[en] Exact and simplified solutions of the equations of motion and continuity for one-dimensional flow through a pipe are considered. For non-stationary processes with changes in the density of the flowing medium the mass velocity becomes a function of space co-ordinates. This effect can be neglected or determined with low efforts and in good approximation for most of the conditions in a pressurized water reactor. In most of the cases, therefore, the expensive exact solution of the linked equations can be renounced. (author)