Hydrogen is an expensive utility in refineries. To reduce costs and save energy, a hydrogen system should be not only optimally designed but also optimally operated. In this paper, a mathematical model is proposed for determining optimal operation of a hydrogen network Changes in the hydrogen demand of a hydrogen sink are assumed to vary with changes in the operational load. In the proposed model, if there is no connection between a source and a sink, the corresponding flow rate will be set as zero. In this way, the model is a linear programming model. A case study shows that the model is effective and very easy to solve.