric constants exceed those of the hydrocarbons previously considered and, by introducing the concept of "effective diameter", to square and nearly-square vertical tanks. Mathematical details are provided in an appendix. The information in this article is subject to a broad, comprehensive disclaimer of liability as expressed in Section 8. Tank loading system designs should be based on a recognized Code of Practice with consideration of all relevant requirements and limitations. The purpose of this article is to provide a detailed description of the theoretical considerations behind the new methodology. The article is of general interest since this is the first practical application of a technically-based "electrostatic" model to the tank loading problem. Of particular interest is how electrostatic theory plus empirical data have been reduced to yield a very simple flow velocity expression. The flow rate solutions and tables are the same as those given in the forthcoming IEC 60079-32-1 Technical Specification and are provided to assist other standards-setting agencies wishing to adopt the same guidance. © 2012 American Institute of Chemical Engineers.