The Hamma (Algeria) desalination plant, briefly described in the paper, was mathematically modelled in order to check its behaviour in ail advanced phase of its design. The plant will employ 288 ERI's PX units to recover the pressure of the brine discarded from the RO membranes. This way of recovering energy is one of the most advanced and efficient now available, and its functioning is also briefly described in the paper. The devices' behaviour in unsteady flow conditions was checked by tests carried out in the hydraulics laboratory of the Politecnico di Milano, Italy. The paper reports on the experimental plant and the results obtained. On the basis of the experimental results, which showed an almost perfect hydraulic disconnection between the high and low pressure lines, the mathematical model or the Hamma plant was built using the momentum and continuity equations integrated by means of the characteristic method. The paper describes the mathematical model and sets out the results, which evidence of the plant's safety even under the worst conditions likely to occur.