In the oil and gas industry, bypass pigs are used to allow part of the production fluids to bypass the pig (Pipeline Inspection Gauge) during a pigging operation as compared to a conventional pig. This has been proven to be beneficial for both cleaning of the pipe and inspection of the pipe wall in both single and multiphase systems. To monitor the propagation of the bypass pig in a pipeline with a ID transient tool (such as OLGA or LedaFlow), the pressure loss coefficient between the bypassing fluid and the pig and the friction coefficient between the pig and the wall needs to be known in advance. The pressure loss coefficient in these tools typically ranges from 1-1.5, and is modelled without taking into account the specific geometry that is used to create the bypass area of the pig. The present CFD study is focused on finding an appropriate value for the pressure loss coefficient. For many bypass pig configurations a deflector plate is attached at the exit of the orifice of the bypass pig. This deflector plate constitutes of a circular disk which is mounted at a specified distance from the bypass pig opening which ensures that the pig gets into motion in the launcher and can help in distributing corrosion inhibitors at the top of the pipeline. In fact by changing the space between the deflector plate and the front of the orifice the bypass opening can be set before launching the pig. In this study the effect of the deflector plate on the flow through the bypass pig in a single phase system is investigated using CFD (Fluent version 14.5). An axisymmetric framework is used in which the bypass pig is assumed to move at a constant velocity. It is found that the pressure loss coefficient of the pig can be as high as 4, which is in contrast to a value of 1 - 1.5 that is commonly used in industry for bypass pigs without a deflector plate. This has significant practical implications as the driving force related to the pressure loss coefficient (which is counteracted by wall friction) determines the travel velocity of the pig through the pipeline. With the help of the obtained CFD results for various parameter ranges (such as the Reynolds number and the bypass opening) existing correlations for pressure loss coefficients used for bypass pigs are reviewed and their range of applicability is discussed for bypass pigs with and without a deflector disk. These results can be used to improve ID modelling tools for bypass pigging.@en