further investigation show that the Limits for line(pipe) lenght are as follows:
Max lenght for simulation to consider pipe as "closed" >0,02mm
Even so: Pipes are generated at shorter lenghts in the model, down to 0.005mm, but at these lenghts they do not seem to be recognized by the simulation software.
Also, when going down from, say, 0,5mm , changing the line lenght to 0,005, the pipe is generated, but once you go below this value and the pipe is not generated, you have to put in a larger value to force the pipe to begenerated again. By larger, I mean 0,007 for instance.