A simplified model nonlinear describing the stable functioning of the single branch PHP is proposed. It is based on the earlier proposed film evaporation-condensation approach and assumes harmonic oscillations of the meniscus displacement and vapor pressure. The model predicts the frequency as a function of the oscillation amplitude and the relationship between the amplitudes of meniscus displacement and pressure. Results of the model are compared to available in the literature experimental data of different groups; they show a good agreement. The added mass caused by the oscillating flow in the liquid plug is discussed.