Abstract Steam accumulators are applied as buffers between steam generators and consumers in cases of different steam production and consumption rates. The steam accumulator is filled with water and steam and it operates with a sliding pressure. Dynamics of the steam accumulator charging and discharging depends on the accumulator volume, initial and boundary process parameters and condensation and evaporation rates. The non-equilibrium numerical model of steam accumulator is developed with the aim of predicting a steam accumulator capacity and as a support to the design of a control system. The model is based on the mass and energy balance equations for each phase and closure laws of phase transitions. The applied methodology is new regarding the commonly used equilibrium approach. The steam accumulator pressure transients are predicted for the case of periodic variable steam supply to a coal drying plant. The influence of the inlet steam enthalpy on the water mass and level change is investigated. The importance of the non-equilibrium evaporation and condensation modelling for the reliable prediction of steam accumulator transients is demonstrated by examples of charging and discharging tests.