Abstract

The transport of natural gas and particulates through non-metallic pipes could lead to the accumulation of static charge on the inner surface of the pipe. Due to the non-conductive nature of the pipe walls, this charge is not dissipated and may create a significant risk of explosion, damage, and injury to persons should it exceed a certain limit and discharge suddenly. Moreover, if the charge conditions across the pipe wall result in an electric field which exceeds the dielectric strength of the pipe material, then the subsequent discharge can melt a hole through the pipe wall, a phenomenon known as pin-holing. This risk has to be properly quantified and mitigated in order to ensure safe utilisation of non-metallic pipes in natural gas service. Current approaches to evaluating the risk of electrostatic discharge rely only on the determination of the flow regime (API/RP 2003 and NFPA 77), often using analytical approximations (e.g. Baker and Mandhane charts): if a mist regime is present, then the risk of electrostatic discharge is declared high. This approach can be over-conservative and mitigation methods to avoid a mist flow regime can be difficult to implement. Instead, in this work, a modelling approach combining heat transfer, computational fluid dynamics, and electrostatics has been developed to provide a quantitative assessment of the risk of electrostatic charge build-up in composite pipes used in natural gas transportation. The modelling approach consists of three levels which become progressively less conservative and the models more detailed. The modelling approach has been validated in laboratory conditions to demonstrate its efficacy and used on real case scenarios from the field. Future developments to the model will look at validating the models over a broader set of conditions and validating the impact of sand particle concentration on the electrostatic charge developed on the non-metallic pipe.