This work proposes a simplified model and a unified model to predict the critical gas velocities of film reversal based on the conservative momentum principle. The simplified model is expressed by an analytical solution of dimensionless form, while the unified model is presented by an iterative solution. The measured datasets of different pipe angles and diameters from the present and published works were used for model evaluation. The proposed simplified model is recommended to predict the film reversal because it can accurately predict the critical velocity with the total relative errors of 10.18% in the laboratory and 27.03% in the field, outperforming the unified model and published models. The unified model has a total relative error of 11.56% in the laboratory and 34.26% in the field. The contributions of the proposed two models are discussed by model comparisons. The results show that the simplified model integrates the merits of simple form and high accuracy; but, owing to the simplifications, the simplified model is not rigorous in the mechanism. In contrast, the unified model is more rigorous because it considers the interior liquid film viscosity, the droplet entrainment, and the non-uniform film thickness distribution in inclined pipes. The superiority of the unified model is discussed by comparing it with previous models in vertical and inclined pipes. The unified model contributes to understanding the underlying mechanism of film reversal. In general, the proposed simplified model and unified model complement each other to improve the prediction of the film reversal practically and theoretically.