The 3D model of conjugate heat transfer from a fire to compressed gas storage cylinder is described. The model predictions of temperature outside and inside the cylinder as well as pressure increase during a fire are compared against a fire test experiment. The simulation reproduced measured in test temperatures and pressures. The original failure criterion of the cylinder in a fire has been applied in the model. This allowed for the prediction of the cylinder catastrophic rupture time with acceptable engineering accuracy. The significance of 3D modelling is demonstrated, and recommendations to improve safety of high-pressure composite tanks are given.