  Granular flows through pipes show interesting phenomena, e.g. clogging and
density waves, 1/f-noise. These things are fairly good studied by
computer-experiments, but there is a lack in theoretical and analytical
consideration. We introduce a simple "minimal" model describing such a flow of
granular particles and examine the stability of an initially homogeneous system
against perturbations. In order to define the collisions between the granular
particles we use two different approaches. For both, the simple and the more
advanced collision definition, the model shows the qualitative same behaviour.
