Current approaches to evaluating the risk of electrostatic discharge rely only on the flow regime (API/RP 2003 and NFPA 77) 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 does not quantify the risk and can be overly conservative. Moreover, mitigation methods to avoid a mist regime are difficult to practically implement.
For these reasons, the NIC is developing a multi-physics modelling-based procedure that quantifies the risk of electrostatic discharge in non-metallic pipes containing flows of gases, mists and particulates. Initial finite element models in COMSOL Multiphysics® v5.5 have extended the work of Walmsley (Walmsley, 1996) to consider aerosol flow regimes and used the work of Matsusaka (Matsusaka, 2010) to consider the charging behaviour of sand particles.
The initial COMSOL models use the Pipe Flow Module to evaluate the flow velocity in the pipe and the AC/DC Module was used to investigate the charging behaviour (see Figure 1). The Particle Tracing Module was used to investigate the behaviour of sand particles (Figure 2) in the fluid flow.
The approach was validated in laboratory conditions for gas flows using a bespoke rig and the models were then used to assess real scenarios from the field. It is now being used by the oil and gas industry to provide risk assessments and optimise designs for safety. Currently, more extensive experimental validation work of the models is being undertaken to consider much higher gas velocities and the effect of water and sand flow rates through industry-wide used pipes (Figure 3). The final model will then be packaged in a user-friendly numerical tool to be used for risk assessment and decision making.
Upon the completion of the current models enhancement at NIC, the models can then be used to compare different ESD mitigation methods, for example, thickening of the pipe wall, choice of pipe material, presence of conductive coatings and additives and the location of grounding points, to allow the choice of the optimum piping and operating conditions to ensure ESD does not occur.